Drop rebound after impact: the role of the receding contact angle

Exploiting intermediate wetting on superhydrophobic surfaces for efficient icing prevention

HYPOTHESIS

Superhydrophobic surfaces can effectively prevent the freezing of supercooled droplets in technological systems. Droplets on superhydrophobic surfaces commonly not only wet the top asperities (Cassie State), but also partially penetrate into microstructure due to surface properties, environment, and droplet impact occurring in real-world applications. Implications on ice nucleation can be expected and are little explored. It remains elusive how anti-icing surfaces can be designed to exploit intermediate wetting phenomena.

EXPERIMENTS

We utilized engineered micro-/nanostructures, specifically micropillars, to modulate the wetting fraction in the microstructure. The behavior of intermediate wetting with supercooling and resulting implications on ice nucleation delay when potential nucleation sites are formed in the microcavities were investigated using experimental, theoretical, and simulation components.

FINDINGS

The temperature-dependent wetting fraction in the microstructure increased at supercooled temperatures, partly activated by condensation in the microcavities. At -10/-20 °C, a critical wetting fraction led to maximum ice nucleation delays, with experimental results consistent with theoretical predictions. This critical wetting fraction minimized the effective contact area solid-to-liquid along the partially wetted microstructure. The study establishes physical relations between ice nucleation delays, geometrical surface parameters and wettability properties in the intermediate wetting regime, providing guidance for the design of ice resistant microstructured surfaces.

PFAS-free superhydrophobic chitosan coating for fabrics

In view of health and environmental concerns, together with the upcoming restrictive regulations on per- and polyfluoroalkyl substances (PFAS), less impactful materials must be explored for the hydrophobization of surfaces. Polysaccharides, and especially chitosan, are being explored for their desirable properties of film formation and ease of modification. We present a PFAS-free chitosan superhydrophobic coating for textiles deposited through a solvent-free method. By contact angle analysis and drop impact, we observe that the coating imparts hydrophobicity to the fabrics, reaching superhydrophobicty (θA = 151°, θR = 136°) with increased amount of coating (from 1.6 g/cm2). This effect is obtained by the combination of chemical water repellency of the modified chitosan and the nano- and micro-roughness, assessed by SEM analysis. We perform a comprehensive study on the durability of the coatings, showing good results especially for acidic soaking where the hydrophobicity is maintained until the 8th cycle of washing. We assess the degradation of the coating by a TGA-IR investigation to define the compounds released with thermal degradation, and we confirm the coating's biodegradability by biochemical oxygen consumption. Finally, we demonstrate its biocompatibility on keratinocytes (HaCaT cell line) and fibroblasts (HFF-1 cell line), confirming that the coating is safe for human skin cells.

Liquid Marbles and Drops on Superhydrophobic Surfaces: Interfacial Aspects and Dynamics of Formation: A Review

Liquid marbles (LMs) are droplets encapsulated with powders presenting varied roughness and wettability. These LMs have garnered a lot of attention due to their dual properties of leakage-free and quick transport on both solid and liquid surfaces. These droplets are in a Cassie-Baxter wetting state sitting on both roughness and air pockets existing between particles. They are also reminiscent of the state of a drop on a superhydrophobic (SH) surface. In this review, LMs and bare droplets on SH surfaces are comparatively investigated in terms of two aspects: interfacial and dynamical. LMs present a fascinating class of soft matter due to their superior interfacial activity and their remarkable stability. Inherently hydrophobic powders form stable LMs by simple rolling; however, particles with defined morphologies and chemistries contribute to the varied stability of LMs. The factors contributing to this interesting robustness with respect to bare droplets are then identified by tests of stability such as evaporation and compression. Next, the dynamics of the impact of a drop on a hydrophobic powder bed to form LMs is studied vis-à̀-vis that of drop impact on flat surfaces. The knowledge from drop impact phenomena on flat surfaces is used to build and complement insights to that of drop impact on powder surfaces. The maximum spread of the drop is empirically understood in terms of dimensionless numbers, and their drawbacks are highlighted. Various stages of drop impact-spreading, retraction and rebound, splashing, and final outcome-are systematically explored on both solid and hard surfaces. The implications of crater formation and energy dissipations are discussed in the case of granular beds. While the drop impact on solid surfaces is extensively reviewed, deep interpretation of the drop impact on granular surfaces needs to be improved. Additionally, the applications of each step in the sequence of drop impact phenomena on both substrates are also identified. Next, the criterion for the formation of peculiar jammed LMs was examined. Finally, the challenges and possible future perspectives are envisaged.

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.

On the rebound of soapy drops

Pure water is known to bounce on super-hydrophobic materials, and we discuss here whether this remains true if the surface tension of water is lowered by either alcohol or surfactants. After determining the threshold in surface tension below which drops stick to the substrate, we show that a decrease of surface tension makes the rebound slower, a consequence of the reduced stiffness of this kind of spring. We also report that water with "slow" surfactants can still bounce despite a static surface tension smaller than the rebound threshold, which is interpreted as an effect of dynamic surface tension. The liquid is substantially deformed at impact, which impoverishes the surfactants at the its surface and thus can trigger repellency for a wetting liquid.

