Add to Google Calendar
Date:  Fri, December 07, 2018
Time:  11:00am-12:00pm
Location:  Holmes 309
Speaker:  Kyongok Kang, Soft Condensed Matter Group (ICS-3)

Abstract:

In this presentation, I will talk about collective micro-dynamics of crowded environments of charged DNA-virus suspensions under external fields, as in electric-field and simple shear flow. First, phase transitions in colloidal systems occur by external electric fields due to interactions between polarization charges and the dissociation/association of condensed ions. For very high frequencies (of the order of MHz), the only polarization mechanism is the dielectric polarization of the interface between the solvent and the colloidal core. Relatively large field strengths (of the order 100-500 V/mm) are needed to induce polarization charge-charge interactions. however, at lower frequencies (in the kHz range), the electric double layer and the layer of condensed ions are polarized, dissociation/association of condensed ions occurs, and field-induced electroosmotic flow is present. Various phases and states are induced by such low-frequency electric fields in suspensions of highly charged DNA-virus, as the phase diagram, at a low ionic strength, and for concentrations where there is an isotropic-nematic coexistence without the electric field. In particular, cyclic dissociation and association of condensed ions explain persistent melting and forming of nematic domains in dynamic states. Second, the frictional forces in suspensions vary depending on the size, shape, and the surface of the particles, which are either charged or neutral. For anisotropic particles with no spatial gradient in the order parameter under external parameters, they exhibit either a continuous phase transition or "freezing" of the order parameter fluctuations, known as the collective soft-mode, which has a finite cutoff dispersion where the relaxation time diverges. The soft-mode is revealed, in the microscopic dynamics, as a minimum spatial coherence length, at the lower I-N binodal concentration with a rotation restoring "twist". Third, a glass-transition is found to be far above the isotropic-nematic coexistence region, at low ionic strength, where thick electric double layers are present. Structural arrest due to particle caging occurs, while the orientation texture freezes at the same glass-transition concentration. Under simple shear-flow, non-uniform 3D flow profiles of such concentration, probed by means of in-situ imaging and Doppler velocimetry. Internal fracture classical shear banding (which requires very strong shear thinning) are observed, in combination with Taylor rolls, although the shear-banding SCC (shear-gradient concentration coupling) mechanism due to shear-gradient induced mass transport may also play a role.

About the Speaker:

Kyongok Kang is a Scientific Staff Member at the Soft Condensed Matter Group (ICS-3) in the Forschungszentrum Julich, Germany, where she joined in 2007 after receiving her PhD in Physics at Kent State University in 2003 and postdoctoral fellowship in Institute of Complex System at the Helmholtz Research Center in Julich, Germany (2003-2007). Her recent research interests include both non-equilibrium phase transitions (in electric field and shear-flow) and the equilibrium orientation dynamics of interacting charged chiral DNA-virus rods, as well as slow dynamics and glass phenomena.

*Contact: Sangwoo Shin (MechE; sangwoos@hawaii.edu, 956-3679)