Coalescence-induced jumping of droplets on superomniphobic surfaces with macrotexture

Damping the jump of coalescing droplets through substrate compliance

Sessile droplets coalescing on superhydrophobic surfaces result in spontaneous droplet jumping. Here, through coalescence experiments and fluid-structure interaction simulations for microliter droplets, we demonstrate that such droplet jumping can be damped if the underlying substrate is designed to be compliant. We show that a compliant superhydrophobic substrate with synergistic combinations of low stiffness and inertia deforms rapidly during the coalescence process to minimize the substrate reaction, thus diminishing the jumping velocity. A spring-mass system model for coalescing water droplets is proposed that successfully captures droplet motion and substrate deformation for a wide range of compliant superhydrophobic substrates. These insights can be leveraged to improve the process efficiency in multiple applications, such as designing compliant superhydrophobic substrates for minimizing the scattering of small, nanoliter-sized droplets during atmospheric water harvesting. Lastly, experiments on an exemplar butterfly wing show that droplet jumping velocity reduction can also manifest on natural superhydrophobic substrates due to their inherent compliance.

Enhancement and Predictable Guidance of Coalescence-Induced Droplet Jumping on V-Shaped Superhydrophobic Surfaces with a Ridge

Coalescence-induced droplet jumping has attracted significant attention in recent years. However, achieving a high jumping velocity while predictably regulating the jumping direction of the merged droplets by simple superhydrophobic structures remains a challenge. In this work, a novel V-shaped superhydrophobic surface with a ridge is conceived for enhanced and predictably guided coalescence-induced droplet jumping. By conducting experiments and lattice Boltzmann simulations, it is found that the presence of a ridge in the V-shaped superhydrophobic surface can modify the fluid dynamics during the droplet coalescence process, resulting in a much higher droplet jumping velocity than that achieved by the V-shaped superhydrophobic surface without a ridge. The enhancement of the droplet jumping velocity is mainly attributed to the combined effect of the earlier and more sufficient impingement between the liquid bridge and the ridge, as well as the accelerated droplet contraction by redirecting the internal liquid flow toward the jumping direction. A high normalized jumping velocity of V j * ≈ 0.71 is achieved by the newly designed surface, with a 930% increase in the energy conversion efficiency in comparison with that on a flat surface. Moreover, adjusting the opening direction of the V-groove at different groove angles is found to be an effective method to regulate the droplet jumping direction and expand the range of the jumping angle. Particularly, the droplet jumping angle can be well predicted based on the rotational angle (ω) and the groove angle (α), i.e., θj,p ≈ 90° - 0.5α - ω.

Injectable 2D Material-Based Sensor Array for Minimally Invasive Neural Implants

Intracranial implants for diagnosis and treatment of brain diseases have been developed over the past few decades. However, the platform of conventional implantable devices still relies on invasive probes and bulky sensors in conjunction with large-area craniotomy and provides only limited biometric information. Here, an implantable multi-modal sensor array that can be injected through a small hole in the skull and inherently spread out for conformal contact with the cortical surface is reported. The injectable sensor array, composed of graphene multi-channel electrodes for neural recording and electrical stimulation and MoS2-based sensors for monitoring intracranial temperature and pressure, is designed based on a mesh structure whose elastic restoring force enables the contracted device to spread out. It is demonstrated that the sensor array injected into a rabbit's head can detect epileptic discharges on the surface of the cortex and mitigate it by electrical stimulation while monitoring both intracranial temperature and pressure. This method provides good potential for implanting a variety of functional devices via minimally invasive surgery.

Hierarchical Structure with Microcrater Covered with Nanograss Enhancing Condensation and Its Antifrosting/Anti-Icing Performance Inspired by Euphorbia helioscopia L

Over an extended period of evolution and natural selection, a multitude of species developed a diverse array of biological interface features with specific functions. These biological structures provide a rich source of inspiration for the design of bionic structures on superhydrophobic surfaces. Understanding the functional mechanism of plant leaves is of paramount importance for the advancement of new engineering materials and the further promotion of engineering applications of bionic research. The hierarchical structure of microcrater-covered nanograss (MCNG) on the surface of E. helioscopia L. leaf provided the inspiration for the bionic MCNG surface, which was successfully prepared on a copper substrate by hybrid laser micromachining technology and chemical etching. The combined action of texture structure and surface chemistry resulted in a contact angle of 169° ± 1° for MCNG surface droplets and a rolling angle of less than 1°. Notably, the condensation-induced adhesion force does not augment with the increase of the temperature difference, which facilitated the shedding of hot droplets from the surface. The microscope observation revealed a high density of condensed droplets on the MCNG surface and the tangible jumping behavior of the droplets. The fabricated MCNG also demonstrated excellent antifrost/anti-icing abilities in low-temperature and high-humidity environments. Finally, the study confirmed the exceptional mechanical durability and reusability of the MCNG surface through various tests, including scratch damage, sandpaper wear, water flow impact and flushing, and condensation-drying cycle tests. The nanograss can be effectively protected within the microcrater structure. This research presents a promising approach for preventing and/or removing unwanted droplets in numerous engineering applications.

Coalescence-Induced Self-Propelled Particle Transport with Asymmetry Arrangement

We experimentally investigated the coalescence-induced droplet-particle jumping phenomenon on a submillimeter scale in symmetric and asymmetric particle arrangements with poly(methyl methacrylate) (PMMA) particles and stainless steel (SS) particles. Coalescence-induced droplet-particle jumping exhibited excellent capability and interesting behavior for both droplet jumping enhancement and particle transport. The particle increased the normalized droplet jumping velocity from 0.250 for no particle case to 0.315 and 0.320 for symmetric and asymmetric particle cases. Compared with similar-sized macrostructures fixed between droplets, better jumping performance with particles may be attributed to avoiding the work of adhesion during droplet-macrostructure separation. Besides, all particles always sunk at the bottom in the symmetric cases, while the stick mode for PMMA particles and sink, wander, and jet modes for SS particles appeared in the asymmetry cases. We revealed that the asymmetric particle arrangement induces an unbalanced surface tension force, which may provide a driving force in the vertical direction. Additionally, a small enough resistive force caused by hydrophobic particles is another necessary condition for the wonder and jet mode. Finally, we realized a significantly superior particle transport in the asymmetric SS particle cases with maximum particle height reaching ∼2.1 mm, ∼12.4 times the particle radius, the most significant vertical self-propelled transport distance currently.

