Final answer:
This is due to cohesion, where windshield treatments repel water as cohesive forces amongst water molecules are stronger than their adhesive forces with the treatment, leading to the formation of droplets that easily roll off. Adhesive forces between water molecules and surfaces like glass are stronger, causing water to spread out.
Step-by-step explanation:
This is due to cohesion, because the windshield treatments are made to repel water (hydrophobic). Since the cohesive water molecules are attracted to each other and not the non-polar surface created by the windshield treatments, the water forms into drops. The drops have less interaction with the surface of the windshield treatment, making it easier to roll off of, as opposed to spreading.
Intermolecular forces (IMFs) play a critical role in determining how water interacts with different surfaces. Adhesive forces, the attraction between different molecules, can be weaker than the cohesive forces holding water molecules together. When cohesive forces outweigh the adhesive forces, water droplets form rather than spread out on surfaces like waxed car surfaces or plastics like polyethylene. This occurs because drops aim to minimize surface area and maximize cohesive interactions. Thus, surface tension maintains these droplets' shape, even allowing for lightweight objects to float on water's surface without breaking through, illustrating water's cohesive property.
On the other hand, when adhesive forces are stronger, like between water and glass, water spreads to form a thin film. This difference in interaction also explains phenomena such as the concave meniscus of water in a glass tube, where water 'wets' the glass and climbs up its sides, or the convex meniscus of mercury which does not wet glass, all guided by the balance between cohesive and adhesive forces.