Ultra‐Soft Liquid‐Ferrofluid Interfaces

Soft interfaces are ubiquitous in nature, governing quintessential hydrodynamics functions, like lubrication, stability, and cargo transport. It is shown here how a magnetic pressure at a magnetic–nonmagnetic fluid interface results in an ultra‐soft interface with nonlinear elasticity and tunable viscous shear properties. The balance between magnetic pressure, viscous stress, and Laplace pressure results in a deformed and stable liquid‐in‐liquid tube with apparent elasticity in the range of 2–10 kPa, possibly extended by a proper choice of liquid properties. Such highly deformable liquid–liquid interfaces of arbitrary shape with vanishing viscous shear open doors to unique microfluidic phenomena, biomaterial flows and complex biosystems mimicking.