Dynamics and contact microstructure of rough colloids

Colloidal particles with hard-sphere interactions have been used to study crystallization and glassy dynamics. Here we synthesize smooth and rough poly(methyl methacrylate) colloids and disperse them in an index-matched solvent, squalene, at volume fractions (Φ) ranging from dilute (Φ=0.1) to random close packing (RCP). We use confocal microscopy to obtain 3D image-stacks of the colloidal suspensions and process them to obtain particle centroids with subpixel precision. We estimate the contact numbers for smooth and rough colloids at RCP to be 6 and 4 respectively, as predicted by simulations of frictional granules. In both the systems, scaling of the form <z>~Φ^a was observed, where parameter 'a' varies with surface roughness. We hypothesize that the power law variations are related to the phase behaviors and associated dynamics (mean-squared displacement) of these particles at various Φ. We believe our approach to connect the contact microstructure to dynamics in suspensions will enable better understanding of the effect of surface anisotropy on colloidal crystallization, rheological phenomena, and osmotic pressure of suspensions.