An electric-field induced dynamical state in dispersions of highly charged colloidal rods: comparison of experiment and theory

Abstract

Concentrated dispersions of highly charged rod-like colloids (fd-virus particles) in isotropic-nematic coexistence exhibit a dynamical state when subjected to low-frequency electric fields [Soft Matter, 2010, 6, 273]. This dynamical state consists of nematic domains which persistently melt and form on time scales typically of the order of seconds. The origin of the dynamical state has been attributed to a field-induced, cyclic dissociation and association of condensed ions [Soft Matter, 2014, 10, 1987, Soft Matter, 2015, 11, 2893]. The ionic strength increases on dissociation of condensed ions, rendering the nematic domains unstable, while the subsequent decrease of the ionic strength due to association of condensed ions leads to a recurrent stabilization of the nematic state. The role of dissociation/association of condensed ions in the phase/state behaviour of charged colloids in electric fields has not been addressed before. The electric field strength that is necessary to dissociate sufficient condensed ions to render a nematic domain unstable, depends critically on the ambient ionic strength of the dispersion without the external field, as well as the rod-concentration. The aim of this paper is to compare experimental results for the location of transition lines and the dynamics of melting and forming of nematic domains at various ionic strengths and rod-concentrations with the ion-dissociation/association model. Phase/state diagrams in the field-amplitude versus frequency plane at two different ambient ionic strengths and various rod-concentrations are presented, and compared to the theory. The time scale on which melting and forming of the nematic domains occurs diverges on approach of the transition line where the dynamical state appears. The corresponding critical exponents have been measured by means of image time-correlation spectroscopy [Eur. Phys. J. E, 2009, 30, 333], and are compared to the theoretical values predicted by the ion-dissociation/association model.

Soft-mode of charged chiral fibrous viruses (fd)

“…Theoretically predicted I-N coexistence concentration regions are 0.66 < [f d] < 0.84 mg/ml and the experimental I-N binodal concentrations are 0.8 ± 0.2 and 1.5 ± 0.4 mg/ml for the ionic strength of 0.032 mM buffer[29].…”

“…There are competing interactions between electrostatics and hydrodynamic interaction, such that, above the ionic strength of 1mM salt solution, thin electric double layers are present, in which screened hydrodynamics is valid [27], however, below 1mM salt solution, relatively larger Debye length is present as com- pared to the bare diameter of fd-virus (of 6.8nm). The location of binodals and spinodals depend both on the Debye screening length, and the effective number of surface charges [29]. Current work is done at a low ionic strength of 0.032mM salt, where Debye length of fd-virus is about 54nm, as 7 times larger than the bare diameter, and the length of fd-virus is 880nm, thus it gives the reduced aspect ratio of 16, shown in Figure 1a.…”

“…with d, as the core diameter, l B , the Bjerrum length, N 0 , the number of immobile charges chemically attached to the surface of a rod, and N c,0 the number of condensed ions of a rod in the absence of any electric fields [29]. The third contribution ∆ twist is the twist contribution, which turned out to be not negligible parameter for charged fdrods, where thick electric double layers are present [31,32],…”

Synchronized oscillations of dimers in biphasic charged fd-virus suspensions

“…For lowering the ionic strength of suspension of highly charged fd-virus, different phase longer electrostatic interactions are exhibited in the phase behaviors [9]. At the very low ionic strength used here, the isotropic-nematic phase boundaries are located at relatively quite low fd-concentrations [10], while no sedimentation occurs of nematic domains in the bi-phasic region over a time span of many hours. PS is used as a tracer method, meaning that they are much diluted as compared to the suspensions of fd-virus so that PS is expected to be not interacting themselves in the medium of fd-virus networks.…”

“…The length of a fd-virus particle is 880 nm, and the core thickness is 6.8 nm, with the persistence length of 2200 − 2500 nm (see the inset of Fig.1b). The ionic strength in the present experiments is far low, as 0.032 mM , corresponding to a relatively large Debye screening length of 54 nm [10]. At this low ionic strength, the effective diameter that measures the range of electrostatic interactions is a few times larger than the bare core diameter [8].…”