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Hydrophilicity is the tendency of a surface to become wet or to absorb water. Generally speaking, when a surface shows hydrophilicity, the molecules of water form bonds with the molecules of the surface. A material is not necessarily either entirely hydrophilic or hydrophobic because these two tendencies are two ends of a spectrum, and a material can fall anywhere along that spectrum.
On a surface that shows hydrophilicity, the bonds between water molecules are readily broken, and molecules of water are able to bond to the molecules in the hydrophilic surface. The bonds between the single oxygen atom and the two hydrogen atoms in a single molecule of water are not broken, but the weaker bonds that occur between hydrogen atoms in different water molecules are. Once these bonds are broken, the molecules in the hydrophilic surface are able to form bonds with the hydrogen atoms on water molecules. Surfaces that are hydrophobic, on the other hand, do not readily bond with water molecules, which allows the water to stay bonded to itself and create droplets.
After water molecules have formed a bond with with a hydrophilic surface, the surface is said to be wet. Once the water has been absorbed on a molecular level, it can be removed from the other molecular structure through the process of evaporation. Simply put, given warm, dry conditions and a certain amount of time, most hydrophilic surfaces will dry out as the liquid water evaporates and escapes into the air.
Though there are ways of determining hydrophilicity that utilize precise mathematical formulas, it is also possible to roughly determine how readily a surface absorbs water by making a few simple observations. If droplets of water remain formed on the surface, and roll off easily when the surface is angled towards the ground, the material is said to be particularly hydrophobic. If, on the other hand, water is absorbed into the surface and that surface becomes wet, it is hydrophilic.
In most cases, hydrophilicity refers to how readily a surface reacts with water molecules. It is also possible to measure the hydrophilicity of a surface when it comes into contact with other liquids. When examining the behavior of a surface with various liquids, its hydrophilicity for one liquid is not necessarily predicted by its behavior with another liquid. Understanding how hydrophilic a surface is can help engineers develop materials that show the proper amount of hydrophilicity for their intended use.
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