Uniform and Persistent Jumping Detachment of Condensed Nanodroplets

Realizing jumping detachment of condensed droplets from solid surfaces at the smallest sizes possible is vital for applications such as antifogging/frosting and heat transfer. For instance, if droplets uniformly jump at sizes smaller than visible light wavelengths of 400-720 nm, antifogging issues could be resolved. In comparison, the smallest droplets experimentally observed so far to jump uniformly were around 16 μm in radius. Here, we show molecular dynamics (MD) simulations of persistent droplet jumping with a uniform radius down to only 3.6 nm on superhydrophobic thin-walled lattice (TWL) nanostructures integrated with superhydrophilic nanospots. The size cutoff is attributed to the preferential cross-lattice coalescence of island droplets. As an application, the MD results exhibit a 10× boost in the heat transfer coefficient (HTC), showing a -1 scaling law with the maximum droplet radius. We provide phase diagrams for jumping and wetting behaviors to guide the design of lattice structures with advanced antidew performance.

Recent progress in understanding the anti-icing behavior of materials

Despite the significant progress in fundamental research in the physics of atmospheric icing or the revolutionary changes in modern materials and coatings achieved due to the recent development of nanotechnology and synthetic chemistry, the problem of reliable protection against atmospheric icing remains a hot topic of surface science. In this paper, we present a brief analysis of the mechanisms of anti-icing behavior that attracted the greatest interest of the scientific community and approaches which realize these mechanisms. We also note the strengths and weaknesses of such approaches and discuss future studies and prospects for the practical application of developed coatings.

Coalescence-Induced Droplet Jumping on Superhydrophobic Surfaces with Annular Wedge-Shaped Micropillar Arrays

The coalescence-induced droplet jumping on superhydrophobic surfaces has extensive application potential in water harvesting, thermal management of electronic devices, and microfluidics. The rational design of the surface structure can influence the interaction between the droplet and the surface, thereby controlling the velocity and direction of the droplet's jumping. In this study, we fabricate the superhydrophobic surface with annular wedge-shaped micropillar arrays, examine the dynamic behavior of condensate droplets on the surface, and measure the temporal and spatial variations of droplet density, average radius, and surface coverage with wedge-shaped micropillars of varying sizes. In addition, the energy analysis of the coalescence-induced droplet jumping reveals that the two primary factors influencing the jumping are the relative size and position of the droplets and micropillars. Further numerical simulations find that the wedge-shaped micropillars cause an asymmetric distribution of pressure within the droplet and at the solid-liquid contact surface, which generates an unbalanced force driving the droplet in the gradient direction of the wedge-shaped micropillar, causing the droplet to jump off the surface with both vertical and gradient-direction velocities. The capacity of the wedge-shaped micropillar surface to transport droplets in the gradient direction increases and then decreases as the relative size of the droplets and micropillars increases. The relative position of the droplet center-of-mass line perpendicular to the bottom edge of the wedge micropillars' trapezoidal shape is more favorable for droplet transport. This work reveals the influence mechanism of surface structure on the velocity and direction of droplet jumping, and the results can guide the microstructure design of superhydrophobic surfaces, which has significant implications for the application of droplet jumping.

Damage Tolerance of Superhydrophobic Coatings with Binary Cooperative Cells for Water Harvesting

Multifunction superhydrophobic coatings that facilitate water harvesting are attractive for addressing the daunting water crisis, yet, they are caught in a double bind when their durability is considered, as durable coatings will require both tough micro-textures to survive concentrated stress and high-surface-energy chemistry to form chemical bonds within the matrix. To date, a universal bulk-phase coating that combines multifunctionality, ultra-durability, and fabrication feasibility remains challenging. Here, a binary cooperative cell design is reported that can solve the contradiction between the multifunctionality and durability requirements of superhydrophobic coatings. In this strategy, mechanochemically tailored cells with releasable nanoseeds are infused in the common matrix, which serves both as a versatile chemical bridge to achieve strong bonds within the coating building blocks, and as an instantaneous self-repairing generator to improve durability. Such a strategy significantly boosted the wear resistance and outdoor stability of the coatings by over 30-100 and 18 folds, respectively, compared with conventional coatings. The coating is applied to the sustainable application, i.e., enhancing the water collection efficiency by at least 1000% even after harsh abrasion. The strategy will broaden the vision in handling the dilemma properties among functional coatings and promote the application of superhydrophobic coatings in extreme environments.

Ultra-durable superhydrophobic cellular coatings

Developing versatile, scalable, and durable coatings that resist the accretion of matters (liquid, vapor, and solid phases) in various operating environments is important to industrial applications, yet has proven challenging. Here, we report a cellular coating that imparts liquid-repellence, vapor-imperviousness, and solid-shedding capabilities without the need for complicated structures and fabrication processes. The key lies in designing basic cells consisting of rigid microshells and releasable nanoseeds, which together serve as a rigid shield and a bridge that chemically bonds with matrix and substrate. The durability and strong resistance to accretion of different matters of our cellular coating are evidenced by strong anti-abrasion, enhanced anti-corrosion against saltwater over 1000 h, and maintaining dry in complicated phase change conditions. The cells can be impregnated into diverse matrixes for facile mass production through scalable spraying. Our strategy provides a generic design blueprint for engineering ultra-durable coatings for a wide range of applications.

Molecularly Capped Omniphobic Polydimethylsiloxane Brushes with Ultra-Fast Contact Line Dynamics

Droplet friction is common and significant in any field where liquids interact with solid surfaces. This study explores the molecular capping of surface-tethered, liquid-like polydimethylsiloxane (PDMS) brushes and its substantial effect on droplet friction and liquid repellency. By exchanging polymer chain terminal silanol groups for methyls using a single-step vapor phase reaction, the contact line relaxation time is decreased by three orders of magnitude-from seconds to milliseconds. This leads to a substantial reduction in the static and kinetic friction of both high- and low-surface tension fluids. Vertical droplet oscillatory imaging confirms the ultra-fast contact line dynamics of capped PDMS brushes, which is corroborated by live contact angle monitoring during fluid flow. This study proposes that truly omniphobic surfaces should not only have very small contact angle hysteresis, but their contact line relaxation time should be significantly shorter than the timescale of their useful application, i.e., a Deborah number less than unity. Capped PDMS brushes that meet these criteria demonstrate complete suppression of the coffee ring effect, excellent anti-fouling behavior, directional droplet transport, increased water harvesting performance, and transparency retention following the evaporation of non-Newtonian fluids.

