Statics and Dynamics of Strongly Charged Soft Matter

Soft matter materials, such as polymers, membranes, proteins, are often electrically charged. This makes them water soluble, which is of great importance in technological application and a prerequisite for biological function. We discuss a few static and dynamic systems that are dominated by charge effects. One class comprises complexation between oppositely charged objects, for example the adsorption of charged ions or charged polymers (such as DNA) on oppositely charged substrates of different geometry. The second class comprises effective interactions between similarly charged objects. Here the main theme is to understand the experimental finding that similarly and highly charged bodies attract each other in the presence of multi-valent counterions. This is demonstrated using field-theoretic arguments as well as Monte-Carlo simulations for the case of two homogeneously charged bodies. Realistic surfaces, on the other hand, are corrugated and also exhibit modulated charge distributions, which is important for static properties such as the counterion-density distribution, but has even more pronounced consequences for dynamic properties such as the counterion mobility. More pronounced dynamic effects are obtained with highly condensed charged systems in strong electric fields. Likewise, an electrostatically collapsed highly charged polymer is unfolded and oriented in strong electric fields. At the end of this review, we give a very brief account of the behavior of water at planar surfaces and demonstrate using ab-initio methods that specific interactions between oppositely charged groups cause ion-specific effects that have recently moved into the focus of interest.Comment: 61 pages, 31 figures, Physics Reports (2005)-in press (high quality figures available from authors

Orientation of elastic rods in homogeneous Stokes flow

Using hydrodynamic simulation methods and scaling arguments, we consider an elastic rod which is moving in a gravitational or electric field through a quiescent fluid in the low-Reynolds-number limit. Hydrodynamic effects lead to rod bending and orientation perpendicular to the direction of motion, similar to what is seen in anomalous electric birefringence experiments on TM and FD viruses or polyelectrolytes. Static and dynamic scaling relations for the mean orientation as a function of rod length and elasticity are established

Orientation of elastic rods in homogeneous Stokes flow

Using hydrodynamic simulation methods and scaling arguments, we consider an elastic rod which is moving in a gravitational or electric field through a quiescent fluid in the low-Reynolds-number limit. Hydrodynamic effects lead to rod bending and orientation perpendicular to the direction of motion, similar to what is seen in anomalous electric birefringence experiments on TM and FD viruses or polyelectrolytes. Static and dynamic scaling relations for the mean orientation as a function of rod length and elasticity are established

Anomalous birefringence and polarizability saturation of charged elastic rods: Field-strength, salt and finite-concentration effects

We consider the effective electric polarizability and electric-field–induced birefringence of a dilute solution of rod-like semiflexible charged polymers using hydrodynamic simulations and scaling arguments. We investigate the influence of polymer length, salt concentration, electric field strength and polyion concentration. We show that the polarizabilty is drastically reduced not only for high but also for very low salt concentrations, i.e. when the ionic clouds of neighboring rods start to overlap. As the electric polarizability (favoring rod orientation parallel to the electric field) decreases, the elasto-hydrodynamic orientation due to rod bending (favoring perpendicular alignment) can take over for not too stiff rods. This furnishes a generic mechanism for the experimentally well-known birefringence anomaly of a wide class of charged rod-like systems

Diffusion of a single semiflexible charged polymer

The diffusion of charged semiflexible polymers is studied by hydrodynamic simulations. The salt-dependent diffusion constant exhibits a shallow minimum at a screening length comparable to the polymer length. Fluorescence correlation spectroscopy data for 394\un{bp} ds-DNA fragments confirm the relative insensitivity of diffusion over a wide range of salt concentrations. A scaling expression for the diffusion constant of a neutral semiflexible chain encompassing the rod, the intermediate ideal and the asymptotic swollen regime is constructed