Capillarity, or capillary action, is a phenomena in which a liquid will spontaneously rise or fall in a small cross-sectional area tube-like passageway.
Surface tension, which is brought on by the cohesion of liquid molecules, and adhesive forces between the liquid and the container wall combine to create capillarity.
1) Surface Tension- Free surface of a fluid behaves like a stretched membrane and tries to minimise its area upto maximum possible extent. This property is known as surface tension.
Note: 1)Free surface is a surface with constant normal stress and zero shear stress. 2) Normal stress is always equal to the pressure and zero shear stress means no external force in tangential direction.
Mathematically, Surface Tension= Force (F)/ Length (l)
2) Adhesive and Cohesive force- Adhesive force is the attraction force between the unlike molecules. Cohesive force is the attraction force between the molecules of same substance.
Note: 1) If adhesive force>> cohesive force, liquid will wet the solid boundary and the liquid is called wetting liquid. Eg: water glass interface 2) If cohesive force >> adhesive force, liquid is called non-wetting liquid. Eg: Glass mercury interface.
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