Freezing Characteristics of a Water Droplet on a Multiscale Superhydrophobic Surface

Superhydrophobic surfaces have the potential to retard ice formation owing to their super water-repellant nature arising from high static contact angle and low contact angle hysteresis. Most of the previous studies have focused on patterned surfaces with mono-scaled prismatic structures. In contrast, the freezing behavior on multiscaled rough superhydrophobic surfaces that are of practical significance is relatively little studied. This article presents, for the first time, the freezing dynamics of a water droplet interacting with multiscale fractal superhydrophobic surfaces which validates well with experimental measurements. It is shown that the dual effects of increased contact angle and poor interfacial conduction due to trapped air cavities within the roughness features of the superhydrophobic surface lead to increasing freezing time with increasing surface hydrophobicity, determined as a function of the fractal surface parameters. A comparison of the freezing dynamics of sessile droplets of identical contact angle on a smooth versus a rough superhydrophobic surface shows that interfacial asperity thermal resistance contributes to over 14% increase in the freeze time. It is further shown that by tailoring the multiscale characteristics, the freeze time may be increased by up to 7-fold compared to freezing on a smooth surface. The application of the numerical model to studying ice formation on several practical superhydrophobic surfaces of a range of metallic materials and fabrication methods is also discussed, which offers guidelines for the design of anti-icing surfaces in practice.

A gravity-driven droplet fluidic point-of-care test

We report a simple droplet fluidic point-of-care test (POCT) that uses gravity to manipulate the sequence, timing, and motion of droplets on a surface. To fabricate this POCT, we first developed a surface coating toolbox of nine different coatings with three levels of wettability and three levels of slipperiness that can be independently tailored. We then fabricated a device that has interconnected fluidic elements-pumps, flow resistors and flow guides-on a highly slippery solid surface to precisely control the timing and sequence of motion of multiple droplets and their interactions on the surface. We then used this device to carry out a multi-step enzymatic assay of a clinically relevant analyte-lactate dehydrogenase (LDH)-to demonstrate the application of this technology for point-of-care diagnosis.

Optimum substrate stiffness in coalescence-induced droplet jumping

When droplets are brought into contact and coalesced on a superhydrophobic surface, the kinetic energy converted from the surface energy enables the merged droplet to jump. Current studies mainly focus on the microstructure of surfaces and the properties of droplets that influence the jumping dynamics. Here, by means of molecular dynamics, we investigate the coalescence-induced jumping of nanodroplets on soft substrates. The optimum stiffness of the substrate is suggested and the mechanism involved is demonstrated through the analysis of the interactions between the droplets and the substrates. The momentum of the droplet is evaluated by integrating the forces from the substrate. The optimum stiffness for jumping velocity is provided by the competition between the impact and the adhesion from the substrate during the process, which are both closely related to the stiffness. The results may inspire fundamental research and applications in a broad scope.

Contaminant Removal from Nature's Self-Cleaning Surfaces

Many organisms in nature have evolved superhydrophobic surfaces that leverage water droplets to clean themselves. While this ubiquitous self-cleaning process has substantial industrial promise, experiments have so far been unable to comprehend the underlying physics. With the aid of molecular simulations, here we rationalize and theoretically explain self-cleaning mechanisms by resolving the complex interplay between particle-droplet and particle-surface interactions, which originate at the nanoscale. We present a universal phase diagram that consolidates (a) observations from previous surface self-cleaning experiments conducted at micro-to-millimeter length scales and (b) our nanoscale particle-droplet simulations. Counterintuitively, our analysis shows that an upper limit for the radius of the droplet exists to remove contaminants of a particular size. We are now able to predict when and how particles of varying scale (from nano-to-micrometer) and adhesive strengths are removed from superhydrophobic surfaces.

Synergetic Role of Nano-/Microscale Structures of the Trifolium Leaf Surface for Self-Cleaning Properties

Wetting has an essential pertinence to surface applications. The exemplary water-repelling and self-cleaning surfaces in nature have stimulated considerable scientific exploration, given their practical leverage in cleaning window glasses, painted surfaces, fabrics, and solar cells. Here, we explored the three-tier hierarchical surface structure of the Trifolium leaf with distinguished self-cleaning characteristics. The leaf remains fresh, withstands adverse weather, thrives throughout the year, and self-cleans itself against mud or dust. Self-cleaning features are attributed to a three-tier hierarchical synergetic design. The leaf surface is explicated by an optical microscope, a scanning electron microscope, a three-dimensional profilometer, and a water contact angle measuring device. Hierarchical base roughness (i.e., nano-/microscale) comprises a fascinating arrangement, which imparts a superhydrophobic feature to the surface. As a result, the contaminants present on the leaf surface are washed with rolling water droplets. We noticed that self-cleaning is a function of impacting or rolling droplets, and the rolling mechanism is identified as efficient. The self-cleaning phenomenon is studied for contaminations of variable sizes, shapes, and compositions. The contaminations are supplied in both dry and aqueous mixtures. Furthermore, we examined the self-cleaning effect of the Trifolium leaf surface by atmospheric water harvesting. The captured water drops fuse, roll, descend, and wash away the contaminating particles. The diversity of contaminants investigated makes this study applicable to different environmental conditions. And, along with other parallel technologies, this investigation could be useful for crafting sustainable self-cleaning surfaces for regions with acute water scarcity.

Interfacial Features Govern Nanoscale Jumping Droplets

The solid surfaces with different profile levels impact the liquid-solid contact nature and hence wetting characteristics, showing a vital role in liquid droplets' mobility and dynamic behaviors. Therefore, engineering nanostructured features ultimately enables tuning and controlling the dynamic motion of droplets. In this study, we demonstrate an approach to manipulate nanodroplets' motion behaviors in contact with a surface through tailoring the surface morphological profile. By tracking the trajectories of water molecules at the interface, we find that the motions of a nanodroplet subjected to different levels of lateral force reveal various modes that are identified as creeping, rolling, and jumping motions. Interestingly, the elastic deformation of the droplet and sudden changes in the receding contact angle provide the mechanistic origin for droplet jumping. Guided by computational simulations, a regime map delineating the droplet motion modes with surface profile levels and applied forces is constructed, providing a design strategy for controlling droplet motions via surface engineering.

Wetting state transitions of individual condensed droplets on pillared textured surfaces

The ability to realize the self-removal of condensed droplets from a surface is of critical importance for science and applications such as water harvesting and thermal engineering. Despite the enormous interest in micro/nanotextured superhydrophobic materials for high-efficiency condensation, a clear picture of the wetting state transition of condensed droplets is missing, particularly, on a single-droplet level of the order of micrometers. Herein, by varying a substantial parameter space of the contact angle and the geometry of the pillared textures, we have quantified the wetting transition of individual droplets during condensation. We found that a droplet is finally either spontaneously removed from the textures due to a Laplace pressure difference or wets the textures; four different wetting state transition modes have been identified numerically and they are classified in a phase diagram. Simple theories have been constructed to correlate the critical conditions of the wetting state transition to the wettability and geometry of the textures, and they were verified experimentally. We found that the self-removal of condensed droplets benefits from the contact angle and the height of the pillars. These findings not only enhance our fundamental understanding of the wetting state transition of condensed droplets but also allow the rational design of micro/nanotextured water-repellent materials for anti-fogging and anti-wetting.

