Contact Line and Adhesion Force of Droplets on Concentric Ring-Textured Hydrophobic Surfaces

Abstract

Advances made in fabrication of patterned surfaces with well-defined dimensions of topographic features and their lateral dissemination drive the progress in interpretation of liquid spreading, adhesion, and retreat on engineered solid surfaces. Despite extensive studies on liquid droplet spreading and adhesion on textured surfaces in recent years, conformation of the three-phase contact line and its effect on macroscopic contact angle and droplet adhesion remain the focus of intensive debate. Here, we investigate the effect of surface topography on the adhesion force of Cassie−Baxter-state droplets on concentric ring-textured hydrophobic surfaces having rings with lateral dimensions of 5, 10, and 45 μm and separated by 5, 6, and 7 μm trenches, respectively, with fixed depth of 15 μm. Unlike mostly tested surfaces textured with straight ridges, pores, and pillars, where the droplet base contact line is anisotropic and its conformation varies along the apparent boundary, concentric rings are symmetrical and reinforce the microscopic contact line to align to a circular one that reflects the shape of the pattern. In this study, adhesion forces were calculated based on surface tension and Laplace pressure forces and were compared with the experimental forces for both water and ethylene glycol droplets having a varying contact diameter on the concentric ring-pattern at the point of maximum adhesion force. Results show that the microscopic contact line of the liquid retains its circular shape controlled by circular rings of the pattern, irrespectively of the droplet base diameter larger than 0.8 mm, and there is a good agreement between the experimental and calculated adhesion forces.

Droplet Characteristics at the Maximum Adhesion on Curved Surfaces

“…There are four distinctive points in the force curve: the spreading point where the droplet spontaneously spreads to a stable shape after contacting with the sample surface (denoted as spreading ), the compression end point where the continuous lift of the sample ends (denoted as compression end ), the maximum adhesion point where a maximum adhesion force is detected (denoted as maximum adhesion ), and the point where the droplet separates from the surface (denoted as pull-off ). Additional examples of a force curve recorded on a number of different but planar surfaces are presented in our previous publications. ,,, …”

“…The exposed surfaces of PET were rinsed with ethanol and deionized water and blown dried with compressed air before experiments. High-performance-liquid chromatography-grade water with a surface tension of 72.4 mN/m (Sigma-Aldrich, St Louis, MO) and ethylene glycol with a surface tension of 46.5 mN/m (VWR International, Radnor, PA) were used in this study, as described in detail by Wang et al 42 2.2. Force and Contact Angle Measurements.…”

Lubricant-Mediated Strong Droplet Adhesion on Lubricant-Impregnated Surfaces

“…Distinct from the superhydrophobic surface, once a droplet comes in contact with the TiO 2 LIS, it propagates immediately onto the surface (Figure d, t 0 – t 1 ), showing a unique “jump-in” feature (Movie S2), which is absent in the droplet adhesion on the superhydrophobic surface (Figure a and Movie S1). Such jump-in behavior originates from the interaction between the lubricant and the droplet, depending on the surface wettability analogous to the droplet adhesion on the hydrophobic or hydrophilic surfaces. − After loading compression (Figure d,e, t 1 – t 2 ), as the droplet rises, the interplay between the droplet and the lubricant induces a maximum adhesive force, F max ≈ 118.8 ± 3.2 μN (Figure d,e, t 3 ), more than 3 times higher than that on the superhydrophobic surface. More importantly, when the droplet detaches from the LIS, it exhibits a “jump-off” behavior, in which the droplet breaks up, leaving a tiny residual droplet resting on the LIS (Movie S2 and Figure e, t 5 ).…”

Direct measurement of surface adhesion between thin films of nanocellulose and urea–formaldehyde resin adhesives

“…However, in this study, the direct adhesion between nanocellulose and UF resins is determined by measuring the adhesion force (Sun et al 2017;Wang et al 2020a) between nanocellulose films and liquid droplets of the UF resins, as well as by calculating the work of adhesion between the films of the nanocelluloses and UF resins using contact angles via the van Oss−Chaudhury−Good (OCG) method (van Oss et al 1988;Gustafsson et al 2012). These two methods were selected because of their convenience and simplicity.…”

“…2) across the liquid-gas interface for a stretched droplet (see Scheme 1 for details). The combination of these two forces can be expressed using the following equation (Sun et al 2017;Wang et al 2020a):…”