Soft-mode of charged chiral fibrous viruses (fd)

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 fluctuation. They are known as the collective soft-mode, which has a finite cutoff dispersion where the relaxation time diverges. From microscopic dynamics of charged chiral fd-viruses, the soft-mode is revealed with a rotation restoring “twist”, obtained from both polarized (VV) and depolarized (VH) small angle dynamic light scattering. Here, I have found the minimum spatial coherence length at a lower I–N binodal concentration, which is due to the reverse of electrostatic repulsive forces with an increase in the concentration of charged chiral rods

Nematic-Isotropic Spinodal Decomposition Kinetics of Rod-like Viruses

We investigate spinodal decomposition kinetics of an initially nematic dispersion of rod-like viruses (fd virus). Quench experiments are performed from a flow-stabilized homogeneous nematic state at high shear rate into the two-phase isotropic-nematic coexistence region at zero shear rate. We present experimental evidence that spinodal decomposition is driven by orientational diffusion, in accordance with a very recent theory.Comment: 17 pages, 6 figures, accepted in Phys. Rev.

Collective dynamics in dispersions of anisometric pigment particles

© 2018 Elsevier B.V. Dynamics of suspensions of solid rodlike pigment particles in a non-polar solvent were studied in a concentration range from the isotropic up to the orientationally ordered nematic-like phase. Using dynamic light scattering and gradient recovery measurements, we studied the rotational and translational diffusion coefficients. We demonstrate that the translational diffusion coefficient in this system is increasing with increasing concentration of the pigment particles in the vicinity of the transition into an ordered phase. This unexpected behaviour can be attributed to the collective interactions between the particles and the alignment effects

Kinetic pathways of the Nematic-Isotropic phase transition as studied by confocal microscopy on rod-like viruses

We investigate the kinetics of phase separation for a mixture of rodlike viruses (fd) and polymer (dextran), which effectively constitutes a system of attractive rods. This dispersion is quenched from a flow-induced fully nematic state into the region where the nematic and the isotropic phase coexist. We show experimental evidence that the kinetic pathway depends on the overall concentration. When the quench is made at high concentrations, the system is meta-stable and we observe typical nucleation-and-growth. For quenches at low concentration the system is unstable and the system undergoes a spinodal decomposition. At intermediate concentrations we see the transition between both demixing processes, where we locate the spinodal point.Comment: 11 pages, 6 figures, accepted in J. Phys.: Condens. Matter as symposium paper for the 6th Liquid Matter Conference in Utrech

Electric-field induced microdynamics of charged rods

Electric-field induced phase/state transitions are observed in AC electric fields with small amplitudes and low frequencies in suspensions of charged fibrous viruses (fd), which are model systems for highly charged rod-like colloids. Texture- and particle-dynamics in these field-induced states, and on crossing transition lines, are explored by image time-correlation and dynamic light scattering, respectively. At relatively low frequencies, starting from a system within the isotropic-nematic coexistence region, a transition from a nematic to a chiral nematic is observed, as well as a dynamical state where nematic domains melt and reform. These transitions are preliminary due to field-induced dissociation/association of condensed ions. At higher frequencies a uniform state is formed that is stabilized by hydrodynamic interactions through field-induced electro-osmotic flow where the rods align along the field direction. There is a point in the field-amplitude vs. frequency plane where various transition lines meet. This point can be identified as a “non-equilibrium critical point,” in the sense that a length scale and a time scale diverge on approach of that point. The microscopic dynamics exhibits discontinuities on crossing transition lines that were identified independently by means of image and signal correlation spectroscopy

