Dynamics of spontaneous initial spreading and spreading of a hydrodynamically driven droplet under the influence of surrounding pressure

  • Author / Creator
    Sumaiya Farzana
  • We investigate experimentally the early time dynamics of spontaneous spreading of silicone oils with various viscosities at elevated surrounding pressures. The surrounding medium pressure is increased in a monotone fashion starting from atmospheric pressure to a maximum value of 30 megapascal (MPa) above atmospheric conditions in 10 MPa increments. We conduct our analysis using four grades of silicone oils namely: D10, N35, S60, and D500 and study their spreading behavior on Polytetrafluoroethylene (PTFE) substrates. Once contact with the substrate is established, the three-phase contact lines which form at the intersection of the solid, liquid, and gas phases advance rapidly to reduce the droplet’s surface energy. Experimental observations which are conducted using a high-speed camera and a drop shape analyzer reveal three distinct regimes of spreading: an initial spreading regime, a short but intermediate transition regime followed by a late-stage viscous regime. The results from these experiments are analyzed in terms of the temporal evolution of the contact radius, contact line velocities, and droplet contact angle. In addition, we analyze the pressure effects on the onset of the transition regime and the empirically obtained spreading exponent. Our observations show that the early time spreading dynamics conform themselves to a power law where the empirically obtained exponent concomitant to spreading is a strong function of pressure. More specifically, a rise in pressure decreases the spreading exponent in the inertial regime. Additionally, an increase of pressure reduces the onset time of the transition regime as well as spreading velocities. This is due to increased viscous dissipation at the contact line which enables a quick conversion to the transition regime. We also propose a theoretical model for investigating the spreading behavior of hydrodynamically driven droplets on a solid substrate under various surrounding pressures. By combining the extended Overall Energy Balance (OEB) approach and the Lucas empirical model for estimating drop viscosity at elevated pressures, we predict the advancement of a liquid droplet on a solid substrate as it undergoes constant mass flux addition while maintaining a spherical cap. For validation, we conduct an experimental investigation on a spreading droplet while considering the addition of mass over time in atmospheric pressure and surrounding gauge pressures of 10 and 20 MPa. We analyze the results of the pressure effects on the spreading droplet whose primary driving force is hydrodynamic in terms of the advancing contact angle, increase of volume over time, and spreading radius of water on four different substrates namely: Polytetrafluoroethylene (PTFE), Aluminium (Al), Copper (Cu), and Poly (methyl methacrylate) (PMMA). The results show that an increase in pressure during the advancing stage lowers the spreading radius and simultaneously increases the contact angle.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.