Molecular dynamics studies of microscopic wetting phenomena on self-assembled monolayers

Molecular dynamics calculations have been used to investigate the behavior of overlayers of water or n-alkane fluids on solid surfaces formed from “self-assembled” monolayers of long-chain hydrocarbons. A microscopic analog of the wetting contact angle is used to measure the surface wetting characteristics. On a nonpolar surface, formed by close packed chains having -CH₃ tailgroups, the water molecules aggregate to form a compact droplet. The calculated contact angle of the droplet is similar to experimental values for macroscopic water droplets. Contrary to intuition, the overlayers of hexadecane or decane form droplets with smaller contact angles on the same surface. However, the calculated contact angles are again in reasonable accord with experimental values.     

Influence of N-alkanes on contact angle and flotability of quartz

Measurements of time dependence of the contact angle were carried out for water drops on a quartz surface in the presence of saturated hydrocarbon vapour as well as on a quartz surface previously wetted with a hydrocarbon. The results obtained were compared with theoretically calculated values on the basis of the modified Young equation and the literature data for the surface free energy components of quartz covered with a monolayer water film. On the basis of the measurements and calculations it was found that the values of the contact angle in the studied systems depend on the kind and thickness of hydrocarbon film present on the quartz surface, as well as on the thickness of water film present on this surface. Changes of the contact angle with the hydrocarbon film thickness are characterized by the maximum and minimum values. This allows previously obtained results of the quartz flotation activity changes in the presence of the aliphatic hydrocarbon films to be explained.     

THE LINE/PSEUDO-LINE TENSION IN THREE-PHASE SYSTEMS

The effect of drop (bubble) size on contact angle was examined over a wide range of drop (bubble) sizes for the CCl₄-toluene solution/NaCl aqueous solution/quartz, ethylene gtycol/CCI₄-toluene solution/quartz, bitumen/ Na₅P₃O₁₀ aqueous solution/quartz, water/kerosene/quartz, heptane/water/quartz, and air bubble/water/polyethylene systems. For most systems, a nonlinear relationship between cosine of the contact angle and reciprocal of drop (bubble) base radius was obtained. All known factors affecting the contact angle/drop size relationship are discussed. It was found that the effect of gravitational forces as well as solid strain in the vicinity of the three-phase contact line are not responsible for the contact angle/drop size relationship for the systems examined. It appears that surface heterogeneities (contaminants) have the greatest effect on the contact angle/drop size relationship as is evident for these results from our laboratory. Also, surface roughness can contribute to the effect of drop size on contact angle. A theoretical background is presented and modifications of both the Cassie Equation and Wenzel Equation are derived.

A linear correlation of cosine of the contact angle vs. reciprocal of drop base radius was obtained for the heptane/waler/quartz systems, when high purity single-component liquids and a smooth, chemically cleaned, quartz plate were used. The line tension was determined for this system to be (5.1 ± 4.3) 10 ^- 9N.     

类荷叶表面疏水结构的材料表面制备

利用纳米/微米复模成型的方法,制备了荷叶表面微乳突状疏水结构的聚乙烯醇(PVA)和聚苯乙烯(PS)阴模模具,并利用阴模复模成型在聚二甲基硅烷(PDMS)表面制备了类荷叶的表面结构.扫描电子显微镜(SEM)的观察表明PDMS材料表面上制得的类荷叶结构与荷叶表面的微乳突结构有较好的一致性,而PVA阴模在保持微观结构上更有优势.通过对水滴在PDMS材料表面接触角的测量,证明了在制备有类荷叶表面结构的PDMS材料表面上,水滴的接触角可以得到显著提高.     

DISPLACEMENT OF PARTICLES DEPOSITED ON SOLID SURFACES BY A MOVING GAS-LIQUID-SOLID INTERFACE

Deposited small particles change their position and can build aggregates on surfaces when wetted/dewetted. The size and form of these aggregates depend on the amount of water condensed, the form of the particles and the contact angles. Experiments with glass spheres and quartz particles on three different surfaces with water as wetting liquid were carried out. Results of the wetting/dewetting experiments are shown and discussed. A model is presented to estimate the magnitude of involved forces and the displacement of the particles taking into account contact angles, amount of condensed water, and size of particles. The model explains, why particles, as observed, tend to gather near the edge of a droplet at small surface contact angles and near the droplet center at high surface contact angles.     