Charged Colloidal Rods Out of Equilibrium

This article is a comprehensive overview of the ongoing research of the author on charged colloid- al rods out of equilibrium, under external electric fields and at high concentrations around the glass transition. The suspensions of fd-virus particles are used as a model system for charged col- loidal rods, which exhibit several disorder-order (and liquid-crystalline) phase transitions. When a low AC electric field is applied to suspensions in isotropic-nematic coexistence concentration, with frequencies that are sufficiently low to polarize the electric double layer and the layer of condensed ions, various phases/states are induced: a chiral nematic, a dynamical state where ne- matic domains persistently melt and form, and a uniform homeotropic phase. A point in the field-amplitude versus frequency diagram, where various transitions lines meet, can be identified as a non-equilibrium critical point. Without an electric field, at high concentrations of charged fd-rods, various self-assembled orientation textures are found beyond the isotropic-nematic coex- istence regions, and a glass transition is observed on approach and within the glass state that are probed. The presented system exhibits transient behaviors of repulsive glasses and slow dynamics out of equilibrium

Non-equilibrium phase transitions and equilibrium textures of charged chiral rods (fd-viruses)

Both non-equilibrium phase transitions and equilibrium textures of interacting charged chiral rods are explored, where thick electric double layers are present in the suspensions of charged chiral fibrous (fd) viruses at a low ionic strength. We first start with the electric phase/state diagram, illustrating dynamic frequency responses at the concentration of isotropic-nematic (I-N) coexistence, leading to various field-induced phases/states. As a low frequency response, two sharp transitions of chiral-nematic phases and dynamical states are induced, while as one transition is found to a homeotropic phases that is stabilized at a high-frequency. The characterizations of field-induced phases/states, and critical slowing down behaviors in the non-equilibrium criticality are discussed, by means of image-time correlation, dynamic light scattering and electric birefringence. For the equilibrium phase behaviors, depolarized optical morphology is studied with their texture dynamics, as an increase of rod concentration for a long equilibration time (80-100 h). Structure arrest has been observed by dynamic light scattering, above a glass transition concentration. Below the glass transition concentration, chiral-nematic textures are equilibrated at low, but above the I-N coexistence concentration. At higher, near to the glass transition concentration, another type of the equilibrium is reached as “domain” textures (of the helical domains) . Whether there will be a thermodynamic access of the density in orientational domains in these equilibrated textures in the interaction of charged chiral rods would be an interesting issue

Charged (Filamentous) DNA-viruses in External Electric Fields and Shear Flow

Bacteriophage DNA-viruses (fd) consist of a DNA strand that is covered by several thousands of fd-coat proteins. The 880 nm long DNA-virus is relatively stiff due to these coat proteins (the persistence length is about 2500 nm). In addition, the coat proteins render a DNA-virus highly charged, such that a large fraction of ions are condensed onto the core of the virus. By varying the ionic strength, the equilibrium phase behaviors are explored well for the stable chiral-mesophases, as well the field-induced new phases in non-equilibrium processes at ow AC electric fields and shear flow. I will address the response of concentrated DNA-virus suspensions to external electric fields (for suspensions in the two-phase isotropic-nematic coexistence region), and to externally applied shear flow (for crowded suspensions in the glassy state). Several phases and dynamical states are induced by electric fields, depending on the field strength and frequency, due to field-induced dissociation/association of condensed ions and hydrodynamic interactions thorugh field-induced electro-osmotic flow. Shear flow induces several kinds of inhomogeneous flow profiles. At low shear rates, the plug flow and fracture are observed, a transition to a shear-banded state is seen on increasing the shear rate, in coexistence with Taylor vorticity banding, while at high shear rates a homogeneously sheared profile is observed where the shear rate is constant throughout the gap of the shear cell. The above phenomena have been observed in bulk, with 1 mm thick cylindrical sample cells. It would be also interesting to investigate the effect of confinement in an arbitrary shape of curvatures, using microfluidic channel flows, allowing the gradients of different ionic strengths. In particular, the role of confinement in both field-induced dynamical states for small (chiral) nematic domains, and the shear-induced fracture in the glassy state can be further conveyed, to which extent the size of (chiral) nematic domains are affected by the dimensions in the confining geometry