Bouncing Regimes of Supercooled Water Droplets Impacting Superhydrophobic Surfaces with Controlled Temperature and Humidity

Superhydrophobic surfaces have shown significant potential for the passive anti-icing application due to their unique water repellency. Reducing the contact time between the impacting droplets and the underlying surfaces with certain textures, especially applying the pancake bouncing mechanism, is expected to eliminate droplet icing upon impingement. However, the anti-icing performance of such superhydrophobic surfaces against the impact of supercooled water droplets has not yet been examined. Therefore, we fabricated a typical post-array superhydrophobic surface (PSHS) and a flat superhydrophobic surface (FSHS), to study the droplet impact dynamics on them with controlled temperature and humidity. The contact time and the bouncing behavior on these surfaces and their dependence on the surface temperature, Weber number, and surface frost were systematically investigated. The conventional rebound and full adhesion were observed on the FSHS, and the adhesion is mainly induced by the penetration of the droplet into the surface micro/nanostructures and the consequent Cassie-to-Wenzel transition. On the PSHS, four distinct regimes including pancake rebound, conventional rebound, partial rebound, and full adhesion were observed, where the contact time increases correspondingly. Over a certain Weber number range, the pancake rebound regime where the droplet bounces off the surface with a dramatically shortened contact time benefits the anti-icing performance. By further decreasing the surface temperature, the pancake rebound turns into the conventional rebound, where the droplet is not levitated after the capillary emptying process. Our scale analysis indicates that the frost between the posts reduces the capillary energy stored during the downward penetration, resulting in the failure of the pancake bouncing. A droplet adheres onto the frosted surface at sufficiently low temperature, especially at larger Weber numbers, on account of the coupling influence of droplet nucleation and wetting transition.

Study on the Bouncing Behaviors of a Non-Newtonian Fluid Droplet Impacting on a Hydrophobic Surface

The control of a droplet bouncing on a substrate is of great importance not only in academic research but also in practical applications. In this work, we focus on a particular type of non-Newtonian fluid known as shear-thinning fluid. The rebound behaviors of shear-thinning fluid droplets impinging on a hydrophobic surface (equilibrium contact angle θ_eq ≈ 108°and contact angle hysteresis Δθ ≈ 20°) have been studied experimentally and numerically. The impact processes of Newtonian fluid droplets with various viscosities and non-Newtonian fluid droplets with dilute xanthan gum solutions were recorded by a high-speed imaging system under a range of Weber numbers (We) from 12 to 208. A numerical model of the droplet impact on the solid substrate was also constructed using a finite element scheme with the phase field method (PFM). The experimental results show that unlike the Newtonian fluid droplets where either partial rebound or deposition occurs, complete rebound behavior was observed for non-Newtonian fluid droplets under a certain range of We. Moreover, the minimum value of We required for complete rebound increases with xanthan concentration. The numerical simulations indicate that the shear-thinning property significantly affects the rebound behavior of the droplets. As the amount of xanthan increases, the high shear rate regions shift to the bottom of the droplet and the receding of the contact line accelerates. Once the high shear rate region appears only near the contact line, the droplet tends to fully rebound even on a hydrophobic surface. Through the impact maps of various droplets, we found that the maximum dimensionless height H_max^* of the droplet increases almost linearly with We as H_max^* ∼ αWe. In addition, a critical value H_max, c^* for the distinction between deposition and rebound for droplets on the hydrophobic surface has been theoretically derived. The prediction of the model shows good consistency with the experimental results.

Chitosan-based coatings with tunable transparency and superhydrophobicity: A solvent-free and fluorine-free approach by stearoyl derivatization

One of the current greatest challenges in materials science and technology is the development of safe- and sustainable-by-design coatings with enhanced functionalities, e.g. to substitute fluorinated substances raising concerns for their potential hazard on human health. Bio-based polymeric coatings represent a promising route with a high potential. In this study, we propose an innovative sustainable method for fabricating coatings based on chitosan with modified functionality, with a fine-tuning of coating properties, namely transparency and superhydrophobicity. The process consists in two main steps: i) fluorine-free modification of chitosan functional groups with stearoyl chloride and freeze-drying to obtain a superhydrophobic powder, ii) coating deposition using a novel solvent-free approach through a thermal treatment. The modified chitosan is characterized to assess its chemico-physical properties and confirm the functionality modification with fatty acid tails. The deposition method enables tuning the coating properties of transparency and superhydrophobicity, maintaining good durability.

Icephobic Performance of Combined Fluorine-Containing Composite Layers on Al-Mg-Mn-Si Alloy Surface

This paper presents the results of an evaluation of anti-icing properties of samples obtained by plasma electrolytic oxidation (PEO) with a subsequent application of superdispersed polytetrafluoroethylene (SPTFE) and polyvinylidenefluoride (PVDF). A combined treatment of the samples with SPTFE and PVDF is also presented. It is revealed that impregnation of a PEO layer with fluoropolymer materials leads to a significant increase in surface relief uniformity. Combined PVDF–SPFTE layers with a ratio of PVDF to SPTFE of 1:4 reveal the best electrochemical characteristics, hydrophobicity and icephobic properties among all of the studied samples. It is shown that the decrease in corrosion current density Ic for PVDF–SPFTE coatings is higher by more than five orders of magnitude in comparison with uncoated aluminum alloy. The contact angle for PVDF–SPFTE coatings attain 160.5°, which allows us to classify the coating as superhydrophobic with promising anti-icing performance. A treatment of a PEO layer with PVDF–SPFTE leads to a decrease in ice adhesion strength by 22.1 times compared to an untreated PEO coating.

