Phase separation and emergent dynamics of paramagnetic suspensions in toggled magnetic fields*

Colloids and nanoparticles self-assemble in external fields, a property which enables functional and smart materials like magnetorheological fluids and potentially new ones in which structure controls the transport of heat, light, or chemical species. A model to study these phenomena are suspensions of paramagnetic colloids. In a strong, steady magnetic field, paramagnetic colloids form system-spanning, kinetically arrested networks similar to a gel. From this state, it is possible to phase separate and condense the suspension by toggling the external field. During its evolution towards the equilibrium state, the suspension undergoes a Rayleigh-Plateau instability for a range of field strengths and toggle frequencies. The particles initially chain together to form a percolated network that coarsens diffusively.  With time, the surface of the growing domains in the network become unstable. The amplitude of the waves eventually reaches a critical value and the columns pinch off and condense into ellipsoidal-like structures. 

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 phase separation of paramagnetic colloids in toggled fields, including the field strength and toggle frequency. The suspension structures observed over many toggle cycles are in good agreement with those predicted by the theoretical and computational work of Sherman et al. (Langmuir 2018, 34, 1029−1041). Surprisingly, however, experiments performed in a narrow range of low duty ratios and toggle frequencies lead to suspension microstructures that connect perpendicularly to the magnetic field over long distances, reminiscent of critical fluctuations. These structures, as well as ellipsoidal-like domains, exhibit rich steady-state emergent dynamics of rotation, breakup, and coalescence.