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In this twodimensional simulation, three colloids with hardsphere interaction are embedded in a solvent. The fluid is modelled by multiparticle collision dynamics (MPC), a particlebased mesoscopic hydrodynamics simulation technique. In this case, there are about 11,000 fluid particles, which are not shown. The fluid velocity field is represented by arrows. Noslip boundary conditions are employed for the fluidcolloid interaction. Since MPC naturally contains thermal fluctuations, the colloids are subject to Brownian motion.
Interactive simulation not yet available
Click on a colloid and drag it with your mouse through the fluid.
An external force proportional to the distance between the colloid and the current mouse position is applied (indicated by the red arrow).
When a colloid is dragged towards another colloid, a flow field emerges and momentum is transferred by the fluid to the second colloid that starts to move well before the hard colloidcolloid interaction becomes active. Due to the inherent manybody character of hydrodynamic interactions, the resulting forces acting on the other colloids depend on the relative positions of all colloids. Note that periodic boundary conditions are used, and the reference frame is comoving with the center of mass of the whole system.
Under nonequilibrium conditions, soft matter systems exhibit both viscous and elastic characteristics  they are viscoelastic. Prominent examples are polymer solutions and melts. For a colloidal suspension in a viscoelastic solvent, e.g., proteins in a cell, or colloids dispersed in polymer solutions, the situation is even more complex  the solvent itself is now a complex fluid. To unravel dynamical and rheological properties of such complex systems requires a simplified mesoscopic model of a viscoelastic solvent.
We have extended the multiparticle collision dynamics (MPC) technique to shearthinning viscoelastic solvents. Here, the normal MPC particles are replaced by dumbbells with harmonic or finiteextensible nonlinear elastic (FENE) springs. We have studied the properties of FENEdumbbell fluids under simple shear flow. Shearthinning behavior is found for shear rates larger than the inverse characteristic dumbbell relaxation time. We have applied this approach to the flow behavior of a colloid in a shearthinning viscoelastic fluid.
Angular momentum is conserved in fluids  with a few exceptions such as ferrofluids in an external field. However, it can be violated locally in fluid simulations to reduce computational costs. We have developed a version of multiparticle collision dynamics (MPC), a particle based simulation method, in which angularmomentum conservation can be switched on or off. To investigate the effects of angularmomentum violation, we have studied circular Couette flows between concentric and eccentric cylinders. Nonphysical torques due to the lack of the angularmomentum conservation are found, whereas the velocity field is not affected. In addition, in simulations of fluids with different viscosities in contact, incorrect angular velocities are shown to occur. These results quantitatively agree with the theoretical predictions based on the macroscopic stress tensor, which contains an antisymmetric contribution in the absence of angular momentum conservation.
The dynamics of complex fluids is often governed by the hydrodynamic behavior of the solvent. Due to a large separation of length and time scales between the atomic scale of the solvent molecules and the mesoscopic scale of the solute, direct simulation approaches with atomistic solvent are prohibitively costly in computer time. This has stimulated the development of several mesoscale simulation techniques in recent years.
MPC is a particlebased mesoscale simulation technique. The fluid is modeled by N point particles. Positions and velocities evolve in discrete increments of time. The algorithm consists of two steps. In the streaming step the particles move ballistically. In the collision step, the particles are sorted into collision boxes, and the velocities relative to the centre of mass motion perform a random rotation.