Lyophobic Surfaces Research Articles

Contact angle calculations for argon and water sessile droplets on planar lyophilic and lyophobic surfaces within molecular dynamics modeling

There exist many ways to calculate the contact angle of a sessile droplet on a planar surface on the basis of molecular modeling data, and the result may depend on the chosen approach. This paper presents molecular dynamics simulations of argon and water sessile droplets on simple model and more complex atomistic substrates. Several approaches for calculating contact angles are described. These approaches are based on an analysis of instantaneous droplet’s configurations and averaged density profiles. The latter are analyzed using density cutoffs and the Sobel filters. Parameters of the interaction between the planar surface and the fluid are being tuned in order to consider model lyophilic and lyophobic substrates. Dependences of the contact angle on the strength of interaction between the fluid and the substrate and on the droplet size are obtained. The apparent line tensions for the observed sessile droplets are evaluated.

Singular jets during the impingement of compound drops upon lyophilic surfaces

An important phenomenon produced during the impingement of drops upon solid surfaces is the formation of singular jet, which is often followed by the pinch-off of satellite droplets. Great efforts have been made to investigate the jetting dynamics of low-viscosity single-phase drops impact upon sufficiently lyophobic surfaces. However, whether such singular jets can be produced during the impact of compound drops and how the liquid properties and surface wettabilities affect the dynamics have remained largely unexplored. Herein, we perform comparative and systematic experiments on the impact dynamics of single-phase water and silicon oil drops, as well as water-in-oil compound drops on lyophilic substrates. We show that singular jets only occur during the impact of compound drops. The critical values in terms of the Weber number depend on both the viscosity of the silicon oil and the volume ratio of the two liquids composing the compound drops. We also show that the singular jets break up and throw out satellite droplets only when they are considerably fast and thin. Power-law correlations between the jet velocities and the jet radii, between the jet neck radius and time, and between the maximum jet height and the jetting time are obtained. A linear correlation between the radii of the jet droplets and those of the singular jets is also found and analyzed.

Impinging blood droplets on different wettable surfaces: Impact phenomena, contact line motion, post-impact oscillation and dried stains

Understanding the underlying hydrodynamics of impinging blood droplets and finding out the physical parameters determining the bloodstain characteristics are of great importance in blood related forensic investigations. In this work, the impact of non-Newtonian blood droplets on solid surfaces ranging from lyophilic to superlyophobic was systematically investigated and compared to that of Newtonian droplets with a similar dynamic shear viscosity. We show that impinging blood droplets behave as low-viscosity Newtonian droplets in the short-time spreading, which is dominated by capillary and inertial forces, but their non-Newtonian viscoelasticity would notably affect the droplet retraction and post-impact oscillation occurring in large timescales. Whereas the strong liquid–solid adhesion and the non-Newtonian elongational viscosity hinder droplet recoiling and thus alter the impact phenomena on lyophobic and superlyophobic surfaces, the shear and elongational viscosities are coupled to result in higher damping coefficients of oscillating blood droplets after deposition, in comparison to that of impinging Newtonian droplets. The size of the dried bloodstain was found to be different from both the maximum spreading radius of the droplet that can reach during impact and the final radius of the deposited droplet after oscillation, and their correlations are highly dependent on the impact velocity and surface wettability. Moreover, the morphologic characteristics of the bloodstains would also be changed by varying either the impact velocity or the surface wettability. We envision that these findings can not only find applications in the bloodstain pattern analysis, but also provide useful information for medical diagnosis based on blood droplet test.

Disjoining pressure in vapor layers near planar and spherical lyophobic surfaces

A classical square-gradient density functional theory is used for describing disjoining pressure in vapor layers between a lyophobic planar smooth solid substrate and stable bulk liquid, and between a lyophobic nanosized solid sphere and stretched bulk liquid. Profiles of local density, of normal and tangential components of the local pressure tensor in the interlayers are calculated and analyzed for planar substrate and spherical particle at several values of the chemical potential of the fluid and parameters controlling wettability of solid. The pressure tensor is found to be significantly anisotropic in the vapor layers near the smooth solid surfaces. Mechanical and thermodynamic definitions of the disjoining pressure in the spherical vapor layer were compared and shown to be consistent. In contrast to case of liquid films, the disjoining pressure in vapor layers near the nanosized lyophobic solid particle is found to be greater than near the planar wall. It has been directly shown that, depending on lyophobicity, the disjoining pressure isotherms change from nonmonotonic to monotonic functions of the vapor layer thickness.