Transparent, self-cleaning, scratch resistance and environment friendly coatings for glass substrate and their potential applications in outdoor and automobile industry

In this research work six novel combinations of Hydroxy Ethyl Meth Acrylate based copolymers have been synthesized and commercial titania, after activation was added by adopting simple strategy to manufacture super-hydrophobic, cost effective, transparent, antifogging, self-cleaning and antimicrobial coating on the glass sheet which will be helpful for outdoor and automobile windscreen. The super-hydrophobic covering was set up by dip covering procedure and coated specimen have been characterized for Wetting behaviour, transparency and SEM analysis. Likewise, the dependability of the coating was evaluated at conditions comparable strengthening at higher temperatures (4–400 °C), illumination by UV spectrum at basic and acidic limits, Results revealed that developed material has good adhesion with glass and shows transparency more than 97%, and water contact edge (CA) of 135 ± 2°. Furthermore, the covering displays astounding self-cleaning property. All the outcomes demonstrated that such kind of coatings could be used many modern level applications on automobile wind screen and glass-windows in building and other glasses where protection from UV radiation, anti-fogging and cleaning is required. Such type of coating material can also be used to preserve architectural work leather and other decoration and artwork. The graphical representation is given in Fig. 1.

Droplet Wetting Propagation on a Hybrid-Wettability Surface

Droplet impinging on the boundary between hydrophilic and hydrophobic regions of a hybrid-wettability surface is studied both experimentally and numerically in the present paper. The interfacial evolution and dynamic feature and the corresponding underlying mechanisms behind are mainly analyzed. Because of the unbalanced surface energy in the vicinity of a boundary, the droplet undergoes spreading-receding in the hydrophobic region before migration toward the hydrophilic region. This results in an increase first but then a decrease in the spreading factor in the hydrophobic region, while it increases continuously in the hydrophilic region. In addition, increasing Weber number leads to the increase in both the spreading factor and migration displacement of the droplet in the hydrophobic region, but the latter decreases in the hydrophilic region, resulting from different momentums of secondary spreading. The experimental determinations are verified in detail by a series of numerical simulations performed based on the single variable method by fixing contact angles in different regions separately and excluding the impact momentum. It is shown that the highly unsymmetrical pressure field is exactly one important reason for droplet migration on the hybrid-wettability surface. Despite the weak dependence of the spreading factor on the hydrophilic contact angle in the hydrophobic region, it has an appreciably positive effect on droplet migration, which is confirmed by the increased pressure gradient with its action area in the hydrophobic region when decreasing the hydrophilic contact angle. This paper advances the fundamental understanding for droplet migration on the hybrid-/gradient-wettability surface.

Magneto-Elastic Effect in Non-Newtonian Ferrofluid Droplets Impacting Superhydrophobic Surfaces

In this article, we propose, with the aid of detailed experiments and scaling analysis, the existence of magneto-elastic effects in the impact hydrodynamics of non-Newtonian ferrofluid droplets on superhydrophobic surfaces in the presence of a magnetic field. The effects of magnetic Bond number (Bom), Weber number (We), polymer concentration, and magnetic nanoparticle (Fe3O4) concentration in the ferrofluids were investigated. In comparison to Newtonian ferrofluid droplets, addition of polymers caused rebound suppression of the droplets relatively at lower Bom for a fixed magnetic nanoparticle concentration and We. We further observed that for a fixed polymer concentration and We, increasing magnetic nanoparticle concentration also triggers earlier rebound suppression with increasing Bom. In the absence of the magnetic nanoparticles, the non-Newtonian droplets do not show rebound suppression for the range of Bom investigated. Likewise, the Newtonian ferrofluids show rebound suppression at large Bom. This intriguing interplay of elastic effects of polymer chains and the magnetic nanoparticles, dubbed as the magneto-elastic effect, is noted to lead to the rebound suppression. We establish a scaling relationship to show that the rebound suppression is observed as a manifestation of the onset of magneto-elastic instability only when the proposed magnetic Weissenberg number (Wim) exceeds unity. We also put forward a phase map to identify the various regimes of impact ferrohydrodynamics of such droplets and the occurrence of the magneto-elastic effect.

The role of oscillation in ellipsoidal drop impact on a solid surface

Ellipsoidal shapes of drops can significantly modify the impact dynamics and suppress the rebound by inducing symmetry breaking in the mass and momentum distributions compared to the axisymmetric dynamics of typical drops. However, the previous works have assumed that the drop oscillation at the moment of impact only slightly affects the post-dynamics although the oscillation must be involved in the spreading. Here, we study the impact dynamics of the oscillating ellipsoidal drops on non-wetting surfaces as a function of the ellipticity, oscillation phase, and Weber number (We) experimentally and numerically. The spreading dynamics show notable hysteretic features in the maximal spreading diameters at the four regions of the oscillation phase. The hysteresis appears more prominently in prolate drops than in oblate drops and becomes remarkably suppressed at the four phases as We increases. Momentum analysis shows that the phases for shaping the drops spherically can drive higher asymmetry in the horizontal momenta than the other phases for shaping the drops ellipsoidally. The momentum asymmetry in the horizontal axes indicates that the oscillation phase as well as the ellipticity can play an important role in altering the hydrodynamics and reducing the bounce magnitude.

