Measurement of droplet gravity-induced mechanical force by optofluidic microbottle resonator with lever model

Whispering gallery mode resonators provide an intriguing platform for precision measurement due to their high responsivity and low detection limit. Here, we propose and demonstrate an optofluidic microbottle resonator (OFMBR) that is utilized to measure droplet gravity-induced mechanical force, realized by establishing a lever model between tapered fiber and OFMBR. The mechanical force can be amplified by adjusting the ratio of the load arm to the effort arm of the lever, which is validated by theoretical simulation. The evolved mechanical force deforms OFMBR morphology and enhances light scattering, resulting in mixed variations in the transmission spectrum, including resonance wavelength, mode linewidth, and signal intensity. Experimentally, the mechanical force is first measured by monitoring resonance wavelength shift, and a responsivity of −56 pm/mN is obtained within the range of 0–0.2 mN. Furthermore, to monitor the mixed variations in the transmission spectrum and obtain the actual mechanical force directly, the optical barcode method is utilized to simultaneously monitor the variations of multimode features. The arbitrary unknown mechanical force is determined by the cross correlation function, and the measurement resolution is about 5 µN. Our scheme provides a thread for characterizing the liquid properties and investigating the dynamics at solid–liquid interfaces.