Water is wet because it has polar molecules that have the ability to adhere to other molecules. This creates an interaction between water and objects, which gives the sensation of being wet.
Water is quite simple: two hydrogen atoms bonded to one oxygen atom (H₂O). But be careful, these atoms do not share their electrons equally! Oxygen attracts the electrons more strongly towards itself, causing it to become slightly negative, while the hydrogen atoms become slightly positive. This unequal distribution of charges is what we call polarity. A water molecule therefore behaves like a tiny magnet, with two different poles. This little peculiarity explains many of the amazing behaviors of water, particularly why it easily adheres to and "wet" objects.
Water loves to cling to surfaces due to its polar nature. Much like a tiny magnet attracted to certain materials, water molecules feel comfortable with surfaces that have a certain affinity for them (hydrophilic). Conversely, on surfaces that repel it, such as certain plant leaves or plastic materials (hydrophobic), water forms round droplets and slides off more easily. It is this close or distant relationship between the surface and the water molecules that explains why your wet hand stays damp for a long time, while on a waterproof jacket, the droplets roll off without lingering.
On the surface, water molecules cling tightly to each other, creating a sort of taut skin. This is due to surface tension, an invisible film that allows insects to walk on water without sinking. When you place your finger in the water, this tension decreases locally and the liquid spreads around: this is precisely what wetting is. The lower the surface tension, the more easily water spreads on a surface. On a surface like glass, water happily spreads its droplets, but on a waxy surface, it prefers to cluster into nice little compact beads, without really wetting.
Water wets due to a discreet tug-of-war between two types of forces: adhesive forces and cohesive forces. Adhesive forces are those that make water molecules want to stick to other materials, like glass or your skin. They are due to the attraction between the charged poles of water molecules and the molecules of the materials they come into contact with. In contrast, cohesive forces are those that push water molecules to prefer staying together, well grouped, thanks to the famous hydrogen bonds. In practical terms, when water touches a surface, if the adhesive forces win over the cohesive forces, water spreads out nicely—it wets more. If the opposite is true, water forms small, well-rounded droplets and slides more easily, like on a lotus leaf.
The surface of a liquid, like water, contains a certain energy, aptly called surface energy. And it is precisely this energy that water naturally seeks to reduce by wetting a surface: when water contacts a solid surface, if it reduces the total energy (essentially, if it benefits everyone energetically), the liquid willingly spreads over it. The higher the surface energy of the solid, the more it will easily attract water molecules to reduce this energy tension together. This small quest for energetic balance explains why water wets certain materials — like glass — by forming a thin film, while it forms nicely rounded droplets on others, like plastic or waxed paper, preferring to keep its energy to itself.
The skin of lotus leaves has a very unique microscopic structure that creates a waterproof effect known as the 'lotus effect.' This prevents water from adhering to the surface of the leaves, allowing droplets to roll off and carry away dust and dirt.
Hot water generally wets better than cold water because its surface tension decreases with an increase in temperature, allowing for better penetration into fabrics or paper.
Most insects that walk on water, such as Gerridae (water striders), use high surface tension as support. Their legs are covered with hydrophobic hairs, which significantly limits wetting and prevents them from sinking.
Detergents or soap are 'wetting agents' that significantly lower the surface tension of water, thereby enhancing its ability to wet materials such as textiles or dishes.
When getting out of the bath or shower, water adheres to the skin due to adhesive forces (the attraction between the water and the molecules on the surface of the skin). These adhesive forces locally surpass the cohesive forces (the attraction between water molecules themselves), causing the water to temporarily remain on the body’s surface.
The shape of water droplets primarily depends on surface tension, the properties of the surface on which they rest (hydrophilicity or hydrophobicity), and any dissolved impurities. High surface tension and a hydrophobic surface will cause the droplets to adopt an almost spherical shape.
Hydrophilic surfaces strongly attract water; therefore, water spreads easily on these surfaces. In contrast, hydrophobic surfaces repel water, preventing it from spreading and causing it to cluster into droplets.
When water is heated, its surface tension slightly decreases, allowing it to spread better and thus wet certain surfaces more effectively. This explains why hot water sometimes seems to 'wet' or clean better than cold water.
Some surfaces are called 'hydrophobic', meaning that they repel water due to their low surface energy. These materials, such as certain plant leaves or special coatings, create a high surface tension that prevents water from adhering well, resulting in the formation of droplets.
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