Contact Line Motion on Nanorough Surfaces: A Thermally Activated Process

Life beyond Fritz: On the Detachment of Electrolytic Bubbles

We present an experimental study on detachment characteristics of hydrogen bubbles during electrolysis. Using a transparent (Pt or Ni) electrode enables us to directly observe the bubble contact line and bubble size. Based on these quantities we determine other parameters such as the contact angle and volume through solutions of the Young-Laplace equation. We observe bubbles without ("pinned bubbles") and with ("spreading bubbles") contact line spreading and find that the latter mode becomes more prevalent if the concentration of HClO4 is ≥0.1 M. The departure radius for spreading bubbles is found to drastically exceed the value predicted by the well-known formula of W. Fritz [Phys. Z. 1935, 36, 379-384] for this case. We show that this is related to the contact line hysteresis, which leads to pinning of the contact line after an initial spreading phase at the receding contact angle. The departure mode is then similar to a pinned bubble and occurs once the contact angle reaches the advancing contact angle of the surface. A prediction for the departure radius based on these findings is found to be consistent with the experimental data.

Spontaneous Rise of Hydrogen Microbubbles in Interfacial Gas Evolution Reaction

Liquid organic hydrogen carrier is a promising option for the transport and storage of hydrogen as a clean energy source. This study examines the stability and behavior of organic drops immobilized on a substrate during an interfacial hydrogen-evolution reaction (HER) at the drop surface and its surrounding aqueous solution. Hydrogen microbubbles form within the drop and rise to the drop apex. The growth rate of the hydrogen in-drop bubble increases with the concentration of the reactant in the surrounding medium. The drop remains stable till the buoyancy acting on the in-drop bubble is large enough to overcome the capillary force and the external viscous drag. The bubble spontaneously rises and carries a portion drop liquid to the solution surface. These spontaneous rising in-drop bubbles are detected in measurements using a high-precision sensor placed on the upper surface of the aqueous solution, reversing the settling phase from phase separation in the reactive emulsion. The finding from this work provides new insights into the behaviors of drops and bubbles in many interfacial gas evolution reactions in clean technologies.

Dynamic wetting of various liquids: Theoretical models, experiments, simulations and applications

Dynamic wetting is a ubiquitous phenomenon and frequently observed in our daily life, as exemplified by the famous lotus effect. It is also an interfacial process of upmost importance involving many cutting-edge applications and has hence received significantly increasing academic and industrial attention for several decades. However, we are still far away to completely understand and predict wetting dynamics for a given system due to the complexity of this dynamic process. The physics of moving contact lines is mainly ascribed to the full coupling with the solid surface on which the liquids contact, the atmosphere surrounding the liquids, and the physico-chemical characteristics of the liquids involved (small-molecule liquids, metal liquids, polymer liquids, and simulated liquids). Therefore, to deepen the understanding and efficiently harness wetting dynamics, we propose to review the major advances in the available literature. After an introduction providing a concise and general background on dynamic wetting, the main theories are presented and critically compared. Next, the dynamic wetting of various liquids ranging from small-molecule liquids to simulated liquids are systematically summarized, in which the new physical concepts (such as surface segregation, contact line fluctuations, etc.) are particularly highlighted. Subsequently, the related emerging applications are briefly presented in this review. Finally, some tentative suggestions and challenges are proposed with the aim to guide future developments.

Growth Rates of Hydrogen Microbubbles in Reacting Femtoliter Droplets

Chemical reactions in small droplets are extensively explored to accelerate the discovery of new materials and increase the efficiency and specificity in catalytic biphasic conversion and high-throughput analytics. In this work, we investigate the local rate of the gas-evolution reaction within femtoliter droplets immobilized on a solid surface. The growth rate of hydrogen microbubbles (≥500 nm in radius) produced from the reaction was measured online with high-resolution confocal microscopic images. The growth rate of bubbles was faster in smaller droplets and near the droplet rim in the same droplet. The results were consistent for both pure and binary reacting droplets and on substrates of different wettability. Our theoretical analysis based on diffusion, chemical reaction, and bubble growth predicted that the concentration of the reactant depended on the droplet size and the bubble location inside the droplet, in good agreement with experimental results. Our results reveal that the reaction rate may be spatially nonuniform in the reacting microdroplets. The findings may have implications for formulating the chemical properties and uses of these droplets.

