Molecular Origin of Slippery Behavior in Tethered Liquid Layers

Abstract

Slippery covalently attached liquid surfaces (SCALS) are a family of nanothin polymer layers with ultralow static droplet friction, characterized by a low contact angle hysteresis (CAH < 5°), which makes them ideally suited for self-cleaning, water harvesting, and antifouling applications. Recently, a Goldilocks zone of lowest CAH has been identified for polydimethylsiloxane (PDMS) SCALS of intermediate thickness (≈4 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 due to a previously overlooked lateral microphase separation occurring in polydisperse brushes, and (2) layer deformation due to the contact line being larger than in thinner layers. The most pronounced slippery behavior occurs for smooth PDMS layers that do not exhibit nanoscale waviness. The converging insights from simulations, experiments, and a CAH theory provide design guidelines for tethered polymer layers with ultralow CAH.