11.8 Cohesion and adhesion in liquids: surface tension and capillary  (Page 4/11)

Adhesion and capillary action

Why is it that water beads up on a waxed car but does not on bare paint? The answer is that the adhesive forces between water and wax are much smaller than those between water and paint. Competition between the forces of adhesion and cohesion are important in the macroscopic behavior of liquids. An important factor in studying the roles of these two forces is the angle $\theta$ between the tangent to the liquid surface and the surface. (See [link] .) The contact angle     $\theta$ is directly related to the relative strength of the cohesive and adhesive forces. The larger the strength of the cohesive force relative to the adhesive force, the larger $\theta$ is, and the more the liquid tends to form a droplet. The smaller $\theta$ is, the smaller the relative strength, so that the adhesive force is able to flatten the drop. [link] lists contact angles for several combinations of liquids and solids.

One important phenomenon related to the relative strength of cohesive and adhesive forces is capillary action    —the tendency of a fluid to be raised or suppressed in a narrow tube, or capillary tube . This action causes blood to be drawn into a small-diameter tube when the tube touches a drop.