Kirstin R. Purdy

Isotropic-nematic phase transition in suspensions of filamentous virus and the neutral polymer Dextran

We present an experimental study of the isotropic-nematic phase transition in an aqueous mixture of charged semiflexible rods ( fd virus) and neutral polymer (Dextran). A complete phase diagram is measured as a function of ionic strength and polymer molecular weight. At high ionic strength we find that adding polymer widens the isotropic-nematic coexistence region with polymers preferentially partitioning into the isotropic phase, while at low ionic strength the added polymer has no effect on the phase transition. The nematic order parameter is determined from birefringence measurements and is found to be independent of polymer concentration (or equivalently the strength of attraction). The experimental results are compared with the existing theoretical predictions for the isotropic-nematic transition in rods with attractive interactions.

Measuring the nematic order of suspensions of colloidal fd virus by x-ray diffraction and optical birefringence

The orientational distribution function of the nematic phase of the semi-flexible rod-like virus fd is measured by x-ray diffraction as a function of concentration and ionic strength. The angular distribution of the scattered intensity from a single-domain nematic phase of fd arises from only the single particle orientational distribution function at high angle but it also includes spatial and orientational correlations at low angle. Experimental measurements of the orientational distribution function from both the interparticle and intraparticle scattering were made to test whether the correlations present in interparticle scatter influence the measurement of the single particle orientational distribution function. It was found that the two types of scatter yield consistent values for the nematic order parameter. It was also found that x-ray diffraction is insensitive to the orientational distribution functions precise form, and the measured angular intensity distribution is described equally well by both Onsagers trial function and a Gaussian. At high ionic strength the order parameter S of the nematic phase coexisting with the isotropic phase approaches theoretical predictions for long semi-flexible rods S=0.55, but deviations from theory increase with decreasing ionic strength. The concentration dependence of the nematic order parameter was also found to better agree with theoretical predictions at high ionic strength, indicating that electrostatic interactions have a measurable effect on the nematic order parameter. The measured x-ray order parameters are also shown to be proportional to the measured birefringence and the saturation birefringence of fd is measured, enabling a simple, inexpensive way to measure the order parameter.

Isotropic-cholesteric phase transition of filamentous virus suspensions as a function of rod length and charge

The viruses studied are genetically engineered, charged, semiflexible filamentous bacteriophages that are structurally identical to M13 virus, but differ either in contour length or surface charge. While varying contour length (L) we assume the persistence length (P) remains constant, and thus we alter the rod flexibility (L/P) . Surface charge is altered both by changing solution pH and by comparing two viruses, fd and M13, which differ only by the substitution of one charged for one neutral amino acid per virus coat protein. We measure both the isotropic and cholesteric coexistence concentrations as well as the nematic order parameter after unwinding the cholesteric phase in a magnetic field as a function of rod surface charge, rod length, solution ionic strength, and solution pH . The isotropic-cholesteric transition experimental results agree semiquantitatively with theoretical predictions for semiflexible, charged rods at high ionic strength, but disagree at low ionic strength.

Nematic phase transitions in mixtures of thin and thick colloidal rods

We report experimental measurements of the phase behavior of mixtures of thin (charged semiflexible fd virus) and thick (fd-PEG, fd virus covalently coated with polyethylene glycol) rods with diameter ratio varying from 3.7 to 1.1. The phase diagrams of the rod mixtures reveal isotropic-nematic, isotropic-nematic-nematic, and nematic-nematic coexisting phases with increasing concentration. In stark contrast to predictions from earlier theoretical work, we observe a nematic-nematic coexistence region bound by a lower critical point. Moreover, we show that a rescaled Onsager-type theory for binary hard-rod mixtures qualitatively describes the observed phase behavior.