# Electrically Tunable Friction through Surface Adsorption Layer Restructuring

## TL;DR

This study shows how electric potential can control friction in lubricants by changing the structure of surfactant layers on metal surfaces.

## Contribution

The study reveals that Na+ counterions, not surfactant anions, are key to potential-driven friction modulation in SDS solutions.

## Key findings

- SDS forms hemicylindrical micelles on steel surfaces due to electrostatic and hydrophobic interactions.
- Friction increases with negative potential when a critical Na+ concentration is achieved.
- Friction modulation depends on the transition from hemicylindrical to hemispherical micelles.

## Abstract

Electric-potential-controlled friction, which manipulates
the frictional
response of lubricants via an applied potential, offers the possibility
of on-demand lubrication. Conventional understanding suggests that
the applied potential influences the adsorption of surfactant ions
on rubbing surfaces, thereby altering friction. This study investigates
the effect of applied potential on the tribological behavior of sodium
dodecyl sulfate (SDS) aqueous solutions in steel–steel contacts
through experiments and molecular simulations. It is shown that SDS,
as an anionic surfactant, readily forms hemicylindrical surface micelles
due to electrostatic and hydrophobic interactions, achieving high
coverage even at low concentrations. Consequently, the adsorbed Na+ counterions are more responsive to the applied potential
than the SDS anions. Contrary to the common belief, friction in steel–steel
contacts is governed by Na+ concentration through its role
in manipulating the structures of the adsorbed SDS aggregates. A critical
Na+ concentrationachieved either through concentrated
SDS solutions or added sodium saltis required for friction
to increase with increasingly negative potential. This friction increase
can be attributed to a transition from hemicylindrical to hemispherical
surface micelles. This work underscores the competing roles of electrostatic
and hydrophobic interactions in surfactant lubrication, suggesting
that an effective electro-responsive additive must balance these interactions
to enable potential-driven modulation. These findings provide key
insights for the design of smart lubricants with potential-tunable
friction properties.