PhD thesis proposal: Electrohydrodynamic lift force

Supervisor: Prof. Alois Würger
Contact: alois.wurger@u-bordeaux.fr

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 the transport properties in these systems . The coupling of the diffuse layer of counter-ions to liquid flow is at the origin of various electro-kinetic and visco-electric effects, leading to several applications ranging from microfluidics to medical analy-sis.

Electro-hydrodynamic coupling has been suggested as an alternative approach to lubrica-tion, which 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. The velocity profile consists of a linear shear flow and a backward
Poiseuille flow, as sketched in the figure for pure water. Since the velocity diminishes with
radial distance, advection in the Poiseuille flow tends to accumulate counter-ions at the back
side of the sphere, thus generating an electric field parallel to the surface and thus a lift force perpendicular to the velocity. Colloidal Probe Atomic Force Microscope (AFM) have shown their ability to probe accurately the electrostatic properties of surfaces, such as sur-face charge and potential, by measuring the equilibrium interaction force between confined Electrostatic Double Layers (EDL). They have also shown their unique power to unravel the
hydrodynamics at solid/liquid interfaces.

This is a collaborative project with the group of Dr Abdelhamid Maali at LOMA. The main objective is a comprehensive study, both experimentally and theoretically, of the dynamic coupling between the electric double layer to fluid flow. More precisely, we will investigate the visco-elastic and electro-dynamic response to lateral oscillations  of the substrate,  and in particular we will measure the lift force perpendicular to the motion. Since the force de-pends on the square value of the velocity, it comprises a static component and one oscillat-ing at twice the driving frequency. The former is obtained by measuring the time-averaged force. We will study both of these two components and their dependence versus distance, viscosity and salinity of the solutions and frequency of the lateral oscillations. 返回小木虫查看更多