Whole Contact Line Pinning for Droplets Impacting on a Hydrophobic Surface Due to Hydrophilic TiO2 Nanoparticle Addition

Controlling droplet deposition on a hydrophobic surface has received much attention due to its wide applications. Addition of certain elements into a working droplet is a feasible way to improve drop deposition, which, however, often leads to a significant change in droplet spreading properties. In this work, we show that adding a small amount of hydrophilic TiO₂ nanoparticles without any surfactant can significantly suppress the droplet rebound and even generate a whole contact line pinning on the hydrophobic surface. The whole contact line pinning is positively related to the Weber number (i.e., impact velocity) and suspension concentration. Specifically, when the suspension concentration exceeds a critical value, the pinning and droplet deposition occur in the same We range. A mechanism is proposed to explain the observed unique pinning and depinning behaviors, according to which the agglomerated TiO₂ particles depositing at the triple line can change the wettability of the local surface, which leads to pinning, while the disturbance of capillary oscillation leads to depinning. Interestingly, a long-time whole contact line pinning for more than a second was observed under certain conditions. This work can be of value for many practical applications such as pesticide deposition and spray cooling.

Pore-Scale Modeling of the Effect of Wettability on Two-Phase Flow Properties for Newtonian and Non-Newtonian Fluids

The Darcy-scale properties of reservoir rocks, such as capillary pressure and relative permeability, are controlled by multiphase flow properties at the pore scale. In the present paper, we implement a volume of fluid (VOF) method coupled with a physically based dynamic contact angle to perform pore-scale simulation of two-phase flow within a porous medium. The numerical model is based on the resolution of the Navier–Stokes equations as well as a phase fraction equation incorporating a dynamic contact angle model with wetting hysteresis effect. After the model is validated for a single phase, a two-phase flow simulation is performed on both a Newtonian and a non-Newtonian fluid; the latter consists of a polymer solution displaying a shear-thinning power law viscosity. To investigate the effects of contact angle hysteresis and the non-Newtonian nature of the fluid, simulations of both drainage and imbibition are carried out in order to analyze water and oil saturation—particularly critical parameters such as initial water saturation (Swi) and residual oil saturation (Sor) are assessed in terms of wettability. Additionally, the model sensitivities to the consistency factor (χ), the flow behavior index (n), and the advancing and receding contact angles are tested. Interestingly, the model correctly retrieves the variation in Sor and wettability and predicts behavior over a wide range of contact angles that are difficult to probe experimentally.

Dynamics of splashed droplets impacting wheat leaves treated with a fungicide

Wheat is threatened by diseases such as leaf rust. One significant mechanism of disease spread is the liberation and dispersal of rust spores due to rainsplash. However, it is unclear to what extent the spore-laden splashed droplets can transmit the disease to neighbouring leaves. Here, we show that splashed droplets either bounce or stick, depending on the orientation of the leaf and whether the surface of the leaf has been treated with a fungicide. A scaling model revealed that bouncing was enabled when the droplet's kinetic energy exceeded its pinning energy to the surface. Our findings indicate that, ironically, the application of fungicide to protect a wheat plant may also facilitate pathogen spread and infection by making leaves sticky to spore-laden droplets.

Rebound of self-lubricating compound drops

Drop impact on solid surfaces is encountered in numerous natural and technological processes. Although the impact of single-phase drops has been widely explored, the impact of compound drops has received little attention. Here, we demonstrate a self-lubrication mechanism for water-in-oil compound drops impacting on a solid surface. Unexpectedly, the core water drop rebounds from the surface below a threshold impact velocity, irrespective of the substrate wettability. This is interpreted as the result of lubrication from the oil shell that prevents contact between the water core and the solid surface. We combine side and bottom view high-speed imaging to demonstrate the correlation between the water core rebound and the oil layer stability. A theoretical model is developed to explain the observed effect of compound drop geometry. This work sets the ground for precise complex drop deposition, with a strong impact on two- and three-dimensional printing technologies and liquid separation.

Optimization of Hybrid Sol-Gel Coating for Dropwise Condensation of Pure Steam

We developed hybrid organic–inorganic sol–gel silica coatings with good durability in harsh environment (high temperatures, high vapor velocities) and with slightly hydrophobic behavior, sufficient to promote dropwise condensation (DWC) of pure steam. DWC is a very promising mechanism in new trends of thermal management and power generation systems to enhance the heat transfer during condensation as compared to film-wise condensation (FWC). The sol–gel coatings have been prepared from methyl triethoxy silane (MTES) and tetraethyl-orthosilicate (TEOS) and deposited on an aluminum substrate. The coatings were optimized in terms of precursor ratio and annealing temperature highlighting potentials and limits of such mixtures. A comprehensive surface characterization before and after saturated steam condensation tests has been performed and related to the thermal measurements for evaluating the heat transfer augmentation as compared to FWC obtained on untreated aluminum surfaces. The results showed that the developed hybrid organic-inorganic sol–gel silica coatings are promising DWC promoters.

Numerical Simulations of Polymer Solution Droplet Impact on Surfaces of Different Wettabilities

This paper presents a physically based numerical model to simulate droplet impact, spreading, and eventually rebound of a viscoelastic droplet. The simulations were based on the volume of fluid (VOF) method in conjunction with a dynamic contact model accounting for the hysteresis between droplet and substrate. The non‐Newtonian nature of the fluid was handled using FENE‐CR constitutive equations which model a polymeric fluid based on its rheological properties. A comparative simulation was carried out between a Newtonian solvent and a viscoelastic dilute polymer solution droplet. Droplet impact analysis was performed on hydrophilic and superhydrophobic substrates, both exhibiting contact angle hysteresis. The effect of substrates’ wettability on droplet impact dynamics was determined the evolution of the spreading diameter. While the kinematic phase of droplet spreading seemed to be independent of both the substrate and fluid rheology, the recoiling phase seemed highly influenced by those operating parameters. Furthermore, our results implied a critical polymer concentration in solution, between 0.25 and 2.5% of polystyrene (PS), above which droplet rebound from a superhydrophobic substrate could be curbed. The present model could be of particular interest for optimized 2D/3D printing of complex fluids.

