title: Multi-scale kinetics of a field-directed colloidal phase transition.
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.
authors.affiliation: Department of Chemical and Biomolecular Engineering and Center for Molecular and Engineering Thermodynamics, Allan P. Colburn Laboratory, University of Delaware, Newark, DE 19716, USA.
authors.forename: James W
authors.initials: JW
authors.lastname: Swan
authors.forename: Paula A
authors.initials: PA
authors.lastname: Vasquez
authors.forename: Peggy A
authors.initials: PA
authors.lastname: Whitson
authors.forename: E Michael
authors.initials: EM
authors.lastname: Fincke
authors.forename: Koichi
authors.initials: K
authors.lastname: Wakata
authors.forename: Sandra H
authors.initials: SH
authors.lastname: Magnus
authors.forename: Frank
authors.initials: F
authors.lastname: De Winne
authors.forename: Michael R
authors.initials: MR
authors.lastname: Barratt
authors.forename: Juan H
authors.initials: JH
authors.lastname: Agui
authors.forename: Robert D
authors.initials: RD
authors.lastname: Green
authors.forename: Nancy R
authors.initials: NR
authors.lastname: Hall
authors.forename: Donna Y
authors.initials: DY
authors.lastname: Bohman
authors.forename: Charles T
authors.initials: CT
authors.lastname: Bunnell
authors.forename: Alice P
authors.initials: AP
authors.lastname: Gast
authors.forename: Eric M
authors.initials: EM
authors.lastname: Furst
mesh_terms: D003102:Colloids / Q000737:chemistry*; D005782:Gels / Q000737:chemistry*; D007700:Kinetics; D008280:Magnetics; D008956:Models, Chemical; D044367:Phase Transition; D012212:Rheology; D014893:Weightlessness
chemical_list: D003102:Colloids; D005782:Gels