State-and-rate friction in contact-line dynamics

TL;DR

This study investigates the dynamics of contact-line motion in sessile drops, revealing a force opposing motion that depends on contact time and withdrawal rate, modeled through a rate-and-state friction framework.

Contribution

It introduces a dynamical model incorporating rate-and-state friction to describe contact-line motion, highlighting the importance of static friction and damping in different liquid/substrate systems.

Findings

Contact-line motion depends on contact time and withdrawal rate.

A rate-and-state friction model captures the observed behavior.

Static friction plays a significant role in certain liquid/substrate pairs.

Abstract

In order to probe the dynamics of contact-line motion, we study the macroscopic properties of sessile drops deposited on and then aspirated from carefully prepared horizontal surfaces. By measuring the contact angle and drop width simultaneously during droplet removal, we determine the changes in the shape of the drop as it depins and recedes. Our data indicate that there is a force which opposes the motion of the contact line that depends both on the amount of time that the drop has been in contact with the surface and on the withdrawal rate. For water on silanized glass, we capture the experimentally observed behavior with an overdamped dynamical model of contact-line motion in which the phenomenological drag coefficient and the assumed equilibrium contact angle are the only inputs. For other liquid/substrate pairs, the observed contact-line motion suggests that a maximum static friction force is important in addition to damping. The dependence on time of contact and withdrawal rate, reminiscent of rate-and-state friction between solid surfaces, is qualitatively consistent across three substrate-liquid pairs.