Nano-droplet jumping due to surface wettability variation: molecular modeling approach

The effect of substrate hydrophobicity on the dynamics of water nano-droplets is computationally studied using the molecular dynamics method. It is observed that the droplet moves upward by increasing the substrate hydrophobicity and then falls down onto the substrate, and during this process, the droplet-substrate interfacial area effectively declines. Moreover, the results indicate that above a critical substrate hydrophobicity, which depends on the droplet radius, the droplet completely detaches from the substrate. The droplet behavior is described at the molecular level using van der Waals attractions for the liquid-solid interface.

On-demand, remote and lossless manipulation of biofluid droplets

The recent global outbreaks of epidemics and pandemics have shown us that we are severely under-prepared to cope with infectious agents. Exposure to infectious agents present in biofluids (e.g., blood, saliva, urine etc.) poses a severe risk to clinical laboratory personnel and healthcare workers, resulting in hundreds of millions of hospital-acquired and laboratory-acquired infections annually. Novel technologies that can minimize human exposure through remote and automated handling of infectious biofluids will mitigate such risk. In this work, we present biofluid manipulators, which allow on-demand, remote and lossless manipulation of virtually any liquid droplet. Our manipulators are designed by integrating thermo-responsive soft actuators with superomniphobic surfaces. Utilizing our manipulators, we demonstrate on-demand, remote and lossless manipulation of biofluid droplets. We envision that our biofluid manipulators will not only reduce manual operations and minimize exposure to infectious agents, but also pave the way for developing inexpensive, simple and portable robotic systems, which can allow point-of-care operations, particularly in developing nations.

Numerical Investigation on Coalescence-Induced Jumping of Centripetal Moving Droplets

Coalescence-induced droplet jumping could promote self-removal of droplets, which has broad potential in related fields such as heat-transfer enhancement, self-cleaning, energy harvesting, electricity generation, radiative cooling, and antifrosting/icing. In practical applications, droplets often have initial velocity under external forces. In this work, the coalescence-induced jumping of centripetal moving droplets on a superhydrophobic plane is experimentally observed using a high-speed photography platform, and the effects of the initial velocity of the moving droplet on jumping velocity, energy conversion, and droplet morphology are numerically investigated. Results show that the jumping velocity decreases and then increases as the We number of the moving droplet increases. The main source of the total kinetic energy of the coalesced droplet switches from the released surface energy to the initial kinetic energy of the moving droplet with the increasing We number, but the proportion of the jumping kinetic energy to the total kinetic energy decreases. Besides, the initial velocity of the moving droplet intensifies the droplet deformation and accelerates the process of coalescence-induced jumping. Through theoretical analysis, it is found that the jumping velocity is affected by two mechanisms: the deformation intensification and the liquid bridge impact enhancement.

Coalescence-Induced Jumping for Removing the Deposited Heterogeneous Droplets: A Molecular Dynamics Simulation Study

The removal of the deposited droplets on a solid surface is crucial to considerable practical applications that require self-cleaning properties. In this work, a strategy of cleaning a deposited droplet ("D-droplet") by coalescing with a heterogeneous and easily jumping droplet ("J-droplet") is proposed. Molecular dynamics simulation studies have shown that the coalescence of these two kinds of droplets would not guarantee the removal of D-droplet, unless the lifting ability of J-droplet is enhanced through the reduction of the solid-liquid interaction. However, this is a bad scenario with low efficiency. Further investigation suggests that by introducing two J-droplets to produce triple-coalescence dynamics, the D-droplet could be successfully jumping from the substrates due to the coalescence-induced effect, which is also verified by the free energy calculation. Moreover, the effects of the size of the droplets and the arrangement mode of these three droplets on the jumping dynamics are both considered. The studies not only help advance our understanding of coalescence-induced jumping of heterogeneous droplets, but also open up new ways to remove the deposited impure droplets, which is expected to guide the fields of self-cleaning.

Dynamics of fog droplets on a harp wire

Fog harps effectively drain small droplets, which prevents clogging and results in more water harvested from fog compared to mesh nets. However, the dynamics of fog droplets coalescing and sliding down a vertical wire remain poorly understood. Here, we develop an analytical model that captures the physics of fog droplets draining down a single vertical wire. The driving forces are gravity and the surface energy released from coalescence events, whereas the dominant resisting forces are revealed to be inertia, contact angle hysteresis, and local viscous dissipation within the droplet's receding wedge. The average sliding velocity of fog droplets on a Teflon-coated wire was only half that of an uncoated stainless steel wire, due to non-coalescence events exclusive to the hydrophobic wire disrupting the momentum of droplet sliding.

Condensation droplet sieve

Large droplets emerging during dropwise condensation impair surface properties such as anti-fogging/frosting ability and heat transfer efficiency. How to spontaneously detach massive randomly distributed droplets with controlled sizes has remained a challenge. Herein, we present a solution called condensation droplet sieve, through fabricating microscale thin-walled lattice structures coated with a superhydrophobic layer. Growing droplets were observed to jump off this surface once becoming slightly larger than the lattices. The maximum radius and residual volume of droplets were strictly confined to 16 μm and 3.2 nl/mm² respectively. We reveal that this droplet radius cut off is attributed to the large tolerance of coalescence mismatch for jumping and effective isolation of droplets between neighboring lattices. Our work brings forth a strategy for the design and fabrication of high-performance anti-dew materials.

Spontaneous droplet jumping and control of dropwise condensation are relevant for water-harvesting, heat transfer and anti-frosting applications. The authors design a superhydrophobic surface with microscale thin-walled lattice structure to achieve effective jumping of droplets with specified radius range.

Coalescence-Induced Droplet Jumping on Honeycomb Bionic Superhydrophobic Surfaces

Condensation-induced jumping of droplets on superhydrophobic surfaces has received extensive attention because of its great potential for applications in areas such as condensation enhancement and self-cleaning. However, the jumping efficiency of droplets on flat superhydrophobic surfaces is very low, and there is no reliable means of achieving efficient droplet jumping on large scales, which greatly limits its application. To this end, we developed a class of honeycomb bionic superhydrophobic surfaces (HBSS) that enable reliable and efficient droplet jumping on a large scale for the first time and performed experimental and simulation studies on droplet condensation and jumping on this kind of surface. Condensation experiments show that condensate droplets on HBSS can be effectively positioned under the influence of gravity and the uniformity of the droplet diameter is ensured, laying the foundation for achieving efficient jumping. The shape and geometric parameters of HBSS have a significant impact on the droplet jumping efficiency, and the maximum dimensionless jumping velocity of droplet jumping was experimentally measured to be 0.747, corresponding to an efficiency of about 45.25%. Combining with the results of simulation calculations, we found that the surface structure of HBSS can promote more of the excess surface energy to net upward kinetic energy along an extremely efficient and simple pathway (direct conversion), thus achieving an energy conversion efficiency of over 45%.

