Magnetically tunable electrokinetic instability and structuring of non-equilibrium nanoparticle gradients

Abstract

Inspired by emergent behaviors of living matter, there is increasing interest in developing approaches to create dynamic patterns and structures in synthetic materials with controllable complexity to enable functionalities that are not possible in thermodynamic equilibrium. Here we show that electrophoretically driven and maintained non-equilibrium gradients of magnetic nanoparticles in non-polar solvent can undergo electrokinetic instabilities (EKI), leading to various electrically controllable spatiotemporally patterned states. These electrokinetic instabilities and patterns can be tuned with a magnetic field via magnetostatic energy reduction mechanism to both increase and decrease the pattern complexity. We reflect the experimental observations on the theoretical electrokinetic and magnetostatic arguments. We further show that small amounts of polar water in the otherwise non-polar system are critical enablers for the electrophoretic mobility of the nanoparticles. Since functionalities of magnetic nanoparticles are widely tunable, we foresee that the combination of dissipative electrokinetic driving and magnetic energy reduction can lead to novel functional dissipative materials.