Wood-Inspired Compressible Superhydrophilic Sponge for Efficient Removal of Micron-Sized Water Droplets from Viscous Oils

Efficient micron-sized droplet separation materials have become a new demand for environmental protection and economic development. However, existing separation methods are difficult to be effectively used for micron-sized water droplets surrounded by viscous oil, and common materials have difficulty maintaining hydrophilicity underoil. Here, inspired by the microstructure of tree xylem, we report a cellulose-polyurethane sponge (CP-Sponge) with wood-like pores and underoil superhydrophilicity using directional freeze-casting. The CP-Sponge has an excellent selective water absorption capacity underoil and compression resilience. This preparation strategy can flexibly control the sponge's dimensional morphology. The designed cylindrical CP-Sponge can be easily installed in the silicone tube of a peristaltic pump. During pump operation, with a simple absorption, compression, and recovery process, the CP-Sponge continuously and effectively removes micron-sized water from crude oil and lubricating oil, reducing residual water in the oil to less than 2 ppm. The absorption-saturated sponge can be dried to continue recycling. Eco-friendly, recyclable, and sustainable artificial porous sponges provide new ideas and inspiration for the practical application of deep dehydration of viscous oils.

Thermally assisted mobility of nanodroplets on surfaces with weak defects

HYPOTHESIS

Thermal activation plays an essential role in contact line dynamics on nanorough surfaces. However, the relation between the aforementioned concept and the sliding motion of nanodroplets remains unclear. As a result, thermally assisted motion of nanodroplets on nanorough surfaces is investigated in this work.

EXPERIMENTS

Steady slide and random motion of nanodroplets on surfaces with weak defects are investigated by Many-body Dissipative Particle Dynamics. The surface roughness is characterized by the slip length acquired from the velocity profile associated with the flowing film.

FINDINGS

The slip length is found to decline with increasing the defect density. The linear relationship between the sliding velocity and driving force gives the mobility and reveals the absence of contact line pinning. On the basis of the Navier condition, a simple relation is derived and states that the mobility is proportional to the slip length and the reciprocal of the product of viscosity and contact area. Our simulation results agree excellently with the theoretical prediction. In the absence of external forces, a two-dimensional Brownian motion of nanodroplets is observed and its mean square displacement decreases with increasing the defect density. The diffusivity is proportional to the mobility, consistent with the Einstein relation. This consequence suggests that thermal fluctuations are able to overcome contact line pinning caused by weak defects.

Taking a closer look: A molecular-dynamics investigation of microscopic and apparent dynamic contact angles

HYPOTHESIS

Molecular dynamics (MD) may be used to investigate the velocity dependence of both the microscopic and apparent dynamic contact angles (θ_m and θ_app).

METHODS

We use large-scale MD to explore the steady displacement of a water-like liquid bridge between two molecularly-smooth solid plates under the influence of an external force F⁰. A coarse-grained model of water reduces the computational demand and the solid-liquid affinity is varied to adjust the equilibrium contact angle θ⁰. Protocols are devised to measure θ_m and θ_app as a function of contact-line velocity U_cl.

FINDINGS

For all θ⁰, θ_m is velocity-dependent and consistent with the molecular-kinetic theory of dynamic wetting (MKT). However, θ_app diverges from θ_m as F⁰ is increased, especially at the receding meniscus. The behavior of θ_app follows that predicted by Voinov: (θ_app)³ = (θ_m)³ + 9Ca·ln(L/L_m), where Ca is the capillary number and L and L_m are suitably-chosen macroscopic and microscopic length scales. For each θ⁰, there is a critical velocity U_crit and contact angle θ_crit at which θ_app→0 and the receding meniscus deposits a liquid film. Setting θ_app=0, θ_m=θ_crit and U_cl=U_crit in the Voinov equation yields the value of L/L_m. The predicted values of θ_app then agree well with those measured from the simulations. Since θ_m obeys the MKT, we have, therefore, demonstrated the utility of the combined model of dynamic wetting proposed by Petrov and Petrov.

