Microfluidic flowmeter using a PDMS-coated capillary

A novel flowmeter based on a whispering gallery modes (WGMs) microbottle resonator is proposed and experimentally demonstrated. WGMs are excited by a tapered fiber coupled perpendicularly to a microbottle resonator formed by polydimethylsiloxane (PDMS) deposited on the surface of the central region of a tapered silica capillary. The tapered capillary works as the microfluidic channel, allowing air to flow through the capillary with varying flow rates. A broadband light source was utilized as a heat source to heat the PDMS microbottle. The air flowing through the capillary causes a temperature decrease, leading to a refractive index (RI) decrease and a size shrinkage of the PDMS microbottle resonator. This change leads to a WGM resonance shift, the value of which can be linked to the airflow rate in the capillary. A sensitivity of 0.881 nm/sccm in the range of flow rates from 0 to 2.52 sccm has been demonstrated in our experiment. The proposed sensor can be used for low-volume flow rate measurement and offers the advantages of simplicity, high sensitivity, and miniature size.