Colloidal Assembly by Directional ice Templating

Here, we investigate the directional ice templating of aqueous colloidal particle dispersions. We coat micron size silica colloids with crosslinkable polymer; we add crosslinker and subject this dispersion to unidirectional freezing. We work at very low colloid concentrations. When the aqueous dispersion freezes, ice crystals force polymer-coated particles and cross-linker into close proximity so that crosslinked clusters are formed at ice crystal boundaries. We varied the particle concentration from 10⁶ /ml to 10⁹ /ml and observed that there is a transition from isolated single particles to increasingly large size clusters. Most of the clusters formed under these conditions are either linear, two particle wide chains or sheet like aggregates. The size distribution (P_N) for the clusters (N<30) is strongly dependent on the particle concentration. The exponent (ν) of the power law (P_N~N^ν) increases with the particle concentration and gets saturated at higher concentration. To understand the system better we performed kinetic simulations that ignore hydrodynamic interactions and instabilities at the growing ice front. We demonstrate that aggregate structure of colloidal assemblies is predominantly governed by the exclusion by growing ice crystals.