A numerical and experimental study of a collapsing bubble-induced droplet ejector

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. © 2009 Springer-Verlag.