Viscous Droplet Impact on Nonwettable Textured Surfaces

Measurement Methods for Droplet Adhesion Characteristics and Micrometer-Scale Quantification of Contact Angle on Superhydrophobic Surfaces: Challenges and Opportunities

Inspired by nature, superhydrophobic surfaces have been widely studied. Usually the wettability of a superhydrophobic surface is quantified by the macroscopic contact angle. However, this method has various limitations, especially for precision micro devices with superhydrophobic surfaces, such as biomimetic artificial compound eyes and biomimetic water strider robots. These precision micro devices with superhydrophobic surfaces proposed a higher demand for the quantification of contact angles, requiring contact angle quantification technology to have micrometer-scale measurement capabilities. In this review, it is proposed to achieve micrometer-scale quantification of superhydrophobic surface contact angles through droplet adhesion characteristics (adhesion force and contact radius). Existing contact angle quantification techniques and droplet characteristics' measurement methods were described in detail. The advancement of micrometer-scale quantification technology for the contact angle of superhydrophobic surfaces will enhance our understanding of superhydrophobic surfaces.

An Experimental Study on Complete Droplet Rebound from Soft Surfaces: Critical Weber Numbers, Maximum Spreading, and Contact Time

Droplet impact on soft surfaces (PDMS) was experimentally studied with particular interest in the complete rebound of droplets. This study focuses on the effect of liquid viscosity and the elastic modulus of the substrate on the critical rebound Weber number, maximum spreading, and contact time. Specifically, the lower and upper critical Weber numbers increase with an increasing droplet viscosity. With decreasing PDMS elastic modulus, the upper critical Weber number increases, while the lower critical Weber number decreases. The PDMS elastic modulus does not significantly affect the maximum spreading time and contact time. An interesting phenomenon of discontinuous contact time was experimentally observed and was theoretically interpreted.

Contact Time of Droplet Impact on Superhydrophobic Cylindrical Surfaces with a Ridge

This study investigates whether adding ridges to a superhydrophobic cylindrical surface can reduce contact times compared to those of ridged flat or cylindrical surfaces, inspired by the shortened contact time achieved by adding ridges to flat surfaces. The study focuses on studying azimuthal ridges on the cylinder through experimentation, emphasizing the impact dynamics and contact time characteristics under varying We (Weber number) and D* (dimensionless droplet diameter). Within the ultralow Weber number range (ULWR), low Weber number range (LWR), and medium Weber number range (MWR), the contact time is longer than on ridged flat surfaces. In the high Weber number range (HWR), the opposite is observed: increased inertial forces lead to the rupture of the liquid film above the ridges due to Rayleigh-Plateau instability. As a result, the primary droplet splits into two sections with curvature effects promoting its recoiling and rebounding. This study introduces a criterion, defined as C = We/D*, and finds that when C exceeds 2.42, not only is the contact time shorter than on ridged flat or cylindrical surfaces, but it also further decreases with an increase in We or a decrease in D*. The contact time characteristics observed in the HWR offer potential applications in areas such as anti-icing.

Impact Dynamics of Non-Newtonian Droplets on Superhydrophobic Surfaces

Droplet impact behavior on a solid surface is critical for many industrial applications such as spray coating, food production, printing, and agriculture. For all of these applications, a common challenge is to modify and control the impact regime and contact time of the droplets. This challenge becomes more critical for non-Newtonian liquids with complex rheology. In this research, we explored the impact dynamics of non-Newtonian liquids (by adding different concentrations of Xanthan into water) on superhydrophobic surfaces. Our experimental results show that by increasing the Xanthan concentration in water, the shapes of the bouncing droplet are dramatically altered, e.g., its shape at the separation moment is changed from a conventional vertical jetting into a "mushroom"-like one. As a result, the contact time of the non-Newtonian droplet could be reduced by up to ∼50%. We compare the impact scenarios of Xanthan liquids with those of glycerol solutions having a similar apparent viscosity, and results show that the differences in the elongation viscosity induce different impact dynamics of the droplets. Finally, we show that by increasing the Weber number for all of the liquids, the contact time is reduced, and the maximum spreading radius is increased.

