Near-critical spreading of droplets

TL;DR

This study investigates droplet spreading near the critical temperature in phase-separated liquids, confirming Tanner's law, identifying a microscopic length scale, and revealing a counter-intuitive thinning of the precursor film.

Contribution

It combines experiments, theory, and simulations to verify Tanner's law near criticality and uncovers the behavior of the precursor film as the critical point is approached.

Findings

Tanner's law is validated near the critical temperature.

A microscopic length scale is identified as the precursor film thickness.

The precursor film thins sharply approaching the critical temperature.

Abstract

We study the spreading of droplets in a near-critical phase-separated liquid mixture, using a combination of experiments, lubrication theory and finite-element numerical simulations. The classical Tanner's law describing the spreading of viscous droplets is robustly verified when the critical temperature is neared. Furthermore, the microscopic cut-off length scale emerging in this law is obtained as a single free parameter for each given temperature. In total-wetting conditions, this length is interpreted as the thickness of the thin precursor film present ahead of the apparent contact line. The collapse of the different evolutions onto a single Tanner-like master curve demonstrates the universality of viscous spreading before entering in the fluctuation-dominated regime. Finally, our results reveal a counter-intuitive and sharp thinning of the precursor film when approaching the critical temperature, which is attributed to the vanishing spreading parameter at the critical point.