Illumination-induced surface charges on thin diamond films at different wavelengths

Contact angle studies of pure water droplets were carried out on thin diamond films. The films were deposited on a fresh Si 111 wafer by plasma assisted chemical vapour deposition (PACVD) and confirmed to be diamond by Raman spectroscopy and scanning electron microscopy. Advancing and receding contact angles were measured as a function of droplet volume by using different incident wavelengths: red, green and violet. Large differences in the contact angles were observed, depending on the wavelength of the incident light. These differences are usually related to the existence of illumination-induced surface charges or electromagnetically active inhomogeneities in the diamond films. The results show that this technique is sufficiently sensitive to correlate changes in the contact angle with changes in the wavelength used.     

Metal solvent droplet stability in silicon whisker growth

We have studied conditions that determine a stable state of the metal solvent droplet in whisker growth through chemical vapor deposition. The results demonstrate that the contact angle of an equilibrium droplet depends on the supersaturation of the vapor phase with the crystallizing substance. Limitations on the contact angle of a stable metal solvent droplet on the tip of Si, SiC, and GaP whiskers have been identified.     

DISPLACEMENT OF PARTICLES DEPOSITED ON SOLID SURFACES BY A MOVING GAS-LIQUID-SOLID INTERFACE

ABSTRACT

Deposited small particles change their position and can build aggregates on surfaces when wetted/dewetted. The size and form of these aggregates depend on the amount of water condensed, the form of the particles and the contact angles. Experiments with glass spheres and quartz particles on three different surfaces with water as wetting liquid were carried out. Results of the wetting/dewetting experiments are shown and discussed. A model is presented to estimate the magnitude of involved forces and the displacement of the particles taking into account contact angles, amount of condensed water, and size of particles. The model explains, why particles, as observed, tend to gather near the edge of a droplet at small surface contact angles and near the droplet center at high surface contact angles.     

Length Scale Selects Directionality of Droplets on Vibrating Pillar Ratchet

Directional control of droplet motion at room temperature is of interest for applications such as microfluidic devices, self‐cleaning coatings, and directional adhesives. Here, arrays of tilted pillars ranging in height from the nanoscale to the microscale are used as structural ratchets to directionally transport water at room temperature. Water droplets deposited onto vibrating chips with a nanostructured ratchet move preferentially in the direction of the feature tilt while the opposite directionality is observed in the case of microstructured ratchets. This remarkable switch in directionality is consistent with changes in the contact angle hysteresis. To glean further insights into the length scale dependent asymmetric contact angle hysteresis, the contact lines formed by a nonvolatile room temperature ionic liquid placed onto the tilted pillar arrays were visualized and analyzed in situ in a scanning electron microscope. The ability to tune droplet directionality by merely changing the length scale of surface features all etched at the same tilt angle would be a versatile tool for manipulating multiphase flows and for selecting droplet directionality in other lap‐on‐chip applications.     

Spreading of a water drop on a marble surface

Measurements of the wetting contact angle for a marble surface were carried out for two systems: dry marble plate-water drop-saturated water vapour and marble wetted by water-water drop-dry air (in the presence of molecular sieves). The marble plate was placed in a measuring chamber and contact angles were measured after different lengths of time; it was found that their values grew to a maximum which was reached after about 30 min. It was found that when the dry marble plate was placed in saturated water vapour for 24 h the contact angle decreased in comparison with its maximal value. To explain the results obtained, theoretical calculations were made. The theoretical calculations and measurements showed that it was possible to obtain a contact angle greater than zero on a marble surface, depending on the structure and thickness of the water film.     

Superhydrophilic textured-surfaces on stainless steel substrates

Aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) is used to produce micro/nano-textured surfaces on stainless steel substrates at low temperatures for altering the wetting property of the substrates. The micro/nano-textured surfaces were characterized using scanning electron microscopy, X-ray spectroscopy, and X-ray diffraction. The wetting properties of the textured surfaces were characterized by water contact angle measurements. It was found that AIC of a-Si changes the apparent contact angles of stainless steel substrates from 90° to about 0°, measured 0.5 s after a water droplet drops on the surfaces. The study also shows that a superhydrophilic textured surface can be converted to a highly hydrophobic surface with an apparent contact angle of 145° by coating the surface with a layer of octadecyltrichlorosilane.     

