Surface tension
In terms of force

Surface tension γ of a liquid is the force per unit length. In the illustration on the right, the rectangular frame, composed of three unmovable sides (black) that form a "U" shape, and a fourth movable side (blue) that can slide to the right. Surface tension will pull the blue bar to the left; the force F required to hold the movable side is proportional to the length L of the immobile side. Thus the ratio F / L depends only on the intrinsic properties of the liquid (composition, temperature, etc.), not on its geometry. For example, if the frame had a more complicated shape, the ratio F / L, with L the length of the movable side and F the force required to stop it from sliding, is found to be the same for all shapes. We therefore define the surface tension as

γ = F 2 L. {\displaystyle \gamma ={\frac {F}{2L}}.} The reason for the 1 / 2 is that the film has two sides (two surfaces), each of which contributes equally to the force; so the force contributed by a single side is γL = F / 2.
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