Droplet Self-Propulsion on Superhydrophobic Microtracks

Enhanced and controlled droplet ejection on magnetic responsive polydimethylsiloxane microarrays

Efficient removal of droplets from solid surfaces is significant in various fields, including fog collection and condensation heat transfer. However, droplets removal on common surfaces with static structures often occurs passively, which limits the possibility of increasing removal efficiency and lacks intelligent controllability. In this paper, an active strategy based on extrusion ejection is proposed and demonstrated on the magnetic responsive polydimethylsiloxane (PDMS) superhydrophobic microplates (MPSM). The MPSM can reversibly transit between the upright and tilted state as the external magnetic field is alternately applied and removed. Under the magnetic field, the direction and trajectories of droplets departure can be intelligently controlled, demonstrating excellent controllability. More importantly, compared with the static structure where the droplet must reach a certain size before departure, droplets can be ejected at smaller sizes as the MPSM is tilted. These advantages are of great significance in many fields, such as a highly efficient fog harvesting system. This strategy of extrusion ejection based on dynamic surface structure control reported in this work may provide fresh ideas for efficient droplet manipulation.

Roll-to-Roll Manufacturing of Breathable Superhydrophobic Membranes

Self-cleaning and anti-biofouling are both advantages for lotus-leaf-like superhydrophobic surfaces. Methods for creating superhydrophobicity, including chemical bonding low surface energy molecular fragments and constructing surface morphology with protrusions, micropores, and trapped micro airbags by traditional physical strategies, unfortunately, have encountered challenges. They often involve complex synthesis processes, stubborn chemical accumulation, brutal degradation, or infeasible calculation and imprecise modulation in fabricating hierarchical surface roughness. Here, a scalable method to prepare high-quality, breathable superhydrophobic membranes is proposed by developing a successive roll-to-roll laser manufacturing technique, which offers advantages over conventional fabrication approaches in enabling automatically large-scale production and ensuring cost-effectiveness. Nanosecond laser writing and femtosecond laser drilling produce surface microstructures and micropore arrays, respectively, endowing the membrane with superior antiwater capability with hierarchical microstructures forming a barrier and blocking water infiltration. The membrane's breathability is carefully optimized by tailoring micropore arrays to allow for the adequate passage of water vapor while maintaining superhydrophobicity. These membranes combine the benefits of anti-aqueous corrosive liquid behaviors, photothermal effects, thermoplastic properties, and stretchable performances as promising comprehensive materials in diverse scenes.

Airflow Triggered Water Film Self-Sculpturing on Femtosecond Laser-Induced Heterogeneously Wetted Micro/Nanostructured Surfaces

Liquid manipulation is essential for daily life and modern industry, and it is widely used in various fields, including seawater desalination, microfluidic robots, and biomedical engineering. Nevertheless, the current research focuses on the manipulation of individual droplets. There are a few projects for water film management. Here, we proposed a facile method of wind-triggered water film self-sculpturing based on a heterogeneous wettability surface, which is achieved by the femtosecond laser direct writing technology and femtosecond laser deposition. Under the conditions of various airflow velocities and water film thicknesses, three distinct behaviors of the water film were analyzed. As a result, when the water film thickness is lower than 4.9 mm, the self-sculpture process will occur until the whole superhydrophobic surface dewetting. Four potential applications are demonstrated, including encryption, oil containers, reconfigurable patterning, and self-splitting devices. This work provides a new approach for manipulating a water film of fluid control engineering.

Control of Water Droplet Transport Using Anodic Porous Alumina with a Wettability Gradient

Water droplets can spontaneously move on surfaces with a wettability gradient, and such surfaces can be used in various functional devices. In this study, surfaces with a wettability gradient were prepared from anodic porous alumina with different pore arrangement regularities. The evaluation of the water droplet transport distance on the surfaces of the prepared samples showed that the pore arrangement regularity of the anodic porous alumina had no effect on the droplet transport distance. It was also shown that the droplet transport distance was affected by the droplet size and the width of the wettability gradient structure. When two water droplets were formed at both ends of the anodic porous alumina with an increasing gradient of pore diameter from the center to both ends, they moved from the ends toward the center and merged. The method reported in this paper shows that by controlling the pore size of anodic porous alumina, one can fabricate a substrate on which droplets can move in any direction. The anodic porous alumina with a wettability gradient as obtained in this study has potential use for various applications such as microfluidic devices and analytical chips.

