As wet as damp

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Bryn Harlech (6), Dunedin, asked: Why is water wet?

Daniel Schumayer, physicist at the University of Otago, responded.

We usually say that something is wet because water has spread on its surface, and is clinging to its surface, but we rarely say that water itself is wet. Why does water spread and cling to a surface? It is a very hard question.

What does "wet", as an adjective, mean? The Oxford English Dictionary lists several distinct, but still current meanings, most already used in Old English. They revolve around the ability of soil or of some object to hold water. The dominant current meaning is "Made damp or moist by exposure to the elements or by falling in water; sprinkled, covered, or permeated with rain, dew, etc." This description seems circular, as it allows the argument: water is wet, because it is covered and permeated with water.

What the definition shows, but does not spell out explicitly is that "wetness" involves two substances, one of which is water and the other substance, e.g., soil, kitchen bench, skin or clothes, etc, which becomes wet via water coming into contact with it and somehow staying on or in it.

Is it always true that an object becomes wet if it comes in contact with water? Not at all. Washing an oily pan is difficult without detergent, because water simply runs off its surface. Also, one may observe spherical water droplets sitting on top of leaves, instead of spreading. Similarly, when you spray water on your skin after applying sunscreen, those tiny droplets roll off. All these surfaces are hydrophobic, i.e., water repellent.

So, looking from this angle, we can say that water is not wet, but it may make other objects wet. I believe this stands closest to our common sense.

Although I wanted to reduce confusion, at the end I feel obliged to increase confusion by mentioning that "dry water" exists. This is a very interesting emulsion of air, water and silica. The water droplets are silica coated. The coating prevents water molecules in one droplet from coming into contact with water molecules in other droplets. Hence, droplets cannot combine and remain separate. You have a molecular cage for water. This interesting substance appears as a white powder, even though 95% of its weight is water. In toy shops you may buy hydrophobic sand and experimenting with it will surprise you.