Card # 21-Angle of contact
When a liquid comes in contact with a solid surface then in the region of contact the surface of liquid is curved. The angle made by the tangent to the liquid surface at any point of contact with the solid surface in the liquid is called angle of contact.
Factors on which angle of contact depends
1. Nature of solid & liquid in contact 2. Medium above the free surface of the liquid 3. On temperature of the liquid 4. Cleanliness of the surface in contact. The value of the angle of contact depends upon the values of the cohesive and adhesive force. If adhesive force > Cohesive force Angle of contact θ 90o obtuse angle Liquid doesn’t we the solid Excess of pressure inside a drop or bubble The surface tension acts along the free surface of the liquid. The surface of liquids is shown in figure below.
In Fig (a) Force on the molecule due to surface tension is zero. The pressure on the liquid side is equal to the pressure on the above the liquid. In Fig (b) the resultant of the surface tension force i.e. R is acting inward into the liquid. Due to this the pressure inside the liquid is higher than that outside it. For the equilibrium of liquid excess of pressure acts in upwards direction. In Fig (c) The resultant surface tension R is directed outwards and the pressure inside the liquid is less than the outside it. In case of liquid in equilibrium the excess of pressure acts in downward direction. * * Excess of pressure inside the drop is P =
2T . A drop has only one free surface. R 4T Excess of pressure inside the soap bubble is P = . A soap bubble has two free R
surfaces because it contains air.