Preparation and Characterization of Modified ZrO2/SiO2/Silicone-Modified Acrylic Emulsion Superhydrophobic Coating

Superhydrophobic coatings have increasingly become the focal point of research due to their distinctive properties like water resistance, wear resistance, and acid-base resilience. In pursuit of maximizing their efficiency, research has primarily revolved around refining the fabrication process and the composition of emulsion/nanoparticle coatings. We innovatively devised a superhydrophobic coating by employing a spraying technique. This involved integrating a γ-Methacryloyloxypropyltrimethoxysilane (KH570)-modified ZrO2/SiO2/silicone-modified acrylic emulsion. A comprehensive evaluation of this coating was undertaken using analytical instruments such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and confocal laser scanning microscopy (CLSM). The coating demonstrated exceptional performance across a range of tests, including wear, immersion, and anti-icing cleaning, showcasing notable wear resistance, sodium chloride corrosion resistance, self-cleaning efficiency, and thermal stability. In particular, one coating exhibited super-hydrophobic properties, with a high contact angle of 158.5 degrees and an impressively low rolling angle of 1.85 degrees. This remarkable combination of properties is attributed to the judicious selection of components, which significantly reinforced the mechanical strength of the coating. These enhancements make it highly suitable for industrial applications where self-cleaning, anti-icing, and anti-contamination capabilities are critical.

High-Performance Waterproof, Breathable, and Radiative Cooling Membranes Based on Nanoarchitectured Fiber/Meshworks

Smart membranes with protection and thermal-wet comfort are highly demanded in various fields. Nevertheless, the existing membranes suffer from a tradeoff dilemma of liquid resistance and moisture permeability, as well as poor thermoregulating ability. Herein, a novel strategy, based on the synchronous occurrence of humidity-induced electrospinning and electromeshing, is developed to synthesize a dual-network structured nanofiber/mesh for personal comfort management. Manipulating the ejection, deformation, and phase separation of spinning jets and charged droplets enables the creation of nanofibrous membranes composed of radiative cooling nanofibers and 2D nanostructured meshworks. With a combination of a true-nanoscale fiber (∼70 nm) in 2D meshworks, a small pore size (0.84 μm), and a superhydrophobic surface (151.9°), the smart membranes present high liquid repellency (95.6 kPa), improved breathability (4.05 kg m-2 d-1), and remarkable cooling performance (7.9 °C cooler than commercial cotton fabrics). This strategy opens up a pathway to the design of advanced smart textiles for personal protection.

Wicking assisted condenser platform with patterned wettability for space application

Vapor condensation is extensively used in applications that demand the exchange of a substantial amount of heat energy or the vapor-liquid phase conversion. In conventional condensers, the condensate removal from a subcooled surface is caused by gravity force. This restricts the use of such condensers in space applications or horizontal orientations. The current study demonstrates proof-of-concept of a novel plate-type condenser platform for passively removing condensate from a horizontally oriented surface to the surrounding wicking reservoir without gravity. The condensing surface is engineered with patterned wettabilities, which enables the continuous migration of condensate from the inner region of the condenser surface to the side edges via surface energy gradient. The surrounding wicking reservoir facilitates the continuous absorption of condensate from the side edges. The condensation dynamics on different substrates with patterned wettabilities are investigated, and their condensation heat transfer performance is compared. The continuous migration of condensate drops from a superhydrophobic to a superhydrophilic area can rejuvenate the nucleation sites in the superhydrophobic area, resulting in increased heat transport. The proposed condenser design with engineered wettability can be used for temperature and humidity management applications in space.

