# Surface forces generated by the action of electric fields across liquid films

## TL;DR

This study investigates how electric fields influence forces across liquid films, revealing complex electrolyte responses that differ from classical predictions and impact colloidal control in soft matter systems.

## Contribution

It provides direct measurements of electric field-induced forces in dielectric and electrolyte films, highlighting non-trivial electrolyte dynamics under AC fields.

## Key findings

- Electrolyte films exhibit larger-than-expected forces under AC fields.
- Force interactions can switch from attractive to repulsive.
- Electrolyte response to AC fields is slow, taking hundreds to thousands of seconds.

## Abstract

We explore the force generation and surface interactions arising when electric fields are applied across fluid films. Using a surface force balance (SFB) we measure directly the force between two electrodes in crossed-cylinder geometry across dielectric and electrolytic fluids. In the case of dielectric films the field between the electrodes exerts a force which can be well explained using classic expressions and with no fitting parameters. However when the electrodes are separated by a film of electrolyte, an alternating electric field induces a force which diverges substantially from the calculated static response of the electrolyte. The magnitude of the force is larger than predicted, and the interaction can switch from attractive to repulsive. Furthermore, the approach to steady state in electrolyte takes place over $10^2$ -- $10^3$~s which is very slow compared to both the charging and viscous timescales of the system. The non-trivial electrolyte response in AC electric fields, measured here directly, is likely to underlie several recent reports of unexpected and bifurcating forces driving colloids in AC fields. Our measurements suggest ways to control colloidal and soft matter using electric fields, as well as providing a direct measure of the length- and time-scales relevant in AC electrochemical and electrokinetic systems.