Self-Accelerating Drops on Silicone-Based Super Liquid-Repellent Surfaces

Design of super liquid-repellent surfaces has relied on an interplay between surface topography and surface energy. Perfluoroalkylated materials are often used, but they are environmentally unsustainable and notorious for building up static charge. Therefore, there is a need for understanding the performance of sustainable low surface energy materials with antistatic properties. Here, we explore drop interactions with perfluoroalkyl- and silicone-based surfaces, focusing on three modes of drop-to-surface interactions. The behavior of drops rolling under gravity is compared to those subjected to lateral and normal forces under constant slide (i.e., friction) and detachment (i.e., adhesion) velocities. We demonstrate that a drop's characteristic and dynamic mobility depends on surface chemistry, with sequential drop interactions being particularly affected. By utilizing force-and-charge instruments, we show how rolling drops are primarily governed by adhesion and its associated electrostatic effects, instead of friction. Perfluoroalkylated surfaces continuously accumulate charges, while silicone surfaces rapidly saturate. Consequently, sequentially contacting drops accumulate significant charges on the former while rapidly diminishing on the latter. The drop charge suppressing behavior of silicones enhances drop mobility despite their higher surface energy compared to perfluoroalkyls. Quantum mechanical density functional theory calculations show significant differences in surface charge distributions at the atomic level. Simulations suggest that variations in the lifetimes of surface hydroxyl ions likely drive the markedly different drop charging behaviors. Our findings demonstrate the critical role of surface chemistry and its coupled electrostatics in drop mobility, providing valuable insights for designing environmentally friendly, antistatic, super liquid-repellent surfaces.