Mixing stuff in very viscous fluids, or better said: at low Reynolds number, is not easy. Here is a microfluidics tool that allows one to do so anyway. It uses a simple small magnet aka stirring bar in a small chamber.
M.J. Workamp, V. Saggiomo, J.A. Dijksman — Journal of Micromechanics and Microengineering 25 (2015)
MultiPhase Matters 