Spreading and receding of oil droplets on silanized glass surfaces in water: Role of three-phase contact line flow direction in spontaneous displacement

HYPOTHESIS

The spontaneous displacement of both spreading and receding droplets on surfaces are extensively involved in numerous technical applications. We hypothesize that the spreading and receding displacement behaviors could be interpreted differently due to opposite flow directions at the three-phase contact line.

EXPERIMENTS

We performed two groups of displacement experiments using different initial setups of oil droplets on silanized glass surfaces in aqueous surroundings.

FINDINGS

The different initial configurations mostly resulted in oil displacement in opposite directions: either spreading or receding of the oil droplet. Different static states were observed at the end of the spreading and receding processes on surfaces with the same wettability due to the contact angle hysteresis. The dynamic displacement was analyzed using the hydrodynamic and molecular kinetic models, which showed distinct applicabilities for the data description of the spreading and receding possesses. The model analysis further indicated the different nature of these possesses, in particular, the resistance to displacement dynamics, which was illustrated by the interpretation of the microscopic slip length and contact line friction in the respective models. This study can shed light on the fundamental role of the displacement direction in the spontaneous liquid-liquid displacement.

Role of surfactants in spontaneous displacement of high viscosity oil droplets from solid surfaces in aqueous solutions

HYPOTHESIS

Displacement of oil droplets receding from solid surfaces in aqueous solutions plays a critical role in many household activities and industrial operations. Surfactants are often involved in these activities to control the displacement process. We hypothesize that the influence of surfactants on the displacement process of oil is highly dependent on the type and dosage of surfactants, with the mechanisms being elucidated by the analysis using appropriate dynamic wetting models.

EXPERIMENTS

We systematically investigated the spontaneous displacement of a high viscosity oil on curved hydrophilic glass surfaces in aqueous solutions of anionic sodium dodecylbenzene sulfonate, cationic hexadecyl trimethyl ammonium bromide, and nonionic Triton^TM X-100 over a wide range of concentrations.

FINDINGS

The rather different oil displacement behaviors were observed with different surfactant additions. The displacement dynamics of the receding oil droplet was found to be inhibited by surfactant additions and followed two distinct models quantitatively: the power-law model describing the temporal evolution of early-stage displacement, and the molecular kinetic model describing the dependence of the three-phase contact line displacement velocity on the dynamic contact angle at the late stage of oil displacement. The model-based data analysis provided insights on the role of surfactants in controlling the oil displacement dynamics.

Counterion-Dictated Self-Cleaning Behavior of Polycation Coating upon Water Action: Macroscopic Dissection of Hydration of Anions

The counterions of polydiallyldimethylammonium (PDADMA) coatings were altered by incubation in aqueous solutions of different electrolytes. Oil de-wetting on the resulting polycationic surfaces upon water action exhibited a straightforward connection with the Jones-Dole viscosity B-coefficient (Bη) sign of surface counteranions. Upon water action, surface counteranions with negative Bη render PDADMA coatings oil-adhering, but those with positive Bη furnish PDADMA coatings with excellent self-cleaning. The oil-adhering PDADMA surfaces can become self-cleaning upon water action in response to the Bη of surface counteranions sign-switching with increasing water temperature. Courtesy of surface counter-anions with Bη>0, self-cleaning PDADMA coatings enable not only conversion of conventional meshes into self-cleaning membranes for oil/water separation, but also regioselective maneuver of oil flow on polycationic surfaces according to the Bη sign of surface counteranions patterned atop.

