Capacitive microdroplet sorting

Physicists at the Hong Kong University of Science and Technology have used a poly-dimethylsiloxane-based conducting composite to add one or more pairs of parallel electrodes to a microfluidic device. The paired electrodes sit on either side of the flow channel and form a capacitor with the liquid being the dielectric material between them. When a droplet having a different dielectric constant is carried by the liquid through the capacitor, a signal is detected, the shape and amplitude of which vary with the droplet’s size and composition. Using a second capacitor reveals the droplet’s velocity. The scientists, led by Weijia Wen, also demonstrated that the droplets can be individually manipulated farther downstream. In the image, oil carries two kinds of droplets in from the left. Once detected and identified, a droplet is given an appropriate charge and subsequently deflected by high-voltage electrodes into its designated path. Here, dark-colored water droplets are directed to the upper branch while lighter-colored droplets of ethylene glycol go to the lower branch. Sorting by size is also possible; the Hong Kong team can look at droplets smaller than a picoliter with a capacitive sensitivity of a picofarad. Wen says not only can the device handle about 10 000 drops per second, which is faster than is possible with optical means, but it is also intrinsically cheaper than the optical equivalent. The goal is to engineer a useful digitally controlled biochemical chip for performing various experiments with nanoliter volumes of reactants or biological samples. (X. Niu et al. , Biomicrofluidics 1 , 044101, 2007 http://dx.doi.org/10.1063/1.2795392 .)