Oblique droplet impact on superhydrophobic surfaces: Jets and bubbles

Previous studies revealed that the perpendicular impact of low-viscosity droplets on sufficiently lyophobic surfaces would stimulate a liquid jet during droplet recoiling, and in some cases, it is accompanied with the entrapment of an air bubble. However, whether such free-surface flow phenomena occur in oblique droplet impact and how surface inclinations influence the dynamics remain open questions. Herein, we perform a comparative investigation on the perpendicular and oblique impact dynamics of water droplets on superhydrophobic surfaces. We show that the inclinations of the target surface do not influence the jet emission and the bubble entrapment in droplet impact. The jetting phenomena, which are triggered by the collapse of the air cavity, can be classified into three types of scenarios, and the jet velocity scales with its size according to two power laws as suggested by the scaling analyses in the previous studies. On the other hand, the air bubble entrapment is caused by the asymmetric cavity collapse when the recoiling speed of the droplet top is close to that of the bottom, and the bubble size can be reduced by the rising jet from the cavity bottom. We also show that some entrapped bubbles on inclined surfaces would move to the droplet surface and eventually burst, shooting out another thin jet. The correlation between the jet velocity and the bubble size is compared with different empirical scaling laws in the literature.

Thermodynamic properties of stable and unstable vapor shells around lyophobic nanoparticles

Existence of stable and unstable spherical shell-like vapor interlayers (or concentric nanobubbles) near lyophobic surfaces has been proved and their thermodynamic properties have been studied within the square gradient density functional theory. The equilibrium density profiles around lyophobic nanoscopic particles (without and with electric charge) in a stretched argon-like liquid have been computed. The combination of the Carnahan–Starling and the mean-field models for fluid–fluid interaction and the total Lennard-Jones potential of interaction between the particle and fluid molecule have been used. The lyophobicity of the particle has been controlled by the energy parameter for attraction of molecules of the particle and the fluid molecules. This parameter has been taken considerably smaller than the energy parameter for fluid–fluid molecular attraction. As a result, two equilibrium radial density profiles corresponding to two concentric vapor shells around particle were found at a fixed value of the condensate chemical potential below its value for the flat equilibrium. It was shown that the smaller shell is related to the minimum of the work of the vapor shell formation and represents a stable nanobubble, while the larger shell corresponds to the maximum of this work and refers to the unstable critical nanobubble. The equimolecular radii of the stable and unstable concentric nanobubbles increase with the radius of the core particle. The curve of the dependence of the chemical potential of fluid molecules in the bubble with the lyophobic core particle on the bubble radius has a minimum, below which heterogeneous nucleation of bubbles becomes thermodynamically barrierless. The appearance of the electric charge on the particle shifts the minimum of the condensate chemical potential deeper and inhibits bubble nucleation. The dependence of the bubble surface tension on the radius of the equimolecular dividing surface and the charge of the particle has been found for bubble at heterogeneous nucleation and compared with that for bubble and droplet at homogeneous nucleation and droplet at heterogeneous nucleation on lyophilic particle.

液滴撞击疏液表面的实验研究

液滴撞击疏液表面的实验研究

Lyophobic slippery surfaces on smooth/hierarchical structured substrates and investigations of their dynamic liquid repellency.

Slippery surfaces were prepared by infusing lubricant into smooth or hierarchical-structured superhydrophobic surfaces (SHS) to compare different surface-free energies. The surfaces obtained showed good repellency towards liquids with various values of surface tension/molecular polarity/viscosity, including hexane, tetradecane, water, ethylene glycol and viscous engine oil. The lyophobicity could be realized on a relatively smooth surface, indicating that the first principle of preparing a lyophobic slippery surface is to perform a low surface-free energy modification. The dynamic liquid repellency was also studied: the sliding speeds of different liquids on lubricant infused SHS showed a negative correlation to their kinematic viscosity values, and a higher surface roughness was favorable for dynamic wettability, whereas for the smooth slippery surface, the travelling speeds showed randomness.

