Molecular origin of slippery behaviour in tethered liquid layers

TL;DR

This study uncovers the molecular mechanisms behind the optimal thickness of tethered liquid layers that exhibit ultra-low contact angle hysteresis, combining simulations and experiments to inform design of slippery surfaces.

Contribution

It provides molecular-level insights into the physical origins of the slippery optimum in tethered liquid layers, linking nanoscale defects and deformation to low hysteresis.

Findings

Nanoscale defects and deformation explain the slippery regime.

Optimal thickness (~3.5 nm) minimizes waviness and defects.

Smooth layers without waviness exhibit the best slippery behaviour.

Abstract

Slippery covalently attached liquid surfaces (SCALS) are a family of nanothin polymer layers with remarkably low static droplet friction, characterised by a low contact angle hysteresis (CAH$< 5 \deg$), which makes them ideally suited to self-cleaning, water harvesting, and anti-fouling applications. Recently, a Goldilocks zone of lowest CAH has been identified for polydimethyl siloxane (PDMS) SCALS of intermediate thickness (3.5 nm), yet, molecular-level insights are missing to reveal the underlying physical mechanism of this elusive, slippery optimum. In this work, the agreement between coarse-grained molecular dynamics simulations and atomic force microscopy data shows that nanoscale defects, as well as deformation for thicker layers, are key to explaining the existence of this `just right' regime. At low thickness values, insufficient substrate coverage gives rise to chemical patchiness; at large thickness values, two features appear: 1) a waviness forms on the surface of the liquid layer due to a previously overlooked lateral microphase separation occurring in polydisperse brushes, and 2) layer deformation due to the contact line is larger than in thinner layers. The most pronounced slippery behaviour occurs for smooth PDMS layers that do not exhibit nanoscale waviness. The converging insights from molecular simulations, experiments, and a contact angle hysteresis theory provide design guidelines for tethered polymer layers with ultra-low contact angle hysteresis.