Janice Van Cleave Super Science Challenges What is the difference between Capillary action and Osmosis? Capillary Capillary action is the result of adhesion and surface tension. Surface tension, in turn, is caused by cohesion. Action Cohesion is force of attraction between molecules of the liquid. The molecules at the surface do not have other like molecules on all sides of them. This makes them cohere more strongly to those directly associated with them on the Solution 1 Solution 2 surface. This forms a surface "film". It is difficult to move an object through this film or to "cut" or "break" this film. There is a kind of upward pushing force caused by the surface molecules sticking together. This phenomenon is called surface tension. Once this film is broken, the object can move freely in the liquid. Selectively Permeable Membrane Osmosis Adhesion is the tendency of the liquid to cling to a surface. When placed in a capillary, the molecules along the walls of the tube cling to the wall. This causes an upward force along the walls of the tube. This upward movement of the molecules along the water plus the surface tension which keeps the molecules at the surface as close together as possible, causes the surface of the liquid to take the shape of a round bottomed vessel. This is called capillary action. Osmosis, on the other hand, has to do with how a fluid passes through a permeable membrane that will allow it to pass, but block the passage of another substance dissolved in it. As the illustration shows, water will move through such a membrane until the concentration of water is the same on both sides. .