Frequency‐Dependent Electrical Response of Polystyrene Microparticle Suspensions on Coplanar Electrodes

The dielectric properties of microparticles, such as bacterial cells, are influenced by several internal factors, including the microparticle’s internal structure, size, and surface modification. These factors create a dielectric fingerprint for the microparticle, which can be exploited for detection using capacitance‐based mechanisms. However, measuring small changes in capacitance due to the presence of specific microparticles in a microfluidic system remains challenging, as many external factors and physical phenomena can influence the measurement. The primary goal of this study is to investigate the capacitance variation of microparticle suspensions placed on top of coplanar electrodes across a wide range of frequencies. Polystyrene particles of sizes 0.29, 0.52, 1.35, 2.26, and 4.79 µm are suspended in distilled water at concentrations from 2.5 to 0.156 wt.%. The study concludes that capacitance spectra are mainly governed by electrode polarization, ionic conduction, and particle/water interfacial polarization at low frequencies and by the intrinsic bulk dielectric characteristic at higher and microwave frequencies. LCR measurements gave a strongly dispersive response, with the apparent capacitance decreasing from about 10–100 nF at 10 Hz to ~3–10 pF at 10 MHz. Increasing polystyrene concentration from 0.156 to 2.5 wt.% and decreasing particle size from 4.79 to 0.29 µm both increased the low‐ and mid‐frequency loss response, yielding higher capacitance, lower resistance, phase angles closer to 0°, and dissipation‐factor peaks of about 6–17 in the 10 4 –10 6 Hz range. The findings of this study help in understanding the capacitance measurement of samples containing suspended microparticles, including biosamples.