1. The problem statement, all variables and given/known data 2. Relevant equations 0=viscous+gravitational+pressure I saw in the solutions that pressure=0 in this case, but why? I also knew that : accumulation= flow in - flow out+generation, why not use this one? 3. The attempt at a solution (their solution) We are interested in getting the velocity profile. I do not understand why they said at x=delta there is no momentum being transferred?is it because it is at the interface? how exactly is the fluid flowing?I want to understand the phenomena Below I am attaching 2 of my sketches, in the solution says that viscous force acting on x surface(which is perpendicular to the plane, and in the z direction). How did they knew that? Which of the below sketches I made are correct, no1 or no2. How can we deduce how is it flowing, given that he gave us the boundaries y=0 as the wall and y=delta as the gas-liquid interface? I would really appreciate it if you could also provide me a better sketch, if mine are wrong. Thank you in advance.