Dr. Yoav Tsori
Modelling of complex fluid systems, effect of electrical field on polymer systems

1. Electric field induced phase separation in liquid mixtures
The effect of electric field on the phase diagram of binary liquid mixtures has been the subject of intense research in the past 50 years. Starting from Debye and Landau and Lifshitz, experiments and theory have shown that uniform electric field can change the critical temperature Tc of a mixture. However, the change to Tc is very small, about 0.01K, and experimentally it always favors mixing. In the Landau mechanism, the change to Tc is brought about by a nonlinear dependence of the dielectric constant ? on mixture concentration. However, a much stronger effect occurs when the field is non- uniform, as we have recently predicted and demonstrated experimentally. In non- uniform fields the change to the coexistence temperature is 2- 100 larger than the same change due to uniform fields. Tc remains unchanged, though. The phase separation is reversible: when field gradients are turned off the mixture becomes homogeneous again. The effect is ideally suited for various nanotechnological applications since it benefits from field gradients near small conducting objects.

2. Control of ordered phases in polymer materials
We have studied theoretically the thin-film Block-copolymers (BCP) morphology and found it to be determined by the complex interplay between interfacial interactions with the substrates, film thickness and mesophase periodicity.
A transitions from lamellar layering parallel to the confining walls to perpendicular layering (as important in applications) occurs at the “right” places in the phase diagram. In a related study we show how a simple surface corrugation can be used to control the orientation of a smectic phase. Here, the elastic penalty of having distortions of a “perfect” bulk phase competes against the preference for a wetting layer. Such a mechanism can be advantageous since it exploits the naturally existing surface roughness.

3. One-molecule heating
Translocation of single macromolecules (e.g. single-stranded DNA) through a hole in a membrane has been extensively studied. We have recently shown that the same system can be utilized to induce heating in a small region of nanoscopic dimensions. A “hot spot” can occur at a nano-pore due large and localized Joule heating. This can be a model system to study various effects in DNA and other molecules.