30 Septembre – Thesis defense - Marcela Rodriguez Matus

10 h30 LOMA - building A4N (Talence campus)

Charge-flow coupling in nanoscale water films in sphere-plane geometry.

The properties of electrolytes in contact with a charged solid surface have been a matter of interest for a long time. Recent progress in experimental and theoretical studies have shown that the liquid flow at micro and nanoscale behave differently from that at the macroscale. When the dimensions are reduced, the surface properties are predominant for the flow behaviour at contact with the surface. For an even smaller thickness when the fluid experiences a high confinement, not only the physico-chemical of the confining surfaces are important, their elastic behaviour should also be taken into account.
This thesis presents a theoretical study of the properties of confined electrolytes and mechanics of electric double layer in a sphere-plane geometry, where the sphere is mounted on a cantilever system that oscillates close to a solid wall. We derive the electrokinetic interactions via the coupling of electric current and Poiseuille flow. The volume and charge flows are closed by Gauss' law and an advection-diffusion equation for the ions.
We obtained the electroviscous force, without applying the linearization approximation, used in previous work. This work provides a brief reminder of Poisson-Boltzmann theory and the static repulsive force. Then we develop the formal apparatus for charge-flow coupling, derive the electroviscous drag coefficient, and compare various analytical approximations with the numerical computation. We make a short study of the visco-elastic response, as a function of the product of the driving frequency and the relaxation time. Finally, we compare the effects of considering a constant charge or a constant surface potential and contrast dynamic-AFM measurements with our theoretical findings.

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