# Salt Water Drops Slide Faster: Ionic Modulation of Drop Friction

## TL;DR

Adding salt to water reduces friction when drops slide on conductive surfaces with thin hydrophobic coatings, making them move faster.

## Contribution

The study reveals that ionic redistribution dynamically couples with electronic charges at nanoscale interfaces to reduce drop friction.

## Key findings

- Increasing NaCl concentration from deionized water to ≥ 0.1 m enhances droplet acceleration by 75%–85%.
- Salt ions reduce kinetic friction by lowering contact angle hysteresis on conductive substrates with hydrophobic films.
- The effect is independent of ion species, substrate doping type, and grounding conditions.

## Abstract

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.

Salt ions markedly accelerate water drops on conductive substrates coated with nanometer‐thin hydrophobic layers. Across diverse electrolytes and surfaces, higher ion concentration consistently lowers kinetic friction by reducing contact angle hysteresis. Rapid ionic redistribution dynamically couples to electronic charges through the thin dielectric, homogenizing interfacial potentials and suppressing dissipation during sliding.