Stick-slip Dynamics in the Forced Wetting of Polymer Brushes

TL;DR

This paper investigates the static and dynamic wetting behavior of polymer brush-covered substrates, revealing a stick-slip motion during the wetting process driven by the interplay of contact angle dynamics and time scale cross-overs.

Contribution

It introduces a mesoscopic hydrodynamic model for wetting on polymer brushes, demonstrating a novel stick-slip behavior linked to contact angle dynamics and time scale analysis.

Findings

Young's law applies at the macroscale for equilibrium contact angles.

A Neumann-type law governs the wetting ridge shape at the mesoscale.

Stick-slip motion occurs during dynamic wetting at certain velocities.

Abstract

We study the static and dynamic wetting of adaptive substrates using a mesoscopic hydrodynamic model for a liquid droplet on a solid substrate covered by a polymer brush. First, we show that on the macroscale Young's law still holds for the equilibrium contact angle and that on the mesoscale a Neumann-type law governs the shape of the wetting ridge. Following an analytic and numeric assessment of the static profiles of droplet and wetting ridge, we examine the dynamics of the wetting ridge for a liquid meniscus that is advanced at constant speed. In other words, we consider an inverse Landau-Levich case where a brush-covered plate is introduced into (and not drawn from) a liquid bath. We find a characteristic stick-slip motion that emerges when the dynamic contact angle of the stationary moving meniscus decreases with increasing velocity, and relate the onset of slip to Gibbs' inequality and to a cross-over in relevant time scales.