Controlling high-speed droplet splashing and superspreading behavior on anisotropic superhydrophobic leaf surfaces by ecofriendly Pseudogemini surfactants

BACKGROUND

Efficient deposition of high-speed droplets on superhydrophobic leaf surfaces remains an important challenge. For anisotropic wired superhydrophobic leaf surfaces, the splashing phenomenon is especially serious because it leads to the low effective utilization of pesticides by biological targets. The lost pesticides cause serious ecological environment pollution, therefore there is an urgent need to develop a green and sustainable cost-effective strategy to achieve efficient deposition of high-speed droplets on anisotropic superhydrophobic leaf surfaces at low dosage.

RESULTS

One type of green pseudogemini surfactant is constructed based on fatty acids and hexamethylenediamine by electrostatic interaction to control the splashing and spreading of high-speed droplets on superhydrophobic surfaces. The formed surfactant can not only achieve complete inhibition of the bouncing of droplets, but also promote rapid spreading on superhydrophobic leaf surfaces at very low usage. The efficient deposition and superspreading phenomenon are attributed to the rapid migration and adsorption of the surfactant from the dynamic spherical micelles at the newly formed solid-liquid interface, the network-like aggregated spherical micelles, and the Marangoni effect caused by the surface tension gradient. Moreover, the surfactant shows an excellent synergistic effect with herbicides to control weeds by inhibiting droplet splashing.

CONCLUSION

This work provides a simpler, more effective and sustainable approach to utilize aggregated spherical micelles rather than conventional vesicles or wormlike micelles to improve the droplet deposition on superhydrophobic leaf surfaces and reduce the impact of surfactants and pesticides on the ecological environment.

Characterizing the Bounce and Separation Dynamics of Janus Drop on Macrotextured Surface

Janus drops are thermodynamically stable when a high-viscosity fluid is imposed on a low-viscosity fluid. To understand physical mechanisms in Janus drop impact on macrotextured surfaces, several challenges in finding parameters or strategies still remain. Here, this study investigates the asymmetric bounce and separation of impinging Janus drops on non-wettable surfaces decorated with a macroridge to explore the effect of the drop size, viscosity ratio, and ridge size on the dynamics. Through numerical simulations, we determine the threshold Weber number, above which separation occurs, by varying drop diameters and viscosity ratios of the Janus drops. We investigate the initial bouncing directions of separated drops as a function of the impact velocity and viscosity ratio. We also predict how the separation efficiency is affected by the ridge’s height and width. The asymmetric impact dynamics of Janus drops on macrotextured surfaces can provide new strategies to control drop bouncing in applications, such as liquid separation and purification.

How an Oxide Layer Influences the Impact Dynamics of Galinstan Droplets on a Superhydrophobic Surface

When exposed to air, gallium-based alloys rapidly form a thin oxide layer with viscoelasticity and high adhesion. Although previous work demonstrated that an oxide layer inhibits liquid metal droplet rebound, there is still a lack of a quantitative study to elaborate how an oxide layer affects the impact dynamics. To address this issue, we experimentally investigate Galinstan droplet impingement on a superhydrophobic CuO nanoblade surface and physically explain the difference in the dynamic characteristics of oxidized and unoxidized droplets. Experimental results show that the effect of an oxide layer becomes prominent during the retraction phase. The high adhesion significantly suppresses retraction and rebound, while the elastic response prevents a droplet from sufficiently stretching and maintains the stability of the morphology. More importantly, we systematically and quantitatively explore the influence of an oxide layer on several critical impact parameters, which contributes to a comprehensive understanding of the impact dynamics of liquid metal droplets. It is indicated that an oxide layer has little effect on the maximum spreading factor and spreading time, whereas it causes a 45% reduction of the restitution coefficient and a 36% increase in contact time. Notably, the scaling laws that describe the critical impact parameters of unoxidized droplets show good agreement with the ones known from ordinary Newtonian fluids.

