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Employing molecular dynamics simulations of a coarse-grained model, the equilibrium swelling behavior of a cross-linked polyelectrolyte gel has been investigated with a bead-spring defect-free network with diamond-like topology under good solvent conditions and close to the theta-point, the counterions were treated explicitly. To examine the validity range of simple scaling arguments we varied the amount of charges in the system, the length of the network chains, the strength of the electrostatic interactions for both solvent qualities over a wide range of parameters. We find that classical theories only apply in the limit of weak electrostatics and suggest corrections based on the microscopic interplay of the systems' components accessible in a simulation, which model successfully our measured data.

A detailed study of the structure and the dynamics of the polystyrene-(PS-) ethylbenzene (EB) polymer-penetrant system is presented. The work combines dual scale (atomistic/coarse grained) simulations and experiments. United atom atomistic NPT molecular dynamics simulations as well as coarse-grained (CG) molecular dynamics simulations have been executed and the ability of the CG simulations to predict the dynamic properties of the polymer/penetrant system is examined. The results are directly compared to pulse-field gradient nuclear magnetic resonance measurements. The coarse-grained simulations, which are much faster than atomistic ones, are capable of describing the diffusion of EB molecules. This opens up the way to study polymer/penetrant systems difficult to reach by experiment but of technological importance.

Using scaling arguments, the equilibrium swelling behaviour of a cross-linked polyelectrolyte gel has been investigated by means of molecular-dynamics simulations of a coarse-grained model; a bead-spring defect-free network with diamond-like topology was considered at the θ-point and under good solvent conditions, where the counterions were treated explicitly. A multitude of systems covering the range between strongly and very weakly charged gels with network chains bearing Nm = 39, . . . , 259 monomers each were examined for both solvent qualities. We compare the resulting structures with mechanical theories describing the swelling behaviour of such systems which, after taking charge renormalization into account, can then be successfully applied to our measured data.

The transition from localized to systemic spreading of bacteria, viruses, and other agents is a fundamental problem that spans medicine, ecology, biology, and agriculture science. We have conducted experiments and simulations in a simple one-dimensional system to determine the spreading of bacterial populations that occurs for an inhomogeneous environment under the influence of external convection. Our system consists of a long channel with growth inhibited by uniform ultraviolet (UV) illumination except in a small "oasis", which is shielded from the UV light. To mimic blood flow or other flow past a localized infection, the oasis is moved with a constant velocity through the UV-illuminated "desert". The experiments are modeled with a convective reaction-diffusion equation. In both the experiment and model, localized or extinct populations are found to develop, depending on conditions, from an initially localized population. The model also yields states where the population grows everywhere. Further, the model reveals that the transitions between localized, extended, and extinct states are continuous and nonhysteretic. However, it does not capture the oscillations of the localized population that are observed in the experiment.