Fig. 1. Schematic diagram of the coupling system，inset：CCD image of the microbubble cavity filled with DI water

Fig. 2. The light field distribution of the fundamental mode，radial 2-order mode and radial 3-order mode with different wall thickness（t），and the inset in the upper right corner is the optical field distribution of the resonance modes in microbubble cavity

Fig. 3. Velocity field distribution and pressure field distribution of microbubble cavity at 100 μL/min

Fig. 4. Relationship between flow rate and pressure for the different ranges of the flow rate and analysis of the dynamic range

Fig. 5. Flow rate sensing experimental set-up

Fig. 6. Q-factor and stability characterization of WGM microbubble cavity

Fig. 7. Experimental results for flow rate sensing