Determination of the peripheral contact angle of sessile drops on solids from the rate of evaporation

A method was developed to determine the initial peripheral contact angle of sessile drops on solid surfaces from the rate of drop evaporation for the case where ₁ < 90°. The constant drop contact radius, the initial weight, and the weight decrease with time should be measured at the ambient temperature for this purpose. When water drops are considered, the relative humidity should also be known. The peripheral contact angle so obtained is regarded as the average of all the various contact angles existing along the circumference of the drop. Thus, each determination yields an average result not unduly influenced by irregularities at a given point on the surface. In addition, the error in personal judgment involved in drawing the tangent to the curved drop profile at the point of contact can be eliminated. The application of this method requires the use of the product of the vapor diffusion coefficient with the vapor pressure at the drop surface temperature. This product can be found experimentally by following the evaporation of fully spherical liquid drops.     

Analysis of evaporating thick liquid films on solids

The diffusion-controlled evaporation of small circular volatile liquid films from solid surfaces was monitored by employing video microscopy from a plan view and then applying digital image analysis techniques. The decrease of the liquid-solid contact area of these films during the last stages of the evaporation was found to be linear with time. This paper presents experimental results of four organic liquid films (n-nonane, n-octane, toluene, n-butanol) on three substrates poly(methyl methacrylate), poly(ethylene terephthalate) (Mylar), and glass. The linear decreases of the surface areas of hanging drops from a polypropylene fiber for the same liquids were also monitored using both plan and side view video cameras for comparison. Analyses of optically recorded liquid film and drop shapes were carried out and a diffusion model depending on the presence or absence of the substrate was developed. By combining the experimental area decrease of a spherical drop due to the diffusion-controlled evaporation with that of a small spherical cap shaped liquid film resting on a solid surface, it is possible to calculate the small contact angles (less than 10°) of the wetting thick liquid films on solids. The relationship between film evaporation rate and the solid-liquid interfacial interactions is also discussed.     

Analysis of evaporating droplets using ellipsoidal cap geometry

The evaporation of small droplets of volatile liquids from solid surfaces depends on whether the initial contact angle is larger or less than 90°. In the latter case, for much of the evaporation time the contact radius remains constant and the contact angle decreases. At equilibrium, the smaller the drop, the more it is possible to neglect gravity and the more the profile is expected to conform to a spherical cap shape. Recently published work suggests that a singular flow progressively develops within the drop during evaporation. This flow might create a pressure gradient and so result in more flattening of the profile as the drop size reduces, in contradiction to expectations based on equilibrium ideas. In either case, it is important to develop methods to quantify confidence in a deduction of elliptical deviations from optically recorded droplet profiles. This paper discusses such methods and illustrates the difficulties that can arise when the drop size changes, but the absolute resolution of the system is fixed. In particular, the difference between local variables, such as contact angle, cap height, and contact diameter, which depend on the precise location of the supporting surface, and global variables such as radii of curvature and eccentricity, is emphasized. The applicability of the ideas developed is not limited to evaporation experiments, but is also relevant to experiments on contact angle variation with drop volume.     

Spreading of Polydimethylsiloxane Drops on Solid Horizontal Surfaces

Effect of the volume of drops, surface energy and roughness of substrate together with temperature and viscosity on the spreading velocity of polydimethylsiloxane (PDMS) drops on solid horizontal surfaces was studied. Spreading velocity was shown to grow with decreasing drop volume, the effect being more pronounced at high viscosities of polymer. The deviation of shape of the spreading drop from that of a spherical segment is more pronounced the higher the surface energy of substrate, the higher the polymer viscosity and the smaller the drop volume. Spreading on a rough surface is slower than on a smooth one owing to the energy barrier created by surface inhomogeneities: the barrier is to be overcome by the spreading liquid. Based on the experimental results a mechanism of spreading of polymer drops is proposed. Changes in potential energy of a drop and in the free surface energy of the system during spreading were compared, allowing a theoretical evaluation of the influence of gravity on the spreading velocity of drops. A theoretical analysis of spreading kinetics of viscous drops is given. The equation proposed agrees well with the experimental results at 90° > θ > 0°.     

Superhydrophobic surfaces: a review on fundamentals,applications, and challenges

Superhydrophobicity is the tendency of a surface to repel water drops. A surface is qualified as a superhydrophobic surface only if the surface possesses a high apparent contact angle (>150°), low contact angle hysteresis (<10°), low sliding angle (<5°) and high stability of Cassie model state. Efforts have been made to mimic the superhydrophobicity found in nature (for example, lotus leaf), so that artificial superhydrophobic surfaces could be prepared for a variety of applications. Due to their versatile use in many applications, such as water-resistant surfaces, antifogging surfaces, anti-icing surfaces, anticorrosion surfaces etc., many methods have been developed to fabricate them. In this article, the fundamental principles of superhydrophobicity, some of the recent works in the preparation of superhydrophobic surfaces, their potential applications, and the challenges confronted in their new applications are reviewed and discussed.     

