Ejecta sheet dynamics

  • Thoraval M
  • Thoroddsen S
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Abstract

Drop impact on a liquid layer has fundamental industrial applications, as in fuel droplets impacting on the walls of a combustion chamber, spray painting, pesticide spraying, and ink-jet printing. Drop impact is also important for an understanding of natural mechanisms (e.g., aerosol formation through atomization and soil erosion by rain). Previous work has identified an ejecta sheet emerging from between the drop and the pool. The purpose of this study is to understand the mechanisms governing the dynamics of this ejecta sheet. Some details of these dynamics were studied with a high speed video camera, able to record up to one million frames per second. Findings have produced the following description of these mechanisms. At lower impact velocities, the jet is thicker and develops into an axisymmetric crown. At higher impact velocities, the jet is thinner and faster. The ejection angle of the ejecta sheet increases in time, explaining its initial bending. It is then deformed by air resistance, developing an elbow behind the thicker leading tip. The air pulled under the elbow creates Bernoulli suction pressure, leading to a catastrophic bending towards the pool. When the elbow impacts the pool, it captures a torus-shaped bubble. The ejecta sheet breaks after contact, and the newly created free end of the sheet is pulled up by surface tension. The free ejecta sheet breaks into microdroplets, which are slingshot horizontally at high speed, up to 30 m/s. By studying the speed of the ejected droplets, we conclude that the thickness of the liquid sheet has reduced to as thin as 300 nanometers when it touches the pool surface and breaks-up. • S. T. Thoroddsen, M.-J. Thoraval, K. Takehara, and T. G. Etoh. “Droplet Splashing by a Slingshot Mechanism.” Physical Review Letters 106: 034501 (2011).

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Thoraval, M.-J., & Thoroddsen, S. T. (2011). Ejecta sheet dynamics. In KAUST Graduate Research Symposium. KAUST. Retrieved from www.kaust.edu.sa/media/symposiums/graduate/Abstracts/PSE/Submitted-Abstracts-PSE-PhD-Poster.pdf#page=7

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