Phase Behavior and Rheology of Attractive Rod Like Particles

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Overview

  • [1] Huang, F., Rotstein, R., Fraden, S., Kasza, K., & Flynn, N. Soft Matter. 5, 2766-2771 (2009).
  • Keywords: Isotropic, Nematic, Viscoelastic, Sol-Gel Transition, Colloidal Rods, Phase Transition

Summary

Huang, Rotstein, Fraden, Kasza, and Flynn study an aqueous solution of rod-shaped particles to look for transitions between isotropic, nematic, liquid, and gel states. The transitions are a function of particle concentration, temperature, and salt concentration in the water. For this experiment, Huang et al. create particles by coating bacteriofage fd with poly(N-isopropylacrylamide) (PNIPAM). The researchers use both rheological measurements and light scattering measurements to characterize the solutions which do not agree with previous theory.

The bacteriofage fd (a polymer with negative surface charge) is itself approximately rod-shaped. The researchers coat the bacteriofage fd with the polymer PNIPAN to give the particle-particle interactions a temperature dependence. The temperature dependence of solubility of PNIPAM and bacteriofage fd are different. The researchers also varied the ion content of the water to change the particle-particle interactions.

Theory predicts that low particle concentrations yield isotropic solutions, high concentrations yield nematic solutions, and intermediate concentrations yield a mixed istropic-nematic phase. Theory also predicts that as temperature increases, the temperature range of this istropic-nematic widens (see figure 1).

Debye Length

Figure 1. A phase diagram of both theory and experiments. Figure 2 from [1]

The researchers compared two samples: a) a low concentration of rod-like particles (9.6 mg/ml) which is isotropic, and b) a higher concentration of rod-like particles (21 mg/ml) which has nematic structure. Both solutions have an ionic strength of 144mM.

-salt and temperature varied -20 degrees to 200 degrees C -debye length -sol-gel transition -comparison to predictions

Soft Matter Details

Experimental Methods:

Dynamic Light Scattering

Rheology

Phase Behavior:

Open Questions/Ongoing Research:

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