Microfluidic electrowetting-based droplet mixing
We present a liquid droplet mixer to enable mixing of the samples and reagents for chemical and biological analysis in micro total analysis systems (μTAS). The droplets in the mixer are actuated based on electrowetting phenomenon. The actuator comprises of two parallel glass plates between which the droplet is actuated on a planar array of electrodes. Mixing of liquid channels in microfluidic systems has been demonstrated for continuous flow systems where the mixing is diffusion-limited due to the laminar flow of liquids, requiring very long, thin channels. In the present paper, mixing is performed on discrete droplets of liquid. When two droplets are brought together, depending on the velocity of the moving droplets, surface tension, viscosity, electrode activation, and volume among other factors, turbulence is created which aids in mixing. The mixing is not limited by diffusion and enhanced by transport. The mixing experiments are performed between fluorescein and plain water droplets whose individual volume is 1.75 μl. Mixing is visualized with a 2-CCD camera setup to observe both the top and side views, with appropriate filters to capture fluorescence. We observed that it takes about 60 seconds for two droplets of 1.75 μl each to mix when their surface tensions are different and it takes about 90 seconds when the two droplets have similar surface tensions. The present mixer stands apart from any current conventional micromixers in that the mixing times occupies much lesser area, mixing does not need any specific architecture on the chip and can be performed on any transport electrodes dynamically assigned to mixing.
