A numerical and experimental study of a collapsing bubble-induced droplet ejector |
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Authors: | Mohammad T Shervani-Tabar Abdolrahman Dadvand Boo Cheong Khoo M R H Nobari |
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Institution: | (1) Faculty of Mechanical Engineering, University of Tabriz, 29 Bahman Boulevard, Tabriz, Iran;(2) Department of Mechanical Engineering, National University of Singapore, Kent Ridge Crescent, Singapore, 119260, Singapore;(3) Department of Mechanical Engineering, Amirkabir University of Technology, Tehran, Iran |
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Abstract: | This paper aims to study a novel drop-on-demand droplet generation mechanism in which the oscillation and deformation of a
non-equilibrium bubble in close proximity to a free surface induce an axisymmetric liquid spike on the free surface. The evolution
of the liquid spike and its deformation due to the effect of surface tension force lead to the formation of a droplet. The
free surface can be accorded by either a circular hole on a horizontal flat plate or by the top opening/nozzle of a vertical
cylinder. A high-speed camera capable of obtaining images at a frame rate of 15,000 fps is utilized to observe the droplet
formation process. Numerical simulations corresponding to the experiments are performed using the boundary integral spatial
solution coupled with the time integration, i.e., a mixed Eulerian–Lagrangian method. In the experiments the bubble is generated
using a very low voltage (only 55 V) in contrast to the relatively much higher voltages usually employed in reported works.
This is very attractive from a safety viewpoint and accords great simplification of the setup. A comparison is made between
the numerical and experimental results. A reasonable agreement has been found. The influences of the main design parameters,
namely, the bubble-free surface distance and the dimension of the hole/nozzle on the bubble dynamics and on the droplet formation
process are discussed and the conditions of the bubble dynamics under which a satellite-free droplet can be generated are
sought. Furthermore, the effects of different geometries, namely, the horizontal flat plate and the vertical cylinder on the
bubble dynamics and on the droplet features are examined. One important feature of the proposed actuation mechanism is the
capability of producing droplets much smaller than the nozzle size. The possible applications of this mechanism are those
where the accurate direction of the ejected droplet is of great importance such as inkjet printing.
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Keywords: | Boundary element method Non-equilibrium bubble Actuator Drop-on-demand Droplet ejector |
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