Viscosity control of the dynamic self-assembly in ferromagnetic suspensions

TL;DR

This paper investigates how the viscosity of the suspending liquid influences the formation of different dynamic structures in ferromagnetic colloids, revealing that viscosity controls the transition between snake-like and aster formations.

Contribution

It provides a combined experimental and theoretical analysis showing viscosity as a key parameter in controlling self-assembly structures in magnetic colloids.

Findings

Lower viscosity favors snake formations.

Higher viscosity promotes aster structures.

Analytic solutions elucidate force balance and flow roles.

Abstract

Recent studies of dynamic self-assembly in ferromagnetic colloids suspended in liquid-air or liquid-liquid interfaces revealed a rich variety of dynamic structures ranging from linear snakes to axisymmetric asters, which exhibit novel morphology of the magnetic ordering accompanied by large-scale hydrodynamic flows. Based on controlled experiments and first principle theory, we argue that the transition from snakes to asters is governed by the viscosity of the suspending liquid where less viscous liquids favor snakes and more viscous, asters. By obtaining analytic solutions of the time-averaged Navier-Stokes equations, we gain insights into the role of mean hydrodynamic flows and an overall balance of forces governing the self-assembly. Our results illustrate that the viscosity can be used to control the outcome of the dynamic self-assembly in magnetic colloidal suspensions.