The splashing morphology of liquid-liquid impacts

Cole, David (2007) The splashing morphology of liquid-liquid impacts. PhD thesis, James Cook University.

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Abstract

In this thesis, a systematic experimental study of the flow behaviour resulting from liquid-liquid impacts has been conducted. Numerous new flow behaviours have been identified including microbubble formation from floating drops, pre-entrapment jetting, multiple primary bubble entrapment, downward jets penetrating the entrapped bubble, the break-up of the downward jets to leave drops entrapped inside the entrapped bubble and small vortex ring formation in the early stages of the post-entrapment jetting regime. These new flow phenomena have been combined with existing flow behaviour to produce the most comprehensive maps (both quantitative and qualitative) describing the splashing morphology of liquid-liquid impacts to date. It was found that six different flow regimes were required to adequately categorise all the flow behaviour.

The physics of the cavity formation and collapse were investigated with high speed video and high framing rate particle image velocimetry. The formation and collapse of the cavity can be described as a six stage process. Initially, the cavity expands due to the inertia of the impact and the majority of the displaced fluid is driven into the wave swell. After the energy of the impact has been dissipated, the side walls of the cavity stagnate and the growth of the wave swell also stagnates. This causes the fluid contained in the wall swell to begin flowing downward under the influence of gravity. As the fluid flows down, the base of the cavity stops growing and begins to retract. These actions give rise to a vortex mid way down the cavity and acts to collapse the cavity. The fluid driven by the vortex then converges at the base of the cavity along the axis of symmetry.

The formation of the vortex was shown to be centred around a stationary line that forms on the cavities interface. Several interesting properties of this stationary line were discovered. The depth at which the stationary line forms is almost constant for the same drop size and is independent of impact velocity. The dimensionless width of the cavity, Dw ' was shown to scale to Fr1 3 . The formation of the stationary line was also shown to influence how the flow converges at the base. The wider the cavity grows, the greater the rotation the fluid undergoes before converging along the axis of symmetry. Thus, for small width cavities the flow tends to converge while the fluid is being directed downward. While for larger width cavities, the flow tends to converge with a strong upward component. This has lead to the formulation of three different flow convergence criteria: downward convergence, parallel convergence and upward convergence. All jetting modes or lack of jetting can be described using one of the three convergence criteria.

For cavities that are small and thus have a downward flow convergence condition, no jetting occurs. This type of flow convergence occurs in the primary vortex ring regime and may assist in the development of strong coherent vortex rings. A parallel flow convergence condition is responsible for forming high-speed jets in both the preentrapment jetting and primary bubble entrapment regimes. Here, the flow is similar to two parallel jets impinging on each other. This action forms a stagnation point and a significant rise in the local pressure around this zone follows. This leads to a strong inertial force that drives a small jet of fluid back up into the cavity. Cavity retraction acceleration was measured as high as 90 000g during this time. The maximum exit velocity of the secondary drops formed from the break up of the thin high-speed jets was measured to be in excess of 30 m/s. In the primary bubble entrapment regime it was postulated that multiple stagnation points would form and interact with each other to produce variable jet velocities across the regime. The retraction velocity of the cavity was also shown to have a direct correlation with the exit velocity of the first drop. An upward flow convergence condition was found to be responsible for the thick slow moving jets observed in the post-entrapment jetting regime.

All modes of bubble entrapment were investigated and the quantity of air each mode can entrap has been estimated. It was found that the most efficient method to produce microbubbles was by forming jets in the post-entrapment jetting regime that pinch off secondary drops with Weber numbers ranging from 6 to 20. This produces drops that fall into the primary microbubble entrapment regime to produce thin films that rupture into thousands of microbubbles. Methods for determining the volume of entrapped air from the break up based on the rupture velocity of the film are presented. The entrapped air in the bulk fluid is equivalent on average to 0.3% of the original drop volume.

Item ID: 2065
Item Type: Thesis (PhD)
Keywords: liquids, splashing morphology, liquid-liquid impacts, microbubbles, drops, jets, waves, vortexes, flow convergence, splashes, bubble entrapment, vortex rings, fluids physics, surfaces
Date Deposited: 20 Jan 2009 04:11
FoR Codes: 02 PHYSICAL SCIENCES @ 0%
03 CHEMICAL SCIENCES > 0306 Physical Chemistry (incl Structural) > 030603 Colloid and Surface Chemistry @ 0%
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