Review of droplet dynamics and dropwise condensation enhancement: Theory, experiments and applications

Droplet dynamics and condensation phenomena are widespread in nature and industrial applications, and the fundamentals of various technological applications. Currently, with the rapid development of interfacial materials, microfluidics, micro/nano fabrication technology, as well as the intersection of fluid mechanics, interfacial mechanics, heat and mass transfer, thermodynamics and reaction kinetics and other disciplines, the preparation and design of various novel functional surfaces have contributed to the local modulation of droplets (including nucleation, jumping and directional migration) and the improvement of condensation heat transfer, further deepening the understanding of relevant mechanisms. The wetting and dynamic characteristics of droplets involve complex solid-liquid interfacial interactions, so that the local modulation of microdroplets and the extension of enhanced condensation heat transfer by means of complex micro/nano structures and hydrophilic/hydrophobic properties is one of the current hot topics in heat and mass transfer research. This work presents a detailed review of several scientific issues related to the droplet dynamics and dropwise condensation heat transfer under the influence of multiple factors (including fluid property, surface structure, wettability, temperature external field, etc.). Firstly, the basic theory of droplet wetting on the solid wall is introduced, and the mechanism of solid-liquid interfacial interaction involving droplet jumping and directional migration on the functional surfaces under the various influencing factors is discussed. Optimizing the surface structure for the local modulation of droplets is of guidance for condensation heat transfer. Secondly, we summarize the existing theoretical models of dropwise condensation applicable to various functional surfaces and briefly outline the current numerical models for simulating dropwise condensation at different scales, as well as the fabricating techniques of coatings and functional surfaces for enhancing heat transfer. Finally, the relevant problems and challenges are summarized and future research is discussed.

Laplace Pressure Difference Enhances Droplet Coalescence Jumping on Superhydrophobic Structures

Coalescence-induced droplet jumping has great prospects in many applications. Nevertheless, the applications are vastly limited by a low jumping velocity. Conventional methods to enhance the droplet coalescence jumping velocity are enabled by protruding structures with superhydrophobic surfaces. However, the jumping velocity improvement is limited by the height of protruding structures. Here, we present rationally designed limitation structures with superhydrophobic surfaces to achieve a dimensionless jumping velocity, V_j^* ≈ 0.64. The mechanism of enhancing the jumping velocity is demonstrated through the study of numerical simulations and geometric parameters of limitation structures, providing guidelines for optimized structures. Experimental and numerical results indicate that the mechanism consists of the combined action of the velocity vectors' redirection and the Laplace pressure difference within deformed droplets trapped in limitation structures. On the basis of previous research on the mechanisms of protruding structures and our study, we successfully exploited those mechanisms to further improve the jumping velocity by combining the limitation structure with the protruding structure. Experimentally, we attained a dimensionless jumping velocity of V_j^* ≈ 0.74 with an energy conversion efficiency of η ≈ 48%, breaking the jumping velocity limit. This work not only demonstrates a new mechanism for achieving a high jumping velocity and energy conversion efficiency but also sheds lights on the effect of limitation structures on coalescence hydrodynamics and elucidates a method to further enhance the jumping velocity based on protruding structures.

Waterborne superamphiphobic coatings with network structure for enhancing mechanical durability

Superamphiphobic coatings may significantly change the wettability of a substrate, and so are attractive for applications in aero/marine engineering, biotechnology, and heat transfer. However, the coatings are caught in a double bind when their durability is considered, as they are vulnerable to mechanical abrasion. Meanwhile, the wide use of organic solvents for preparing the coatings generates environmental pollution. Here, we present a waterborne superamphiphobic coating through one-step spraying that repels a wide range of liquids. By tailoring the repellence of the nano-silica to waterborne resin, a network structure is constructed to protect the embedded nano-silica from damage. Thus, the coatings are durable against 725 cycles of friction tester abrasion under a load of 250 g, showing a significant improvement in the mechanical durability by 3-25 times. Moreover, our coating also shows excellent comprehensive durability, including resistance to oil-flow erosion, falling sand impact, chemical attack, thermal treatment, etc. This strategy can be introduced to various waterborne resins, demonstrating its universality, and may offer a new insight to design sustainable superamphiphobic coatings for long-term practical applications.

Exploring the Role of Initial Droplet Position in Coalescence-Induced Droplet Jumping: Lattice Boltzmann Simulations

Coalescence-induced droplet jumping on superhydrophobic surfaces with different initial positions was numerically simulated using the 2D multi-relaxation-time (MRT) Lattice Boltzmann method (LBM). Simulation results show that for coalesced droplets with radii close to the structure length scale, the change of initial droplet positions leads to a significant deviation of jumping velocity and direction. By finely tuning the initial droplet positions on a flat-pillared surface, perpendicular jumping, oblique jumping, and non-jumping are successively observed on the same structured surface. Droplet morphologies and vector diagrams at different moments are considered. It is revealed that the asymmetric droplet detachment from the structured surface leads to the directional transport of liquid mass in the droplet and further results in the oblique jumping of the coalesced droplet. In order to eliminate the influence of initial droplet position on droplet jumping probability, a surface with pointed micropillars is designed. It is demonstrated that compared to flat-topped micropillars, a surface with pointed micropillars can suppress the initial droplet position effects and enhance droplet jumping probability. Furthermore, the effect of droplet/structure scale on droplet jumping is investigated. The influence of initial positions on coalescence-induced droplet jumping from the refined surface can be ignored when the droplet scale is larger than three times the structure scale. This study illustrates the role of initial droplet position in coalescence-induced droplet jumping and provides guidelines for the rational design of structured surfaces with enhanced droplet self-shedding for energy and heat transfer applications.