Antibuoyancy and Unidirectional Gas Evolution by Janus Electrodes with Asymmetric Wettability

The bubbles electrochemically generated by gas evolution reactions are commonly driven off the electrode by buoyancy, a weak force used to overcome bubble adhesion barriers, leading to low gas-transporting efficiency. Herein, a Janus electrode with asymmetric wettability has been prepared by modifying two sides of a porous stainless-steel mesh electrode, with superhydrophobic polytetrafluoroethylene (PTFE) and Pt/C (or Ir/C) catalyst with well-balanced hydrophobicity, respectively, affording unidirectional transportation of as-formed gaseous hydrogen and oxygen from the catalyst side to the gas-collecting side during water splitting. "Bubble-free" electrolysis was realized while "floating" the Janus electrode on the electrolyte. Antibuoyancy through-mesh bubble transportation was observed while immersing the electrode with the PTFE side downward. The wettability gradient within the electrode endowed sticky states of bubbles on the catalyst side, resulting in efficient bubble-free gas transportation with 15-fold higher current density than submerged states.

Interaction Between the Cyclopentane Hydrate Particle and Water Droplet in Hydrocarbon Oil

The presence of immiscible water drops in bulk hydrocarbon is likely to bridge hydrate particles to cause hydrate agglomeration, leading to potential pipeline blockage. This can become a major challenge for flow assurance in offshore petroleum transportation. To avoid hydrate aggregation, the attachment between hydrate and water drops should be avoided. In this study, we used our home-designed integrated thin film drainage apparatus to investigate the interactions between a hydrate particle and a water drop inside model oil (i.e., mixture of cyclopentane and toluene with a volumetric ratio of 1:1). Our experiments showed that asphaltenes, a natural component in crude oil, were an effective inhibitor for the attachment between water drops and hydrate particles. Without asphaltenes in the system, the water drop adhered to the hydrate particle immediately after the two surfaces contacted. By adding 0.03 g/L asphaltenes into the oil phase, the attachment was delayed by 0.7 s when the applied preload force was set to around 0.05 mN. By increasing the asphaltenes addition to 0.05 g/L, the attachment between the hydrate and water drop was prevented even when the contact time lasted up to 25 s. This phenomenon could be explained by the adsorption of an asphaltenes layer along the interface between the aqueous drop and hydrocarbon. Measurements of the dynamic interfacial tension and crumping ratio confirmed the presence of the adsorption layer. The addition of 0.6 mol/L NaCl or 0.3 mol/L CaCl₂ in the aqueous drop could further enhance the strength of the adsorption layer. Results of this research provide understanding of the benefits of asphaltenes and salt in preventing hydrate agglomeration.

How Surfactants Affect Droplet Wetting on Hydrophobic Microstructures

Surfactants, as amphiphilic molecules, adsorb easily at interfaces and can detrimentally destroy the useful, gas-trapping wetting state (Cassie-Baxter, CB) of a drop on superhydrophobic surfaces. Here, we provide a quantitative understanding of how surfactants alter the wetting state and contact angle of aqueous drops on hydrophobic microstructures of different roughness (r) and solid fraction (ϕ). Experimentally, at low surfactant concentrations (C), some drops attain a homogeneous wetting state (Wenzel, W), while others attain the CB state whose large contact angles can be predicted by a thermodynamic model. In contrast, all of our high-C drops attain the Wenzel state. To explain this observed transition, we consider the free energy and find that, theoretically, for our surfaces the W state is always preferred, while the CB state is metastable at low C, consistent with experimental results. Furthermore, we provide a beneficial blueprint for stable CB states for applications exploiting superhydrophobicity.

Robust Oil-Fouling Resistance of Amorphous Cellulose Surface Underwater: A Wetting Study and Application

