Colloidal systems in three-dimensional microchannels: lattice control via channel width and external force

Driven binary colloidal mixture in a 2D narrow channel with hard walls

We have investigated the properties of a driven equi-molar binary colloidal mixture confined to a two-dimensional narrow channel. The walls are hard and periodic boundary condition is applied along the channel. Colloidal particles perform Brownian motion in a solvent having a fixed temperature and interact with each other via a Debye-Hückel Coulombic interaction (Yukawa potential). A constant external force drives the colloids along the channel. Two species move oppositely to each other. Hydrodynamic interactions are neglected and the dynamics is assumed to be over-damped. The flow increases nonlinearly with the external force but does not exhibit a notable dependence on channel width. Above a critical driving force the system undergoes a homogeneous-to-laning transition. It is shown that the mean lane width as well as the laning order parameter increases with the channel width. The reentrance effect is observed in the narrow channel geometry.

Segregation of oppositely driven colloidal particles in hard-walled channels: A finite-size study

We investigate segregation phenomena of particles driven in opposite directions with Brownian dynamics simulations. The particles interact via a repulsive potential and are confined in three-dimensional hard-walled pipes with quadratic cross sections. In a systematic finite-size study, the pipe length is varied. Ordering on finite length scales can be observed, but global segregation seems to vanish for infinite channel length, as it was recently found for lane formation in two dimensions. As an additional effect of the finite hard-walled boundary conditions, interface vibrations and longitudinal demixing are found.

Hematocrit and flow rate regulate the adhesion of platelets to von Willebrand factor

Primary hemostasis and blood clotting is known to be influenced by the red blood cell volume fraction (hematocrit) in blood. Depressed or elevated levels of red blood cells can lead to vascular perfusion problems ranging from bleeding to thrombus formation. The early stage of hemostasis and thus blood clotting in all vessel sections from the arterial to the venous system involves the adhesion of platelets to von Willebrand factor. Here we present experimental and theoretical results showing that the adhesion probability of platelets to von Willebrand factor is strongly and nonlinearly dependent on hematocrit and flow rate. Interestingly, the actual binding forces are not markedly different, which suggest that the origin of such behavior is in the distribution of platelets. Using hydrodynamic simulations of a simple model, we explicitly show that the higher the hematocrit and the flow rate, the larger the amount of platelets residing close to the wall. Our simulation results, which are in excellent agreement with the experimental observations, explain why such phenomena occur. We believe that the nonhomogeneous red blood cell distribution as well as the shear dependent hydrodynamic interaction is key for the accumulation of platelets on the vessel wall. The work we present here is an important step forward from our earlier work on single molecules and extends into the collective cellular behavior of whole blood. It sheds new light on the correlation between hematocrit and the initial steps in hemostasis and thrombosis, and outlines advances for the treatment of vascular diseases associated with high levels of red blood cells. These results are not only highly relevant for the field of hemostasis and the physics of blood clotting but are also of powerful impact in applied science most obviously in drug delivery and colloidal science.

Effects of confinement and external fields on structure and transport in colloidal dispersions in reduced dimensionality

In this work, we focus on low-dimensional colloidal model systems, via simulation studies and also some complementary experiments, in order to elucidate the interplay between phase behavior, geometric structures and transport properties. In particular, we try to investigate the (nonlinear!) response of these very soft colloidal systems to various perturbations: uniform and uniaxial pressure, laser fields, shear due to moving boundaries and randomly quenched disorder. We study ordering phenomena on surfaces or in monolayers by Monte Carlo computer simulations of binary hard-disk mixtures, the influence of a substrate being modeled by an external potential. Weak external fields allow a controlled tuning of the miscibility of the mixture. We discuss the laser induced de-mixing for the three different possible couplings to the external potential. The structural behavior of hard spheres interacting with repulsive screened Coulomb or dipolar interaction in 2D and 3D narrow constrictions is investigated using Brownian dynamics simulations. Due to misfits between multiples of the lattice parameter and the channel widths, a variety of ordered and disordered lattice structures have been observed. The resulting local lattice structures and defect probabilities are studied for various cross sections. The influence of a self-organized order within the system is reflected in the velocity of the particles and their diffusive behavior. Additionally, in an experimental system of dipolar colloidal particles confined by gravity on a solid substrate we investigate the effect of pinning on the dynamics of a two-dimensional colloidal liquid. This work contains sections reviewing previous work by the authors as well as new, unpublished results. Among the latter are detailed studies of the phase boundaries of the de-mixing regime in binary systems in external light fields, configurations for shear induced effects at structured walls, studies on the effect of confinement on the structures and defect densities in three-dimensional systems, the effect of confinement and barriers on two-dimensional flow and diffusion, and the effect of pinning sites on the diffusion.