Spreading Dynamics and the Residence Time of Ellipsoidal Drops on a Solid Surface

Controlling bouncing drops on solid surfaces has gained significant attention because of the benefit of low residence time in anti-icing and self-cleaning strategies. Given that the drop shape at the moment of impact is classically assumed to be spherical, the residence time on a flat surface is bounded by a theoretical Rayleigh limit. In this study, we investigated the impact dynamics of oblate and prolate ellipsoidal drops to demonstrate the concept of modifying the residence time by shaping like raindrops. Experimental and numerical studies show that the initial shape plays a vital role in an increase or reduction in bounce speed of the drop, which is explained by scaling the maximum spreading time. The hydrodynamic features of ellipsoidal drops are analyzed by quantifying the temporal variations in diameters, heights, velocity fields, momenta, and energy dissipation. We believe that the ellipsoidal drop impact can provide an efficient pathway for controlling the residence time in practical applications.

Ultra-Porous Nanocellulose Foams: A Facile and Scalable Fabrication Approach

Cellulose nanofibril foams are cellulose-based porous materials with outstanding mechanical properties, resulting from the high strength-to-weight ratio of nanofibrils. Here we report the development of an optimized fabrication process for highly porous cellulose foams, based on a well-controlled freeze-thawing-drying (FTD) process at ambient pressure. This process enables the fabrication of foams with ultra-high porosity, up to 99.4%, density of 10 mg/cm³, and liquid (such as oil) absorption capacity of 100 L/kg. The proposed approach is based on the ice-templating of nanocellulose suspension in water, followed by thawing in ethanol and drying at environmental pressures. As such, the proposed fabrication route overcomes one of the major bottle-necks of the classical freeze-drying approach, by eliminating the energy-demanding vacuum drying step required to avoid wet foam collapse upon drying. As a result, the process is simple, environmentally friendly, and easily scalable. Details of the foam development fabrication process and functionalization are thoroughly discussed, highlighting the main parameters affecting the process, e.g., the concentration of nanocellulose and additives used to control the ice nucleation. The foams are also characterized by mechanical tests and oil absorption measurements, which are used to assess the foam absorption capability as well as the foam porosity. Compound water-in-oil drop impact experiments are used to demonstrate the potential of immiscible liquid separation using cellulose foams.

Droplet impact dynamics on textiles

The development of textiles that repel droplets following droplet impact at a high velocity is a common requirement in a number of applications, ranging from waterproof clothing to inkjet printing, yet the underpinning physical mechanisms are not entirely understood. The impact of a droplet on the surface of a textile produces two simultaneous yet separate flows, occurring above and below the surface, and which are associated with the spreading and penetration dynamics. In this paper, we study the temporal evolution of the lateral spreading diameter of a droplet impacting both hydrophobic and hydrophilic textiles. We show that the impact on textiles at short timescales involves no deformation of the droplet shape if the textile's porosity is sufficiently low. We show that the early-stage impact penetration is solely driven by inertia and no lamella is visible. We also show that for hydrophilic textiles, depending on the impact conditions, a droplet can be captured by the textile or penetrate it. We show by balancing the dynamic impact and capillary pressures that the penetration behaviour is governed by a threshold pore size, the liquid characteristics and the droplet diameter. Our conclusions highlight that the ability of a textile to repel water is controlled by the mesh size. Our experiments and analysis were carried out on coated hydrophobic and non-coated hydrophilic textiles with four corresponding mesh sizes, and are in agreement with the previous findings on hydrophobic metallic (copper) meshes.

Reducing the Bounce Height during Truncated Spherical Drop Impact on a Solid Surface

Controlling drop dynamics on solid surfaces is an important challenge. In many strategies for efficient drop deposition, drop dynamics is generally assumed to be axisymmetrical. We demonstrate shape-dependent impact dynamics that can considerably modify the dynamics by deforming the drop into a truncated spherical shape at the impact moment. Experimental and numerical studies show the exceptional rim dynamics that lead to reduced bounce heights compared with spherical drops. We investigate the impact dynamics of truncated spherical drops as a function of the truncation depth, surface wettability, and impact velocity numerically. The bounce height of the truncated drop reduces by 56% below spherical drops. To elucidate the mechanism for the reduction in the bounce height, we conduct the horizontal and vertical momentum analyses of truncated drops. The truncated drop impact can potentially open up new opportunities for enhancing drop deposition in practical applications, such as surface coating and spray cooling.

Controlling the residence time of a bouncing drop with asymmetric shaping

The bouncing of a drop on non-wetted surfaces has received substantial attention because of the minimum residence time of a drop on a surface. Drop dynamics is classically limited to circular symmetry and a theoretical residence time. Thus, altering the time has been a challenging task. In this study, we investigate the bouncing dynamics of egg-shaped footprint drops to prove the concept of controlling the residence time with asymmetric shaping in an electrohydrodynamic device. The asymmetry and ellipticity of the shape provide an efficient pathway for reducing the residence time by nearly 28% below a spherical shape. The exceptional impact dynamics and the reduced contact time are characterized in terms of the geometric parameters of the shape model, which is rationalized by quantitative momentum analysis in the simulation. The distinct bounce features of the asymmetric drop can offer potential for diverse applications, such as maintaining dryness, anti-icing, and self-cleaning.

