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Colloids and nanoparticles self-assemble in external fields, a characteristic which enables functional and smart materials like magnetorheological fluids and potentially new ones in which structures control the transport of heat, light, or chemical species. [1-3] In studies of paramagnetic colloids in magnetic fields, suspensions form system-spanning and kinetically arrested networks analogous to a gel. From this state, it is possible for the suspension to phase separate by toggling the field.

In experiments conducted in the microgravity environment of the International Space Station and earth-based experiments, we find that several factors govern the kinetics of this phase separation and the steady-state structures that paramagnetic suspensions form. These include the field strength, toggle frequency, and duty ratio. The steady-state suspension structures found over many toggle cycles are in good agreement with those predicted by the theoretical and computational work of Sherman et al. [4,5] including a dependence on the field duty.

Our results are important tests for models of dissipative self-assembly, processes in which ordered structures form far from equilibrium by continuously absorbing energy and dissipating it into the surroundings. Such active assembly processes offer promising methods to generate complex structures and circumvent arrest in undesirable metastable states.

[1] M. Grzelczak, et al., ACS Nano 2010, 4, 3591–3605.
[2] J. D. Forster, et al., ACS Nano 2011, 8, 6695.
[3] P. J. Beltramo, et al., Phys. Rev. Lett. 2014, 113, 205503.
[4] Z. M. Sherman, H. Rosenthal, J. W. Swan, Langmuir 2018, 34, 1029.
[5] H. Kim, M. Sau, E. M. Furst. Langmuir 2020, 36, 9926--9934.

Eric M. Furst is the William H. Severns Jr. Distinguished Chair of Chemical Engineering in the Department of Chemical and Biomolecular Engineering at the University of Delaware. His research and teaching interests are at the intersections of soft matter, colloid and interface science, and rheology. He recently co-authored the book Microrheology for Oxford University Press. Furst received his BS with University Honors in Chemical Engineering from Carnegie Mellon University and his PhD from Stanford University. Prior to joining the faculty at Delaware, he studied biophysics at Institut Curie, Paris. Furst is the recipient of the 2013 Soft Matter Lectureship Award, the NASA Exceptional Scientific Achievement Medal, the University Excellence in Undergraduate Advising and Mentoring Award, and he is a Fellow of the American Chemical Society and the American Association for the Advancement of Science.