SUPERHYDROPHOBIC SURFACES FOR WATER-REPELLENT OR SELF-CLEANING BEHAVIOR: CHEMICAL EFFECT

The liquid drop captured at the capillary end, which is observed in capillary valve and pendant drop technique, is investigated theoretically and experimentally. Because of contact line pinning of the lower meniscus, the lower contact angle is able to rise from the intrinsic contact angle (θ*) so that the external force acting on the drop can be balanced by the capillary force. In the absence of contact angle hysteresis (CAH), the upper contact angle remains at θ*. However, in the presence of CAH, the upper contact angle can descend to provide more capillary force. The coupling between the lower and upper contact angles determines the equilibrium shape of the captured drop. In a capillary valve, the pinned contact line can move across the edge as the pressure difference exceeds the valving pressure, which depends on the geometrical characteristic and wetting property of the valve opening. When CAH is considered, the valving pressure is elevated because the capillary force is enhanced by the receding contact angle. For a pendant drop under gravity, the maximal capillary force is achieved as the lower contact angle reaches 180° in the absence of CAH. However, in the presence of CAH, four regimes can be identified by three critical drop volumes. The lower contact angle can exceed 180°, and therefore the drop takes on the shape of a light bulb, which does not exist in the absence of CAH. The comparisons between Surface Evolver simulations and experiments are quite well.

5 - Thermodynamics of contact angles on rough, heterogeneous surfaces

On the effect of static and dynamic contact angles on humid air condensation heat transfer

The intrinsic contact angle between liquid metal and a solid substrate is important for interpreting the contact interaction between them. However, the apparent contact angle revealed in a wetting test does not always coincide with the intrinsic angle. In this study, the intrinsic contact angle of liquid silver on monocrystalline and polycrytalline graphite substrates was investigated by means of a sessile drop method under a 10%H2−Ar atmosphere at 1273 K. The intrinsic contact angles were estimated to be 124° for the monocrystalline (C surface) and 127° for the polycrystalline.

Evaluation of unusual change in contact angle between MgO and molten magnesium

Bouncing droplets on solid surfaces is of great significance in diversified applications such as anti-icing and self-cleaning. It is important to establish a unified model to predict whether an impacting droplet can rebound from a surface or not. This work focuses on the rebound dynamic of a droplet impacting a hydrophobic surface via theoretical methods. Based on energy conservation, a new theoretical model to predict the rebound behavior of an impacting droplet is established. For an ideal surface, the contact angle hysteresis Δθ can be ignored and the rebound condition is θ ≥ θc,i, where θ is the equilibrium contact angle and θc,i is the critical rebounding contact angle (CRCA) of an ideal surface. For a real surface, Δθ is considered and the rebound condition is θr ≥ θc,r, where θr is the receding contact angle and θc,r is CRCA of a real surface. Especially, when Δθ is not large enough, the rebound condition for a real surface can be expressed as θr ≥ θc,i. This work is the first to establish the theoretical model considering both the energy dissipation throughout the impact process and the contact angle hysteresis, which shows a higher consistency with the previous works.

Liquid drops on vertical and inclined surfaces: I. An experimental study of drop geometry