Molecular dynamics simulation of wetting and evaporation characteristics for sessile nanofluid nanodroplets

Wetting and evaporation characteristics of sessile nanofluid nanodroplet (SNFND) were investigated by molecular dynamics simulation. Results show that for lyophilic surface, with the increase of nanoparticle energy coefficient, contact angle of SNFND increases and total evaporation decreases. Effects of nanoparticle volume percentage on the wetting and evaporation behaviors of SNFND are related to surface wettability. For lyophilic surface, the greater the nanoparticle volume percentage, the greater the contact angle, and the slower the evaporation. For neutral and lyophobic surfaces, nanoparticle volume percentage has obviously strengthening effect on evaporation, and the strengthening effect decreases as nanoparticle volume percentage increases.

Impact of viscous droplets on different wettable surfaces: Impact phenomena, the maximum spreading factor, spreading time and post-impact oscillation

In this paper, we experimentally investigated the impact dynamics of different viscous droplets on solid surfaces with diverse wettabilities. We show that the outcome of an impinging droplet is dependent on the physical property of the droplet and the wettability of the surface. Whereas only deposition was observed on lyophilic surfaces, more impact phenomena were identified on lyophobic and superlyophobic surfaces. It was found that none of the existing theoretical models can well describe the maximum spreading factor, revealing the complexity of the droplet impact dynamics and suggesting that more factors need to be considered in the theory. By using the modified capillary-inertial time, which considers the effects of liquid viscosity and surface wettability on droplet spreading, a universal scaling law describing the spreading time was obtained. Finally, we analyzed the post-impact droplet oscillation with the theory for damped harmonic oscillators and interpreted the effects of liquid viscosity and surface wettability on the oscillation by simple scaling analyses.

Influence of interface wettability on normal and explosive boiling of ultra‐thin liquid films on a heated substrate in nanoscale: a molecular dynamics study

Wettability, as one of the important properties of solid surface, may influence heat and mass transfer in boiling process. Molecular dynamics method is employed to investigate the effects of wettability on normal and explosive boiling of ultra-thin liquid argon film absorbed on a heated solid aluminium surface in a confined space in present work. The initial three-phase molecular system is comprised of solid aluminium wall, liquid argon and vapour argon and is run for three different solid–liquid interfacial wettability (lyophilic, lyophobic and neutral surfaces), which achieves a balance at 90 K. After equilibrium period, two different jump temperatures degree, 150 and 350 K, are set on heat source layers separately to characterise the boiling phenomena, namely, low-temperature degree for normal boiling and high-temperature degree for explosive boiling in which temperature of solid wall is far beyond the critical temperature of liquid argon. The simulation results indicate that the wetting condition of solid–liquid interfacial surface have significant effects on both cases of boiling phenomena. Furthermore, the heat transfer rate with good wettability (lyophilic) is much higher than bad wettability (lyophobic) with same jump temperature degree.

Spatially Uniform Thin-Film Formation of Polymeric Organic Semiconductors on Lyophobic Gate Insulator Surfaces by Self-Assisted Flow-Coating

Surface hydrophobization by self-assembled monolayer formation is a powerful technique for improving the performance of organic field-effect transistors (OFETs). However, organic thin-film formation on such a surface by solution processing often fails due to the repellent property of the surface against common organic solvents. Here, a scalable unidirectional coating technique that can solve this problem, named self-assisted flow-coating, is reported. Producing a specially designed lyophobic-lyophilic pattern on the lyophobic surface enables organic thin-film formation in the lyophobic surface areas by flow-coating. To demonstrate the usefulness of this technique, OFET arrays with an active layer of poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene) are fabricated. The ideal transfer curves without hysteresis behavior are obtained for all OFETs. The average field-effect hole mobility in the saturation regime is 0.273 and 0.221 cm2·V-1·s-1 for the OFETs with the channels parallel and perpendicular to the flow-coating direction, respectively, and the device-to-device variation is less than 3% for each OFET set. Very small device-to-device variation is also obtained for the on-state current, threshold voltage, and subthreshold swing. These results indicate that the self-assisted flow-coating is a promising coating technique to form spatially uniform thin films of polymeric organic semiconductors on lyophobic gate insulator surfaces.

