Ambient-environment dependence of dynamic contact angles: Droplet tilting vs. captive bubble methods

Abstract

Measuring the contact angle of a water droplet on a solid surface in air is one of the simplest and most widely used methods for evaluating surface wettability across a wide range of research fields. Wettability can also be evaluated in aqueous environments using the captive bubble method, in which an air bubble is attached to a solid surface. However, it has not yet been experimentally verified whether dynamic contact angles measured by this approach correspond to those obtained in air. In this study, we constructed an experimental system based on the captive bubble method. Dynamic contact angles were measured both in air and in water for smooth polymer surfaces, sandpaper polished surfaces, and hydrophobic surfaces with microstructures. For smooth surfaces, the dynamic contact angles obtained in air and water were nearly identical. Similar agreement was also observed for sandpaper polished surfaces, which exhibited the Wenzel state in air and the reversed gas liquid Wenzel state in water, indicating that comparable dynamic contact angles can be obtained in air and water by the captive bubble method. In contrast, microstructured PMMA surfaces that showed hydrophobic behavior in air exhibited hydrophilic behavior with very small hysteresis in water under degassed conditions. These results provide new insights into wettability in aqueous environments.

Figures (9)

• Figure 1: Schematic illustrations of the wetting states on a rough surface. (a) Wenzel state in air. (b) Reversed gas–liquid Wenzel state in water.
• Figure 2: A schematic illustration of the microstructured hydrophobic PMMA surface.
• Figure 3: Optical microscope(Nikon Ts2R) images of the microstructured PMMA surface (a) before and (b) after ultrasonic treatment in water. Air bubbles trapped in the grooves are observed before the treatment, whereas they disappear after ultrasonic degassing.
• Figure 4: Experimental setups used to measure dynamic contact angles. (a) Measurement system for bubbles in water based on the captive bubble method. (b) Measurement system for droplets in air using the tilting stage method. The components indicated by arrows are (A) stepping motor, (B) water tank, (C) syringe, (D) sample, and (E) rotaly stage.
• Figure 5: Representative measured data of dynamic contact angles on a PET surface. (a) Receding contact angle ($\theta_r$) of a bubble during compression against the surface. (b) Advancing contact angle ($\theta_a$) of a bubble during detachment from the surface. (c) Advancing and receding contact angles ($\theta_a$ and $\theta_r$) of a water droplet measured in air just before sliding. (d) Time variation of dynamic the contact angle of bubble during the compression and detachment processes of a bubble on a PET surface.