Numerical simulations of drop impact and spreading on horizontal and inclined surfaces

The phenomenon of drop spreading is important to several process engineering applications. In the present work, numerical simulations of the dynamics of drop impact and spreading on horizontal and inclined surfaces were carried out using the volume of fluid (VOF) method. For the horizontal surfaces, the dynamics of impact and spreading of glycerin drops on wax and glass surfaces was investigated for which the experimental measurements were available [Šikalo, Š., Tropea, C., Ganic, E.N., 2005a. Dynamic wetting angle of a spreading droplet. Experimental Thermal and Fluid Science 29, 795-802; Šikalo, Š., Tropea, C., Ganic, E.N., 2005b. Impact of droplets onto inclined surfaces. Journal of Colloid and Interface Science 286, 661-669]. The influence of surface wetting characteristics was investigated by using static contact angle (SCA) and dynamic contact angle (DCA) models. The dynamics of drop impact and spreading on inclined surfaces and the different regimes of drop impact and spreading process were also investigated. In particular, the effects of surface inclination, surface wetting characteristics, liquid properties and impact velocity on the dynamics of drop impact and spreading were investigated numerically and the results were verified experimentally. It was found that the SCA model can predict the drop impact and spreading behavior in quantitative agreement with the experiments for less wettable surfaces (SCA>90^°). However, for more wettable surfaces (SCA<90^°), the DCA observed at initial contact times were order of magnitude higher than SCA values and therefore the DCA model is needed for the accurate prediction of the spreading behavior.     

Uncertainty in contact angle measurements from the tangent method

The uncertainty in contact angles from sessile drops measured by the tangent method was estimated using a standard error propagation technique involving partial derivatives. If contact angles are <60°, then uncertainty of the tangent method appears to be quite small,≤ ± 2°. However, as θ values approach 90°, uncertainty increases asymptotically and can exceed ±5°.     

Ice adhesion to rubber materials

The aim of this study was to investigate the interfacial shear strength between ice and rubbers. Different rubber materials containing only a polymer and curing agent (peroxide) were tested with regard to surface wettability and interfacial shear strength. The effect of different grades and amounts of carbon black filler was also studied. The wettability was determined from contact angles, using water and diiodomethane as test liquids, measured on carefully cleaned and mirror smooth rubber sheets. The test showed that there is a correlation between ice adhesion and rubber substrate wettability. Below a water contact angle of 90°, the interfacial shear strength of ice decreases linearly with increasing contact angle. For contact angles above 90°, the interfacial shear strength of ice stays practically the same. The presence of high surface energy additives such as reinforcing carbon black (e.g. N220 ISAF) significantly increases the interfacial shear strength. The highly hydrophobic behaviour of different plant surface textures was also investigated regarding ice adhesion strength. The combination of a submicrometer textured surface and a hydrophobic surface characteristic showed an abrupt decrease in the adhesion force of a water droplet at measured macroscopic contact angles above approximately 150°. Despite this water repellency, the ice adhesion strength is not nil. However, it was among the lowest values experienced in the test.     

Low-rate dynamic contact angles on polystyrene and the determination of solid surface tensions

Low-rate dynamic contact angles of 13 liquids on a polystyrene polymer are measured by an automated axisymmetric drop shape analysis – profile (ADSA-P). It is found that 7 liquids yielded non-constant contact angles, and/or dissolved the polymer on contact. From the experimental contact angles of the other 6 liquids, it is found that the liquid-vapor surface tension times cosine of the contact angle changes smoothly with the liquid-vapor surface tension, i.e. γ_lvcosθ depends only on γ_lv for a given solid surface (or solid surface tension). This contact angle pattern is in harmony with those from other inert and non-inert (polar and non-polar) surfaces (7–13, 24–26). The solid-vapor surface tension calculated from the equation-of-state approach for solid-liquid interfacial tensions (33) is found to be 29.8 mJ/m², with a 95% confidence limit of ±0.5 mJ/m² from the experimental contact angles of 6 liquids.     

Ultra-repellency of Al surfaces: design and evaluation

Aluminum (Al) surfaces with ultra-repellency as well as desirable robustness were designed and fabricated. With photolithographic patterning of a thick SU-8 layer and sputtering of a thin Al film, re-entrant micro-pillar textured Al surfaces were prepared. After derivatization with perfluoroalkyl phosphoric acid (FPA), the textured Al surfaces showed ultra-repellency for a wide variety of liquids. The contact angles (CAs) of deionized (DI) water, hexadecane and dodecane were larger than 150°, and those of methanol and ethanol were larger than 100°. The sliding angles (SAs) of DI water, hexadecane and dodecane were 5°, 10°, and 10°, respectively, showing excellent superamphiphobicity. The SAs of methanol and ethanol were in the range of 20°–30°. The robustness of the ultra-repellent Al surface was evaluated by three parameters: robust height (H*), robust angle (T*) and robust factor (A*). For the DI water probing, the values of the parameters are H* ≈ 403, T* ≈ 119 and A* ≈ 92, respectively, indicative of a desirable robustness. We clarified that only re-entrant structures can support composite liquid–solid–vapor interfaces when the corresponding Young’s CAs are smaller than 90°, and the function of the nanometer structures of the hierarchical textures which were widely adopted to fabricate superamphiphobic surfaces is to help construct re-entrant structures. FPA derivatization is effective in lowering the surface energy of Al surfaces, combining with re-entrant textures to provide a simple and high throughput approach to ultra-repellency for a wide variety of liquids.