Nematic droplets in aqueous dispersions of carbon nanotubes

TL;DR

This paper reports the formation of micron-sized nematic droplets in aqueous dispersions of carbon nanotubes, revealing their shape, aspect ratio, and surface tension properties, and providing insights into the phase transition behavior.

Contribution

It demonstrates the existence of tactoids in nanotube dispersions and analyzes their shape and surface tension, offering new understanding of liquid-crystalline phase behavior in such systems.

Findings

Nematic droplets are spindle-shaped with an aspect ratio of about four.

The director field in tactoids is uniform, not bipolar.

Surface tension of coexisting phases is extremely low, around nanonewtons per meter.

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.