Electric-field induced phase transitions of highly charged rod-like colloids

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In the first part of this talk I will give a short introduction to colloids. In the second part, the phases and dynamical states that are induced by external electric fields in a system of very long and thin and highly charged rod-like colloids will be discussed. Experimental results on suspensions of fd-virus particles within the two-phase isotropic-nematic coexistence region will be presented. These fd-virus particles serve as model rod-like colloids, which consist of ds-DNA strands covered with about 6000 coat proteins, they have a length of 880 nm, a thickness of 7 nm, with a persistence length of about 2500 nm. Depending on the electric field strength and the frequency, several phase/state-transitions are induced: a transition from nematic to chiral nematic, from a nematic to a homeotropic state, and a transition to a dynamical state where nematic domains persistently melt and reform. The time scale on which melting and reforming occurs and the size of the domains both diverge at what could be identified as a “non-equilibrium critical point”. An explanation of these phenomena is presented, both on an intuitive level and based on the Smoluchowski equation, which is an equation of motion for the probability density function for the positions and orientations of the rods.