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What do "cohesion" and "adhesion" mean?

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"Cohesion" and "adhesion" are terms that refer to the interaction of molecules with one another and with other types of molecules. Cohesion is the attraction of molecules to one another, what makes them stick together and makes possible, when dealing with water molecules or the molecules of other liquids, a droplet. Adhesion is the attraction of molecules to other types of molecules, such as in the case of a water droplet hanging from the tip of a leaf.

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Chemistry and electricity are two factors that make a substance such as water so cohesive. The hydrogen and oxygen atoms in water molecules contain positive and negative charges that pull them together. The two hydrogen atoms in a water molecule are aligned on one side of the oxygen atom, leaving each molecule with a positive charge on one side and a negative charge on the other. Two molecules of water create a bond when the positive side of one attaches to the negative side of the other.

The three main types of adhesion between substances with different types of molecules are mechanical, chemical and dispersive. The most common type of these in surface adhesion is dispersive, and it occurs when substances are held together by "van der Waals forces," or the attraction of the dissimilar molecules because of positive and negative charges.

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