Measuring contact angle and meniscus shape with a reflected laser beam

TL;DR

This paper introduces a novel laser reflection technique to accurately measure contact angles and meniscus shapes, overcoming limitations of side-view imaging, and demonstrates its effectiveness on various surfaces and liquids.

Contribution

A new laser reflection method for contact angle measurement that improves accuracy and allows meniscus shape reconstruction, validated through experiments on different surfaces.

Findings

Successfully measured contact angles on cylinders with the new method.

Revealed dependence of receding contact angle on speed for specific solutions.

Validated technique on both smooth and rough surfaces.

Abstract

Side-view imaging of the contact angle between an extended planar solid surface and a liquid is problematic. Even when aligning the view perfectly parallel to the contact line, focusing one point of the contact line is not possible. We describe a new measurement technique for determining contact angles with the reflection of a widened laser sheet on a moving contact line. We verified this new technique measuring the contact angle on a cylinder, rotating partially immersed in a liquid. A laser sheet is inclined under an angle $\varphi$ to the unperturbed liquid surface and is reflected off the meniscus. Collected on a screen, the reflection image contains information to determine the contact angle. When dividing the laser sheet into an array of laser rays by placing a mesh into the beam path, the shape of the meniscus can be reconstructed from the reflection image. We verified the method by measuring the receding contact angle versus speed for aqueous cetyltrimethyl ammonium bromide solutions on a smooth hydrophobized as well as on a rough polystyrene surface.