Maximum Spreading and Rebound of a Droplet Impacting onto a Spherical Surface at Low Weber Numbers

The spreading and rebound patterns of low-viscous droplets upon impacting spherical solid surfaces are investigated numerically. The studied cases consider a droplet impinging onto hydrophobic and superhydrophobic surfaces with various parameters varied throughout the study, and their effects on the postimpingement behavior are discussed. These parameters include impact Weber number (through varying the surface tension and impingement velocity), the size ratio of the droplet to the solid surface, and the surface contact angle. According to the findings, the maximum spreading diameter increases with the impact velocity, with an increase of the sphere diameter, with a lower surface wettability, and with a lower surface tension. Typical outcomes of the impact include (1) complete rebound, (2) splash, and (3) a final deposition stage after a series of spreading and recoiling phases. Finally, a novel, practical model is proposed, which can reasonably predict the maximum deformation of low Reynolds number impact of droplets onto hydrophobic or superhydrophobic spherical solid surfaces.

A new model to predict the influence of surface temperature on contact angle

The measurement of the equilibrium contact angle (ECA) of a weakly evaporating sessile drop becomes very challenging when the temperatures are higher than ambient temperature. Since the ECA is a critical input parameter for numerical simulations of diabatic processes, it is relevant to know the variation of the ECA with the fluid and wall temperatures. Several research groups have studied the effect of temperature on ECA either experimentally, with direct measures, or numerically, using molecular dynamic simulations. However, there is some disagreement between the authors. In this paper two possible theoretical models are presented, describing how the ECA varies with the surface temperature. These two models (called Decreasing Trend Model and Unsymmetrical Trend Model, respectively) are compared with experimental measurements. Within the experimental errors, the equilibrium contact angle shows a decrease with increasing surface temperatures on the hydrophilic surface. Conversely the ECA appears approximately constant on hydrophobic surfaces for increasing wall temperatures. The two conclusions for practical applications for weakly evaporating conditions are that (i) the higher the ECA, the smaller is the effect of the surface temperature, (ii) a good evaluation of the decrease of the ECA with the surface temperature can be obtained by the proposed DTM approach.

Drop impact dynamics on slippery liquid-infused porous surfaces: influence of oil thickness

Slippery liquid-infused porous surfaces (SLIPS) are porous nanostructures impregnated with a low surface tension lubricant. They have recently shown great promise in various applications that require non-wettable superhydrophobic surfaces. In this paper, we investigate experimentally the influence of the oil thickness on the wetting properties and drop impact dynamics of new SLIPS. By tuning the thickness of the oil layer deposited through spin-coating, we show that a sufficiently thick layer of oil is necessary to avoid dewetting spots on the porous nanostructure and thus increasing the homogeneity of the liquid distribution. Drop impact on these surfaces is investigated with a particular emphasis on the spreading and rebound dynamics when varying the oil thickness and the Weber number.

Maximum Deformation Ratio of Droplets of Water-Based Paint Impact on a Flat Surface

In this research, the maximum deformation ratio of water-based paint droplets impacting and spreading onto a flat solid surface was investigated numerically based on the Navier–Stokes equation coupled with the level set method. The effects of droplet size, impact velocity, and equilibrium contact angle are taken into account. The maximum deformation ratio increases as droplet size and impact velocity increase, and can scale as We1/4, where We is the Weber number, for the case of the effect of the droplet size. Finally, the effect of equilibrium contact angle is investigated, and the result shows that spreading radius decreases with the increase in equilibrium contact angle, whereas the height increases. When the dimensionless time t* < 0.3, there is a linear relationship between the dimensionless spreading radius and the dimensionless time to the 1/2 power. For the case of 80° ≤ θe ≤ 120°, where θe is the equilibrium contact angle, the simulation result of the maximum deformation ratio follows the fitting result. The research on the maximum deformation ratio of water-based paint is useful for water-based paint applications in the automobile industry, as well as in the biomedical industry and the real estate industry. Please check all the part in the whole passage that highlighted in blue whether retains meaning before.

Understanding the drop impact on moving hydrophilic and hydrophobic surfaces

In this paper, a systematic study was performed to understand the drop impact on hydrophilic and hydrophobic surfaces that were moving in the horizontal direction. Drops (D₀ = 2.5 mm) of liquids with three different viscosities were used. Wide ranges of drop normal velocity (0.5 to 3.4 m s^-1) and surface velocity (0 to 17 m s^-1) were studied. High speed imaging from the top and side was used to capture the impact phenomena. It was found that drop impact behavior on a moving surface significantly differs from that on a stationary surface at both the lamella extension stage (i.e. t ≤ t_max) and the retraction stage (t > t_max). Starting with the lamella extension stage, it was observed that the drop spreads asymmetrically over a moving surface. It was also found that the splashing behavior of the drop upon impact on a moving surface, unlike the understanding in the literature, is azimuthally different along the lamella contact line. In the case of the drop spreading over a moving surface, the surface movement stretches the expanded lamella in the direction of the surface motion. For hydrophilic surfaces, the stretched lamella pins to the surface and moves with the surface velocity; however, for hydrophobic surfaces, the lamella recoils during such stretching. A new model was developed to determine the splashing threshold of the drop impact on a moving surface. The model is capable of describing the azimuthally different behavior of the splashing which is a function of normal capillary and Weber numbers, surface velocity, and surface wettability. It was also found that the increase of the viscosity decreases the splashing threshold. Finally, comprehensive regime maps of the drop impact outcome on a moving surface were provided for both t ≤ t_max and t > t_max stages.