The present study evaluated the contact angles (CAs) of four denture base materials subjected to different surface treatments using deionized water and saliva substitutes. A total of 32 rectangular specimens were manufactured using four different denture base materials: heat-cured compression molded Lucitone 199 (C), milled Lucitone 199 (M), Formlabs Denture Base RP (P), and SR Ivocap High Impact (I). The CA of the surface of the unaltered, mechanically polished, and sandblasted surface specimens was evaluated after the application of five saliva substitutes: Biotene, VEGA, Spry, Moi-Stir, Dentilube, and ionized water. Ten droplet measurements were obtained for each group, with each droplet analyzed for advancing contact angle (ACA), receding contact angle (RCA), and the contact angle hysteresis (CAH) was calculated. The data of the experiment was analyzed using 2-way ANOVA, (α=0.05) with Tukey's test. CAH was demonstrated to have statistically significant differences among the denture bases (p<0.05), with unaltered 3D printed exhibiting the largest CAH, followed by unaltered milled. The unaltered denture bases exhibited higher CAH than the polished, and there were no significant differences in CAH among the polished denture bases (p>0.05). Sandblasting increased the ACA of the milled and conventional bases. The saliva substitutes exhibited differences in ACA, with Spry and VEGA having the highest ACA, and Biotene had the lowest CA of all the saliva substitutes evaluated. The manufacturing methods of denture bases influences the CAH, while the chemical composition of the denture base specimens does not appear to affect CAH. Sandblasting increases the ACA for the milled and conventional groups. Saliva substitutes do impact the ACA. Drawing from previous research, it is hypothesized that a 3D-printed denture base or sandblasting a milled denture base may offer greater resistance to dislodgement.

The effect of liquid properties to contact angle hysteresis

A simple model for Taylor flow induced contact angle hysteresis and capillary pressure inside mini/micro-scale capillary tubes

Influence of temperature on wettability of micro hierarchical rough surfaces: A three-dimensional thermodynamic model

A theoretical study was conducted to investigate the wetting behavior of liquid meniscus on a vertical plate with three-dimensional random characteristics of heterogeneity and roughness. The thermodynamic stable condition was derived by considering the minimum of system free energy. The local stable condition leads to a result similar to that obtained for a plate with two-dimensional characteristics, i.e., the system has many meta-stable states. For the stable condition of the whole system, a relation was derived between macroscopically observed contact angle and the surface characteristics. The product of cosine of the contact angle and liquid surface tension is equal to the energy difference for the liquid to wet the plate by apparent unit area. If the liquid wets the solid surface reversibly, there is only one contact angle observed macroscopically. This fact suggests that the contact angle hysteresis is caused by the irreversible motion when the liquid advances or recedes on the solid surface. The well-known Cassie and Wenzel's contact angles are explained as those corresponding to thermodynamically stable condition when the liquid wets the solid reversibly.

This paper presents a method to fabricate the ordered arrays of microstructures on the metal surface using mechanical micro machining technology. With specific designed micro square pillar arrays, the wettability of a solid surface can be tuned. The samples were fabricated by high speed micromilling machine tool on the aluminum alloy surface. The apparent contact angle (CA) and microstructure of the surface were measured and the surface hydrophobic mechanism was also analysed. The results of the experiment indicate that the CA has been increased. The height-to-length ratio hr (h/a) is one of the key parameters to change the wettability of the material surface. Compared with the intrinsic CA (about 51°), the largest apparent CA is up to 102.88° when the height-to-length ratio hr (h/a) is 3/7. Furthermore, by controlling the size and layout of the micropillars, the aluminum alloy surface could be changed from hydrophilic to hydrophobic.

Tuning wettability and getting superhydrophobic surface by controlling surface roughness with well-designed microstructures

Superhydrophobic surfaces are crucial in many applications because of their unique wetting properties. The present study proposes a three-dimensional thermodynamic model, inspired by the morphologies of springtails' skin and mosquitoes' compound eyes in their natural habitats. The model uses thermodynamic principles to analyze the superhydrophobicity of hexagonal columnar microstructures on solid surfaces. From a theoretical standpoint, the variation rules of normalized free energy and normalized free energy barrier with contact angle in two distinct states of wetting were examined. It then calculated the equilibrium contact angle and contact angle hysteresis using the model. The model elucidates the effects of microcolumn height and center distance on free energy and free energy barriers, as well as the transition between composite and non-composite states. Furthermore, it provides the criterion for the transition between the two wetting states in thermodynamic analysis and theoretical equations. Additionally, observations were conducted on the surface of the rough structure. The thermodynamic analysis and theoretical equation calculations of this model facilitate the design of geometric parameters for rough surfaces. The findings provide a reference point for the design of superhydrophobic surfaces with a stable wetting state.