Editing Talk:Capillary waves
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I may be wrong, but looking at your derivation it seems the boundary conditions are not correctly | I may be wrong, but looking at your derivation it seems the boundary conditions are not correctly | ||
described. If the system is fixed to some immobile frame, only <math>\sin</math> terms should appear in the modes, not <math>\cos</math>. If, on the other hand, periodic boundary conditions are applied, the opposite applies: only <math>\cos</math>, not <math>\sin</math>. This may explain the factor of <math>2</math> that's missing... but I still have to think more carefully about this. --[[User:Dduque|Dduque]] 09:58, 3 February 2009 (CET) | described. If the system is fixed to some immobile frame, only <math>\sin</math> terms should appear in the modes, not <math>\cos</math>. If, on the other hand, periodic boundary conditions are applied, the opposite applies: only <math>\cos</math>, not <math>\sin</math>. This may explain the factor of <math>2</math> that's missing... but I still have to think more carefully about this. --[[User:Dduque|Dduque]] 09:58, 3 February 2009 (CET) | ||
:Thank you. Yes, I didn't appreciate the importance of boundary conditions. I think it is quite reasonable to take something like <math>\frac{\partial h(x,y)}{\partial \mathbf{n}} \Big|_{wall}=0</math>, with <math>\mathbf{n}</math> normal to the wall. I suppose it is valid at least for more or less long waves (several minimal wavelengths), which contribute most to <math>\langle h \rangle</math>. It is likely there is no need to study molecular interaction between liquid and solid surface in this case — boundary conditions will not be affected by the material of the walls at least for real-life systems. So we get <math>\frac{1}{2} k_B T</math> for each mode. Still I'll think it over again and will wait for your reply. [[Special:Contributions/91.76.179.246|91.76.179.246]] 12:53 | :Thank you. Yes, I didn't appreciate the importance of boundary conditions. I think it is quite reasonable to take something like <math>\frac{\partial h(x,y)}{\partial \mathbf{n}} \Big|_{wall}=0</math>, with <math>\mathbf{n}</math> normal to the wall. I suppose it is valid at least for more or less long waves (several minimal wavelengths), which contribute most to <math>\langle h \rangle</math>. It is likely there is no need to study molecular interaction between liquid and solid surface in this case — boundary conditions will not be affected by the material of the walls at least for real-life systems. So we get <math>\frac{1}{2} k_B T</math> for each mode. Still I'll think it over again and will wait for your reply. [[Special:Contributions/91.76.179.246|91.76.179.246]] 12:53, 3 February 2009 (CET) |