Arrested phase separation of colloidal rod-sphere mixtures

Sterically stabilised colloidal rod-sphere mixtures were prepared by mixing sepiolite rods of an average length L 860 nm and aspect ratio L/D 40 with silica spheres of average diameter d = 620 nm. Whereas most previous studies have addressed low or high L/d ratios, the present study has an intermediate ratio of 1.4. Samples were studied at rod concentrations from 3 to 9 wt%, in the isotropic-nematic coexistence region. No dramatic effects were seen on adding spheres, except for two samples at low rod concentrations where a rapid (local) phase separation resulted. This is ascribed to formation of nematic tactoids, separated by layers of spheres. Samples at higher rod concentrations did not show any rapid phase separation. Microscopy using fluorescent rods however showed a fine network of rods formed in this case. Macroscopic phase separation into sphere-rich and rod-rich phases was not observed in any sample, nor was any rapid clustering of spheres as seen previously in mixtures with L/d 0.3. The late stage sediment density can be described well by approximating the osmotic pressure of the colloidal rods at second virial level. Whilst the absence of macroscopic phase separation suggests that these mixtures do not reach chemical equilibrium and instead remain stuck in a range of long-lived metastable states, the observations on the sediment density show that nevertheless local mechanical equilibrium is attained.