Fluid Drag Reduction by Magnetic Confinement

A solid interface with a viscous liquid flow results in large drag, related to frictional forces. Reducing the drag by more than a few tens of percent remains elusive. Here, we use magnetic forces to stabilize a ferrofluid encapsulating a transported immiscible viscous liquid. This liquid-in-liquid flow exhibits drag reduction from 80% to more than 99%, tuneable by the viscosity ratio between the two liquids, over a range where the transported liquid has a viscosity larger or smaller than that of the encapsulating liquid. Our findings are explained by a laminar flow model that matches data for our whole range of experimental conditions.