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Structural Arrest and Texture Dynamics in Suspensions of Charged Colloidal Rods

Structural arrest of charged colloidal rods
Very little is known about the glass transition of very anisometric colloidal particles. We discuss structural arrest of very long and thin rod-like colloids at low ionic strength. A glass is formed due to “cages” that result from overlap of electrical double layers (so-called “Wigner glasses”). As a model system, we use fd-virus particles (880 nm long, diameter 6.8 nm). They form a nematic at 3.0 mg/ml. Structural particle arrest is probed by means of dynamic light scattering, and occurs at 11.7 mg/ml. This is far into the full nematic state, where there is a texture of many (chiral) nematic domains. The dynamics of the texture is quantified with “video correlation spectroscopy”. Image-correlation functions have a time-decay constant (of 250 hours) that is independent of concentration, up to structural particle arrest, above which the time constant discontinuously drops to zero. The glass transition where particle arrest occurs thus coincides with the point where the macroscopic dynamics of the nematic texture freezes. The figure shows images of the texture in a cuvette of 2 cm diameter, for two concentrations: just below and above the glass concentration. Right after filling the cuvette, the nematic texture is flow-aligned (upper two images). The shear-aligned texture breaks down into smaller domains after about 50 hours for the concentration below the glass transition (left lower image). Just above the glass concentration, however, the shear-aligned texture does not relax (lower image on the right).

Soft Condensed Matter:
K. Kang and J. K. G. Dhont

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