Polarizable colloids are expected to form crystalline equilibrium phases when exposed to a steady, uniform field. However, when colloids become localized this field-induced phase transition arrests and the suspension persists indefinitely as a kinetically trapped, percolated structure. We anneal such gels formed from magnetorheological fluids by toggling the field strength at varied frequencies. This processing allows the arrested structure to relax periodically to equilibrium-colloid-rich, cylindrical columns. Two distinct growth regimes are observed: one in which particle domains ripen through diffusive relaxation of the gel, and the other where the system-spanning structure collapses and columnar domains coalesce apparently through field-driven interactions. There is a stark boundary as a function of magnetic field strength and toggle frequency distinguishing the two regimes. These results demonstrate how kinetic barriers to a colloidal phase transition are subverted through measured, periodic variation of driving forces. Such directed assembly may be harnessed to create unique materials from dispersions of colloids.magneto-rheological fluid | microgravity science | complex fluids S mart fluids, colloidal dispersions actuated by external magnetic or electric fields, have diverse applications. In buildings, magneto-rheological (MR) dampers are used to absorb the energy of earthquakes, automobiles and trucks are equipped with active MR shock absorbers, and electro-rheological (ER) fluids enable haptic controllers and tactile displays in microelectronics devices. It is the ability to rapidly and reversibly change the rheological properties of smart fluids that makes them so attractive. Understanding the mechanisms that govern the formation and dissolution of structures in such materials is essential (1, 2). Fieldinduced interactions between particles is the primary mechanism of ER and MR fluids, and is driven chiefly by the mutual attraction or repulsion of induced dipoles.Upon application of a steady field, ER and MR fluids respond by forming particulate chains along the field direction, imparting enhanced viscosity and the ability to resist transverse mechanical stresses. Following chain formation, thermal fluctuations create lateral attractive forces between neighboring chains and cause microstructural coarsening (3-5). Continued cross-linking between chains eventually damps the thermal fluctuations that drive coarsening in a self-retarding fashion. A kinetically arrested percolated structure results. However, investigations of the equilibrium thermodynamic properties of dipolar fluids show that the coarsened state is actually an arrested phase transition. Such dipolar fluids are predicted to form two coexisting phases: a particle rich, body-centered-tetragonal crystalline phase and a dilute fluid phase (6, 7). This contradiction with experimental observation is due to kinetic limitations; at typical field strengths (tens to hundreds of times stronger than the Boltzmann energy) the time scales over which the ...
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