Entropically driven interaction between colloids and their self -assembly

We have made the first direct measurements of entropic interactions of colloidal spheres in suspensions of rods (fd virus). We investigate the influence of sphere size, rod concentration, and ionic strength on these interactions. Although the results compare favorably with a recent calculation, small discrepancies reveal entropic effects due to rod flexibility. Fits to the data with a bent rod model were excellent, provided we used the persistence length less than 1 μm, smaller than the commonly reported value of 2.2 μm for fd-virus. At high salt concentrations, the potential turned repulsive as a result of viral adsorption on the spheres and viral bridging between the spheres. We also investigated the self-assembly of colloidal spheres on periodically patterned templates. The surface potentials and the surface phases were produced entropically by the presence of non-adsorbing polymers in suspension. A rich variety of two-dimensional fluid- and solid-like phases were observed to form on template potentials with both one- and two-dimensional symmetry. The same methodology was then used to nucleate an oriented single FCC crystal more than 30 layers thick on a commensurate substrate. We observed surface-induced freezing of the hard sphere fluid due to the patterned surface of expanding FCC(100) lattice at volume fraction lower than bulk freezing point 54.5%. The bulk osmotic pressure of hard sphere determines the phases above the patterned substrate. The commensurate-incommensurate transitions occurs as the system osmotic pressure increases. At very high osmotic pressure, the system exhibits random hexagonal packed structures despite under-lying square template structures. The template approach provides a new route for directed self-assembly of novel mesoscopic structures