A New Online Monitoring Method for Water-in-Oil Droplet Based Microfluidic Devices

The online water-in-oil (w/o) droplet monitoring is essential for the uniformity of the droplet based microfluidic devices. In this work, a contactless impedance detection (CID) method for online w/o droplet monitoring in microfluidic devices is proposed. In the proposed method, a new CID sensor, which can work at low working frequency and overcome the unfavorable influence of coupling capacitance, is developed to obtain the impedance information of the w/o droplet. Based on the obtained impedance information, measurement models are established to realize online measurement of drop length and dispersed phase holdup. Then the coefficients of variation (CVs) of the two measured parameters are calculated to evaluate the uniformity of the droplets for online monitoring. Online monitoring experiments of w/o droplet were carried out in a microfluidic device with a 500
$\mu \text{m}\,\,\times $
500
$\mu \text{m}$
microchannel. The experimental results verify the effectiveness of the proposed droplet monitoring method. The developed CID sensor can realize impedance measurement at a working frequency lower than 50kHz. The drop length and dispersed phase holdup are useful parameters for online monitoring of droplets. With the established measurement models, the maximum relative error of droplet length measurement is less than 10% in the droplet length range of 1.5mm-3.5mm. And the maximum error of dispersed phase holdup measurement is less than 5% in the dispersed phase holdup range of 0.20-0.67.