Structural rheology of focal conic domains: a stress-quench experiment.

We study the dynamics of focal conic domain (FCD) formation in a thermotropic smectic phase under shear stress. It is known that increasing the shear stress induces a non-equilibrium phase transition from a smectic phase with FCDs (SmAI) to another smectic phase (SmAII) in which the layers are oriented. By quenching the shear stress from the SmAII phase to the SmAI phase, we find three characteristic modes in the FCD formation process. The first mode is attributed to the edge dislocation dynamics induced by climb motions. The second mode results from FCD formation. The first and second modes show slowing down close to the smectic-nematic transition temperature, implying that the dynamics are dominated by dislocation unbinding. The third mode originates from the alignment of FCDs which form oily streaks. Such an alignment occurs when the shear stress balances the line tension of the oily streaks.

Sizes of multilamellar vesicles in shear.

The dynamics of the multilamellar vesicle (MLV) is analyzed theoretically, where membrane interaction squeezes the solvent to flow between the neighboring membranes. With the applied affine shear, the dynamic free energy density of the MLV develops a minima, which selects the MLV size. The model predicts a terminal shear rate, below which the metastable MLV exists. The scaling relations for the MLV size and the terminal shear are both consistent with the experiments.

Viscoelasticity of two-layer vesicles in solution.

The dynamic shape relaxation of the two-layer vesicle is calculated. In addition to the undulation relaxation where the two bilayers move in the same direction, the squeezing mode appears when the gap between the two bilayers is small. At a large gap, the inner vesicle relaxes much faster, whereas the slow mode is mainly due to the outer-layer relaxation. We have calculated the viscoelasticity of the dilute two-layer-vesicle suspension. It is found that for a small gap, the applied shear drives the undulation mode strongly while the slow squeezing mode is not much excited. In this limit, the complex viscosity is dominated by the fast-mode contribution. On the other hand, the slow mode is strongly driven by shear for a larger gap. We have determined the crossover gap, which depends on the interaction between the two bilayers. For a series of samples where the gap is changed systematically, it is possible to observe the two amplitude switchings.

Theory of the polyelectrolyte dielectric function.

A simple double-layer polarization theory is developed for the flexible polyelectrolyte solution. Under the applied electric field, the double layer of the polyelectrolyte induces the excess line fluxes of charge and salt within the double layer. The off-diagonal Onsager coefficient couples the charge and the neutral salt dynamics so that the salt distribution is perturbed by the applied electric field. The nonuniform salt then drives the excess double-layer electric current. We show that the dielectric function can be expressed as the free energy storage and loss within the electric field and the salt distribution. At the leading Born approximation, the dielectric function depends on the chain configuration through the chain structure function. We use a simple mean-field structure function to calculate the dielectric function, where a closed-form expression is obtained. A detailed prediction is made for the full range of the polyelectrolyte concentration.

Phase separation of thin-film polymer mixtures under in-plane electric fields.

Phase-separation dynamics of polymer thin-film mixtures of polystyrene (PS) and poly(methyl methacrylate) (PMMA) are observed while an in-plane electric field is applied, instead of the out-of-plane fields usually employed previously. The phase separation is accompanied by the formation of PS dewetting holes at zero or weak fields. The dewetting velocity at 0.25 µm/min is a few times slower than that seen in regular bilayer dewetting. With the increasing of the field strength, we observe the formation of PS droplets in PMMA matrix, a reversal from zero- or low-field conditions. The PS dewetting holes are also suppressed. At further increased fields, PS droplets quickly penetrate up to the top of the PMMA matrix, leading to smaller and more irregular final PS droplets. This is manifested in the dramatic decrease in the growth exponent of the droplet size L from L∼t1.5 to L∼t0.1. These morphology changes are explained by the electrostatic energy resulted from the PS and PMMA dielectric contrast.