M.S. Thesis Presentation by Bojan Vukasinovic
Thursday, March 28, 2002

(Dr. Ari Glezer, advisor)

"Vibration-Induced Droplet Atomization"

Abstract

The atomization of liquid drops is investigated experimentally using laser vibrometry, high-speed imaging, and particle tracking techniques.  The spray is generated by a novel vibration-induced droplet atomization (VIDA) process in which a sessile drop is atomized by an underlying vibrating thin metal diaphragm that results in rapid ejection of small secondary droplets from the free surface of the primary drop.  Under some conditions, the primary drop can be atomized extremely rapidly by a bursting-like mechanism (e.g., 0.1 ml water drop can be atomized within 0.4 seconds).

The present research has focused on four major areas: the global characteristics of VIDA process, the instability modes and free surface dynamics of the forced drop, the mechanisms of the interface breakup, and the parametric characterization of the ensuing spray.  Prior to the atomization, the drop free surface undergoes three primary transitions from axisymmetric standing waves, to azimuthal waves, followed by a newly-observed lattice mode, and then to a disordered pre-ejection state.  The secondary droplet ejection results from the localized collapse of surface troughs and the initiation and ultimate breakup of momentary liquid spikes.  The breakup begins with a capillary pinch-off from the tip of the spike and can be followed by additional pinching of liquid droplets.  For a relatively low-viscosity liquid, e.g., water, a capillary-wave instability of the spike is observed in some cases, while for a very viscous liquid, e.g., a glycerin/water solution, the first breakup occurs near the stem of the spike, with or without a subsequent breakup of the detached, elongated thread.  The mechanisms that dominate the primary breakup of the spike separate the low- and high-viscosity ejection regimes.  When ejection of the secondary droplets is triggered, the evolution and rate of atomization depend on the coupled dynamics of the primary drop and the vibrating diaphragm.  Due to these dynamics, the VIDA process can be either self-intensifying or self-decaying.  The resulting VIDA spray is axisymmetric, with initial velocity comparable with the peak velocity of the diaphragm during the steady operation.  Active control of the spray velocity is achieved by modulation of the driving signal.