Importance of Spray–Wall Interaction and Post-Deposition Liquid Motion in the Transport and Delivery of Pharmaceutical Nasal Sprays

Nasal sprays, which produce relatively large pharmaceutical droplets and have high momentum, are primarily used to deliver locally acting drugs to the nasal mucosa. Depending on spray pump administration conditions and insertion angles, nasal sprays may interact with the nasal surface in ways that creates complex droplet–wall interactions followed by significant liquid motion after initial wall contact. Additionally, liquid motion can occur after deposition as the spray liquid moves in bulk along the nasal surface. It is difficult or impossible to capture these conditions with commonly used computational fluid dynamics (CFD) models of spray droplet transport that typically employ a deposit-on-touch boundary condition. Hence, an updated CFD framework with a new spray–wall interaction (SWI) model in tandem with a post-deposition liquid motion (PDLM) model was developed and applied to evaluate nasal spray delivery for Flonase and Flonase Sensimist products. For both nasal spray products, CFD revealed significant effects of the spray momentum on surface liquid motion, as well as motion of the surface film due to airflow generated shear stress and gravity. With Flonase, these factors substantially influenced the final resting place of the liquid. For Flonase Sensimist, anterior and posterior liquid movements were approximately balanced over time. As a result, comparisons with concurrent in vitro experimental results were substantially improved for Flonase compared with the traditional deposit-on-touch boundary condition. The new SWI-PDLM model highlights the dynamicenvironment that occurs when a nasal spray interacts with a nasal wall surface and can be used to better understand the delivery of current nasal spray products as well as to develop new nasal drug delivery strategies with improved regional targeting.

Contact Time of Droplet Impact on Inclined Ridged Superhydrophobic Surfaces

Superhydrophobic surfaces decorated with macrostructures have attracted extensive attention due to their excellent performance of reducing the contact time of impacting droplets. In many practical applications, the surface is not perpendicular to the droplet impact direction, but the impacting dynamics in such scenarios still remain mysterious. Here, we experimentally investigate the dynamics of droplet impact on inclined ridged superhydrophobic surfaces and reveal the effect of We_n (the normal Weber number) and α (the inclination angle) on the contact time τ. As We_n increases, τ first decreases rapidly until a platform is reached; if We_n continues to increase, τ further reduces to a lower platform, indicating a three-stage variation of τ in low, middle, and high We_n regions. In the middle and high We_n regions, the contact time is reduced by 30 and 50%, respectively, and is dominated by droplet spreading/retraction in the tangential and lateral directions, respectively. A quantitative analysis demonstrates that τ in the middle and high We_n regions is independent of We_n and α, while the range of middle and high We_n regions is related to α. When α < 30°, increasing α narrows the middle We_n region and enlarges the high We_n region; when α ≥ 30°, the two We_n regions remain unchanged. In addition, droplet sliding is hindered by the friction and is affected by the droplet morphology in the high We_n region. Overall, the synergistic effect of the surface inclination and macrostructures effectively promotes the detachment of impacting droplets on superhydrophobic surfaces, which provides guidance for applications of superhydrophobic surfaces.

Enhanced Surface Icephobicity on an Elastic Substrate

Ice accumulation on exposed surfaces is unavoidable as time elapses and the temperature decreases sufficiently. To mitigate icing problems, various types of icephobic substrates have been rationally designed, including superhydrophobic substrates (SHSs), aqueous lubricating layers, organic lubricating layers, organogels, polyelectrolyte brush layers, electrolyte-based hydrogels, elastic substrates, and multicrack initiator-promoted surfaces. Among these surfaces, elastic substrates show excellent enhanced surface icephobicity during dynamic processes (i.e., water-impacting and de-icing tests). Herein, we summarize recent progress in elastic icephobic substrates and discuss the reasons that surface icephobicity can be enhanced on elastic substrates in terms of enhanced water repellency and further lowering the ice adhesion strength. For enhanced water repellency, we focus on reducing the contact time of water impacting such that water droplets can be easily shed from an elastic substrate before ice occurs. Reducing the contact time of water impacting various substrates (i.e., micro/nanostructured rigid SHSs, macrotextured rigid SHSs, and elastic SHSs) is discussed, followed by exploring their mechanisms. We argue that the ice adhesion strength can be further lowered on an elastic substrate by rationally tuning the elastic modulus and surface textures (i.e., surface textured and hollow subsurface textured) and combining elastic substrate with other passive anti-icing strategies (or functioning passive icephobic substrates with an electrothermal or photothermal stimulus). In short, the introduction of an elastic substrate into a passive or active icephobicity surface opens an avenue toward designing a versatile icephobic surface, providing great potential for outdoor anti-icing applications.