The antioil-fouling characteristic of an amorphous cellulose (a-cellulose) surface was elucidated using the sessile droplet method (static) and a modified Wilhelmy plate technique (dynamic). As compared to other hydrophilic surfaces (cellulose acetate, poly(vinyl alcohol), and glass), the oil (poly(dimethylsiloxane)) contact angle on the a-cellulose surface underwater shows the largest value (170.5 ± 5.0°), having the smallest deviation from its theoretical value (180.0°) as estimated by Young's equation. Also, the a-cellulose surface demonstrates the strongest affinity with water in an oil medium (stable hydrophilicity). Moreover, the work of adhesion between the receding oil phase and a-cellulose underwater is quantified to be 10.3 mN/m, approximately one-fourth of that in air (42.0 mN/m). The overall wetting study suggests a rather low oil/solid/water three-phase contact line (TPL) friction in the direction that water displaces oil. A proposed mechanism attributes these phenomena to the water-accessible rigid cellulose chains and supramolecular structure of a-cellulose. The former hinders molecular rearrangement during processing or upon exposure to oil, such that its polar hydroxyl groups are readily accessible to water, thereby retaining its hydrophilicity. The latter allows water to diffuse across the TPL, forming the hydration shells that weaken the van der Waals interactions between oil and cellulose chains. Such findings of the a-cellulose surface can be exploited to fabricate mesh membranes with high water permeation flux (375.4 ± 13.5 L m^-2 h^-1 Pa^-1), high oil/water separation efficiency (93-98%), and long-lasting stability, which is suitable for offshore oil spill remediation.

Spontaneous Displacement of High Viscosity Micrometer Size Oil Droplets from a Curved Solid in Aqueous Solutions

Spontaneous displacement of high viscosity (∼10³ Pa·s) micrometer size oil droplets from a curved solid in aqueous solutions was investigated. For high viscosity oils, the dynamic droplet shape was found to deviate significantly from a spherical cap shape due to the considerable viscous force in the oil phase. The displacement dynamics of high viscosity droplets were analyzed using molecular kinetic and hydrodynamic models. The molecular kinetic model was found to describe the dynamic displacement well for the droplets of small departure from the spherical cap shape, while the hydrodynamic model is more applicable to the droplets of higher three-phase contact line displacement velocities and hence larger deviation of the droplets from the spherical cap shape.

Controlled defects to link wetting properties to surface heterogeneity

Small-scale heterogeneities have long been understood to give rise to contact angle hysteresis. More recently, the question of how they influence contact line dynamics has generated interest. Models that express the hysteresis or dynamics in terms of defect properties exist but have yet to be conclusively tested by experiment. Here we produce heterogeneous surfaces and thoroughly characterize individual defects. Precise dynamical measurements allow us to conclude that there is no evidence of a thermally activated dynamics due to their presence, but that the hysteresis scales with their concentration and the square of their height, as predicted by some simple models.

Probing Boundary Conditions at Hydrophobic Solid-Water Interfaces by Dynamic Film Drainage Measurement

A newly developed dynamic force apparatus was used to determine hydrodynamic boundary conditions of a liquid on a hydrophobic silica surface. For a given approach velocity of bubble to solid surfaces in an electrolyte solution, a reduced dimple formation and faster film drainage were observed by increasing the hydrophobicity of silica surfaces, indicating a significant change in hydrodynamic boundary conditions of water molecules from an immobile to a mobile water-hydrophobic silica interface. By comparing the measured film profiles with the predictions from the Stokes-Reynolds-Young-Laplace model, the slippage boundary condition of water on the hydrophobic silica surface of surface nanoroughness was quantified. Increasing the surface hydrophobicity was found to increase the mobility of water in the thin liquid film, promoting faster drainage of the liquid. For a given hydrophobicity of solids, the mobility of water occurred only above a critical bubble approach velocity and increased with increasing bubble approach velocity. In contrast, similar experiments with hydrophobized mica surfaces showed no-slip boundary condition of water at the molecularly smooth hydrophobic surface. The results collectively suggest that the observed mobility of water with more than 100 nm in thickness on the studied hydrophobic silica surfaces was due to the nanoroughness of hydrophobic surfaces. Such finding sheds light on one possible way of reducing the friction of water on hydrophobic solid surfaces by creating nanostructured surface of nanoroughness.

