NOTE: If there are no voids between the molecules, it is called continuous distribution of mass.
Knudsen Number (Ku) Knudsen number= mean free path/ characteristic length
The distance between two consecutive collision of molecule is called mean free path.
The length in which we are analysing the fluid is called Characteristic length.
Case 1) If Ku< 0.01, we can say there is continuous distribution of mass, it is called no slip condition which means the fluid at boundary has same velocity equal to boundary itself.
Case 2) When 0.01<Ku<10, we can say it is slip flow i.e. if molecule stick to the boundary it has different velocity than the velocity of boundary itself.
Mechsiast 