How Coalescing Bubbles Depart from a Wall

Bubble evolution plays a fundamental role in boiling and gas-evolving electrochemical systems. One key stage is bubble departure, which is traditionally considered to be buoyancy-driven. However, conventional understanding cannot provide the full physical picture, especially for departure events with small bubble sizes commonly observed in water splitting and high heat flux boiling experiments. Here, we report a new regime of bubble departure owing to the coalescence of two bubbles, where the departure diameter can be much smaller than the conventional buoyancy limit. We show the significant reduction of the bubble base area due to the dynamics of the three-phase contact line during coalescence, which promotes bubble departure. More importantly, combining buoyancy-driven and coalescence-induced bubble departure modes, we demonstrate a unified relationship between the departure diameter and nucleation site density. By elucidating how coalescing bubbles depart from a wall, our work provides design guidelines for energy systems which can largely benefit from efficient bubble departure.

How Superhydrophobic Grooves Drive Single-Droplet Jumping

Rapid shedding of microdroplets enhances the performance of self-cleaning, anti-icing, water-harvesting, and condensation heat-transfer surfaces. Coalescence-induced droplet jumping represents one of the most efficient microdroplet shedding approaches and is fundamentally limited by weak fluid-substrate dynamics, resulting in a departure velocity smaller than 0.3u, where u is the capillary-inertia-scaled droplet velocity. Laplace pressure-driven single-droplet jumping from rationally designed superhydrophobic grooves has been shown to break conventional capillary-inertia energy transfer paradigms by squeezing and launching single droplets independent of coalescence. However, this interesting droplet shedding mechanism remains poorly understood. Here, we investigate single-droplet jumping from superhydrophobic grooves by examining its dependence upon surface and droplet configurations. Using a volume of fluid (VOF) simulation framework benchmarked with optical visualizations, we verify the Laplace pressure contrast established within the groove-confined droplet that governs single-droplet jumping. An optimal departure velocity of 1.13u is achieved, well beyond what is currently available using condensation on homogeneous or hierarchical superhydrophobic structures. We further develop a jumping/non-jumping regime map in terms of surface wettability and initial droplet volume and demonstrate directional jumping under asymmetric confinement. Our work reveals key fluid-structure interactions required for the tuning of droplet jumping dynamics and guides the design of interfaces and materials for enhanced microdroplet shedding for a plethora of applications.

High-Efficiency Directional Ejection of Coalesced Drops on a Circular Groove

Coalescence-induced drop jumping has received significant attention in the past decade. However, its application remains challenging as a result of the low energy conversion efficiency and uncontrollable drop jumping direction. In this work, we report the high-efficiency coalescence-induced drop jumping with tunable jumping direction via rationally designed millimeter-sized circular grooves. By increasing the surface-droplet impact site area and restricting the oscillatory deformation, the energy conversion efficiency of the jumping droplet reaches 43.5%, 600% as high as the conventional superhydrophobic surfaces. The droplet jumping direction can be tuned from 90° to 60° by varying the principal curvature of the circular groove, while the energy conversion efficiency remains unchanged. We show through theoretical analysis and numerical simulations that the directional jumping mainly originates from reallocation of droplet momentum enabled by the asymmetric liquid bridge impact. Our study demonstrates a simple yet effective method for fast, efficient, and directional droplet removal, which warrants promising applications in jumping droplet condensation, water harvesting, anti-icing, and self-cleaning.

A Deep Learning Perspective on Dropwise Condensation

Condensation is ubiquitous in nature and industry. Heterogeneous condensation on surfaces is typified by the continuous cycle of droplet nucleation, growth, and departure. Central to the mechanistic understanding of the thermofluidic processes governing condensation is the rapid and high-fidelity extraction of interpretable physical descriptors from the highly transient droplet population. However, extracting quantifiable measures out of dynamic objects with conventional imaging technologies poses a challenge to researchers. Here, an intelligent vision-based framework is demonstrated that unites classical thermofluidic imaging techniques with deep learning to fundamentally address this challenge. The deep learning framework can autonomously harness physical descriptors and quantify thermal performance at extreme spatio-temporal resolutions of 300 nm and 200 ms, respectively. The data-centric analysis conclusively shows that contrary to classical understanding, the overall condensation performance is governed by a key tradeoff between heat transfer rate per individual droplet and droplet population density. The vision-based approach presents a powerful tool for the study of not only phase-change processes but also any nucleation-based process within and beyond the thermal science community through the harnessing of big data.

Enhancement and Guidance of Coalescence-Induced Jumping of Droplets on Superhydrophobic Surfaces with a U-Groove

Coalescence-induced droplet jumping has received considerable attention owing to its potential to enhance performance in various applications. However, the energy conversion efficiency of droplet coalescence jumping is very low and the jumping direction is uncontrollable, which vastly limits the application of droplet coalescence jumping. In this work, we used superhydrophobic surfaces with a U-groove to experimentally achieve a high dimensionless jumping velocity V_j^* ≈ 0.70, with an energy conversion efficiency η ≈ 43%, about a 900% increase in energy conversion efficiency compared to droplet coalescence jumping on flat superhydrophobic surfaces. Numerical simulation and experimental data indicated that a higher jumping velocity arises from the redirection of in-plane velocity vectors to out-of-plane velocity vectors, which is a joint effect resulting from the redirection of velocity vectors in the coalescence direction and the redirection of velocity vectors of the liquid bridge by limiting maximum deformation of the liquid bridge. Furthermore, the jumping direction of merged droplets could be easily controlled ranging from 17 to 90^° by adjusting the opening direction of the U-groove, with a jumping velocity V_j^* ≥ 0.70. When the opening direction is 60^°, the jumping direction shows a deviation as low as 17^° from the horizontal surface with a jumping velocity V_j^* ≈ 0.73 and corresponding energy conversion efficiency η ≈ 46%. This work not only improves jumping velocity and energy conversion efficiency but also demonstrates the effect of the U-groove on coalescence dynamics and demonstrates a method to further control the droplet jumping direction for enhanced performance in applications.

Suppressing the Universal Occurrence of Microscopic Liquid Residues on Super-Liquid-Repellent Surfaces

Super-liquid-repellent (SLR) surfaces based on surface micro/nanotextures are generally regarded as "non-wettable", though careful examination shows that residual microdroplets remain atop surface textures upon drop shedding-off. Despite its great importance, the origin of microscopic liquid residues remains poorly explored, and how to suppress residue formation is an open question. Herein, on the basis of high-speed microscopic imaging and numerical simulations, we resolve the fast formation dynamics of liquid residues on micropillared SLR surfaces and show that the competition of contact line receding on micropillars and the pinch-off of microcapillary bridges governs residue formation. The local receding angle can temporarily reduce to be drastically lower than the intrinsic one accompanying occurrence of accelerated contact line receding, inevitably leading to capillary bridge pinch-off and residue formation. We further show a liquid-like coating can delay capillary bridge pinch-off and reduce residue volume on SLR surfaces by more than 80% compared to those with conventional perfluoroalkylsilane coatings.