Effect of Viscosity on Bouncing Dynamics of Elliptical Footprint Drops on Non-Wettable Ridged Surfaces

An initial drop shape can alter the bouncing dynamics and significantly decrease the residence time on superhydrophobic surfaces. Elliptical footprint drops show asymmetric dynamics owing to a pronounced flow driven by the initial drop shape. However, the fundamental understanding of the effect of viscosity on the asymmetric dynamics has yet to be investigated, although viscous liquid drop impact on textured surfaces is of scientific and industrial importance. Here, the current study focuses on the impact of elliptical footprint drops with various liquid properties (density, surface tension, and viscosity), drop sizes, and impact velocities to study the bouncing dynamics and residence time on non-wettable ridged surfaces numerically by using a volume-of-fluid method. The underlying mechanism behind the variation in residence time is interpreted by analyzing the shape evolution, and the results are discussed in terms of the spreading, retraction, and bouncing. This study provides an insight on possible outcomes of viscous drops impinging on non-wettable surfaces and will help to design the desired spraying devices and macro-textured surfaces under different impact conditions, such as icephobic surfaces for freezing rain or viscous liquids.

Impact Dynamics and Freezing Behavior of Surfactant-Laden Droplets on Non-Wettable Coatings at Subzero Temperatures

The present study investigates the impact and freezing behavior of the droplets of surfactant solutions on non-wettable coatings at very low temperatures of -10 to -30 °C. Our goal is to elucidate the critical role of concentration, molecular weight, and ionic nature of surfactants on these phenomena. To achieve this goal, we used sodium dodecyl sulfate (anionic), hexadecyltrimethylammonium bromide (cationic), and n-decanoyl-n-methylglucamine (nonionic) at four concentrations ranging from 0 to 2 × CMC (critical micelle concentration). We captured the impact-freezing of the droplets on superhydrophobic alkyl ketene dimer coatings using a high-speed camera at 5000 frames per second. The results show that the ability of the droplets to spread and retract on the coatings is a function of concentration, ionic nature, and molecular weight of the surfactants, as well as the temperature-dependent viscosity of the solutions. Additionally, surfactant-laden droplets generally demonstrated an accelerated freezing compared to pure water. This might be due to the fact that the presence of surfactants can promote both heterogeneous ice nucleation from within the liquid and a larger solid-liquid interfacial area by filling the air pockets of the surface, leading to enhanced heat transfer. The behavior of the cationic surfactant at certain concentrations was, however, an exception leading to a freezing delay, for which a mechanism will be proposed.

On the Mechanism of Human Saliva Interaction with Environmental Dust in Relation to Spreading of Viruses

Environmental effects such as dust mitigation can amplify the spread of viruses via inhaling infected dust particles. Infusion and the spreading rate of human saliva over the dust particles can play a critical role in contiguous virus spread. In the present study, mechanical and chemical interactions of human saliva with environmental dust particles are considered. The saliva droplet impact on dust particles is examined while mimicking saliva droplet spreading during coughing in a dusty ambience. The mechanisms of saliva infusion and cloaking on the dust particles are explored. The characteristics of saliva droplet normal and oblique impacts on a dust particle are examined experimentally and numerically to evaluate the amount of saliva residues on the impacted particle surface. The findings reveal that the saliva liquid infuses and cloaks the dust particle surfaces. The saliva droplet impact on the dust particles leaves a considerable amount of saliva residues on the impacted surfaces, which remain undried for a prolonged period in indoor environments. Weak adhesion of the saliva-infected dust particles on surfaces, such as glass surfaces, enables saliva-infected dust particles to rejoin neighboring ambient air while possessing a high potential for virus spreading.

