Using surface charge to control slip of yield-stress fluids at solid surfaces

Many yield-stress fluids exhibit slip on solid surfaces. This significantly impacts their flow and can dramatically decrease pressure losses by reducing wall friction. Hence, promoting slip can be beneficial to reduce the energy consumption of industrial processes. It can also help to control the structure of certain products, improving their texture and taste. However, the physical origin of slip in yield-stress fluids remains poorly understood. In this talk, 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 which repels the negatively charged Carbopol network, results in a decreased yield threshold τ_s to overcome the adhesion of fluid particles to the wall, and a monotonic increase of the slip coefficient α. These parameters determine the increased velocity at the wall due to slip, u_s = α (τ_w - τ_s), where τ_w is the wall shear stress. We relate this charge-induced variation in slip to that observed for triboelectric coatings such as Teflon and PDMS.