hiiii everyone ... i wish u can help me in this .. i have a mass transport project and it is about couette flow .. in the cylinder i have two fluid water and oil .. my dr. ask me a aquetstion to look after it he says why the water rotates around the oil why not the oppisite he give me a hint he says it is to minimize the energy but he want the princibple that states that ? .. can u help me do u have any idea about what principle he is talking about !