Salt Water Drops Slide Faster: Ionic Modulation of Drop Friction

The friction of drops on solid surfaces governs processes in microfluidics, energy devices, and surface engineering. In recent years, it has been established that slide electrification on insulating substrates leads to a substantial increase in drop friction. Here we present experimental evidence suggesting that such spontaneous charging effects impede the motion of drops not only on insulating substrates, but also on conductive substrates coated with nanometer-thin hydrophobic films. We demonstrate addition of salts reduces drop friction. On PFOTS-coated Si wafers and thiol-functionalized Au, increasing NaCl concentration from deionized water to ≥ 0.1 m enhanced droplet acceleration by 75%-85%, corresponding to a 13%-25% reduction in friction force. This observation goes beyond electrostatic drop retardation by slide electrification, which had only been identified for insulating films much thicker than 0.1 µm. This phenomenon occurs independently of ion species, substrate doping type, and grounding conditions, and is not explained by changes in interfacial energy, viscosity, or electrostatic interactions alone. Instead, rapid ionic redistribution and electrohydrodynamic interactions at the interface dynamically couple droplet ions with electronic charges in the conductor, reducing contact angle hysteresis. Our findings provide a new framework for tailoring hydrodynamic behavior through charge carrier engineering at nanoscale interfaces.