Probing interfacial effects with thermocapillary flows

TL;DR

This study investigates how thermocapillary flows can thin supported liquid films, revealing a master curve for the thinning dynamics and deviations at molecular scales, enabling probing of molecular interactions.

Contribution

It introduces a method to analyze interfacial effects in thin liquid films using thermocapillary-driven thinning and characterizes deviations related to molecular interactions.

Findings

Thinning dynamics follow a master curve dependent on liquid properties and thermal gradient.

Deviations occur at nanometer scales, differing between oil types.

Numerical simulations confirm the role of disjoining pressures.

Abstract

We report on the thinning of supported liquid films driven by thermocapillarity. The liquids are oil films, of initial thicknesses of a few tens of microns. A local and moderate heating of the glass substrate on which they are spread on induces a thermocapillary flow, which allows the formation of ultra-thin films. We show that, within a given time range, the thinning dynamics of submicron thick films is governed by the thermocapillary stress. It results in a simple dependency of the thickness profiles with time, which is evidenced by the collapse of the data onto a master curve. The master curve only depends on the liquid properties and on the thermal gradient, and allows a measurement of the latter. As the films further thin down to thicknesses within the range of molecular interactions, a deviation from the master curve appears. Although all investigated oils are supposedly fully wetting glass, the nature of the deviation differs between alkanes and silicone oils. We attribute the observed behaviors to differing signs of disjoining pressures, and this picture is confirmed by numerical resolution of the thin-film equation. We suggest thermocapillary flows can be used to finely probe molecular interactions.