Anne Juel

Models of flow and slip of yield-stress fluids in confined geometries

Yield-stress fluid flow occurs in many industrial applications, including in porous systems during enhanced oil recovery and membrane emulsification. However, the high computational cost of computational fluid dynamics (CFD) simulations for yield-stress fluids hinders the study of these large-scale systems. In this work, we develop a fully-predictive network model for flow through 2D disordered porous media that obtains a solution in minutes on a single CPU core, considerably faster than the days or weeks required by CFD with multiple CPU cores. Our model generates network nodes using a Voronoi tessellation, extracts geometric parameters accounting for the topology of the domain, and numerically solves mass and momentum conservation equations with a Newton-Raphson solver. We accurately predict the pressure drop and velocity field of flows with and without wall slip, validating our results against CFD produced in-house and from the literature.

Wall slip is ubiquitous in real-world flows of these materials, dramatically decreasing friction at solid-fluid interfaces and changing the overall resistance in porous networks. However, the physical origin of slip in yield-stress fluids remains poorly understood. We demonstrate that the slip of a Carbopol microgel on a nanometrically smooth gold surface can be controlled by surface charge imposed electrochemically. When imposing negative charge at the gold surface, the formation of an electrical double layer is accompanied by electrostatic repulsion which results in a decreased yield threshold to overcome the adhesion of fluid particles to the wall. We relate this charge-induced variation in slip to that observed for triboelectric coatings.