Theory and Simulation of Complex Fluids (F. Varnik)
Group Mission. The group, which is located both at ICAMS and at the Max Planck Institute, studies the mechanical properties of complex multiphase and colloidal fluids. In many cases, such fluids are easily deformed under the action of rather weak forces such as in the case of shear melting. This effect is often accompanied by a drop in shear viscosity upon increasing shear rate (shear thinning). The relation between the stress and deformation for a complex fluid is often non-linear. There is a wide field of applications of complex fluid mechanics, for instance in the soft matter processing industry, in biology, and in metallurgical casting and solidification processes [61,62]. As a modeling tool, the group uses the lattice Boltzmann method (LBM) and multiphase variants thereof [63-69]. LBM is a powerful tool for the numerical calculation of fluid flow, heat, and solute transport. Unlike Navier-Stokes solvers, the LBM mimics flows as collections of pseudo-particles that are represented by a velocity distribution function. These fluid portions reside and interact on the nodes of a grid. System dynamics emerge by the repeated application of local rules for the motion, collision, and re-distribution of the fluid particles. The method is an ideal approach for mesoscale and scale-bridging simulations because it has high computational efficiency, good versatility in the constitutive description of its pseudo-particles, and simplicity in coding. In particular, LBM exhibits good numerical stability for simulating complex fluids, such as multi-phase and multi-component flow phenomena under complicated boundary conditions. Since LBM describes fluid motion at the level of the distribution functions, it can be naturally coupled with related simulation techniques such as cellular automata or phase field models [61,62].
Research Highlights 2009-2010. Using the LBM, the group investigates problems in micro-fluidics such as inhomogeneous diffusive broadening , droplet and contact dynamics on chemically  and topographically [67,68] patterned substrates as well as flow between topographically rough walls [63,64]. On the nano-scale, on the other hand, the group focuses on the effects of thermal fluctuations on droplet dynamics. Furthermore, the group has also developed efficient parallel multiphase LBM variants that are recently particularly used for the study of blood flow mechanics [65,69]. These studies have proved very fruitful with a number of interesting observations as well as theoretical predictions, the latter being verified by independent computer simulations. To name just a few examples, we mention the observation of instantaneous droplet motion on a gradient of texture, and the prediction of genuinely new wetting states in the case of small droplets with a size comparable to the roughness scale [67,68].
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