Minimizing particle induced fluid motion in a vertically rotating system*

Liquid rotates as a rigid body in a confined system rotating vertically, though introducing particles in the solution generates fluid motion due to particle sedimentation. Based on rotational speed three different regimes (microfluidic mixing, particle suspension, and settling due to centrifugal acceleration) can be established for a certain particle-solution selection. The goal of our investigation is to demonstrate the existence of long-term particle suspension regime and, thus, characterize colloidal behavior. After a certain time, if other parameters remain constant, system reach its constant microfluidic motion state in a certain rotational speed at a negligible centrifugal acceleration. At that equilibrium motion state, we correlated particle-induced fluid motion per rotation with a proposed non-dimensional number which is the ratio of inertia forces on fluid cell caused by particles to the viscous resistance. We quantified average fluid motion at different particle concentration and at different rotational speeds and compared it to the proposed non-dimensional number. Further, particle distribution was observed at different rotational speeds to quantify the effect of centrifugal acceleration.