Mixing control by frequency variable magnetic micropillar

Mixing of fluids in microchannels is a crucial yet challenging task in lab-on-a-chip devices. Here we demonstrate a mixing enhancement method of two streams of fluids based on actuation of a single micropillar embedded with paramagnetic nanoparticles. The actuation is controlled by an external rotating magnetic field and has variable frequency up to 160 Hz. We systematically study the effects of actuation frequency and flow rate on the mixing efficiency, which can almost reach the highest value corresponding to the completely mixed state. We find that the resulting concentration profile of the mixed fluids downstream of the actuated pillar can be described by the convection-diffusion equation with an effective coefficient that depends only on one control parameter: the number of actuation cycles of the micropillar during the fluid's transit time. Our results indicate that the proposed approach may be adapted and scaled up to enhance the mixing efficiency with high degree of control in other lab-on-chip devices. We demonstrate an active mixing enhancement method based on actuation of a single magnetic micropillar with variable beating frequency.