接触角和质量分数对液滴冻结过程的影响

本文通过FLUENT软件的凝固/熔化模型,模拟了接触角及质量分数对纯水和氯化钠溶液在冷表面冻结过程的影响,选择铜片为亲水表面,纳米膜表面为疏水表面,对液滴在不同表面特性条件下的冻结过程进行实验研究。结果表明：液滴在冷表面的冻结特性与接触角、质量分数有关。当溶液质量分数一定时,接触角越小,液滴冻结速度越快,完全冻结时间越短;在冻结过程的初始时刻,接触角越小,液滴底部温度越低;当冻结时刻相同、液滴高度一致时,液滴表面的温度和液相分数均比液滴内部低;接触角相同时,溶液质量分数与液滴的开始冻结温度成反比,与完全冻结时间成正比。对比实验结果与模拟可知,不同质量分数的氯化钠液滴在接触角为60°和100°时,冻结时间的变化趋势一致,但实验值大于模拟值。     

Molecularly Capped Omniphobic Polydimethylsiloxane Brushes with Ultra-Fast Contact Line Dynamics

Droplet friction is common and significant in any field where liquids interact with solid surfaces. This study explores the molecular capping of surface-tethered, liquid-like polydimethylsiloxane (PDMS) brushes and its substantial effect on droplet friction and liquid repellency. By exchanging polymer chain terminal silanol groups for methyls using a single-step vapor phase reaction, the contact line relaxation time is decreased by three orders of magnitude–from seconds to milliseconds. This leads to a substantial reduction in the static and kinetic friction of both high- and low-surface tension fluids. Vertical droplet oscillatory imaging confirms the ultra-fast contact line dynamics of capped PDMS brushes, which is corroborated by live contact angle monitoring during fluid flow. This study proposes that truly omniphobic surfaces should not only have very small contact angle hysteresis, but their contact line relaxation time should be significantly shorter than the timescale of their useful application, i.e., a Deborah number less than unity. Capped PDMS brushes that meet these criteria demonstrate complete suppression of the coffee ring effect, excellent anti-fouling behavior, directional droplet transport, increased water harvesting performance, and transparency retention following the evaporation of non-Newtonian fluids.     

Hierarchical silicon etched structures for controlled hydrophobicity/superhydrophobicity

Silicon surface hydrophobicity has been varied by using silane treatments on silicon pyramid surfaces generated by KOH anisotropic etching. Results demonstrated that by altering the surface hydrophobicity, the apparent contact angle changed in accord with the Wenzel equation for surface structures with inclined side walls. Hierarchical structures were also constructed from Si pyramids where nanostructures were added by Au-assisted electroless HF/H2O2 etching. Surface hydrophobicity and superhydrophobicity were achieved by surface modification with a variety of silanes. Stability of the Cassie state of superhydrophobicity is described with respect to the Laplace pressure as indicated by the water droplet meniscus in contact with the hierarchical structures. The contact angle hysteresis observed is also discussed with respect to water/substrate adhesion.     

Effects of temperature,size of water droplets,and surface roughness on nanowetting properties investigated using molecular dynamics simulation

The wetting properties of water nanodroplets on a gold substrate are studied using molecular dynamics (MD) simulations. The effects of temperature, droplet size, and surface roughness are evaluated in terms of molecular trajectories, internal energy, dynamic contact angle, and the radial distribution function. The simulation results show that the wetting ability and spreading speed of water greatly increases with increasing temperature. The dynamic contact angle of water on the gold substrate decreases with increasing temperature and decreasing droplet size and surface roughness, which leads to an increase in wetting ability. The compactness of a water droplet increases with decreasing temperature and droplet size, and slightly increases with degree of roughness. The internal energy of a water droplet decreases with increasing surface roughness, indicating that droplets form more stably on a rough surface.     

