Assembly and disorder dissipation in superparamagnetic nanoparticle chains in a rotating magnetic field
We investigate the formation of chains of superparamagnetic iron oxide nanoparticles (SPIONs) in a rotating magnetic field, combining two well-explored chain-forming systems: larger micron-scale beads in a rotating magnetic field, and SPIONs in a static field. This simple combination is interesting because it features self-assembly that occurs both far from equilibrium and at a finite temperature, with the better-explored systems constituting respectively its zero temperature and near-equilibrium limits. Theories applicable to either of the two limits qualitatively predict the chain length distributions, except that chains in our experiments are shorter, which we attribute to the simultaneous presence of thermal fluctuations and fluid shear forces that work in concert to break chains apart. Our most striking result is that the disorder in the SPION chains gradually dissipates over a timescale of tens of minutes, about two orders of magnitude slower than the characteristic chain assembly time. The disorder dissipation can be sped up by increasing particle concentration and solution ionic strength, both of which increase the speed of chain assembly. This strongly suggests that the improvement in chain order with time is not due to thermal fluctuations but rather to energy imparted by the self-assembly process, which continually causes chains to grow and break apart, even when a steady state distribution has obtained. More generally, our results indicate that self-assembly away from equilibrium may sometimes lead to better ordered assemblies than under near-equilibrium conditions.