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Meaning of surface tension: The pulling force between any two adjacent parts on the surface of a liquid, along the unit-length boundary line perpendicular to them. The formation of surface tension is closely related to the special force conditions experienced by the molecules in the thin layer at the liquid surface. The presence of surface tension gives rise to a series of special phenomena that can be observed in everyday life. ①The direction of surface tension is tangent to the liquid surface and perpendicular to the boundary line between the two portions; if the liquid surface is flat, then the surface tension lies on that plane. If the liquid surface is curved, the surface tension lies on the tangent plane of this curve. ②Surface tension is a manifestation of intermolecular forces. It occurs at the boundary where a liquid and a gas come into contact. It is determined by the special conditions of the liquid molecules in the surface layer. The molecules within a liquid are almost in close contact with one another; they maintain a balanced distance between them. When they are slightly farther apart, they attract each other, while when they are slightly closer together, they repel each other. This is why liquid molecules cannot spread infinitely like gas molecules, but can only vibrate and rotate around their equilibrium positions. Molecules near the liquid surface are subjected to uneven forces, as they are only significantly affected by the molecules inside the liquid; this allows molecules with higher speeds to easily escape to the surface and turn into vapor. As a result, the molecular distribution in the surface layer of the liquid (the thin layer in contact with the gas) is sparser than that in the interior parts of the liquid. Compared to the distribution of molecules within the liquid, they are in a special situation. The repulsive force between molecules in the surface layer decreases as the distance between them increases; in this special layer, the attractive forces between molecules become dominant. Therefore, if a dividing line MN is drawn arbitrarily on the liquid surface to divide it into parts a and b. F represents the attraction between the molecules in the surface layer of part a and those in part b, while F6 represents the attraction between the molecules in the surface layer of the right-hand part and those in part a. The forces exerted by these two pairs of molecules are always equal in magnitude but opposite in direction. The mutual pulling force between any two parts of this surface layer drives the liquid surface layer to contract. Due to surface tension, the liquid surface always tends to minimize its size; as a result, small droplets in the air usually take a spherical shape. ③The magnitude of the surface tension F is proportional to the length of the dividing line MN. It can be written as F=σL or σ=F/L. The ratio σ is called the surface tension coefficient, and its unit is commonly dyn/cm. Numerically, the surface tension coefficient equals the force per unit length of attraction between adjacent portions of a liquid surface. Surface tension coefficient of the liquid film = Surface energy of the liquid film / Area of the liquid film = F_surface tension / (2 * length of the selected line segment). The surface tension coefficient is related to the properties of the liquid, and not to the size of the liquid surface.