Nematic droplets in aqueous dispersions of carbon nanotubes

Abstract

Aqueous dispersions of exfoliated, bile-salt stabilized single-wall carbon nanotubes exhibit a first order transition to a nematic liquid-crystalline phase. The nematic phase presents itself in the form of micron-sized nematic droplets also known as tactoids, freely floating in the isotropic host dispersion. The nematic droplets are spindle shaped and have an aspect ratio of about four, irrespective of their size. We attribute this to a director field that is uniform rather than bipolar, which is confirmed by polarization microscopy. It follows that the ratio of the anchoring strength and the surface tension must be about four, which is quite larger than predicted theoretically but in line with earlier observations of bipolar tactoids. From the scatter in the data we deduce that the surface tension of the coexisting isotropic and nematic phases must be extremely low, that is, of the order of nN/m. Carbon nanotubes or CNTs are colloidal particles with a very large aspect ratio, typically in the range from many tens to hundreds up to even thousands. Hence, it is not surprising that, provided they are properly stabilized against aggregation, fluid dispersions containing CNTs exhibit an Onsagertype isotropic-nematic transition ͓1-6͔. This happens at concentrations in excess of a critical value that depends on the aspect ratio of the rods. The relevant concentration scale here is the volume or packing fraction because the driving force for the spontaneous alignment is the anisotropic volume exclusion between the particles. For the nematic to become stable the volume fraction of CNTs should be in excess of a few times the reciprocal of some average of their aspect ratios ͓7͔. It follows that the nematic transition must occur at very low concentrations of, say, one per cent of CNTs. This, by and large, is in agreement with experimental observation, allowing for instance for the effects of polydispersity ͓1-6͔.Often, before isotropic-nematic phase separation occurs on a macroscopic scale in dispersions of elongated colloidal particles, the nematic phase establishes itself in the form of droplets called tactoids. Tactoids have been observed in many dispersions, such as tobacco mosaic virus ͓8,9͔, boehmite rods ͓10͔, poly͑butyl glutamate͒, self-assembled chromonics ͓11͔, fd virus ͓12͔, f-actin ͓13͔, and vanadium pentoxide ͓14,15͔. These droplets have in common their unusual elongated, spindlelike shape. This shape can be explained by the preferential planar anchoring of the nematic director at the interface with the isotropic phase. The competition between surface tension and the elastic deformation of the bipolar director field that accommodates this preferential planar anchoring plausibly determines the optimal aspect ratio ͑Fig. 1͒. If the director field of a tactoid is indeed bipolar with field lines connecting two boojum surface defects, then its shape, as described, e.g., by the aspect ratio or the tip angle, depends on its physical dimensions because the surface and bulk elastic energies scale differently with drop...

Experimental realization of crossover in shape and director field of nematic tactoids

“…The only study that showed homogenous tactoids is that of bile salt-stabilized CNTs in water by Puech et al [9]. However, their experimental system was 10-20 μm wide and only yielded small tactoids below the crossover size with homogenous director fields [19].…”

“…1(b)] [2][3][4][5][6]10,11], with the exception of the recent report of homogeneous tactoids [uniform director field, Fig. 1(c)] in aqueous carbon nanotube (CNT) suspensions [9]. Yet, macroscopic theory predicts that tactoids should display a continuous range of configurations from homogeneous toward bipolar, including intermediate configurations, where the boojums are outside the droplet [virtual boojums, Fig.…”

“…The director-field (n(r)) configurations were determined by imaging tactoids at different orientations between crossed polarizers and classifying the optical textures based on literature on bipolar [2,10] and homogenous [9] tactoids, as well as Jones matrix calculations [12] of tactoids with representative configurations. Optical textures were simulated using size (R) and shape (R/r) measurements of the tactoids at 1000 ppm in Figs.…”

“…Tactoids have been observed for over a hundred years in fluids containing highly elongated colloids or macromolecules [2][3][4][5][6][7][8][9]. So far, the observed tactoids have displayed only a bipolar configuration, where the director field (representative of the local molecular orientation) is parallel to the interface and converges at two surface point defects called boojums [ Fig.…”

Deformable homeotropic nematic droplets in a magnetic field

“…[1][2][3][4][5][6][7] These so-called tactoids have been investigated quite intensively over the last few decades, not least because of their interesting and sometimes unusual shapes and internal structures and relevance to display technology. 2 These include oblate and prolate droplet shapes with round and sharp edges, and director-field structures involving surface and bulk point defects as well as ring-shaped disclination lines and bipolar twisted "parity-broken" director fields, all depending on the size of the drops and the shape of the colloids.…”