Digital Droplet Sorting.

A new microfluidic lab-on-a-chip setup forms tiny droplets, passes them through a pair of electrodes which can perform an identification of the droplets, passes them through a second pair which gives them a charge, and then through a third pair which sorts the drops according to their properties. Basically the charge imparted to the droplet is proportional to the droplet size, and the charge is gauged by the effect it has when passing through the first set of capacitor electrodes.

Scientists at the Hong Kong University of Science and Technology form a supply of drops moving in a microchannel by having the fluid of interest (in one channel) merge with a running rivulet of oil (silicon or sunflower oil) in a second channel (see the figure at http://www.aip.org/png/2007/290.htm). By regulating the flow rate of the fluid and the oil, droplets of many sizes and rates can be formed. The Hong Kong Scientists currently can look at droplets smaller than a pico-liter (10^-12 liter) in size with a capacitive sensitivity of a pico-Farad (10^-12 F).

The detection rate right now is about 10,000 drops per second, which is already pretty high. According to one of the researchers, Weijia Wen (phwen@ust.hk), this capacitance-based detection rate is better than that can be accomplished with optical means (such as with a CCD camera), and the capacitance method is intrinsically cheaper than the optical equivalent.

In the Hong Kong approach the detection and the sorting are both performed electrostatically: sorting happens when an electric field sends the higher-charged drops into one channel, and the lesser-charged drops into another channel. In this way nano- or micro-particles can be sorted digitally.The goal is to furnish a useful digitally-controlled bio-chemical chip for performing various experiments with nano-liter volumes of reactants or biological samples. (Niu et al., Biomicrofluidics, Oct-Dec 2007)