Microfluidic flowmeter based on a liquid crystal-filled nested capillary

Microfluidic flowmeters are a powerful and highly accurate tool, enabling precise monitoring and measurements of flows of gases and fluids in a range of applications. Here we proposed and experimentally demonstrated a whispering gallery modes flowmeter composed of a liquid crystal-filled nested capillary. Whispering gallery modes are excited by a tapered fiber coupled perpendicularly to the nested capillary. The air flowing through the capillary cools it down, which leads to a temperature-induced change of the refractive index of the nematic liquid crystals. This change in turn leads to a spectral shift of the whispering gallery modes resonances, which can be linked to the airflow rate in the capillary. The temperature change in the liquid crystals was simulated considering the heat transfer between the liquid crystals and airflow in the capillary, which indicated that the liquid crystals temperature decreases in a nonlinear manner with the increase of the airflow rate. A flowmeter with the maximum sensitivity of 0.3423 nm·min·mL −1 in the flowrate range from 0 to 2.52 nm·min·mL −1 and a resolution of 5.72 pm was demonstrated in our experiment. The proposed sensor provides a platform for whispering gallery modes flowmeters and offers the advantages of good stability, high sensitivity, and miniature size.