Molecular Origin of Slippery Behavior in Tethered Liquid Layers

Area of Science:

• Surface Science
• Polymer Chemistry
• Nanotechnology

Background:

• Slippery covalently attached liquid surfaces (SCALS) exhibit ultralow static droplet friction and low contact angle hysteresis (CAH < 5°).
• These properties make SCALS suitable for self-cleaning, water harvesting, and antifouling applications.
• A specific thickness range (around 4 nm) for polydimethylsiloxane (PDMS) SCALS shows optimal low CAH, but the underlying mechanism is not fully understood.

Purpose of the Study:

• To elucidate the molecular-level physical mechanisms governing the optimal slippery behavior of PDMS SCALS.
• To identify the key factors contributing to the observed "Goldilocks zone" of lowest contact angle hysteresis.
• To provide design guidelines for creating tethered polymer layers with ultralow CAH.

Main Methods:

• Coarse-grained molecular dynamics simulations were employed to model PDMS SCALS.
• Atomic force microscopy (AFM) was used to experimentally characterize the surface properties.
• Data from simulations and experiments were compared with contact angle hysteresis (CAH) theory.

Main Results:

• Agreement between simulations and AFM data highlights the role of nanoscale defects and layer deformation in determining CAH.
• At low thicknesses, insufficient substrate coverage leads to chemical patchiness.
• At larger thicknesses, waviness from microphase separation and layer deformation contribute to increased CAH.

Conclusions:

• The optimal slippery regime in PDMS SCALS is achieved with smooth layers that lack nanoscale waviness.
• Nanoscale defects and layer deformation are critical factors influencing the contact angle hysteresis.
• The study provides a framework for designing advanced SCALS with tailored surface properties for specific applications.