Arrested phase separation of colloidal rod–sphere mixtures

Abstract

Sterically stabilised colloidal rod-sphere mixtures were prepared by mixing sepiolite rods of an average length L ¼ 860 nm and aspect ratio L/D ¼ 40 with silica spheres of average diameter d ¼ 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 isotropicnematic 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 ¼ 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.

Isotropic–nematic phase transition in aqueous sepiolite suspensions

“…The particles tend to be quite elongated with lengths ranging from a few hundred nm to a few micron, and diameters in the range 10-30 nm. [13] The porous channels in sepiolite make this mineral of interest as an absorbent and also allow incorporation of dye molecules such as indigo [15] or acridine orange [16]. Sepiolite is known to be very hygroscopic and so is used abundantly as an animal litter absorbent.…”

Increasing entropy for colloidal stabilization

“…3a), and demixing occurred, which can be attributed to depletion attraction. Similar to the demixing mechanism for a colloidal rod–plate mixture26, the origin of demixing may be the excess excluded volume of platelet–sphere pair compared with platelet–platelet and sphere–sphere pairs. The TEM photographs of the supernatant and sedimentation of sample 12 are shown in Fig.…”

“…Although the entropically driven phase behaviours of binary colloidal dispersion systems have garnered considerable attention181920212223242526, these works have focused on phase demixing2627, and the function of entropy in the improvement of colloidal stability has been scarcely reported until now.…”

Orientational order and translational dynamics of magnetic particle assemblies in liquid crystals

“…In particular, the phase behavior of such systems has been investigated by experiments, 25,26 theoretical studies [27][28][29][30] and computer simulations [31][32][33][34] revealing a rich polymorphism that includes re-entrance phase transitions as well as multiphase equilibria between isotropic, nematic, smectic and columnar phases. Our intention here is concentrated in fully miscible binary rod-sphere mixtures in various LC states.…”