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Numerical Investigation of Motion of Nanodroplets on Wetting Gradient Surfaces

Mohammadi, Ahmad | 2012

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 43225 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Moosavi, Ali
  7. Abstract:
  8. A droplet placed on the boundary of two solids with different wettabilities will move to the more wettable part. This is a well-known phenomenon and has been extensively used in a variety of processes and applications ranging from biological systems and ink jet printing to the commercial lab-on-a-chip. Because of its importance, many studies are conducted around this phenomenon. The difference between the equilibrium contact angles of the droplet on the two parts gives rise to an interfacial driving force which moves the droplet. Such a motion is not continuous as the droplet slides to the more wettable area, the driving force diminishes and consequently the droplet stops. A non-stop motion could be achieved utilizing a continuous wettability gradient. Such surfaces are called chemically heterogeneous surfaces which are used for transfer of fluid droplets.
    In this study, the dynamics of nano scale droplets on chemically heterogeneous surfaces has been studied both numerically and theoretically. Most of the previous studies are limited to special cases of wettability gradients, material type or size of the droplets. In this research, the influence of droplet size and wettablity gradient on the velocity of nano droplets was investigated. Also, effect of slip boundary condition on the dynamic of nano droplet has been considered, as the no-slip boundary condition is not valid in the nano scale flows. A comparison between simulation results of 2D and 3D droplets is presented. In the case of 2D, an analytical method is also developed to calculate the velocity of nano droplets and the results has been compared with numerical outcomes.
    Both 2D and 3D simulations indicate that the larger the droplet the higher its velocity. Also increasing the slip length will increase the velocity of droplets significantly. The results reveal that the velocity of 3D nano droplets is more than 2D nano droplets. In this study, one interesting difference between the nano scale and micro scale is also realized. Dynamics of nano droplets is a function of some parameters that become important only at the nano scale dimensions and the behavior of the system can be completely different with those observed at macro and micro scales
  9. Keywords:
  10. Nanodroplets ; Wettability ; Equilibrium Contact Angle ; Chemical Heterogeneous Substrate ; Wetting Gradient

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