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Title Splashing and Breakup of Droplets Impacting on a Solid Surface
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Publication Date
Date Available
Degree Level doctoral
University/Publisher University of Toronto
Abstract

Two new mechanisms of droplet splashing and breakup during impact have been identified and analyzed. One is the internal rupture of spreading droplet film through formation of holes, and the other is the splashing of droplet due to its freezing during spreading. The mechanism of film rupture was investigated by two different methods. In the first method, circular water films were produced by directing a 1 mm diameter water jet onto a flat, horizontal plate for 10 ms. In the second method, films were produced by making 0.6 mm water droplets impact a solid surface mounted on the rim of a rotating flywheel. Substrate wettability was varied over a wide range, including superhydrophobic. In both cases, the tendency to film rupture first increased and then decreased with contact angle. A thermodynamic stability analysis predicted this behavior by showing that films would be stable at very small or very large contact angle, but unstable in between. Film rupture was also found to be promoted by increasing surface roughness or decreasing film thickness. To study the effect of solidification, the impact of molten tin droplets (0.6 mm diameter) on solid surfaces was observed for a range of impact velocities (10 to 30 m/s), substrate temperatures (25 to 200°C) and substrate materials (stainless steel, aluminum and glass) using the rotating flywheel apparatus. Droplets splashed extensively on a cold surface but on a hot surface there was no splashing. Splashing could be completely suppressed by either increasing the substrate temperature or reducing its thermal diffusivity. An analytical model was developed to predict this splashing behavior. The above two theories of freezing-induced splashing and film rupture were combined to predict the morphology of splats typically observed in a thermal spray process. A dimensionless solidification parameter, which takes into account factors such as the droplet diameter and velocity, substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, was developed. Predictions from the model were compared with a wide range of experimental data and found to agree well.

PhD

Subjects/Keywords Fluid Dynamics; Heat Transfer; Droplet Impact; Droplet Splashing; Thermal Spray Coating; Water Jet Impact; 0548
Contributors Chandra, Sanjeev; Mechanical and Industrial Engineering
Language en
Country of Publication ca
Record ID handle:1807/17753
Repository toronto-diss
Date Retrieved
Date Indexed 2020-03-09

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…thickness~25 μm) produced by the impact of a 550 μm water droplet on a mirror-polished stainless steel surface at 40 m/s [5]: (a) Formation of holes, (b) growth of holes………………………………………..6 Figure 1.5: Splats formed by plasma…

…arrangement to study liquid jet impact on a pin………………….22 Figure 2.8: Jet tip displacement measurements with time…………………………………...23 Figure 2.9: A schematic diagram of molten tin droplet generator…………………………...26 Figure 2.10: Photograph of the water droplet

…generator…………………………………….27 Figure 2.11: A schematic diagram of the droplet impact apparatus…………………………29 Figure 2.12: A photograph (top view) of the rotating flywheel mechanism…………………30 Figure 2.13: A typical sequence of droplet impact, Do =600 μm…

…Formation and growth of a hole due to the impact of a droplet onto an otherwise stable water film on smooth wax surface. By contrast, film on Plexiglas surface still ix remains stable. Impact Re = 2800………………………………………………….46 Figure 3.7: Formation of holes…

Impact velocity: 10, 20 & 30 m/s. Data of Mehdizadeh [42]……………72 Figure 4.7: Modeling droplet impact on a solid surface: (a) before impact, t=0, (b) at an intermediate time, t, (c) at maximum spread, t=tc…

…Figure 5.6: Impact of molten tin drops with velocity 10 m/s on substrates of different materials at an initial temperature, Ts,I=25oC. The last picture in each column is the final solidified shape of the droplet. Re = 21818, We = 795………………..94 xii…

droplet-substrate interface………………………………………………………………………..100 Figure 6.1: Impact of 40 μm plasma-sprayed (a) molybdenum (Vo=135 m/s), and (b) zirconia (Vo=200 m/s) particles onto glass surfaces at Ts,i=27oC [4, 6…

…x5D;..……………….103 Figure 6.2: Critical Reynolds number above which a thin film created by droplet impact will rupture as a function of receding liquid-solid contact angle. The initial hole is assumed have a diameter 0.1 times that of the droplet

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