Thermally activated motion of a contact line over defects

At the nanometer scale, the motion of a contact line separating a dry from a wet region is limited by the presence of surface heterogeneities that pin it. Here we revisit the seminal model proposed by Joanny and de Gennes to include the influence of thermal noise and viscosity using a Langevin model with two degrees of freedom: the average position of the contact line and its distortion. We identify the conditions under which the dynamics in a velocity-driven experiment can in fact be described by a constant forcing at small scale. We then relate the asymptotic properties of the relation between force and contact line velocity to the properties of the defects. In particular, we show that Kramers' approximation misses the strong asymmetry between advancing and receding directions. Finally, we show how to use the model to fit experimental data and extract the salient features of the surface energy landscape.

The Role of Controlled Surface Topography and Chemistry on Mouse Embryonic Stem Cell Attachment, Growth and Self-Renewal

The success of stem cell therapies relies heavily on our ability to control their fate in vitro during expansion to ensure an appropriate supply. The biophysical properties of the cell culture environment have been recognised as a potent stimuli influencing cellular behaviour. In this work we used advanced plasma-based techniques to generate model culture substrates with controlled nanotopographical features of 16 nm, 38 nm and 68 nm in magnitude, and three differently tailored surface chemical functionalities. The effect of these two surface properties on the adhesion, spreading, and self-renewal of mouse embryonic stem cells (mESCs) were assessed. The results demonstrated that physical and chemical cues influenced the behaviour of these stem cells in in vitro culture in different ways. The size of the nanotopographical features impacted on the cell adhesion, spreading and proliferation, while the chemistry influenced the cell self-renewal and differentiation.

Oleofoams: Properties of Crystal-Coated Bubbles from Whipped Oleogels-Evidence for Pickering Stabilization

Interfacially stabilized nonaqueous lipid-based foams, which we name here oleofoams, are rarely encountered as opposed to the large number of aqueous foams stabilized by molecular or particulate emulsifiers. There is no case well described in the literature with a convincing characterization of the interfacial contribution to oleofoam stability. Methods for filling this gap are described here, which reach out to a large part of the lipid phase diagram. We bring here complete evidence that lipidic crystals made of a high fraction of fully soluble monoglyceride (MG) in oil do not only adsorb at the oil-air interface but also can easily form a jammed, closely packed layer of crystals around the bubbles of a foam produced by whipping (Pickering effect). Very fine bubbles, soft textures, or firmer ones such as for shaving foams could be obtained, with a high air fraction (up to 75%), which is unprecedented. A thin, jammed layer of crystals on bubbles can cause bubbles to retain nonspherical shapes in the absence of bulk effects for times much longer than the characteristic capillary relaxation time for bare bubbles, which is actual evidence for Pickering-type interfacial stabilization. By comparing to foams obtained by depressurization, we show that whipping is necessary for bubble wrapping with a layer of crystals. The origin of high stability against Ostwald ripening at long times is also discussed. Furthermore, we show that these Pickering whipped foams have rheological properties dominated by interfacial or film contributions, which is of high interest for food and cosmetics applications because of their high moduli. This system can be considered to be a model of the crystallization behavior of MG in oil, which is similar to that in many fats. Our methods are very general in the context of lipid-based foaming, in particular, from food materials, and were used in patent applications.

Effects of the Molecular Structure of a Self-Assembled Monolayer on the Formation and Morphology of Surface Nanodroplets

The formation and morphology of microscopic droplets on a chemically modified surface are important for many droplet-related applications. In this study, we examined the formation and morphological characteristics of nanodroplets produced in the same process of solvent exchange on a gold surface coated with a methyl-terminated alkanethiol monolayer. From atomic force microscopy images, we obtained the contact angles of polymerized nanodroplets in 12 combinations of the length of a straight alkyl chain and the type of droplet liquid. Our results show a significant decrease in the number density of the droplets as the number of methyl groups extends from 8 to 12 or 14. The contact angle of the droplets on octanethiol is significantly larger than that on dodecanethiol or tetradecanethiol, possibly because of the screening effect from the monolayer. Our results demonstrate that under the same solution conditions the morphology of surface nanodroplets is sensitive to the detailed molecular structures of the monolayer on the substrate. This finding has important implications for understanding static wetting on the microscopic scale and the origin of three-phase contact line pinning.

