Electroferrofluids with nonequilibrium voltage-controlled magnetism, diffuse interfaces, and patterns

It has been recognized that driving matter to nonequilibrium states can lead to emergent behaviors and functionalities. Here, we show that uniform colloidal dispersions can be driven into dissipative nonuniform states with emerging behaviors. We experimentally demonstrate this with electrically driven weakly charged superparamagnetic iron oxide nanoparticles in a nonpolar solvent. The driving leads to formation of nonequilibrium concentration gradients that further translate to nonequilibrium magnetism, including voltage-controlled magnetization and susceptibility. The concentration gradients also serve as diffuse interfaces that respond to external magnetic fields, leading to novel dissipative patterns. We identify the closest nondissipative analogs, discuss the differences, and highlight the ability to directly quantify the dissipation and link it to the pattern formation. Beyond voltage-controlled magnetism, we foresee that the concept can be generalized to other functional colloids to create, e.g., optical, electrical, catalytic, and mechanical responses that are not possible in thermodynamic equilibrium.