Rotational Drag and Hydrodynamics of Single Dipolar Colloids and Linear Colloidal Strings under an Oscillating Drive*

Fluid dynamics plays an essential role in the mechanics of colloidal dispersions. While much attention is paid to high-density systems, low-density systems have received much less attention. This could be attributed to the fact that the impact of hydrodynamics on colloidal mechanics and assemblies in low densities is only very localized and, thus, less pronounced effects might be expected; but it is equally true that quantities such as mechanical drag and flow fields at the microscale remain difficult to examine for many micron-sized objects and in non-homogeneous environments. Here, we investigate the rotational drag and the role of hydrodynamics of dispersed magnetic Janus beads close to a wall. For single Janus particles as well as particles in small, linear assemblies, we demonstrate how the Stokes coefficient of rotational drag can be obtained experimentally from driven torsional oscillations. Furthermore, we simulate the fluid around the oscillating single particles and particle strings close to a wall using fluctuating Lattice Boltzmann simulations. Comparing experimental and numerical results, we are able to estimate the impact of hydrodynamic effects in such low-density systems.