Due to the effect of cohesion, the surface of the liquid always tends to shrink into a spherical shape, but in reality, liquid cannot exist freely. Its surface always has to contact with solid, gas or other liquid, and it is also affected by gravity, so it is not easy to see the liquid surface with an accurate spherical surface.
Since the liquid cannot maintain a fixed shape, when it contacts the solid, the shape of the contact surface is determined by the solid surface. The interaction between the molecules on the contact surface is mainly related to our study of sealing. There is an interaction between gas molecules and liquid surface molecules on the contact surface of the liquid and gas, but because the density of gas molecules is small, the influence of mutual attraction is also small. At this time, the shape of the liquid surface is also affected by the surrounding solid surface in contact with it and the effect of gravity, so it is usually a curved surface. Since the cohesion is directed to the inside of the liquid in the normal direction of the curved surface, surface tension appears on the curved liquid surface (note that only when the surface is a curved surface), and the liquid surface is like a taut membrane.
Surface tension is a force along the tangent direction of the surface, but its direction cannot be specifically pointed out. Only when a certain line on the liquid surface is specifically determined can we determine that the direction of the surface tension on it is perpendicular to the line and along the tangent direction of the surface. No matter how the line on the liquid surface is selected, the surface tension always exists in the perpendicular direction and is equal and opposite at the same time, and the direction of its resultant force is exactly the direction of the normal pointing inward.
