A pushing-pulling captive bubble method for precise measurement of dynamic contact angles underwater

TL;DR

This paper introduces a pushing-pulling captive bubble method for stable, precise measurement of dynamic contact angles underwater, overcoming limitations of traditional methods and enabling new insights into surface wettability.

Contribution

A novel pushing-pulling captive bubble technique that improves stability and accuracy in measuring underwater dynamic contact angles without altering bubble volume.

Findings

Method achieves stable contact line observation and suppresses bubble deformation.

Dynamic contact angles in water and air are similar for smooth and Wenzel surfaces.

Ultrasonic degassing allows measurements on microstructured surfaces with trapped air layers.

Abstract

Accurate measurement of dynamic contact angles in aqueous environments is essential for evaluating surface wettability. However, conventional captive bubble methods often suffer from limitations such as bubble instability and interference from needle wetting. In this study, we develop a pushing-pulling captive bubble method that enables stable and precise measurement of dynamic contact angles underwater without directly changing the bubble volume. In this method, a bubble is pushed against and detached from a surface by controlled vertical motion. This procedure allows stable observation of the contact line while suppressing bubble deformation and lateral movement. Dynamic contact angles were measured in both air and water using three types of surfaces: smooth surfaces, sandpaper-polished surfaces prepared to exhibit the Wenzel state in air and the reversed gas-liquid Wenzel state in water, and microstructured surfaces exhibiting hydrophobicity in air. For smooth and Wenzel surfaces, the dynamic contact angles measured in air and water showed similar values. Moreover, the modified captive bubble method exhibited reproducibility comparable to or higher than that of conventional captive bubble methods. For microstructured surfaces, dynamic contact angle measurements in water had previously been difficult because an air layer remained trapped on the surface. In this study, ultrasonic degassing enabled dynamic contact angle measurements under fully wetted conditions, revealing behavior that differed significantly from that observed in air.