High quality (Q) factor whispering gallery modes (WGMs) can induce nonlinear effects in liquid droplets through mechanisms such as radiation pressure, light scattering, thermocapillarity, Kerr nonlinearity, and thermal effect. However, such nonlinear effects have yet to be thoroughly investigated and compared in the literature. In this study, we first investigate a micron-sized liquid spherical resonator and present an approximated solution for the resonator interface deformation due to the radiation pressure. We then derive an analytical approach that can exactly calculate the droplet deformation induced by the radiation pressure. The accuracy of the analytical solution is confirmed through numerical analyses based on the boundary element method. We show that the nonlinear optofluidic effect induced by the radiation pressure is stronger than the Kerr effect and the thermal effect under a large variety of realistic conditions. Using liquids with ultra-low and experimentally attainable interfacial tension, we further confirm the prediction that it may only take a few photons to produce measurable WGM resonance shift through radiation pressure induced droplet deformation.

Similar to the radiation pressure, the scattering force in the droplet can induce a rotational fluid motion which also leads to the interface deformation. The interface deformation can also be produced by the thermocapillarity as a result of the WGM energy absorption and temperature increase. In this study, we provide a numerical scheme to calculate the fluid motion and quantify the nonlinearity induced by the optical scattering force and thermocapillarity. The magnitude of the optofluidic nonlinearities induced by the radiation pressure, thermocapillary effect, light scattering and Kerr effect are compared. We show that the radiation pressure due to the WGM produces the strongest nonlinear optofluidic effect.