On-Chip Liquid Manipulation via a Flexible Dual-Layered Channel Possessing Hydrophilic/Hydrophobic Dichotomy

The hydrophilic/hydrophobic cooperative interface provides a smart platform to control liquid distribution and delivery. Through the fusion of flexibility and complex structure, we present a manipulable, open, and dual-layered liquid channel (MODLC) for on-demand mechanical control of fluid delivery. Driven by anisotropic Laplace pressure, the mechano-controllable asymmetric channel of MODLC can propel the directional slipping of liquid located between the paired tracks. Upon a single press, the longest transport distance can reach 10 cm with an average speed of ∼3 cm/s. The liquid on the MODLC can be immediately manipulated by pressing or dragging processes, and versatile liquid-manipulating processes on hierarchical MODLC chips have been achieved, including remote droplet magneto-control, continuous liquid distributor, and gas-producing chip. The flexible hydrophilic/hydrophobic interface and its assembly can extend the function and applications of the wettability-patterned interface, which should update our understanding of complex systems for sophisticated liquid transport.

Curvature Adjustable Liquid Transport on Anisotropic Microstructured Elastic Film

Directional liquid transport is expected via adjusting chemical components, surface morphology, and external stimuli and is critical for practical applications. Although many studies have been conducted, there are still challenges to achieving real-time transformation of liquid transport direction on the material surface. Herein, we demonstrate a strategy to achieve curvature responsive anisotropic wetting on the elastic film with V-shaped prism microarray (VPM) microstructure, which can be used to control the direction of liquid transport. The results reveal that the curvature change of an elastic film can adjust the arrangement of V-shaped prisms on the elastic film. Correspondingly, the liquid wetting trend will change and even the moving direction reverses with varying arrangements of the V-shaped prisms on the elastic film. Meanwhile, surface hydrophobicity of the VPM elastic film also affects the liquid wetting trend and even shows the opposite transport direction of the liquid, which is up to the water wetting state on the VPM elastic film. Based on these results, the VPM elastic film can serve as a valve to control the liquid transport direction and is promising in the application of liquid directional harvest, chemical reaction, microfluidic, etc.

Wetting-State-Induced Turning of Water Droplet Moving Direction on the Surface

The spontaneous directional movement of water droplets on a wedge-shaped groove has gained extensive attention due to the advantage of not requiring energy input and its potential wide applications. However, manipulating the direction of movement of water droplets on a wedge-shaped groove has been not fully achieved, and the fundamental understanding of its underlying mechanism remains unclear. Here, molecular dynamics simulations and theoretical analyses are combined to reveal the mechanism of movement in opposite directions of a water droplet at the same position on the wedge-shaped groove interface. It is shown that the moving direction of the water droplet is related to its wetting state on the surface, i.e., the Wenzel and the Cassie states. A water droplet initially in the Wenzel and Cassie states will move toward the diverging and the converging ends, respectively. This phenomenon is attributed to the opposite roles played by the groove substrate and the upper layers in the two wetting states. Moreover, it is found that the water droplet is likely to move faster on a surface with a higher groove, larger opening angle and stronger hydrophobicity. These findings are expected to be of benefit for fully understanding droplet movement and shedding light on the regulation of the direction of movement of the droplets on the groove surface.

Out-of-Plane Biphilic Surface Structuring for Enhanced Capillary-Driven Dropwise Condensation

Rapid and sustained condensate droplet departure from a surface is key toward achieving high heat-transfer rates in condensation, a physical process critical to a broad range of industrial and societal applications. Despite the progress in enhancing condensation heat transfer through inducing its dropwise mode with hydrophobic materials, sophisticated surface engineering methods that can lead to further enhancement of heat transfer are still highly desirable. Here, by employing a three-dimensional, multiphase computational approach, we present an effective out-of-plane biphilic surface topography, which reveals an unexplored capillarity-driven departure mechanism of condensate droplets. This texture consists of biphilic diverging microcavities wherein a matrix of small hydrophilic spots is placed at their bottom, that is, among the pyramid-shaped, superhydrophobic microtextures forming the cavities. We show that an optimal combination of the hydrophilic spots and the angles of the pyramidal structures can achieve high deformational stretching of the droplets, eventually realizing an impressive "slingshot-like" droplet ejection process from the texture. Such a droplet departure mechanism has the potential to reduce the droplet ejection volume and thus enhance the overall condensation efficiency, compared to coalescence-initiated droplet jumping from other state-of-the-art surfaces. Simulations have shown that optimal pyramid-shaped biphilic microstructures can provoke droplet self-ejection at low volumes, up to 56% lower than superhydrophobic straight pillars, revealing a promising new surface microtexture design strategy toward enhancing the condensation heat-transfer efficiency and water harvesting capabilities.

