Cohesion And Adhesion

As  cohesive and adhesive forces are related with bulk (or macroscopic) features, they are not applicable to atomic and molecular properties. Both cohesive and adhesive forces act on a liquid when it comes into touch with a surface (such as the walls of a graduated cylinder or a tabletop). The shape that the liquid takes is determined by these forces. Wetting is the process of liquid on a surface spreading out to form a thin, somewhat homogeneous film over the surface due to the effects of adhesive forces. In the presence of strong cohesive forces, the liquid can also divide into a number of small, roughly spherical beads that stand on the surface while keeping minimal contact with it.

Adhesion and Cohesion Forces

The phrase “cohesive forces” refers to the collective intermolecular interactions (such as hydrogen bonding and van der Waals forces) that are responsible for liquids’ bulk resistance to separation. These attraction forces exist specifically between molecules of the same material. Because water has great cohesion, which pulls its molecules closely together to form droplets, rain falls in droplets rather than a thin mist. Due to the molecules’ disdain for their environment, this force tends to join liquid molecules, collecting them into relatively big clusters.

Similar to mechanical forces (sticking together) and electrostatic forces, the term “adhesive forces” refers to the attractive forces between dissimilar substances (attraction due to opposing charges). Adhesion causes a liquid to cling to the surface it is resting on in the case of a liquid wetting agent. When water is placed on a clean glass, it spreads out and forms a thin, homogeneous layer on the surface. Because the adhesive forces between water and glass are strong enough, the water molecules are pulled out of their spherical configuration and held against the glass surface, eliminating repulsion between different  molecules.

Macroscopic Effect  on Cohesive and Adhesive Forces

The relative strengths of the cohesive and adhesive forces acting on a liquid influence the shape it will assume when deposited on a smooth surface (and whether or not it will wet the surface). When the adhesive forces between a liquid and a surface are stronger, the liquid will be pulled down, wetting the surface. If the liquid’s cohesive forces are stronger, however, they will resist adherence and cause the liquid to maintain its spherical shape and bead the surface.

Case I. The Meniscus 

The curvature of a liquid’s surface within a container, such as a graduated cylinder, is known as the meniscus. However, before we can explain why some liquids have a concave up meniscus and others have a concave down meniscus, we must first comprehend surface tension adhesive forces at work. For example, water is a polar molecule with a partial positive charge on hydrogens and a partial negative charge on oxygen. Each molecule’s partial positive charge is attracted to its neighbor’s partial negative charge in liquid water. This is where the water’s cohesive forces come from. The water molecules buried inside the liquid are then evenly pulled and pushed in all directions, resulting in no net pull. Meanwhile, because there are no upward pushing forces, the molecules on the liquid’s surface have a net downward pull.

So, how can this cohesive force produce both a concave up and down surface? The interaction between the adhesive forces between the water molecules and the container’s surface provides the answer. When the liquid’s cohesive force is greater than the liquid’s adhesive force to the wall, the liquid concaves down to avoid contact with the wall’s surface. When the liquid’s adhesive force to the wall exceeds its cohesive force, the liquid is more attracted to the wall than its neighbors, resulting in upward concavity.

Case 2: Wine Tears

Droplets of wine appear to “float” above the liquid’s meniscus and form “tears” in agitated glasses of wine. Surface tension, as well as cohesive and adhesive forces, cause this age-old phenomenon. Alcohol has a higher vapor pressure than water. As a result, “alcohol evaporation causes a surface tension gradient that drives a thin film up the side of a wine glass” (Adamson). The “solutal Marangoni effect” is the name for this phenomenon.  Some water clings to the glass walls due to adhesive forces. The “tears” form as a result of the water’s cohesive forces holding it together. The surface tension gradient is “the driving force for the mobility of the liquid,” according to Gugliotti, while the tears themselves are formed by cohesive and adhesive forces.

Conclusion  

We conclude that  the property of like molecules (of the same substance) sticking together due to mutual attraction is known as cohesion. The ability of distinct molecules or surfaces to attach to one another is known as adhesion. Solids, for example, have excellent cohesive qualities and do not adhere to the surfaces with which they come into contact.