Detection and sizing of single droplets flowing in a lab-on-a-chip device by measuring impedance fluctuations

This paper is devoted to the evaluation of the electrochemical noise technique consisting in measuring the fluctuations of the electrolyte resistance (ER) between two metallic electrodes immerged in a conductive electrolyte to detect and characterize single particles circulating in a microfluidic device, without the help of optical measurements that require good visibility of the detection region. Numerical simulations were performed with the finite element method to study the influence of the dimensions of the channel and the electrodes on the ER. Measurements of the ER variations due to the passage of oil droplets and plugs passing between the electrodes were carried out. Excellent agreement was obtained between the theoretical and experimental ER transients, which allowed the velocity and diameter of the oil droplets to be estimated with an accuracy of a few percents in the case of droplet diameters ranging from 60 to 100 μm. According to the numerical simulations and the amplitude of the background noise, oil droplets of diameter larger than 20-25 μm can be detected in the microchannel used (cross section of 100 μm × 100 μm and 100 μm × 100 μm electrodes separated by a gap of 100 μm). Developments of smaller microfluidic devices are under progress to detect and characterize particles of a few micrometers, such as biological cells for example.