Preparation of Anodic Porous Alumina with Gradient Hole Size for Directional Droplet Transport

A surface with a wettability gradient can be used as a surface where water droplets transport in one direction. In this report, a surface with a wettability gradient was formed by Al anodization and subsequent gradient etching. Since the surface wettability of anodic porous alumina formed by the Al anodization was dependent on hole size, anodic porous alumina with gradient hole size was prepared to form a surface with a wettability gradient. The surface obtained by this method showed continuously changing wettability, and it was possible to control the range and amount of change in the contact angle of the water droplets by adjusting the hole size. In addition, water droplets were observed to transport spontaneously from an area with a high contact angle to that with a low contact angle on the surface of anodic porous alumina with gradient hole size. The obtained surface with a wettability gradient can be used in various applications, such as microfluidic devices, water collection, and oil-water separation.

Wetting Ridge-Guided Directional Water Self-Transport

Directional water self-transport plays a crucial role in diverse applications such as biosensing and water harvesting. Despite extensive progress, current strategies for directional water self-transport are restricted to a short self-driving distance, single function, and complicated fabrication methods. Here, a lubricant-infused heterogeneous superwettability surface (LIHSS) for directional water self-transport is proposed on polyimide (PI) film through femtosecond laser direct writing and lubricant infusion. By tuning the parameters of the femtosecond laser, the wettability of PI film can be transformed into superhydrophobic or superhydrophilic. After trapping water droplets on the superhydrophilic surface and depositing excess lubricant, the asymmetrical wetting ridge drives water droplets by an attractive capillary force on the LIHSS. Notably, the maximum droplet self-driving distance can approach ≈3 mm, which is nearly twice as long as the previously reported strategies for direction water self-transport. Significantly, it is demonstrated that this strategy makes it possible to achieve water self-transport, anti-gravity pumping, and chemical microreaction on a tilted LIHSS. This work provides an efficient method to fabricate a promising platform for realizing directional water self-transport.

Patterning Wettability for Open-Surface Fluidic Manipulation: Fundamentals and Applications

Effective manipulation of liquids on open surfaces without external energy input is indispensable for the advancement of point-of-care diagnostic devices. Open-surface microfluidics has the potential to benefit health care, especially in the developing world. This review highlights the prospects for harnessing capillary forces on surface-microfluidic platforms, chiefly by inducing smooth gradients or sharp steps of wettability on substrates, to elicit passive liquid transport and higher-order fluidic manipulations without off-the-chip energy sources. A broad spectrum of the recent progress in the emerging field of passive surface microfluidics is highlighted, and its promise for developing facile, low-cost, easy-to-operate microfluidic devices is discussed in light of recent applications, not only in the domain of biomedical microfluidics but also in the general areas of energy and water conservation.

Regular Self-Actuation of Liquid Metal Nanodroplets in Radial Texture Gradient Surfaces

Liquid metal movement in microfluidic devices generally requires an external stimulus to achieve its motion, which results in many difficulties to precisely manipulate its motion at a nanoscale. Therefore, there is an attempt to control the motion of a liquid metal droplet without the input of an external force. In this paper, we report an approach to achieve the self-actuation of a gallium nanodroplet in radial texture gradients on substrates. The results have proved the validity of this method. It is suggested that there are four stages in the self-motion of the droplet and that the precursor film forming on the second stage plays a pivotal role in the motion. Furthermore, how the impact velocity affects the self-actuation of the nanodroplet on the gradient surface is also studied. We find that the moderate impacting velocity hinders the self-actuation of the gallium nanodroplet. This study is very helpful to regulate the self-actuation on patterned substrates and facilitate their applications in the fields of microfluidics devices, soft robots, and liquid sensors.

