We discuss dissipative particle dynamics as a thermostat to molecular dynamics, and highlight some of its virtues: (i) universal applicability irrespective of the interatomic potential; (ii) correct and unscreened reproduction of hydrodynamic correlations; (iii) stabilization of the numerical integration of the equations of motion; and (iv) the avoidance of a profile bias in boundary-driven nonequilibrium simulations of shear flow. Numerical results on a repulsive Lennard-Jones fluid illustrate our arguments.

We present a simple but versatile off-lattice model for computer simulation studies of amphiphilic systems, constructed mainly for the purpose of computational efficiency. The surfactant molecules are modeled as A-B dimers, where unlike species repel each other, while identical species are also subject to an attraction whose strength drives the various ordering phenomena. This latter potential has been tuned for a good match of interparticle distances, while its short range facilitates fast force calculations. The most important properties of the model are investigated by Molecular Dynamics simulation. In particular, we study the stability of the fluid ordered lamellar phase, as well as the unmixing of the binary fluid of pure A and B.PACS. 82.70.Uv Surfactants, micellar solutions, vesicles, lamellae, amphiphilic systems (hydrophilic and hydrophobic interactions) -61.20.Ja Computer simulation of liquid structure -64.70.Md Transitions in liquid crystals -64.75.+g Solubility, segregation, and mixing; phase separation

Molecular dynamics simulations, integral equation theory, and wide-angle X-ray scattering experiments are used to study the structure of a poly(dimethylsiloxane) (PDMS) melt. Two different chain models are employed for the molecular dynamics: a united-atom class I potential and an explicit-atom class II potential. The effects of partial charges and attractions on the intermolecular pair correlation functions are studied for the united atom model. Good qualitative agreement between the polymer reference interaction site model (PRISM theory) and the simulations is found for the united atom model with charges and attractive dispersion interactions turned off. Simulation results for both the unitedatom and explicit-atom models for the structure factor are in excellent agreement with the X-ray scattering measurements for high wavevector. However, the explicit-atom model gives significantly better agreement for low wavevector, particularly in the vicinity of the first peak.

Experiments on a variety of systems have shown that layered liquids are unstable under shear even if the liquid layers are planes of constant velocity. We investigate the stability of smectic- A like liquids under shear using Molecular Dynamics simulations and a macroscopic hydrodynamic theory (including the layer normal and the director as independent variables). Both methods show an instability of the layers, which sets in above a critical shear rate. We find a remarkable qualitative and reasonable quantitative agreement between both methods for the spatial homogeneous state and the onset of the instability.

We study the shear-induced alignment transition from transverse to the shear plane to parallel and perpendicular in a lamellar or smectics system. A recently proposed simple continuum model for amphiphiles is studied by large-scale nonequilibrium molecular dynamics simulation. We find a shear-rate dependence of the alignment transition under shear flow, parallel at low and perpendicular alignment at high shear rates. To identify the alignment and degree of order of these shear-induced orientations by scattering, a 3D analysis of the scattering data is needed.