Effect of a Superhydrophobic Surface Structure on Droplet Jumping Velocity

The coalescence-induced droplet jumping on superhydrophobic surfaces is fundamentally significant from an academic or practical viewpoint. However, approaches to enhance droplet jumping velocity are very limited. In this work, the effect of structural parameters of the triangular prism on droplet jumping is studied systematically. The results indicate that droplet jumping velocity can be greatly increased by exploiting structure effects, which is a promising reinforcement method. When the height and apex angle of the triangular prism are fixed, the droplet jumping velocity increases with the length of the triangular prism until a plateau is reached. The ratio of translational kinetic energy to released surface energy during droplet jumping is determined by the apex angle and the height of the triangular prism, which is more effective with a smaller apex angle and a larger height. The results are supposed to provide guidelines for optimization of superhydrophobic surfaces.

Coalescence-Induced Droplet Jumping

When two or more droplets coalesce on a superhydrophobic surface, the merged droplet can jump spontaneously from the surface without requiring any external energy. This phenomenon is defined as coalescence-induced droplet jumping and has received significant attention due to its potential applications in a variety of self-cleaning, anti-icing, antifrosting, and condensation heat-transfer enhancement uses. This article reviews the research and applications of coalescence-induced droplet jumping behavior in recent years, including the influence of droplet parameters on coalescence-induced droplet jumping, such as the droplet size, number, and initial velocity, to name a few. The main structure types and influence mechanism of the superhydrophobic substrates for coalescence-induced droplet jumping are described, and the potential application areas of coalescence-induced droplet jumping are summarized and forecasted.

Triple-Scale Superhydrophobic Surface with Excellent Anti-Icing and Icephobic Performance via Ultrafast Laser Hybrid Fabrication

Passive anti-icing or icephobic superhydrophobic surfaces have attracted great interest due to their potential multifaceted implications for the prevention and/or easy removal of undesired ice in many applications. However, a superhydrophobic surface with both excellent anti-icing and icephobic performances has rarely been reported due to difficulties in sustaining a good Cassie state stability. This is the case especially under high humidity and freezing environment conditions. In the present study, a new triple-scale micro/nanostructured superhydrophobic surface with both excellent anti-icing and icephobic properties has been designed via a hybrid method, combining ultrafast laser ablation and chemical oxidation. The novel surface structure is composed of periodical microcone arrays covered with densely grown nanograsses and dispersedly distributed microflowers. This surface exhibits an excellent Cassie state stability with its critical Laplace pressure reaching up to 1450 Pa, which is essential for good anti-icing and icephobic performances. The anti-icing feature of the prepared superhydrophobic surface is achieved by a rapid rolling-off of the impacting droplets. Moreover, an excellent resistance to the impact of high humidity has been achieved via hierarchical condensation, coalescence-induced jumping, and upward moving. A good delay of the heterogeneous nucleation at the solid-liquid interface under freezing condition has been registered as well, due to the presence of stable air pockets within the surface structures. In addition, the ice adhesion strength of the prepared superhydrophobic surface can be as low as 1.7 kPa, which is the lowest value when compared with the state-of-the-art superhydrophobic surfaces. Such a low ice adhesion strength allows the ice to be easily removed by its own weight and demonstrates an excellent icephobic performance. The repeated icing-deicing tests indicate a decent deicing robustness of the synthesized superhydrophobic surface. Thus, this triple-scale superhydrophobic surface exhibits a good anti-icing and icephobic performance with an excellent Cassie state stability, high humidity resistance, and good deicing durability. We hypothesize that the proposed fabrication strategy and associated basic findings will shed new light on the design of robust ice-resistant superhydrophobic surfaces and contribute to a better understanding of the relationship between superhydrophobicity and ice resistance.

Ultimate jumping of coalesced droplets on superhydrophobic surfaces

HYPOTHESIS

Jumping of coalesced droplets on superhydrophobic surfaces (SHSs) is widely used for enhanced condensation, anti-icing/frosting, and self-cleaning due to its superior droplet transport capability. However, because only a tiny fraction (about 5%) of the released excess surface energy during coalescence can be transformed into jumping kinetic energy, the jumping is very weak, limiting its application.

METHODS

We experimentally propose enhanced jumping methods, use machine learning to design structures that achieve ultimate jumping, and finally combine experiments and simulations to investigate the mechanism of the enhanced jumping.

FINDING

We find that a more orderly flow inside the droplets through the structure is the key to improve energy transfer efficiency and that the egg tray-like structure enables the droplet to jump with an energy transfer efficiency 10.6 times higher than that of jumping on flat surfaces. This energy transfer efficiency is very close to the theoretical limit, i.e., almost all the released excess surface energy is transformed into jumping kinetic energy after overcoming viscous dissipation. The ultimate jumping enhances the application of water droplet jumping and enables other low surface energy fluid such as R22, R134a, Gasoline, and Ethanol, which cannot jump on a flat surface, to jump.

Controlling the Jumping Angle of Coalescing Droplets Using Surface Structures

The jumping direction is an essential characteristic of jumping droplets, but it is poorly understood and uncontrollable at present. In this work, we present a method to control the jumping direction by surface structures, where the jumping direction is controlled by changing the inclination angle of the structure. The underlying mechanism is analyzed experimentally, with numerical simulations, and using a theoretical model developed to relate the jumping direction and the inclination angle for a few cases with a specific distribution. Because random droplet distributions are more common on actual condensation surfaces, a more comprehensive prediction model was developed based on a convolution neural network (CNN) to predict the jumping direction for more general cases. The input to the CNN is an image of droplets with various distribution features, which are detected by the neural network and used to predict the jumping angle. SHapley Additive exPlanations methods were then used to analyze the feature importance and to give human-understandable insights from the prediction model. This work offers avenues for improving cooling rates, anti-icing/freezing characteristics, and self-cleaning attributes and contributes to a better understanding of the jumping direction.

Laplace Pressure Driven Single-Droplet Jumping on Structured Surfaces

Droplet transport on, and shedding from, surfaces is ubiquitous in nature and is a key phenomenon governing applications including biofluidics, self-cleaning, anti-icing, water harvesting, and electronics thermal management. Conventional methods to achieve spontaneous droplet shedding enabled by surface-droplet interactions suffer from low droplet transport velocities and energy conversion efficiencies. Here, by spatially confining the growing droplet and enabling relaxation via rationally designed grooves, we achieve single-droplet jumping of micrometer and millimeter droplets with dimensionless jumping velocities v* approaching 0.95, significantly higher than conventional passive approaches such as coalescence-induced droplet jumping (v* ≈ 0.2-0.3). The mechanisms governing single-droplet jumping are elucidated through the study of groove geometry and local pinning, providing guidelines for optimized surface design. We show that rational design of grooves enables flexible control of droplet-jumping velocity, direction, and size via tailoring of local pinning and Laplace pressure differences. We successfully exploit this previously unobserved mechanism as a means for rapid removal of droplets during steam condensation. Our study demonstrates a passive method for fast, efficient, directional, and surface-pinning-tolerant transport and shedding of droplets having micrometer to millimeter length scales.