Prediction of Droplet Sliding on the Continuity of the Three-Phase Contact Line

Many animals and plants have evolved wonderful hydrophobic abilities to adapt to the complex climate environment. The microstructure design of a superhydrophobic surface focuses on bionics and will be restricted by processing technology. Although certain functions can be achieved, there is a lack of unified conclusion on the wetting mechanism and a few quantitative analyses of the continuity of the three-phase contact line. Therefore, the relationship between the surface microstructure of the lattice pattern and the critical sliding angle of the water droplet in the Cassie state was investigated in this paper, and we proposed a method to quantitatively analyze the continuity of the three-phase contact line by a dimensionless length f. The results showed that the three-phase contact line was an important factor to determine the sliding performance of the droplet. The upward traction force generated by the surface tension through the force analysis on the three-phase contact line can enhance the sliding ability of the droplet on the solid surface. There was a good negative linear correlation between the critical sliding angle and dimensionless length, which provided a guiding basis for the optimal design of superhydrophobic surfaces.

Architecture-Driven Fast Droplet Transport without Mass Loss

Spontaneous droplet transport without mass loss has great potential applications in the fields of energy and biotechnology, but it remains challenging due to the difficulty in obtaining a sufficient driving force for the transport while suppressing droplet mass loss. Learning from the slippery peristome of Nepenthes alata and wedge topology of a shorebird beak that can spontaneously feed water against gravity, a combined system consisting of two face-to-face hydrophilic slippery liquid-infused porous surfaces (SLIPS) with variable beak-like opening and spacing was proposed to constrain the droplet in-between and initiate fast droplet transport over a long distance of 75 mm with a maximum speed of 12.2 mm·s^-1 without mass loss by taking advantage of the Laplace pressure gradient induced by the asymmetric shape of the constrained droplet. The theoretical model based on the Navier-Stokes equation was developed to interpret the corresponding mechanism of the droplet transport process. In addition, in situ sophisticated droplet manipulations such as droplet mixing are readily feasible when applying flexible 304 stainless foil as the substrate of SLIPS. It is believed that extended research would contribute to new references for the precise and fast droplet motion control intended for energy harvest and water collection devices.

Spatio-temporal maneuvering of impacting drops

Controlling impacting drops on nonwetting surfaces is desired in multifarious processes. Efforts have been made to solely spatially control the drop movement after the impact or solely temporally reduce liquid-solid contact via surface design. We present a fin-stripe nonwetting surface that enables spatial offset maximization and temporal contact minimization simultaneously, just via structure design without the need for external energies. The wetting stripe provides a large wettability gradient for lateral movement, while the macroscale nonwetting fin restricts the drop movement direction and confines drop spreading for contact time reduction. The fin-stripe surface can decrease the contact time by roughly 30% and provide a normalized lateral distance of roughly 20, an order of magnitude larger than the reported values. Our surface enables effective spatio-temporal maneuvering of impacting drops, essential for various applications that involve liquid transport.

A new scaling number reveals droplet dynamics on vibratory surfaces

HYPOTHESIS

Droplet spreading on surfaces is a ubiquitous phenomenon in nature and is relevant with a wide range of applications. In practical scenarios, surfaces are usually associated with certain levels of vibration. Although vertical or horizontal modes of vibration have been used to promote droplet dewetting, bouncing from immiscible medium, directional transport, etc., a quantitative understanding of how external vibration mediates the droplet behaviors remains to be revealed.

METHODS

We studied droplets impacting on stationary and vibratory surfaces, respectively. In analogy to the Weber number We=ρU_i²D₀/γWe = ρUi2D0/γ, we define the vibration Weber number We^*=ρU_v²D₀/γ to quantitively analyze the vibration-induced dynamic pressure on droplet behaviors on vibratory surfaces, where ρ,γ,D₀,U_iandU_v are liquid density, surface tension, initial droplet diameter, impact velocity of the droplet, and velocity amplitude of vibration, respectively.

FINDINGS

We demonstrate that the effect of vibration on promoting droplet spreading can be captured by a new scaling number expressed as We^*/[We^1\2sin(θ/2)], leading to (D_m - D_m0)/D_m0 ∝ We^*/[We^1\2sin(θ/2)], where θ is the contact angle, and D_m0 and D_m are the maximum diameter of the droplet on stationary and vibratory surfaces, respectively. The scaling number illustrates the relative importance of vibration-induced dynamic pressure compared to inertial force and surface tension. Together with other well-established non-dimensional numbers, this scaling number provides a new dimension and framework for understanding and controlling droplet dynamics. Our findings can also find applications such as improving the power generation efficiency, intensifying the deposition of paint, and enhancing the heat transfer of droplets.

