Electrodynamical diffuse layers – EDDL

Nanoscale flows of electrolytes in confined geometry are of primary importance for active matter, energy storage devices, harvesting of waste energy, desalinization, and actuation and signal detection in micromechanical systems. Surface properties are predominant for the flow behavior and ionic transport in these systems.

Solid surfaces in contact with electrolyte solutions are mostly charged. The coupling of the diffuse layer of counter-ions to liquid flow is at the origin of various electro-kinetic and visco-electric effects. Besides classical applications of capillary electrophoresis ranging from microfluidics to medical analysis, we mention the assembly of active materials through charge-induced electro-osmosis, and electrophoresis through nano-pores as a promising technique for DNA sequencing.

On the other hand, electro-hydrodynamic coupling has been suggested as an alternative approach to lubrication, which does not rely on the fluid’s viscosity but depends on the non-equilibrium properties of the electric double layer. Indeed, advection of the diffuse layer results in an additional dynamic electric disjoining pressure, which may by far exceed the electrostatic repulsion and induce a normal force on a sphere moving along a surface. Complex electro-hydro-mechanical couplings have been specifically investigated in ionic liquids (IL) which are ultimately concentrated electrolytes, with the theoretical prediction of solidification under transverse electric field, and the experimental observation of quantized friction under high confinement.

These coupling effects between ion transport and flow are also intimately linked to the nature of the surfaces and the electrical and hydrodynamic boundary conditions imposed by these surfaces. The Surface Force Apparatus (SFA) and the colloidal Probe Atomic Force Microscope (AFM) have shown their ability to probe accurately the electrostatic properties of surfaces, such as surface charge and potential, by measuring the equilibrium interaction force (a mechanical property) between confined Electrostatic Double Layers (EDL). They have also shown their unique power to unravel the hydrodynamics at solid/liquid interfaces.

The present project aims at a comprehensive study, both experimentally and theoretically, by measuring time-resolved forces in EDLs driven out-of-equilibrium. We will develop together experimental and theoretical investigations in order to extract from dynamic force measurements a thorough understanding of interfacial electro-mechanical couplings and their impact on charge transport. More precisely, we will investigate the visco-elastic and electro-dynamic response induced by non-equilibrium electric double layers, the development and characterization of charge-tunable surfaces, and the dynamic response of confined concentrated electrolytes (ionic liquids).

We consider two generic situations, a sphere moving in normal or parallel direction with respect to a solid boundary. In our experiments, the sphere-plane geometry is realized either by a colloidal sphere mounted on an AFM close to solid boundary, or by SFA. We will build on the open geometry of our experiments to realize charge-tunable surfaces, using buried electrodes able to polarize thin dielectric coatings.

The velocity profile between the two surfaces advects the mobile ions in the diffuse layer, which in turn gives rise to an electric field parallel to the surfaces. Depending on the boundary conditions this enhances the viscous drag on the sphere, or results in an additional electrostatic disjoining pressure. In the quasi-static limit the diffuse layer is hardly modified by the external driving, whereas at high frequency, the mobile ions no longer follow this motion, resulting in retardation and visco-elastic or complex response functions.