Magnetic field effects on tactoids of plate-like colloids

Abstract

We investigate the effect of a magnetic field on the shape and director field of nematic droplets in dispersions of sterically stabilized and charge-stabilized colloidal gibbsite platelets with a negative diamagnetic anisotropy. Depending on the magnetic field strength and tactoid size, we observe with polarized light microscopy several interesting structures, with different shapes and director fields both with and without defects. In particular, our findings provide the first experimental evidence for the existence of the split-core defect structure predicted ten years ago by Mkaddem and Gartland [Phys. Rev. E 62, 6694 (2000)]. The split-core structure is a metastable director-field configuration that can be stabilized by a sufficiently strong externally applied magnetic field but only if the diamagnetic anisotropy of the particles is negative. To account for our observations, we present a calculation of the stability regions of different shapes and director-field structures as a function of tactoid size, anchoring conditions, surface tension, elastic constants, and magnetic field strength. By fitting the experimental data to the theoretically predicted structures, we are able to extract values for the splay elastic constant, interfacial tension, and anchoring strength. Remarkably, we find significant differences between the two systems studied: for sterically stabilized gibbsite in bromotoluene the anchoring strength is one order of magnitude larger than that of aqueous gibbsite, with the latter exhibiting weak and the former strong anchoring of the director field to the interface. The splay elastic constants that we obtain are in agreement with earlier experiments, simulations, and theory, while the interfacial tension and anchoring strength are considerably larger than what was found in earlier experiments.

Deformable homeotropic nematic droplets in a magnetic field

“…However, this would only be possible if the nematogens have a negative diamagnetic susceptibility, which is the case for gibbsite. So, this corresponds exactly to the situation that was experimentally investigated, 30 and confirmed that the split-core configuration can be stabilized by the magnetic field.…”

“…13,[22][23][24] Tactoids have been observed in dispersions of sterically and charge-stabilized gibbsite platelets. 22,25,30,31 For these systems both uniform and radial director fields have been observed, the latter as a result of the tendency of plate-like particles to align homeotropically to interfaces, again for entropic reasons. 31 The radial director field is characterized by a hedgehog point defect in the center of the drop, 2,[25][26][27] which in fact could also be a small ring defect.…”

“…For the case of the sterically stabilized gibbsite this stretching was followed by an elongation of the drop itself for yet stronger fields, whereas in the aqueous system the drops remained spherical. 30 In fact, the existence of a split-core line defect was predicted theoretically about a decade ago by Mkaddem and Gartland, 29 indicating that it may be stabilized by an externally applied magnetic field. However, this would only be possible if the nematogens have a negative diamagnetic susceptibility, which is the case for gibbsite.…”

“…Extensive experimental studies have recently been done on tactoids of gibbsite platelets in the presence of an externally applied magnetic field. 30,31 Two types of system were studied: one where the particles were stabilized sterically and dispersed in bromotoluene, and another in an aqueous solution where the platelets were charge-stabilized. Surprisingly, the type of solvent turned out to cause markedly different behavior.…”

Probing a self-assembledfdvirus membrane with a microtubule

“…[24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]), colloidal membranes and smectic stacks have been the subject of more recent modeling (e.g. [4,5,7,[9][10][11][40][41][42][43][44][45][46][47]).…”