On the Contact Characteristics between Droplet and Microchip/Binding Site for Self-Alignment

The contact characteristics between a droplet and a microchip/binding site strongly affect the accuracy of self-alignment in the self-assembly of micro-electronic-mechanical systems. This study is mainly to implement the Surface Evolver Program, which is commonly adopted for studying surface shaped by surface tension and other energies, to investigate comprehensively the contact characteristics between the small droplet and the microchip/binding site. The details of changes in the contact line and the contact area when the microchip is subjected to translation, compression, yawing and rolling are drawn. The three-dimensional deformation of the droplet between the microchip and the binding site is also presented. The restoring force and restoring torque that are induced on the microchip under those motions are calculated accurately. The critical value that the microchip can return to its aligned position under respective motion is therefore predicted. Lastly, under the effects of the droplet surface tension and the contact angle for the droplet on the microchip/binding site, the regions of overflow and no wet area on different microchip shapes are demonstrated. The computed results agree excellently with those by experiments. Based on the simulation techniques developed, the self-alignment of the microchip with the binding site can be estimated accurately.     

Sessile Drop Wettability in Normal and Reduced Gravity

This paper presents initial work performed to develop a database of contact angles of sessile drops in reduced gravity. Currently, there is no database of wettability of sessile drops in reduced gravity. The creation of such a database is imperative for continued investigations of heat and/or mass transfer in reduced gravity and future engineering designs. In this research, liquid drops of water and ethanol were created on aluminum and PTFE substrates. The formed drops were characterized by their dimensions including contact angle, wetted perimeter and droplet shape in both normal gravity and reduced gravity. The droplets were recorded during testing with high definition video and the images obtained digitally analyzed, post-test, to determine their characteristics as a function of the experimental parameters. The Queensland University of Technology (QUT) Drop Tower Facility was utilized for the reduced gravity experimentation. For droplets with diameters above their capillary length, the changes in drop dimensions and/or wettability was observed. The Young-Laplace equation was validated to accurately predict the contact angle in reduced gravity for small droplets, however it was not adequate to describe the contact angle for larger drops (above the drops associated capillary length).     

Electrowetting on dielectric experiments using graphene

We report electrowetting on dielectric (EWOD) experiments using graphene; a transparent, flexible and stretchable nanomaterial. Graphene sheets were synthesized by chemical vapor deposition, and transferred to various substrates (including glass slides and PET films). Reversible contact angle changes were observed on the Teflon-coated graphene electrode with both AC and DC voltages. Nyquist plots of the EWOD reveal that the graphene electrode has higher capacitive impedance than gold electrodes under otherwise identical conditions, suggesting a lower density of pin-holes and defects in the Teflon/graphene electrode than in the Teflon/gold electrode. Furthermore, we have observed reduced electrolysis of the electrolyte and smaller leakage current in the dielectric layer (Teflon) on graphene electrodes than on Au electrodes at the same Teflon thickness and applied voltage. We expect that the improved EWOD properties using graphene as an electrode material will open the door to various applications, including flexible displays and droplet manipulation in three-dimensional microfluidics.     

A liquid droplet measurement technique as a means of assessing the interlaminar shear strength of fibre-reinforced composites

A liquid droplet measurement technique was used to measure the contact angle for low modulus, medium modulus and high modulus pitch-based carbon fibres. A phase transfer catalytic oxidation treatment and an electrochemical oxidation treatment both resulted in falls in the observed glycerol contact angle. Increases in surface energy values were estimated using an equation-of-state approach. The electrochemical oxidation treatment increased the concentration of oxygen- and nitrogen-bearing groups on the fibre surface (as shown by Auger electron spectroscopy), resulting in a fall in the surface energy values estimated using the droplet measurement technique, and corresponding improvements in composite interlaminar shear strength (ilss) were observed. Thus it was shown that the surface energy values determined using this method could be used to assess changes in ilss due to surface treatment of the fibre, provided only chemical changes resulted from the fibre treatment.     

固含量对聚砜膜透湿性能的影响

选择廉价易得的聚砜为原料,以相转化法制备了聚砜膜.测量了膜的湿交换效率、传质系数、渗透速率、水蒸气吸附量、静态接触角等透湿性能;讨论了铸膜液中固含量的变化对膜透湿性能的影响.研究结果表明,膜水蒸气渗透的湿交换效率随着聚砜浓度的增大而降低;在保持铸膜液中聚砜不变的情况下,加入添加剂PVP可以使孔隙率增大,加入季铵盐能有效提高膜表面的亲水性,从而改善其透湿性能.