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April 14, 2012 6:52pm UTC

Simulation of a superhydrophobic surface using level set method

Hello,

I am a BE student of Mechanical engineering. My final year project is to simulate a superhydrophobic surface. I have been trying to do that for the past two-three months. After several failed approaches I decided to adopt the following method.

I tried to adapt the Rising Bubble model (using level set method) which is provided with COMSOL 4.2. First I made the whole model as per the steps described. This was simple and gave the said results. Then I decided to exchange the roles of the two fluids. So instead of the bubble of oil in the water layer, I made a droplet of water in the oil layer. I kept all the other settings as it is. Solving this model also gave appropriate results, i.e, the drop of water fell from the oil layer onto the water layer and mixed with it. Next I contemplated that if I simply replace the oil (material) with air I should obtain similar result. Replacing the material to air also meant that I had to change the surface tension parameter to liquid/gas from liquid/liquid. After doing this when I solved the model the solution failed to converge and reached a singularity. Just being curious I solved the model again, but this time I neglected the surface tension between the air/water interface. The solution did converge this time but my analysis remains incomplete without using the surface tension as it is an important parameter for simulating the contact angle on a hydrophobic surface. So the problem lies somewhere with the usage of the surface tension.

One more interesting observation was that if I make a model where a spherical droplet is touching a wetted wall (of some contact angle) in the two phase level set mode, and if I don't neglect the surface tension of the water/air interface, then the solution converges.However, when I run the animation, the size of the droplet keeps on decreasing and eventually it almost completely disappears.

I would really appreciate if someone can help me out with this.

Thank You!

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July 6, 2012 3:57pm UTC in response to Chloe Wu

Re: Simulation of a superhydrophobic surface using level set method

Hi Chloe,

Sorry, I wasn't able to complete my simulation and still do not know what I can do to make it work. I suspect that it may be some inherent problem with COMSOL, as I have found several threads which discussed a similar problem. None of those threads came up with a solution, as far as I know. I guess someone from the COMSOL team should look into this and come up with some sort of tutorial or otherwise correct the problem itself.

Regards,
Nitant

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July 6, 2012 9:45pm UTC in response to Ivar Kjelberg

Re: Simulation of a superhydrophobic surface using level set method

Hi,

It is harder to get convergence with an air bubble compared to a water bubble in an oil medium. One reason is that surface tension exerts a pressure on the bubble. For incompressible water, that pressure will not result in any significant volume change. For air however, it may cause a severe transient if it is different from the initial pressure in the bubble. Therefore, try to adjust the initial pressure to reduce that transient.

Nagi Elabbasi
Veryst Engineering

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