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  • Title: Multi-scale kinetics of a field-directed colloidal phase transition.
    Author: Swan JW, Vasquez PA, Whitson PA, Fincke EM, Wakata K, Magnus SH, De Winne F, Barratt MR, Agui JH, Green RD, Hall NR, Bohman DY, Bunnell CT, Gast AP, Furst EM.
    Journal: Proc Natl Acad Sci U S A; 2012 Oct 02; 109(40):16023-8. PubMed ID: 22988079.
    Abstract:
    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 magneto-rheological 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.
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