Tactoids of Plate-Like Particles: Size, Shape, and Director Field

Abstract

We studied, by means of polarized light microscopy, the shape and director field of nematic tactoids as a function of their size in dispersions of colloidal gibbsite platelets in polar and apolar solvents. Because of the homeotropic anchoring of the platelets to the interface, we found large tactoids to be spherical with a radial director field, whereas small tactoids turn out to have an oblate shape and a homogeneous director field, in accordance with theoretical predictions. The transition from a radial to a homogeneous director field seems to proceed via two different routes depending in our case on the solvent. In one route, the what presumably is a hedgehog point defect in the center of the tactoid transforms into a ring defect with a radius that presumably goes to infinity with decreasing drop size. In the other route, the hedgehog defect is displaced from the center to the edge of the tactoid, where it becomes virtual again going to infinity with decreasing drop size. Furthermore, quantitative analysis of the tactoid properties provides us with useful information on the ratio of the splay elastic constant and the anchoring strength and the ratio of the anchoring strength and the surface tension.

Magnetic field effects on tactoids of plate-like colloids

“…However, if we model a 3D radial director field emanating from a point defect that is off center, then it turns out that this configuration is always energetically more expensive than the central hedgehog or the lens with a uniform field. 35 Therefore, we conclude that the tactoid with boundary defect must be a metastable configuration and because of its relatively high free energy is absent in our phase diagram. We speculate that the observed configuration is stabilized by the flow field caused by the sedimentation.…”

“…Figure 2 shows how two large tactoids with a radius of ≈19 and 30 μm are perturbed by an externally applied magnetic field. At zero field, the tactoids are spherical and the extinction pattern corresponds to that of a 3D radial director field emanating from a hedgehog point defect in the center of the tactoid, 23,35 also see the schematic representation of Fig. 7(e).…”

“…This shall be discussed more extensively in a forthcoming paper. 35 The director field of the particular tactoid studied here is schematically depicted in Fig. 7(c) and represents the route where the point defect opens up to become a ring defect with a radius that increases as the drop size decreases.…”

“…We intend to discuss in a forthcoming publication a detailed investigation of tactoid properties as a function of tactoid size in both gibbsite systems that results in comparable values for K 1 /w that we find here. 35 …”

Deformable homeotropic nematic droplets in a 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.…”