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Seminar by Dr. Kyong Kang

ICS-3, Forschungszentrum Jülich (Germany)

27 Apr 2016 11:00
27 Apr 2016 12:00
Lecture room 2009, Jülich GRS building (16.15)

Highly charged fd-coat bacteriophage viruses (fd) at a low ionic strength are used a model system for charged colloidal rods. The phase behavior of these systems, non-equilibrium processes, and dynamics can be probed by means of light scattering, image-time correlation, depolarization microscopy, and Fluorescence Correlation Spectroscopy (FCS). In this presentation, I will discuss (i) diffusion of a small colloidal particle through the quasi-static network formed by the very long and thin fd-rods, (ii) the response to AC electric fields, (iii) as well as the formation of a glass state at very high concentrations.
(i) The reduction of the long-time self-diffusion coefficient of a small colloidal sphere (BSA protein) in fd-rod networks is due to screened hydrodynamic interactions with the network and to electrostatic interactions. By lowering the ionic strength of the solvent, leading to a larger Debye screening length, electrostatic interactions become more important. The increasing range of electrostatic interactions leads to effectively thicker rods and a larger size of the diffusing sphere. FCS experiments will be discussed for isotropic and nematic fd-networks, for various ionic strengths, together with a theoretical description of the diffusion process.
(ii) At a low ionic strength and for fd-concentrations within the isotropic-nematic coexistence region, alternating electric fields are shown to induce various new phases and dynamical states. Of special interest is a dynamical state where nematic domains persistently melt and reform. This is attributed to alternating dissociation and association of condensed ions. A theory will be shortly discussed that semi-quantitatively reproduces experimental findings.
(iii) With an increase of concentration of charged fd-rods (in the absence of electric fields) a glass transition is observed, where the system freezes into a long-lived non-equilibrium, dynamically arrested state. The glass transition is observed far into the full nematic state. Light scattering experiments reveal that particles are dynamically arrested, while image-time correlation reveals freezing of the nematic texture at the same concentration. In addition, below the glass concentration, there is a transition from a non-chiral nematic to a chiral nematic. The chirality of the nematic is due to the fd core-core interactions, which are chiral structure of the constituting DNA.