Aggregation of superparamagnetic colloids in magnetic fields: the quest for the equilibrium state

TL;DR

This study reveals that superparamagnetic colloids can reach an equilibrium size distribution in magnetic fields, contrasting with the previously observed irreversible growth, supported by simulations and thermodynamic theory.

Contribution

It demonstrates, through simulations and theory, that superparamagnetic colloids can form equilibrium aggregates, unlike the irreversible chains seen in prior studies.

Findings

Aggregates reach an equilibrium size distribution.

The equilibrium size depends on a dimensionless parameter.

Conditions for observing this behavior are identified.

Abstract

Experimental and simulation studies of superparamagnetic colloids in strong external fields have systematically shown an irreversible aggregation process in which chains of particles steadily grow and the average size increases with time as a power-law. Here we show, by employing Langevin dynamics simulations the existence of a different aggregation behavior: aggregates form during a transient period and the system attains an equilibrium distribution of aggregate sizes. A thermodynamic self-assembly theory supports the simulation results and it also predicts that the average aggregate size in the equilibrium state depends only on a dimensionless parameter combining the volume fraction of colloids phi0 and the magnetic coupling parameter Gamma. The conditions under which this new behavior can be observed are discussed.