Drop Impacts Under Extreme Conditions on Thin Liquid Films or Solid Walls

  • Abdulrahman Barakat Aljedaani

Student thesis: Doctoral Thesis

Abstract

Drop impacts play a key role in many industrial applications, from spray coating of surfaces, to splashing of fuel-droplets within combustion chambers. Splashing, or break-up during ink-jet printing, can cross-contaminate biological assays, or degrade the quality of ink-jet printed products. Crime scene studies of blood splatter can give vital clues for the police. Spreading of plant diseases between nearby leaves by splashing depends on the velocity and trajectory of secondary droplets. In this dissertation, I study the early dynamics of splashing and the dynamics of ejecta sheets under extreme impact conditions, using ultra-high-speed video imaging at up to 5 million fps. In the first part, I show the effect of the surface tension differences on the break-up of the Edgerton crown, I verify that individual droplets hit the crown wall and generated Marangoni holes, thereby causing the crown wall to rupture at multiple locations. In the second part, I investigate the splashing of a drop impacting onto a solid substrate with high impact velocity, I show that for sufficiently high Re, splashing can no longer be suppressed by only reducing the surrounding air pressure. Furthermore, I tracked the earliest splashed spray droplets to catch their maximum velocity. Surprisingly, the splashed droplets can travel at extremely high speed of up to 1 km/s, which is 50 times faster than the impact speed. The influence of viscosity on the lamellar spreading along the substrate was investigated. I find that the intact lamella, following the fine spray, spreads as R(t) ~〖 t〗^(1/3) , while the maximum spreading radius of the drop was shown to be a strong function of viscosity, scaling as β_max∝〖Re〗^0.175. The data did not show a strong effect of surface tension on β_max over a wide range. Therefore, I concluded that surface tension at this parameter space does not play a major role in both splashing nor spreading. In the third part, I study extreme splashing dynamics of the Ejecta sheet when a drop impacts on a thin liquid film with very large impact velocities using the same device, at up to ~ 22 m/s. For this purpose, we have constructed a novel experimental device consisting of a 26-m-tall vacuum tube. I investigate the interplay between viscosity, the surrounding ambient air pressure, and surface tension, on the ejecta shapes and break-up. I show how the bending of the ejecta sheet is primarily produced by air-resistance. This is supported by an analytical and numerical model to quantify the effect of the surrounding air pressure on the sheet bending and touch-down.
Date of AwardOct 2019
Original languageEnglish (US)
Awarding Institution
  • Physical Science and Engineering
SupervisorSigurdur Thoroddsen (Supervisor)

Keywords

  • splashing
  • Drop impact
  • Marangoni
  • Capillary flow
  • Ejecta sheet

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