Microfluidic flowmeter based on liquid crystal filled nested capillary

A novel flowmeter composed of a liquid crystal-filled nested capillary is proposed and experimentally demonstrated. Whispering gallery modes (WGMs) in the nested capillary are excited by a tapered fiber coupled perpendicularly to the nested capillary. The WGM transmission spectrum of the fiber taper was optimized to achieve the highest possible quality (Q) factor by moving the capillary along the axis of the fiber taper. The air flowing through the capillary cools it down, which leads to a temperature-induced change of the refractive index of the nematic liquid crystal. This change in turn leads to a spectral shift of the WGM resonances, which can be linked to the airflow speed in capillary. A sensitivity of 0.242 nm/sccm has been demonstrated in our experiment. The proposed sensor provides a new platform for WGM flowmeters and offers the advantages of high sensitivity and miniature size.