Design and Fabrication of the Lyophobic Slippery Surface and Its Application in Anti-Icing

A suspension that can be sprayed onto substrates was developed to form a superhydrophobic/oleophilic surface. Lyophobic slippery surfaces were prepared by infusing perfluorinated lubricants into the superhydrophobic coating to repel almost all liquids with low surface tension values, including hexane, kerosene, and diesel oil, showing a transition between superoleophilicity and lyophobicity. In addition, the traveling speeds of liquids appeared to be negatively correlated with the kinematic viscosity. In the anti-icing tests, the droplet was pinned after contacting a 0 °C textured superhydrophobic surface for a few seconds because of the meniscus caused by the condensation of atmospheric humidity; by contrast, on the lyophobic slippery surface, a water droplet could easily slide even at −20 °C, demonstrating superior icing resistance.

Thermohydrodynamics of boiling in binary compressible fluids.

We numerically study the thermohydrodynamics of boiling for a CO(2) + ethanol mixture on lyophilic and lyophobic surfaces in both closed and open systems, based on a diffuse interface model for a two-component system. The corresponding wetting boundary conditions for an isothermal system are proposed and verified in this paper. New phenomena due to the addition of another component, mainly the preferential evaporation of the more volatile component, are observed. In the open system and the closed system, the physical process shows very different characteristics. In the open system, except for the movement of the contact line, the qualitative features are rather similar for lyophobic and lyophilic surfaces. In the closed system, the vortices that are observed on a lyophobic surface are not seen on a lyophilic surface. More sophisticated wetting boundary conditions for nonisothermal, two-component systems might need to be further developed, taking into account the variations of density, temperature, and surface tension near the wall, while numerical results show that the boundary conditions proposed here also work well even in boiling, where the temperature is nonuniform.

Combined nitrogen, hexane, and benzene adsorption characterization of pores and surfaces of lyophobic mesoporous silicas.

For lyophobic porous surfaces, structural analysis by vapor adsorption is complicated due to weak adsorbate-adsorbent interactions and limited wetting of the pores (nonzero contact angles). To investigate further, adsorption isotherms of three distinct adsorbates (nitrogen - 77 K, n-hexane and benzene - 298 K) were studied for SBA-15 ordered mesoporous silica where the surface was functionalized with lyophobic perfluoroalkyl groups (C6F13 termini). The results demonstrated a clear advantage of the combined use of the adsorption isotherms of less surface sensitive (nitrogen) and more surface sensitive (hydrocarbons) adsorbates. The adsorption of nitrogen provided basic structural characteristics like surface area, pore volume, and pore size distribution, while the isotherms of benzene and n-hexane were used to characterize wetting (contact angles) and surface energy of the C6F13 surfaces within the pores. For the first time, the statistical film thickness for nitrogen, benzene, and n-hexane are being reported for the adsorption on fluorinated surfaces, thereby providing critical data for the pore size and the contact angle determination of the lyophobic materials.

The sliding drop method for optimizing surface energies for patterning in a roll-to-roll process

We describe a method of assessing and optimizing the de-wetting properties of a lyophobic substrate by using a sliding liquid drop to determine the maximum speed of de-wetting of an aqueous liquid on a range of lyophobic surfaces. The sliding drop method has direct relevance to the use of surface energy patterns on flexible substrates, to create discrete liquid patterns in a continuous, roll-to-roll manner. Surface energy patterning makes use of a substrate pre-patterned with lyophilic and lyophobic regions which is over-coated with a liquid; the liquid withdraws from the lyophobic areas and collects on the lyophilic ones. The overall speed of the patterning process is determined by the rate with which liquid de-wets from the lyophobic regions and is therefore a strong function of the static advancing contact angle. However, measurement of the static contact angle alone is not enough to fully characterize the de-wetting behaviour on a range of different lyophobic materials. In contrast, the sliding drop method depends upon the dynamic contact angle of a liquid as it de-wets, and so allows for a direct measurement of the maximum speed of de-wetting of a given liquid on a particular surface. Use of the technique to screen a range of lyophobic materials applied by different methods to determine their suitability for surface energy patterning is shown.

