Question

Small bodies, such as straw or pieces of cork, floating on the surface of a liquid often attract each other and collect together in clusters. To investigate the theory of this effect, let us suppose that A and B (fig.) are two parallel vertical plates immersed in a liquid which wets both of them, the liquid will stand at a higher level between the plates than it does outside.

as far the horizontal forces are concerned from liquid on sides, we may suppose the surfaces of the liquid flat, and represented by the dotted lines in fig. Considering now the forces acting on the plate A, the pulls exerted by the surface tension at P and u are equal and opposite; on the left the plate is acted on by the atmospheric pressure, on the right by the pressure in the liquid. Now the pressure in the liquid at any point is less than the atmospheric pressure by an amount proportional to the height of the point above the level of the undisturbed liquid; thus the pressure on A tending to push it towards B is greater than the pressure tending to push it away from B, and thus plates are pulled together.