Nanoscale View of Dewetting and Coating on Partially Wetted Solids

There remain significant gaps in our ability to predict dewetting and wetting despite the extensive study over the past century. An important reason is the absence of nanoscopic knowledge about the processes near the moving contact line. This experimental study for the first time obtained the liquid morphology within 10 nm of the contact line, which was receding at low speed (U < 50 nm/s). The results put an end to long-standing debate about the microscopic contact angle, which turned out to be varying with the speed as opposed to the constant-angle assumption that has been frequently employed in modeling. Moreover, a residual film of nanometer thickness ubiquitously remained on the solid after the receding contact line passed. This microscopic residual film modified the solid surface and thus made dewetting far from a simple reverse of wetting. A complete scenario for dewetting and coating is provided.

Defects at the Nanoscale Impact Contact Line Motion at all Scales

The contact angle of a liquid drop moving on a real solid surface depends on the speed and direction of motion of the three-phase contact line. Many experiments have demonstrated that pinning on surface defects, thermal activation and viscous dissipation impact contact line dynamics, but so far, efforts have failed to disentangle the role of each of these dissipation channels. Here, we propose a unifying multiscale approach that provides a single quantitative framework. We use this approach to successfully account for the dynamics measured in a classic dip-coating experiment performed over an unprecedentedly wide range of velocity. We show that the full contact line dynamics up to the liquid film entrainment threshold can be parametrized by the size, amplitude and density of nanometer-scale defects. This leads us to reinterpret the contact angle hysteresis as a dynamical crossover rather than a depinning transition.

Tuning and predicting the wetting of nanoengineered material surface

The wetting of a material can be tuned by changing the roughness on its surface. Recent advances in the field of nanotechnology open exciting opportunities to control macroscopic wetting behaviour. Yet, the benchmark theories used to describe the wettability of macroscopically rough surfaces fail to fully describe the wetting behaviour of systems with topographical features at the nanoscale. To shed light on the events occurring at the nanoscale we have utilised model gradient substrata where surface nanotopography was tailored in a controlled and robust manner. The intrinsic wettability of the coatings was varied from hydrophilic to hydrophobic. The measured water contact angle could not be described by the classical theories. We developed an empirical model that effectively captures the experimental data, and further enables us to predict the wetting of surfaces with nanoscale roughness by considering the physical and chemical properties of the material. The fundamental insights presented here are important for the rational design of advanced materials having tailored surface nanotopography with predictable wettability.

Liquid-Solid Nanofriction and Interfacial Wetting

Using atomic force microscopy, the nanofriction coefficient was measured systematically for a series of liquids on planar graphite, silica and mica surfaces. This allows us to explore the quantitative interplay between nanofriction at liquid-solid interfaces and interfacial wetting. A corresponding states theory analysis shows that the nanofriction coefficient, μ = dF(F)/dF(N), where FF is the friction force and FN is the normal force, is a function of three dimensionless parameters that reflect the intermolecular forces involved and the structure of the solid substrate. Of these, we show that one parameter in particular, β = ρ(s)Δ(s)σ(ls)(2), where ρ(s) is the atomic density of the solid, Δ(s) is the spacing between layers of solid atoms, and σ(ls) is the molecular diameter that characterizes the liquid-substrate interaction, is very important in determining the friction coefficient. This parameter β, which we term the structure adhesion parameter, provides a measure of the intermolecular interaction between a liquid molecule and the substrate and also of the surface area of contact of the liquid molecule with the substrate. We find a linear dependence of μ on the structure adhesion parameter for the systems studied. We also find that increasing β leads to an increase in the vertical adhesion forces FA (the attractive force exerted by the solid surface on the liquid film). Our quantitative relationship between the nanofriction coefficient and the key parameter β which governs the vertical adhesive strength, opens up an opportunity for describing liquid flows on solid surfaces at the molecular level, with implications for the development of membrane and nanofluidic devices.

