Vortex ring droplet formation of immiscible fluids and effects of density and viscosity ratio

Micro-gravity provides an ideal environment to experimentally study the dynamics of fluid interfaces. Of particular interest is the dynamics of droplet formation by vortical flows when surface tension is the dominant stabilizing mechanism. These flow phenomena arise in a wide variety of engineering problems both in space systems and on Earth. For example, most earth-based atomization systems operate under surface tension dominated dynamics because the drop size is very small. Liquid atomization is a critical component of many systems including: aerosol generators for drug delivery, fuel injectors, and inkjet printers. For these applications surface tension is the dominant stabilizing force of the liquid interface. In micro-gravity, these flow processes can be studied at much larger scale, which facilitates experimental observation.; In this work, new micro-gravity data on the propagation of a vortex ring through both an air/liquid interface and interfaces between immiscible liquids of similar density and viscosity are presented. The micro-gravity experiments were conducted in the Drop Tower facility at NASA's John Glenn Research Center. The experimental results revealed unexpected effects of density and viscosity ratio under surface tension-dominated interface dynamics. The results show that air/liquid interfaces can not be modeled with small values of the density ratio for computational expediency as has been proposed. The viscosity ratio was also found to be a key element in formation of vortex ring droplet.; For the case of a liquid/liquid interface with density and viscosity ratio of one, the interaction results in the formation of a vortex ring droplet. The droplet shape is approximately spherical and the fluid inside the droplet carries some vorticity from the initial vortex ring. PIV tests were also conducted for this case in 1-G condition in order to study details of interaction between vortex ring and liquid interface.; In contrast, the result from an air/liquid interface shows the vortex ring propagating through the interface and forming an elongated liquid column. The difference from liquid/liquid interface case can only be attributed to density and viscosity ratio effects at the interface.; As an illustration of Bond number similarity, it is interesting to compare the flow evolution captured in micro-gravity of air/liquid interface with images of liquid spray and drop ejection in inkjet print-heads obtained using ultrahigh-speed video for micro-devices. In all cases the interface dynamics is controlled by surface tension. However, there is a three order of magnitude change in the geometrical and time scale. This illustrates the power of micro-gravity testing to study liquid atomization relevant to Earth-based systems.