Vibrating orifice droplet generator for precision optical studies

Abstract

A special purpose vibrating orifice droplet generator is described possessing improved short-term monodispersity (instantaneous diameter fluctuations of 2×10−5 and a differential drift of 10−5/min.). We demonstrate that this is sufficient to allow cw laser excitation of specific morphology-dependent resonances (MDRs). Improved performance results from (1) the liquid sample being direct pressure fed to the vibrating orifice from a closed pressure reservoir rather than by a conventional gear driven syringe pump, and (2) the vibrating orifice is driven by a periodic square wave voltage source having a frequency which is four orders of magnitude more constant (1 part in 108 per day) than sources normally used. A novel variational size spectroscopy is also described that is made possible by programmed frequency ramping of the voltage source driving the vibrating orifice. By monitoring elastic scattering during ramped size changes we have been able to infer precise values of droplet index of refraction (to ±0.001) and size (to ±3 parts in 104) at any orifice operating frequency by comparison of the resulting experimental pattern to Lorentz–Mie calculations. This aerosol source permits the next generation of optical experiments (e.g., double resonance, fluorescence rate inhibition and enhancement, cw nonlinear optical, accurate chemical speciation, etc.) to be performed on droplets with diameters of 10–80 μm.