Contactless Transport and Mixing of Liquids on Self-Sustained Sublimating Coatings

Controlled handling of liquids and colloidal suspensions as they interact with surfaces, targeting a broad palette of related functionalities, is of great importance in science, technology, and nature. When small liquid volumes (drops on the order of microliters or nanoliters) need to be processed in microfluidic devices, contamination on the solid/liquid interface and loss of liquid due to adhesion on the transport channels are two very common problems that can significantly alter the process outcome, for example, the chemical reaction efficiency or the purity and the final concentration of a suspension. It is, therefore, no surprise that both levitation and minimal contact transport methods-including nonwetting surfaces-have been developed to minimize the interactions between liquids and surfaces. Here, we demonstrate contactless surface levitation and transport of liquid drops, realized by harnessing and sustaining the natural sublimation of a solid-carbon-dioxide-coated substrate to generate a continuous supporting vapor layer. The capability and limitations of this technique in handling liquids of extreme surface tension and kinematic viscosity values are investigated both experimentally and theoretically. The sublimating coating is capable of repelling many viscous and low-surface-tension liquids, colloidal suspensions, and non-Newtonian fluids as well, displaying outstanding omniphobic properties. Finally, we demonstrate how sublimation can be used for liquid transport, mixing, and drop coalescence, with a sublimating layer coated on an underlying substrate with prefabricated channels, conferring omniphobicity using a simple physical approach (i.e., phase change) rather than a chemical one. The independence of the surface levitation principle from material properties, such as electromagnetic, thermal or optical, surface energy, adhesion, or mechanical properties, renders this method attractive for a wide range of potential applications.

Water Touch-and-Bounce from a Soft Viscoelastic Substrate: Wetting, Dewetting, and Rebound on Bitumen

Understanding the interaction between liquids and deformable solid surfaces is a fascinating fundamental problem, in which interaction and coupling of capillary and viscoelastic effects, due to solid substrate deformation, give rise to complex wetting mechanisms. Here we investigated as a model case the behavior of water drops on two smooth bitumen substrates with different rheological properties, defined as hard and soft (with complex shear moduli in the order of 10(7) and 10(5) Pa, respectively, at 1 Hz), focusing both on wetting and on dewetting behavior. By means of classical quasi-static contact angle measurements and drop impact tests, we show that the water drop behavior can significantly change from the quasi-static to the dynamic regime on soft viscoelastic surfaces, with the transition being defined by the substrate rheological properties. As a result, we also show that on the hard substrate, where the elastic response is dominant under all investigated conditions, classical quasi-static contact angle measurements provide consistent results that can be used to predict the drop dynamic wetting behavior, such as drop deposition or rebound after impact, as typically observed for nondeformable substrates. Differently, on soft surfaces, the formation of wetting ridges did not allow to define uniquely the substrate intrinsic advancing and receding contact angles. In addition, despite showing a high adhesion to the soft surface in quasi-static measurements, the drop was surprisingly able to rebound and escape from the surface after impact, as it is typically observed for hydrophobic surfaces. These results highlight that measurements of wetting properties for viscoelastic substrates need to be critically used and that wetting behavior of a liquid on viscoelastic surfaces is a function of the characteristic time scales.

Rebound suppression of a droplet impacting on an oscillating horizontal surface

The behavior of a droplet impinging onto a solid substrate can be influenced significantly by the horizontal motion of the substrate. The coupled interactions between the moving wall and the impacting droplet may result in various outcomes, which may be different from the usual normal droplet impact on a stationary wall. In this paper, we present a method to suppress drop rebound on hydrophobic surfaces via transverse wall oscillations, normal to the impact direction. The numerical investigation shows that the suppression of droplet rebound has a direct relationship with the oscillation phase, amplitude, and frequency. For a particular range of oscillation frequencies and amplitudes, a lateral shifting of the droplet position is observed along the oscillating direction. While large oscillation amplitude favors the process of droplet deposition, a high frequency promotes droplet rebound from the oscillating wall. A linear trend in the transition region between deposition and rebound is observed from a scaled phase diagram of the oscillation amplitude versus frequency. We provide a systematic investigation of drop deposition and elucidate the mechanism of rebound suppression through the temporal evolution of the nonaxial kinetic energy and the velocity flow field.

Is a Knowledge of Surface Topology and Contact Angles Enough to Define the Drop Impact Outcome?