Influence of Organic Friction Modifier on Liquid Slip: A New Mechanism of Organic Friction Modifier Action

Recent work using a number of experimental techniques has shown that simple Newtonian liquids can slip against very smooth, lyophobic surfaces. Most previous work has examined lyophobic surfaces produced by forming self-assembled monolayers on smooth substrates. However it has also been shown that octadecanoic (stearic) acid solution in hydrocarbon can promote slip of the liquid against smooth surfaces. This raises the intriguing possibility that one mechanism of action of organic friction modifier additives may be to promote slip in hydrodynamic contacts and thus reduce friction. To test this conjecture, experiments have been carried using a low-load, hydrodynamic friction tester in which a glass cylinder is loaded against a very smooth sapphire flat which has been previously immersed in stearic acid solution. The contact is supplied with a solution of stearic acid in hexadecane and the glass cylinder is rotated at a speed sufficient to generate a full hydrodynamic film. It is found that this system gives significantly lower friction than when using hexadecane alone or when the sapphire is slightly roughened. The results are consistent with the occurrence of liquid slip at the sapphire surface.

Drying Process of Micro-Scale Polymer Solution Droplets Deposited on Lyophobic Surfaces

A micro-scale polymer solution droplet is deposited on a lyophobic surface using an inkjet printing method, and then dried to form a thin polymer film. The shapes of the films formed from xylene-polystyrene (PS) droplets are changed from ring-like to dot-like as the initial PS concentration decreases. Detailed investigations of the drying processes reveal that the pinning time of the contact line dominates the shape and dimension of the polymer film. For ethyl acetate/acetophenone-PS solutions, ethyl acetate rapidly evaporates at the initial stage, but it plays an important role in delaying the pinning time. As a result, this binary solvent forms a dot-like polymer film.

An experimental investigation of the reversibility and hysteresis of the condensation curves

An experimental study is performed on the transition characteristics concerning the reversibility and the hysteresis loop of the condensation curve. From the results of condensation experiments using propylene glycol vapor on lyophobic surfaces, having three different edge conditions of wettable, partially non-wettable and non-wettable, it is proved that the transition modes are greatly affected by these conditions. Under a large overall cooling-side conductance which realizes the continuous curve during the process of increasing surface subcooling, the reversible and/or hysteresis curves are obtained. The transition modes during the process of decreasing subcooling are closely related to the aspects of condensation in increasing subcooling. Although the modes for increasing subcooling are determined by the instability of the overall heat transfer system, they are strongly influenced by the edge conditions and the vapor force as well. Under a small overall cooling-side conductance which brings about the jump mode, the hysteresis loops appear in all cases.

Theoretical studies on the standard but misleading adsorption isotherm II: Modification of the t method to determine the true monolayer capacity and the surface area

Although the t curve is one of the most convenient methods for determining the surface area of solids, it may be misleading in that it does not accomodate volume filling for microporous solids or partly lyophobic surfaces. The method is modified to determine the true surface area to avoid volume filling and the partly lyophobic surfaces which may be present in the microporous and partly lyophobic solids. Also V 1- n R plots (where V 1 (ml g -1) is the amount of adsorbate at a definite relative pressure and n R is the number of layers adsorbed on the non-porous adsorbent at the same relative pressure) are derived from V 1- t plots to determine the monolayer capacity. The V 1- n R plot has an advantage over the t curve in surface area determinations because it avoids the calculation of the statistical thickness of the adsorbate, which is criticized by many investigators. Applications of the modified V 1- t and V 1- n R plots on previously studied partly hydrophobic and microporous silica gels give the true surface areas. The present methods could easily be applied to determine the true surface area of any solid, independent of the nature of the adsorbents.