279507 A Hexatic-to-Disorder Transition in Colloidal Crystals near Electrodes: Stronger Flow Yields Less Order

Wednesday, October 31, 2012: 9:10 AM
406 (Convention Center )
Cari S. Dutcher, Nicholas H. Talken and William D. Ristenpart, Chemical Engineering and Materials Science, University of California Davis, Davis, CA

Colloids are known to form two-dimensional, hexagonal closed packed (HCP) crystals near electrodes in response to electrohydrodynamic (EHD) flow.  Previous work by several groups has established that the strength of the EHD flow increases as the applied AC frequency decreases, suggesting that the driving force for crystallization should increase at lower frequencies.  Here we report that the HCP crystals instead undergo an order-to-disorder transition at sufficiently low frequencies, despite the increase in the attractive EHD driving force. At large frequencies (~500 Hz), monodisperse suspensions of micron-scale particles are observed to arrange into planar HCP crystals, consistent with prior work.  As the frequency is decreased below approximately 250 Hz, however, the crystalline structure transitions to randomly close packed (RCP) with an orientational order parameter, ψ6, significantly less than one.    The transition is reversible and second order with respect to frequency, and independent measurements of the EHD aggregation rate confirm that the EHD driving force is indeed higher at the lower frequencies.   We present evidence that the order-disorder transition is instead caused by an increased particle diffusivity associated with a corresponding increase in the particle height over the electrode induced at lower frequencies.  The observations reported here thus represent a rare instance of an electrically tunable HCP-to-RCP transition, with broad implications for the use of time-varying frequencies to facilitate annealing of planar colloidal crystals.

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