Nanoscale Electroviscous Lift Force

TL;DR

This study directly measures the electroviscous lift force on charged particles near a wall in an electrolyte, revealing its dependency on various parameters and confirming a saturation effect at high velocities, which was previously unverified.

Contribution

The paper provides the first direct measurement of the electroviscous lift force and develops a new analytical model that accurately predicts its behavior under various conditions.

Findings

Lift force depends on distance, velocity, and particle size.

Experimental results agree with the new theoretical model.

Lift force saturates at high velocities.

Abstract

About forty years ago, it has been predicted that a charged particle, moving parallel to a charged wall in an electrolyte, should experience a lift force that, contrarily to electrostatic forces, is not screened at large distances. Up to now, such electroviscous lift force has not been directly measured. Here, we use Atomic Force Microscopy to directly measure the electroviscous lift force and quantify its dependency with the distance to the wall, the translation velocity or the particle's size. Observing that existing theories exhibit large discrepancies with our experimental observations, we develop an analytical approach combining lubrication theory to a previously introduced formalism for small screening length. The experimentally observed lift forces are in good agreement with our theoretical predictions and reveal, for the first time, a saturation of the lift force for increasing velocities. Altogether, our results characterize, through direct measurements and analytical approach, the properties of electroviscous forces between charged particles in viscous electrolytes in non-equilibrium conditions.