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Numerical Study of Anisotropic Wetting of Nano-Droplets on Chemically Patterned Surfaces

Rajabpour, Shahram | 2013

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 45237 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Moosavi, Ali
  7. Abstract:
  8. This study investigates three-dimensional modeling of nanodroplets over chemically patterned surfaces. The pattern can be established by changing the contact angle on the surface. The problem is studied numerically by solving the thin film equations.Both the long-range and short-range inter-molecular interactions are considered. The numerical procedure is validated by using the theoretical studies. The effects of parameters such as the size of the droplets, the gradient of the surface, and the slip coefficient on the dynamics and speed of droplets and the shape of them during the motion are inspected.By examining the dynamics of the system it is revealed that the velocity of the droplets increase initially but then decrease. Also the shape of droplets changes through the motion and loses its height. This reduction is fast initially and then slows down. The velocity rises by increasing the size of droplets. This behavior can be due to the fact that the larger droplets experience a greater contact angle difference compared to the small droplets. Therefore a larger driving force is applied to the larger droplets. The effect of the gradient of the surface is investigated and,as expected, the results show that by increasing the intensity of the gradient the velocity increases.In addition, the results indicate that increasing the slip coefficient enhances the dynamics. In other words, in order to increase the speed of the droplets, the slip coefficient can be increased. nvestigating the effect of slip is very important since for small-sized droplets no-slip assumption may be invalid.In a different study, the dynamics is investigated for droplets on inclined surfaces with an asymmetric pattern, instead of the flat surfaces. In order to make the issue more asymmetric, a tilt/ deviation angle is considered between the bars of the surface and the inclined surface. The results have revealed how the speed of the droplets is oscillatory and a sinusoidal-like curve is observed. The reason can be due to the fact that when the droplets move from a bar to another, the incoming forces are from one side of the stimulus and there is no other driving force on the other side. Thus, the balance between these forces keeps the droplets speed almost constant. In addition,by comparing the result of this part with other experiments that have been done for this issue but for larger sizes, the size effect can be determined.Keywords: Wetting, Numerical Modeling, chemical radient, Thin film method,Nano-droplet, slip coefficient
  9. Keywords:
  10. Numerical Modeling ; Nanodroplets ; Wetting ; Chemical Gradient ; Thin Film Method ; Slip Coefficient

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