Experimental and theoretical investigation of contact-angle variation for water–ethanol mixture droplets on a low-surface-energy solid

Abstract

Consider a situation where a droplet is deposited on a solid surface. If the droplet volume decreases, the droplet exhibits complex behavior, such as hysteresis and size dependence of its contact angle. In this study, we perform experimental and theoretical studies of the wettability of water–ethanol droplets on a low-surface-energy solid. In the experiment, the droplet behavior is examined with decreasing volume as a result of microsyringe extraction or natural evaporation. In particular, we analytically model the adsorption of liquid molecules at the solid–liquid interface and the change in the surface energy density of the liquid during the volume-change process. These models are validated with experimental data. From the experimental results, the droplet behavior resulting from microsyringe extraction and natural evaporation differ considerably with increasing ethanol concentration. Our results suggest that the complex behavior of the droplets during the volume change is related to liquid molecules adsorbed at the solid–liquid interface and the change in the surface energy density of the liquid induced by the concentration change resulting from evaporation of the binary liquid mixture near the contact line.