Wetting hysteresis induced by nanodefects

Wetting of actual surfaces involves diverse hysteretic phenomena stemming from ever-present imperfections. Here, we clarify the origin of wetting hysteresis for a liquid front advancing or receding across an isolated defect of nanometric size. Various kinds of chemical and topographical nanodefects, which represent salient features of actual heterogeneous surfaces, are investigated. The most probable wetting path across surface heterogeneities is identified by combining, within an innovative approach, microscopic classical density functional theory and the string method devised for the study of rare events. The computed rugged free-energy landscape demonstrates that hysteresis emerges as a consequence of metastable pinning of the liquid front at the defects; the barriers for thermally activated defect crossing, the pinning force, and hysteresis are quantified and related to the geometry and chemistry of the defects allowing for the occurrence of nanoscopic effects. The main result of our calculations is that even weak nanoscale defects, which are difficult to characterize in generic microfluidic experiments, can be the source of a plethora of hysteretical phenomena, including the pinning of nanobubbles.

Current opinion in superspreading mechanisms

BACKGROUND

An aqueous solution of trisiloxane-ethoxylate surfactants (superspreaders) has fascinating surface properties that promote rapid spreading over a large area of difficult-to-wet substrates. The overall spread area achieved by an aqueous droplet containing superspreaders can be as much as 50 times greater than water, and 25 times more effective than a conventional surfactant on a leaf's surface. The phenomenon that drives superspreading is still not well understood and it is under continuous discussion.

CONCEPTS AND MODELS

The goal of this paper is to review the data published to elucidate the concepts and mechanisms presented in the literature and to propose a model for superspreading.

CONCLUSIONS

A simple model was proposed concerning the surface tension gradient (or Marangoni flow) over the curved droplet surface when the value of the wetting angle was considered. The model predicted the optimum rate of spreading vs. angle at θ = 60° and was in a satisfactory agreement with the experimental observations.

Superhydrophobic nanocoatings: from materials to fabrications and to applications

Superhydrophobic nanocoatings, a combination of nanotechnology and superhydrophobic surfaces, have received extraordinary attention recently, focusing both on novel preparation strategies and on investigations of their unique properties. In the past few decades, inspired by the lotus leaf, the discovery of nano- and micro-hierarchical structures has brought about great change in the superhydrophobic nanocoatings field. In this paper we review the contributions to this field reported in recent literature, mainly including materials, fabrication and applications. In order to facilitate comparison, materials are divided into 3 categories as follows: inorganic materials, organic materials, and inorganic-organic materials. Each kind of materials has itself merits and demerits, as well as fabrication techniques. The process of each technique is illustrated simply through a few classical examples. There is, to some extent, an association between various fabrication techniques, but many are different. So, it is important to choose appropriate preparation strategies, according to conditions and purposes. The peculiar properties of superhydrophobic nanocoatings, such as self-cleaning, anti-bacteria, anti-icing, corrosion resistance and so on, are the most dramatic. Not only do we introduce application examples, but also try to briefly discuss the principle behind the phenomenon. Finally, some challenges and potential promising breakthroughs in this field are also succinctly highlighted.

Thermally enhanced self-propelled droplet motion on gradient surfaces

Enhanced droplet movement at elevated temperatures.

Unorthodox bubbles when boiling in cold water

High-speed movies are taken when bubbles grow at gold surfaces heated spotwise with a near-infrared laser beam heating water below the boiling point (60-70 °C) with heating powers spanning the range from very low to so high that water fails to rewet the surface after bubbles detach. Roughly half the bubbles are conventional: They grow symmetrically through evaporation until buoyancy lifts them away. Others have unorthodox shapes and appear to contribute disproportionately to heat transfer efficiency: mushroom cloud shapes, violently explosive bubbles, and cavitation events, probably stimulated by a combination of superheating, convection, turbulence, and surface dewetting during the initial bubble growth. Moreover, bubbles often follow one another in complex sequences, often beginning with an unorthodox bubble that stirs the water, followed by several conventional bubbles. This large dataset is analyzed and discussed with emphasis on how explosive phenomena such as cavitation induce discrepancies from classical expectations about boiling.