Ultrafast drop impact dynamics on unheated and heated flat surfaces

• Author / Creator

  Yichi Zhang

• Motivated by relevant applications in spray cooling, coating, and inkjet printing, droplet impact dynamics on a solid surface is investigated both numerically and experimentally by varying several major control parameters such as impact velocity, contact angle, and surface temperature.

  First, computational fluid dynamics (CFD) simulations are carried out to study a water droplet impact onto a flat and unheated surface with different surface wettability, e.g., hydrophilic, hydrophobic, and ultrahydrophobic surfaces parameterized using the droplet’s static contact angle, θw. In the CFD study, the governing equations consist of the Navier- Stokes equations with a level-set method used to track the interface between the two fluid phases: water and air. The corresponding Weber (We) number, defined as the droplet’s kinetic to surface energy ratio, ranges from 4 to 510. The numerical model successfully reproduces the experimental impact outcomes with good agreement, especially in the low We (< 30) regime. The primary impact outcomes include spreading, rebound, jetting, and splashing as impact velocity VI is increased. Most of the simulation results show a universal scaling law of maximum spreading factor, βmax ∼ We1/4, especially on hydrophobic and ultraphydrophobic surfaces. The slip length b, accounting for the frictional force exerted by a flat solid surface on a droplet, is found to be a key factor controlling dissipation and thus impact dynamics and should be varied with surface wettability. As VI increases, the generation of entrapped air bubbles, jetting, and splashing occur and are caused by the interplay between pressure variation, droplet deformation, and surface tension.

  Second, we carry out experiments of water drop impact on a heated flat surface using a high-speed camera, an Infrared (IR) camera, and thermocouples to record both ultrafast drop dynamics and temperature field. As the increase of surface temperature Ts (100○C to 450○C) and We (1.6 to 129), four specific dynamical events are observed: spreading, totally rebound, spreading with atomization, and splashing break up with atomization. The Leidenfrost effect, with an insulating vapor layer formed underneath the droplet, is observed at high Ts (> 350○C) but low We (< 10). The droplet temperature is analyzed with the IR camera results and shows good agreement with a theoretical model at a low surface temperature region (Ts ≈ 100○C). The surface temperature changing ∣ΔTs∣, i.e., the surface cooling rate by impacting droplet, is analyzed from the thermocouple results. ∣ΔTs∣ is increased in low-Ts regime but decreased in high-Ts regime. Spreading is the typical event at low-Ts, with similar contact area, higher Ts leads larger ∣ΔTs∣. A total droplet rebound observed at high-Ts regime with a vapor film underneath avoids droplet directly contact with the surface; therefore, the change of surface temperature ∣ΔTs∣ is small.

  Our simulation and experimental results demonstrate the importance of surface wettability on high-speed drop impact dynamics on a flat and unheated surface. Furthermore, our experimental results of water drop impact on a heated, flat surface reveal the interplay of both fluid motion and heat transfer on the drop impact outcomes and the dynamic Leidenfrost point depending on We.

• Subjects / Keywords

  • Drop impact
  • CFD
  • Surface wettability
  • Heated surfaces

• Graduation date

  Fall 2021

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-rk1t-ge81

• 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.