This project aims to develop a new method to measure the fluid properties of tiny liquid droplets by using light to deform the shape of the drop. The method involves using a laser and an optical fiber to inject photons into the drop. These photons form a whispering gallery mode, which is an optical wave that is reflected by the droplet interface and circulates near the equator of the drop. The radiation pressure from the optical wave causes the interface to bulge, which, in turn, causes a change in the frequency of the whispering gallery mode. By detecting the change in frequency, extraordinarily small changes in the shape of the drop can be measured. These shape changes can then be used to deduce the surface tension of the fluid interface and the viscosity of the internal fluid. There are no other comparable methods for measuring these properties in micron-size drops. Thus, the project will provide scientists and engineers with a new tool for measuring physical properties that can be applied to colloidal systems, emulsions, aerosols and other suspensions that are formed in many manufacturing processes and biological systems.
The presence of high-Q whispering gallery modes within the liquid drops will be established through optical spectrum measurements. The optical force that induces drop deformation will be experimentally confirmed and characterized. Drop deformation will be determined by measuring shifts in the whispering gallery mode resonance frequency. Results will be verified by measuring deformation with white light interferometry that is capable of detecting interface movement with nanometer scale resolution. The roles of surface tension and viscosity will be investigated by comparing the measured drop deformation and rate of deformation with boundary element solutions of the Stokes equations for the drops. Experiments will be carried out using simple fluids such as water and silicone oils. A system consisting of nanoparticles suspended in liquid will be characterized to simulate whispering gallery mode induced drop deformation in a more complex fluid.
