Free surface topography of capillary flows using spatiotemporal phase shifting profilometry

TL;DR

This paper introduces a new optical profilometry technique adapted for capillary flows, enabling detailed surface topography measurements that align well with theoretical models and visualizations.

Contribution

The study adapts the Spatio-Temporal Phase Shifting Profilometry method for capillary flows, providing a novel, accurate, and high signal-to-noise approach for free surface characterization.

Findings

Accurate surface profiles of various capillary flows achieved

Good correlation with side-view visualizations for droplets

Reconstructed rivulet flows match theoretical predictions

Abstract

We present a novel experimental technique to characterize the free surface of capillary flows adapted from the Spatio-Temporal Phase Shifting Profilometry (ST-PSP) method introduced by Ri et al. (Journal of Optics, 2019) for solid bodies. The present study is focused on various regimes of capillary flows over inclined surfaces, including drops, rivulets, meanders, and braided films. A calibration process is carried out using a solid wedge to determine the optical distances required for the phase-to-height relationship. The optimal dye concentration necessary for accurately reconstructing the free surface of a dyed water flow is explored. The ST-PSP method is then applied to profile different liquid flows and objects, achieving large signal-to-noise ratios across all experiments. Notably, the analysis of a sessile droplet shows a promising correlation between the ST-PSP results and side-view visualizations, as evidenced by the accurate recovery of its apparent contact angle. Furthermore, free surface reconstructions of rivulet flows exhibit good agreement with the theoretical predictions of Duffy and Moffatt (The Chemical Engineering Journal and the Biochemical Engineering Journal, 1995). These results suggest that the ST-PSP method is highly effective for obtaining accurate height maps of capillary flows and objects, making it a valuable tool for validating theoretical models in the future.