It is well known that a superhydrophobic surface may not be able to repel impacting droplets because of the so-called Cassie-to-Wenzel transition. It has been proven that a critical value of the receding contact angle (θR) exists for the complete rebound of water, recently experimentally measured to be 100° for a large range of impact velocities. On the contrary, in the present work, no rebound was observed when low-surface-tension liquids such as hexadecane (σ = 27.5 mN/m at 25 °C) are concerned, even for very low impact velocities and very high values of θR and low contact angle hysteresis. Therefore, the critical threshold of θR ≈ 100° does not sound acceptable for all liquids and for all hydrophobic surfaces. For the same Weber numbers, a Cassie-to-Wenzel state transition occurs after the impact as a result of the easier penetration of low-surface-tension fluids in the surface structure. Hence, a criterion for the drop rebound of low-surface-tension liquids must consider not only the contact angle values with surfaces but also their surface tension and viscosity. This suggests that, even if it is possible to produce surfaces with enhanced static repellence against oils and organics, generally the realization of synthetic materials with self-cleaning and antisticking abilities in dynamic phenomena, such as spray impact, remains an unsolved task. Moreover, it is demonstrated that the chemistry of the surface, the physicochemical interactions with the liquid drops, and the possible wettability gradient of the surface asperity also play important roles in determining the critical Weber number above which impalement occurs. Therefore, the classical numerical simulations of drop impact on dry surfaces are definitively not able to capture the final outcomes of the impact for all possible fluids if the surface topology and chemistry and/or the wettability gradient in the surface structure are not properly reflected.

Stretchable Superhydrophobicity from Monolithic, Three-Dimensional Hierarchical Wrinkles

We report the design of three-dimensional (3D) hierarchical wrinkle substrates that can maintain their superhydrophobicity even after being repeatedly stretched. Monolithic poly(dimethysiloxane) with multiscale features showed wetting properties characteristic of static superhydrophobicity with water contact angles (>160°) and very low contact angle hysteresis (<5°). To examine how superhydrophobicity was maintained as the substrate was stretched, we investigated the dynamic wetting behavior of bouncing and splashing upon droplet impact with the surface. On hierarchical wrinkles consisting of three different length scales, superhydrophobic bouncing was observed. The substrate remained superhydrophobic up to 100% stretching with no structural defects after 1000 cycles of stretching and releasing. Stretchable superhydrophobicity was possible because of the monolithic nature of the hierarchical wrinkles as well as partial preservation of nanoscale structures under stretching.

Revisiting the effect of hierarchical structure on the superhydrophobicity

We have studied the wetting behaviors of surfaces with a single micro-scale structure and a double micro/nano hierarchical structure, respectively. We have found the delayed wetting phenomenon on the single micro scale surface, which indicates that the wetting state transits from the initial Cassie state to the Cassie impregnating one. Furthermore, the droplet rebound becomes incomplete on the single micro scale surface when the impact velocity exceeds a critical value. On the contrary, complete rebound can still be observed when impacting on the micro/nano hierarchical structure. We proposed that, under static deposition the wetting transition occurs though the contact line depinning mechanism, whereas it occurs via sagging mechanism under a dynamic impact. Our results may be helpful for the understanding of superhydrophobicity and the wetting transition on complex structures.

Single etch fabrication and characterization of robust nanoparticle tipped bi-level superhydrophobic surfaces

Though hierarchical roughness gives best anti-wetting surfaces, they fail even under small mechanical stresses. In contrast dual level surfaces fabricated using a single etch step provides robust superhydrophobicity.

Complex Drop Impact Morphology

The aim of this work is to understand the changes in the observed phenomena during particle-laden drop impact. The impact of millimeter-size drops was investigated onto hydrophilic (glass) and hydrophobic (polycarbonate) substrates. The drops were dispersions of water and spherical and nearly iso-dense hydrophobic particles with diameters of 200 and 500 μm. The impact was studied by side and bottom view images in the range 150 ≤ We ≤ 750 and 7100 ≤ Re ≤ 16400. The particles suppressed the appearance of singular jetting and drop partial rebound but promoted splashing, receding breakup, and rupture. The drops with 200 μm particles spread in two phases: fast and slow, caused by inertial and capillary forces, respectively. Also, the increase in volume fraction of 200 μm particle led to a linear decrease in the maximum spreading factor caused by the inertia force on both hydrophilic and hydrophobic substrates. The explanation of this reduction was argued to be the result of energy dissipation through frictional losses between particles and the substrate.

Polymer Brush Surfaces Showing Superhydrophobicity and Air-Bubble Repellency in a Variety of Organic Liquids

Silicon (Si) substrates were modified with polyalkyl methacrylate brushes having different alkyl chain lengths (C(n), where n = 1, 4, 8, and 18) using ARGET-ATRP at ambient temperature without purging the reaction solution of oxygen. The dynamic hydrophobicity of these polymer brush-covered Si surfaces when submerged in a variety of organic solvents (1-butanol, dichloromethane, toluene, n-hexane) depended markedly on the alkyl chain length and to a lesser extent polymer solubility. Long-chain poly(stearyl methacrylate) brushes (C(n) = 18) submerged in toluene showed excellent water-repellant properties, having large advancing/receding contact angles (CAs) of 169°/168° with negligible CA hysteresis (1°). Whereas polymer brushes with short alkyl-chain (C(n) ≤ 4) had significantly worse water drop mobility because of small CAs (as low as 125°/55°) and large CA hysteresis (up to 70°). However, such poor dynamic dewetting behavior of these surfaces was found to significantly improve when water drops impacted onto the surfaces at moderate velocities. Under these conditions, all brush surfaces were able to expel water drops from their surface. In addition, our brush surfaces were also highly repellant toward air bubbles under all conditions, irrespective of C(n) or polymer solubility. These excellent surface properties were found to be vastly superior to the performance of conventional perfluoroalkylsilane-derived surfaces.