Studying Macro- and Mesoscopic Wetting Dynamics of a Spreading Oil Droplet Using Multiple Wavelength Interferometry

TL;DR

This paper introduces a multi-wavelength interferometric method to accurately measure the transient free surface shape and contact angles of spreading oil droplets at nanoliter scale, enhancing understanding of wetting dynamics.

Contribution

A novel interferometric technique using multiple wavelengths for precise, unambiguous measurement of droplet surface contours and film thicknesses during spreading.

Findings

Accurate measurement of film thicknesses down to 0.1 μm.

Capability to determine three-dimensional droplet shapes.

Reliable time-resolved contact angle measurements.

Abstract

In this study we present an interferometric technique based on multiple wavelengths to capture the transient free surface contour of nanoliter drops spreading on a wettable surface, in particular close to the three-phase contact line. Various data analysis procedures are evaluated in terms of error and noise sensitivity. The technique allows an unambiguous determination of the local liquid film thickness for optical path differences up to $\Delta s \approx 3.19\,\mathrm{\mu m}$ without the need of a known reference height. Film thicknesses as low as $0.1\,\mathrm{\mu m}$ can be measured with the present optical configuration. The entire three-dimensional droplet shape is investigated for different capillary numbers, allowing also reliable measurements of the time-resolved contact angle.