Impedance Monitoring of Droplets in a Microfluidic Chip

We present the use of impedance monitoring to determine the geometrical characteristics and the ionic concentration of droplets in a microchannel. A compact impedance monitoring system is developed based on an embedded computer that can provide an adjustable excitation signal from 2 KHz to 2 MHz. Aqueous droplets are continuously formed by hydrodynamic focusing and then directed into a primary channel for monitoring with embedded electrodes in a microfluidic chip. With the monitoring system, the correlation between the electrical amplitude and the ionic concentration in droplets is investigated and the correlation coefficient is 0.98. Moveover the dependency of the impedance response on the geometrical features of droplet is analyzed. According to the experimental results, an algorithm for the quantitative estimation of droplet length is established. Compared with the results from optical microscopy, the relative difference of the estimated droplet length is less than 10%.