Axial spreading of droplet impact on ridged superhydrophobic surfaces

HYPOTHESIS

Due to the complex hydrodynamics of droplet impact on ridged superhydrophobic surfaces, quantitative droplet spreading characteristics are unrevealed, limiting the practical applications of ridged superhydrophobic surfaces. During droplet impacting, the size ratio (the ratio of the ridge diameter to the droplet diameter) is an important factor that affects droplet spreading dynamics.

EXPERIMENTS

We fabricated ridged superhydrophobic surfaces with size ratios ranging from zero to one, and conduct water droplet impact experiments on these surfaces at varied Weber numbers. Aided by the numerical simulations and theoretical analysis, we illustrate the droplet spreading dynamics and reveal the law on the maximum axial spreading coefficient.

FINDS

The results show that the droplet spreading and retraction dynamics on ridged superhydrophobic surfaces are significantly asymmetric in the axial and spanwise directions. Focusing on the maximum axial spreading coefficient, we find it decreases first and then increases with increasing size ratios, indicating the existence of the critical size ratio. The maximum axial spreading coefficient can be reduced by 25-40% at the critical size ratio compared with that on flat surfaces. To predict the maximum axial spreading coefficient, two theoretical models are proposed respectively for size ratios smaller and larger than the critical size ratio.

Impacting Water Droplets Can Alleviate Dust from Slanted Hydrophobic Surfaces

Water droplet impacting on a slanted dusty hydrophobic surface is examined in relation to dust mitigation from surfaces. Impacting droplet characteristics including droplet spreading/retraction rates, slipping length, and rebound heights are analyzed via high-speed recording and a tracker program. The environmental dust characteristics in terms of size, shape, elemental composition, and surface free energy are evaluated by adopting the analytical methods. The findings reveal that the dynamic characteristics of the impacting droplet on the slanted hydrophobic surface are significantly influenced by the dust particles. The maximum droplet spreading over the dusty surface becomes smaller than that of the nondusty surface. The presence of the dust particles on the slanted hydrophobic surface increases energy dissipation, and the water droplet slipping length over the surface becomes less than that corresponding to the nondusty surface. Impacting droplet fluid infuses over the dust particle surface, which enables mitigation of dust from the surface to the droplet fluid. A dust-mitigated area on the slanted surface is larger than that corresponding to the horizontal surface; in which case, the area ratio becomes almost six-fold, which slightly reduces with increasing Weber number. The optical transmittance of the dust-mitigated surface by the impacting droplet remains high.

Further Step toward a Comprehensive Understanding of the Effect of Surfactant Additions on Altering the Impact Dynamics of Water Droplets

The addition of surfactants to pure water for specific applications has made controlling the impact dynamics of surfactant-laden droplets a complex phenomenon. This work investigates the influence of the molecular weight (MW), concentration, and ionic nature of the surfactants as well as the substrate surface characteristics on the impact dynamics of surfactant-laden droplets using a high-speed camera at 10 000 frames per second. Sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, and n-decanoyl-n-methylglucamine were used as anionic, cationic, and nonionic surfactants, respectively. We used hydrophilic glass slides, hydrophobic polytetrafluoroethylene, and superhydrophobic alkyl ketene dimer (AKD) as substrates. The results show that the efficiency of the surfactant addition in increasing the maximum spreading diameter is significantly influenced by the molecular weight and ionic nature of the solutions as well as the nonwettability of the substrate. Among all of the surfaces examined, the concentration and ionic nature of the solutions were found to be more dominant parameters in determining the energy dissipation in the retraction phase of the droplet impact on the superhydrophobic AKD surfaces. As the concentration decreases or positive charges are present in the solution, it is more likely to observe a similar retraction dynamic to pure water when the droplet hits the superhydrophobic AKD having negatively charged surface sites. Finally, in terms of the impact outcomes of the surfactant-laden droplets on the superhydrophobic AKD, it is shown that the influence of the surfactant addition is more noticeable at lower Weber numbers, where the droplet tries to rebound by overcoming the energy loss that occurred in the spreading.