![]() Water Droplet on Leaf: When a water droplet forms on a leaf, the cohesive forces between the water molecules are greater than the adhesive forces between the water and leaf surface. If the cohesive forces are greater than the adhesive forces, the resulting contact angles will be large and will form a more circular drop. If liquid molecules are strongly attracted to the molecules of the solid surface (adhesive forces > cohesive forces), the drop will tend to spread out and the contact angle will be close to zero degrees. The contact angle is measured in the liquid and depends on the relative strength of cohesive forces in the liquid and adhesive forces between the liquid and interface materials. This results in a contact angle at the interface. Where the surfaces meet, forces must be in equilibrium. The surface of a liquid in a container is an interface between the liquid, the air, and the container. Adhesion describes the attractive force between molecules of different types. Therefore, the surface tension will be a property of the interface rather than simply the liquid. The surface of a liquid is an interface between another fluid, a solid body, or both. The molecules at the surface feel a greater attractive force toward the bulk material than the interface material. In the bulk of the liquid, the molecules are pulled equally in all directions. In both cases, the vertical component of the surface tension is enough to support the weight of the object. Surface Tension FBD: Force diagrams showing the direction of forces for water supporting a water strider (insect) foot and a pin. Also, a paper clip or pin can be supported by the surface tension at a water air interface. This effect is visible in nature with water strider insects that are able to walk on water. This effect is a result of cohesion of the molecules of the liquid causing the surface of the liquid to contract to the smallest area possible. Surface tension is the tendency of a liquid surface to resist forces applied to it.
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