Diffusion of spheres in isotropic and nematic networks of rods: Electrostatic interactions and hydrodynamic screening

Abstract

Translational diffusion of a small charged tracer sphere in isotropic and nematic suspensions of long and thin charged rods is investigated as a function of ionic strength and rod concentration. A theory for the diffusive properties of a small sphere is developed, where both (screened) hydrodynamic interactions and charge interactions between the tracer sphere and the rod network are analyzed. Hydrodynamic interactions are formulated in terms of the hydrodynamic screening length. As yet, there are no independent theoretical predictions for the hydrodynamic screening length for rod networks. Experimental tracer-diffusion data are presented for various ionic strengths as a function of the rod concentration, both in the isotropic and nematic states. Orientational order parameters are measured for the same ionic strengths as a function of the rod concentration. The hydrodynamic screening length is determined from these experimental data and scaling relations obtained from the above mentioned theory. For the isotropic networks, a master curve is found for the hydrodynamic screening length as a function of the rod concentration. For the nematic networks the screening length turns out to be a very sensitive function of the orientational order parameter.

Image time-correlation, dynamic light scattering, and birefringence for the study of the response of anisometric colloids to external fields

“…Bacteriophage fd is a rod-like macromolecule with a length of L = 880 nm, a bare diameter of D = 6.6 nm, a persistence length at high salt concentration of P = 2200 nm, and a molecular weight of M = 1.64 × 10 7 g/mol. [25][26][27] The response of fd-virus suspensions to an electric field has been shown to be dependent on the ionic strength. [27][28][29] In this paper, we use suspensions of fd-virus particles at a low ionic strength to ensure thick electric double layers.…”

“…[25][26][27] The response of fd-virus suspensions to an electric field has been shown to be dependent on the ionic strength. [27][28][29] In this paper, we use suspensions of fd-virus particles at a low ionic strength to ensure thick electric double layers. Slow dynamics within dynamical states on a large length scale are characterized by means of image time-correlation functions.…”

Efficient simulation of noncrossing fibers and chains in a hydrodynamic solvent

“…Coupling between fluid flow and fiber dynamics may be especially important when the probe bead is smaller than the mesh size of the network. 5 Even for probe sizes in between the mesh size and the fiber contour length interesting new mechanisms may be observed. 31 Computer simulations in which HIs, entanglements, and Brownian motion are treated on an equal footing may help in gaining insight in the dynamics of Brownian fiber suspensions.…”

“…18,19 On the other hand, it is believed that HIs are effectively screened in very concentrated suspensions and to a certain extent also in semidilute suspensions in equilibrium situations. 5,12,20 The onset of the semidilute regime already occurs at lower concentrations for rigid rods than for flexible chains of equal contour length. 12 This corresponds to a smaller dynamic correlation length in a semidilute suspension of rigid rods than in an equally concentrated suspension of flexible chains.…”

“…Examples include carbon nanotubes, 1 fd-virus, [2][3][4][5] and biologically relevant polymers such as actin [6][7][8][9] and tubulin. 10,11 Already at surprisingly low concentrations, uncrossability in such systems leads to a temporary and anisotropic "cage" or tube from which the rod or fiber can only escape through anisotropic motion ͑reptation͒ 12 or through collective motion, as exemplified by the collective reorientation observed in sheared concentrated rod suspensions.…”