Molecular physics of jumping nanodroplets

Next-generation processor-chip cooling devices and self-cleaning surfaces can be enhanced by a passive process that requires little to no electrical input, through coalescence-induced nanodroplet jumping. Here, we describe the crucial impact thermal capillary waves and ambient gas rarefaction have on enhancing/limiting the jumping speeds of nanodroplets on low adhesion surfaces. By using high-fidelity non-equilibrium molecular dynamics simulations in conjunction with well-resolved volume-of-fluid continuum calculations, we are able to quantify the different dissipation mechanisms that govern nanodroplet jumping at length scales that are currently difficult to access experimentally. We find that interfacial thermal capillary waves contribute to a large statistical spread of nanodroplet jumping speeds that range from 0-30 m s-1, where the typical jumping speeds of micro/millimeter sized droplets are only up to a few m s-1. As the gas surrounding these liquid droplets is no longer in thermodynamic equilibrium, we also show how the reduced external drag leads to increased jumping speeds. This work demonstrates that, in the viscous-dominated regime, the Ohnesorge number and viscosity ratio between the two phases alone are not sufficient, but that the thermal fluctuation number (Th) and the Knudsen number (Kn) are both needed to recover the relevant molecular physics at nanoscales. Our results and analysis suggest that these dimensionless parameters would be relevant for many other free-surface flow processes and applications that operate at the nanoscale.

Breaking Droplet Jumping Energy Conversion Limits with Superhydrophobic Microgrooves

Coalescence-induced droplet jumping has the potential to enhance the performance of a variety of applications including condensation heat transfer, surface self-cleaning, anti-icing, and defrosting to name a few. Here, we study droplet jumping on hierarchical microgrooved and nanostructured smooth superhydrophobic surfaces. We show that the confined microgroove structures play a key role in tailoring droplet coalescence hydrodynamics, which in turn affects the droplet jumping velocity and energy conversion efficiency. We observed self-jumping of individual deformed droplets within microgrooves having maximum surface-to-kinetic energy conversion efficiency of 8%. Furthermore, various coalescence-induced jumping modes were observed on the hierarchical microgrooved superhydrophobic surface. The microgroove structure enabled high droplet jumping velocity (≈0.74U) and energy conversion efficiency (≈46%) by enabling the coalescence of deformed droplets in microgrooves with undeformed droplets on adjacent plateaus. The jumping velocity and energy conversion efficiency enhancements are 1.93× and 6.67× higher than traditional coalescence-induced droplet jumping on smooth superhydrophobic surfaces. This work not only demonstrates high droplet jumping velocity and energy conversion efficiency but also demonstrates the key role played by macroscale structures on coalescence hydrodynamics and elucidates a method to further control droplet jumping physics for a plethora of applications.

Lifting a sessile oil drop from a superamphiphobic surface with an impacting one

Colliding drops are encountered in everyday technologies and natural processes, from combustion engines and commodity sprays to raindrops and cloud formation. The outcome of a collision depends on many factors, including the impact velocity and the degree of alignment, and intrinsic properties like surface tension. Yet, little is known on binary impact dynamics of low-surface-tension drops on a low-wetting surface. We investigate the dynamics of an oil drop impacting an identical sessile drop sitting on a superamphiphobic surface. We observe five rebound scenarios, four of which do not involve coalescence. We describe two previously unexplored cases for sessile drop liftoff, resulting from drop-on-drop impact. Numerical simulations quantitatively reproduce the rebound scenarios and enable quantification of velocity profiles, energy transfer, and viscous dissipation. Our results illustrate how varying the offset from head-on alignment and the impact velocity results in controllable rebound dynamics for oil drop collisions on superamphiphobic surfaces.

Self-Enhancement of Coalescence-Induced Droplet Jumping on Superhydrophobic Surfaces with an Asymmetric V-Groove

Coalescence-induced droplet jumping on superhydrophobic surfaces have recently received significant attention owing to their potential in a variety of applications. Previous studies demonstrated that the self-jumping process is inherently inefficient, with an energy conversion efficiency η ≤ 6% and dimensionless jumping velocity V_j* ≤ 0.23. To realize a quick removal of droplets, increasing effort has been devoted to breaking the jumping velocity limit and inducing droplets sweeping. In this work, we used superhydrophobic surfaces with an asymmetric V-groove to experimentally achieve an enhanced coalescence-induced jumping velocity V_j* ≈ 0.61, i.e., more than 700% increase in energy conversion efficiency compared with droplets jumping on flat superhydrophobic surfaces, which is the highest efficiency reported thus far. Moreover, the enhanced jumping direction shows a deviation as high as 60° from the substrate normal. The induced in-plane motion is conducive to remove a considerable number of droplets along the sweeping path and significantly increase the speed of droplet removal. Numerical simulation indicated that the jumping enhancement is a joint effect resulting from the impact of the liquid bridge on the corner of the V-groove and the suppression of droplet expansion by the sidewall of the V-groove. The transient variation of the droplet velocity and the driving force of the coalescing droplets on a surface with and without the asymmetric V-groove were revealed and discussed. Furthermore, effects of groove angle, droplet pair positions, and size mismatches on the jumping velocity and direction have been studied. The novel mechanism of simultaneously increasing the coalescence-induced droplet jumping velocity and changing the jumping direction can be further studied to enhance the efficiency of various applications.

Steiner triangular drop dynamics

Steiner's circumellipse is the unique geometric regularization of any triangle to a circumscribed ellipse with the same centroid, a regularization that motivates our introduction of the Steiner triangle as a minimal model for liquid droplet dynamics. The Steiner drop is a deforming triangle with one side making sliding contact against a planar basal support. The center of mass of the triangle is governed by Newton's law. The resulting dynamical system lives in a four dimensional phase space and exhibits a rich one-parameter family of dynamics. Two invariant manifolds are identified with "bouncing" and "rocking" periodic motions; these intersect at the stable equilibrium and are surrounded by nested quasiperiodic motions. We study the inherently interesting dynamics and also find that this model, however minimal, can capture space-time symmetries of more realistic continuum drop models.