Directed colloidal self-assembly in toggled magnetic fields.

Suspensions of paramagnetic colloids are driven to phase separate and self-assemble by a toggled magnetic field. Initially, all suspensions form network structures that span the sample cell. When the magnetic field is toggled, this network structure coarsens diffusively for a time that scales exponentially with frequency. Beyond this break through time, suspensions cease diffusive coarsening and undergo an apparent instability. The magnetic field drives suspensions to condense into dispersed, domains of bodycentered tetragonal crystals. Within these domains the crystalline order depends on the pulse frequency. Because the scaling of the break through time with respect to frequency is exponential, the steady state limit corresponding to an infinite pulse frequency is kinetically arrested and the equilibrium state is unreachable. These experiments show that there is an out-of-equilibrium pathway that can be used to escape a kinetically arrested state as well as a diverging time scale for phase separation as the critical frequency for condensation is approached. Rather than fine tuning the strength of the interactions among particles, a simple annealing scheme - toggling of the magnetic field - is used to create a broad envelope for assembly of ordered particle structures.