Density distribution within the water contact layer determines slippage: Beyond surface wettability

Abstract

Hypothesis

Both hydrophobic and hydrophilic surfaces have been shown to promote solid-liquid boundary slippage, which leading to a controversy regarding the role of surface wettability in liquid slippage. We hypothesize that liquid slippage is determined by a more fundamental physical mechanism beyond surface wettability effect.

Simulations

Through equilibrium (EMD) and non-equilibrium molecular dynamics (NEMD) simulations, we investigate the effect of solid-liquid Lennard-Jones (L-J) interactions, solid lattice constants, surface charges, and roughness on water wetting and slippage on solid surfaces.

Findings

We demonstrate that the density distribution within the water contact layer on solid surfaces determines water slippage, beyond surface wettability. The new concept of Roughness of Contact Density (RCD) is proposed to quantitatively describe this effect and revealing a power-law relationship between RCD and slip length. The slip length increases as the RCD decreases, with significant slippage and a slip length of 10 nm occurring when the RCD equals 0.2. Our work not only clarifies the persistent ambiguous role of surface wettability in liquid slippage but also provides a new physical mechanism for liquid slippage.