Initial-position-driven opposite directional transport of a water droplet on a wedge-shaped groove

The transport direction of water droplets on a functionalized surface is of great significance due to its wide applications in microfluidics technology. The prevailing view is that a water droplet on a wedge-shaped groove always moves towards the wider end. In this paper, however, molecular dynamics simulations show that a water droplet can move towards the narrower end if placed at specific positions. It is found that the direction of water droplet transport on a grooved surface is related to its initial position. The water droplet moves towards the wider end only when it is placed near the wider end initially. If the water droplet is placed near the narrower end, it will move in the opposite direction. The novel phenomenon is attributed to the opposite interactions of the groove substrate and the groove upper layers with water droplets. Two effective models are proposed to exploit the physical origin of different transport directions of water droplets on a wedge-shaped groove surface. The study provides an insight into the design of nanostructured surfaces to effectively control the droplet motion.

Unidirectional water transport on a two-dimensional hydrophilic channel with anisotropic superhydrophobic barriers

Many creatures have a unique anisotropic structure and special wettability on their skins, presenting intriguing water transporting properties. Inspired by the biosphere, a two-dimensional titanium dioxide-based hydrophilic channel possessing anisotropic superhydrophobic barriers was synthesized. This channel demonstrates unidirectional water transporting properties. When water is injected into the channel, fluid tends to spread in a specific direction. An asymmetric spreading resistance is generated by the different interaction modes between the liquid and superhydrophobic barriers. The superhydrophobic barriers are designed as two main styles: line and curve. As for line barriers, the included angle between barrier and horizontal is the key parameter for the unidirectional water transporting ability whereas, for curve barriers, the radius is an important variable. The best design scheme for unidirectional water transporting properties could be found by varying the parameters of these two types of barriers in the channel. Overall, this study is expected to have a significant implication in the water transporting field.

Collective Entrainment and Confinement Amplify Transport by Schooling Microswimmers

Microswimmers can serve as cargo carriers that move deep inside complex flow networks. When a school collectively entrains the surrounding fluid, their transport capacity can be enhanced. This effect is quantified with good agreement between experiments with self-propelled droplets and a confined Brinkman squirmer model. The volume of liquid entrained can be much larger than the droplet itself, amplifying the effective cargo capacity over an order of magnitude, even for dilute schools. Hence, biological and engineered swimmers can efficiently transport materials into confined environments.

Self-Propulsion of Boiling Droplets on Thin Heated Oil Films

We report on the self-propulsion of boiling droplets which, despite their contact with viscous, immiscible oil films, attain high velocities comparable to those of levitating Leidenfrost droplets. Experiments and model reveal that droplet propulsion originates from a coupling between seemingly disparate short and long timescale phenomena due to microsecond fluctuations induced by boiling events at the droplet-oil interface. This interplay of phenomena leads to continuous asymmetric vapor release and momentum transfer for high droplet velocities.

Rapid construct superhydrophobic microcracks on the open-surface platform for droplet manipulations

Droplet-based transport driven by surface tension has been explored as an automated pumping source for several biomedical applications. This paper presented a simple and fast superhydrophobic modify and patterning approach to fabricate various open-surface platforms to manipulate droplets to achieve transport, mixing, concentration, and rebounding control. Several commercial reagents were tested in our approach, and the Glaco reagent was selected to create a superhydrophobic layer; laser cutters are utilized to scan on these superhydrophobic surface to create gradient hydrophilic micro-patterns. Implementing back-and-forth vibrations on the predetermined parallel patterns, droplets can be transported and mixed successfully. Colorimetry of horseradish peroxidase (HRP) mixing with substrates also reduced the reaction time by more than 5-times with the help of superhydrophobic patterned chips. Besides, patterned superhydrophobic chips can significantly improve the sensitivity of colorimetric glucose-sensing by more than 10 times. Moreover, all bioassays were distributed homogeneously within the region of hydrophilic micropatterns without the coffee-ring effect. In addition, to discuss further applications of the surface wettability, the way of controlling the droplet impacting and rebounding phenomenon was also demonstrated. This work reports a rapid approach to modify and patterning superhydrophobic films to perform droplet-based manipulations with a lower technical barrier, higher efficiency, and easier operation. It holds the potential to broaden the applications of open microfluidics in the future.