Colloidal Phase Transition Dynamics

Abstract

In this work, we present experimental observations on the colloidal phase transitions (from liquid phase to solid phase) involving evaporation at different time scales. Two experimental setups have been used: vertical deposition configuration (large duration and low evaporation) and spincoating (short duration and high evaporation) to carry out various research objectives. (a) We study patterns formation in vertical deposition of colloids on applying weak AC fields at various temperatures. Diluted suspension of negatively charged polystyrene particles of 1.3 µm in diameter suspended in ultra pure water are used to perform several experiments at different evaporation in external weak AC fields (0.8 V/mm to 1.2 V/mm and of frequency from 1 Hz to 3 Hz). At room temperature the application of AC field leads to the formation of one dimensional clusters array at the interface. The dynamical flows of the system during clusters formation in external AC fields have been studied by PIV analysis. Also, we demonstrate the effect of increasing initial particle concentration (0.5% to 1.1% (w/w)) on the clusters evolution. At higher temperature (63 ◦C) we obtain ordered columnar deposits of these colloidal suspensions in AC fields. We study the effect of field strengths on the behavior of columns formation. (b) We obtain polycrystals of mono-layer on photo-patterned substrates by spin-coating using concentrated colloidal suspension of 458 nm diameter silica spheres dispersed in ethanol 95% of 20% (V/V) concentration. We break the axial symmetry imposed by spin axis and eliminate the emerging OCP character (6– or 4–arms). The effect of scaling spacing of surface patterning on the different structures formation of the spin-coated deposits has been studied extensively. We analyze the different structures in depth with bond order parameters and with Minkowski structure metrics. Also, we investigate the influence of external magnetic fields on the flow dynamics of mixed colloidal particles suspension (carbonyl iron and silica particles of micrometer size) by spin-coating and to manipulate the magneto-rheology in terms of occupation factor of the resulting deposits. We develop a generalized model by modifying Cregan and O’Brien model [85] to measure the change in relative viscosity of the mixed colloids in magnetic fields.