Date on Master's Thesis/Doctoral Dissertation

5-2019

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Mechanical Engineering

Degree Program

Mechanical Engineering, PhD

Committee Chair

Lian, Yongsheng

Committee Co-Chair (if applicable)

Brehob, Ellen

Committee Member

Brehob, Ellen

Committee Member

Cobourn, Geoffrey

Committee Member

Berson, Eric

Author's Keywords

CFD; multiphase flow; droplet; coalescence; energy; jumping

Abstract

When small drops coalesce on a superhydrophobic surface, the merged drop can jump away from the surface due to the surface energy released during the coalescence. This self-propelled behavior has been observed on various superhydrophobic surfaces and has potential applications in areas related to the heat and mass transfer, such as heat exchangers, anti-icing and anti-frost devices, thermal management and water harvesting. The jumping velocity model was obtained based on published experimental data and the balance of various energy terms described in previous studies. However, the self-propelled mechanism is still not fully understood. In this study, the self-propelled droplet phenomenon upon droplet coalescence was numerically studied to understand the mechanism. A multiphase flow solver was used to solve the three-dimensional Navier-Stokes equations. The liquid-air interface was captured using the moment of fluid (MOF) along with a direction splitting method applied to advect the interface. An approximate projection method was used to decouple the calculation of velocity and pressure. Both static and dynamic contact angle models were used to represent the surface wettability. The droplet jumping process was accurately captured by the multiphase flow solver. Both simulated droplet deformation and the vertical axis length matched the experimental results. Two cases with and without contact substrate were compared to investigate the jumping mechanism. With contact substrate, the droplet had double the time of acceleration in the upward direction. A high-pressure area appeared at the bottom of merged droplet and extended the acceleration. During the detachment the merged droplet with contact substrate also had a smaller surface area which indicates that more surface energy was converted into kinetic energy. The effects of droplet size, surface tension, and droplet density were studied. The jumping speed generally obeyed the capillary-inertial scaling law. The effect of approaching speed was also investigated. With lower approaching speed, the surface tension dominates while with higher approaching speed, the inertia force dominates the jumping process. The effect of substrate curvature was studied, and the numerical results revealed that droplet peripheries were formed on the symmetric sides of the wedge. The peripheries forced the droplet transferring more surface energy into kinetic energy in the upward direction. The jumping velocity increased by increasing the surface curvature. The droplet size was studied on the wedged surface and it obeyed the capillary-inertial scaling law. Our study also showed that with a lower contact angle, the droplet jumping velocity decreased. And the droplet was unable to jump away from substrate if the contact angle was below certain value.

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