Nematic phase transitions in mixtures of thin and thick colloidal rods

TL;DR

This study experimentally investigates phase transitions in mixtures of thin charged and thick sterically stabilized colloidal rods, revealing new coexistence phenomena and confirming theoretical predictions.

Contribution

First experimental measurement of phase behavior in mixtures of charged and sterically stabilized rods, identifying a lower critical point in nematic-nematic coexistence.

Findings

Observation of isotropic, nematic, and coexistence phases

Identification of a lower critical point in nematic-nematic coexistence

Qualitative agreement with rescaled Onsager theory

Abstract

We present the first experimental measurements of the isotropic and nematic phases of mixtures of thin, charged semiflexible fd virus, and thick, fd-PEG created by covalently grafting poly-(ethylene glycol) to the the surface of fd, rods. The fd-PEG is sterically stabilized and its phase behavior is independent of ionic strength. The fd is charged. Therefore, by varying the ionic strength of a mixture of fd and fd-PEG, only the effective diameter of the bare fd rods changes, subsequently varying the effective diameter ratio (d=D_fd-PEG/D_fd) from 3.7 to 1. In solution, binary mixtures of fd and fd-PEG are shown to exhibit isotropic-nematic, isotropic-nematic-nematic and nematic-nematic coexisting phases with increasing concentration. We measure the binary phase diagrams as a function of composition, total concentration, and ionic strength. We find a lower critical point in the nematic-nematic coexistence region which has not been observed previously. These experimental results resolve a controversy in the literature concerning the evolution of the nematic-nematic phase separation with concentration. The experimental results are qualitatively described by a rescaled Onsager-type theory for the phase behavior of binary rod mixtures.