Two-fluid mixing inside a sessile micro droplet using magnetic beads actuation

The present work reports the mixing between two fluids within a sessile droplet using actuation of magnetic microspheres due to the cyclic magnetic field produced by four electromagnets. Gaussmeter measurements and COMSOL simulation have been used for characterization of the magnetic field. Mixing experiments are conducted for different strength of actuation current and actuation frequencies to the electromagnets. Mixing effectiveness between the fluids is evaluated using the “μ-LIF technique”. Micro-PIV technique is used to measure the flow field inside the droplet in the presence of magnetic field in order to explain the mixing mechanism. A recirculating flow in synchronization with the applied periodic magnetic field is observed inside the droplet that enhances the viscous interaction between the magnetic microspheres and fluids leading to efficient mixing of fluids. The strength of the actuation current and actuation frequency are observed to be the control parameters for effective mixing inside a droplet. A critical actuation frequency is observed above which mixing between the two fluids is degraded with increase in frequency to the electromagnets.