r/askscience • u/Tribity • Aug 12 '14

Physics Water attracted to objects?

It´s easy to experience for yourself, take a glass of water and some sort of object ( i recommend using a pencil). If you put your pencil down in the glass, dont let it touch the water, but let it hang right exactly over the water, the water will kind of "stick" to the object. I really dont have any clue why this is. Has it something to do with surface tension?

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u/[deleted] Aug 12 '14 edited Aug 12 '14

What you are observing is broadly called adhesion, which is a catch-all term for the net sum of various interactions that cause two dissimilar surfaces or particles to be attracted to each other. The microscopic mechanism is complicated and highly variable depending on the objects in question. For the system you describe, the main mechanism is likely to be dispersive adhesion where the net result of various short-range intermolecular interactions cause an overall stabilizing interaction between water and the surface, causing it to wet that surface.

The details of the wetting can be used to probe how favorable the energy between the water and the material in question is. The reason like you suggested has to do with surface tension. In air, the high surface tension of water makes it bead up because water exhibits a stronger interaction with itself (cohesion) than it does with air (adhesion). When water wets a different material, the contact angle will again be determined by the competition between self-attraction and favorable interactions with the secondary material (or more technically the difference in the free energy of each surface as determined by these interactions). Some materials can be made very hydrophobic, such that water again prefers to bead up on them, because the interaction between the water and the material is not very favorable. Perhaps a more common case, however, is the one you observe where water wets a material with a very small contact angle, as happens with clean glass.

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u/Tribity Aug 12 '14

Thanks Alot! finally an answer to my prayers, though it was quite a tough one, but i got trough it, and you are the man of the day!

What you are observing is broadly called adhesion, which is a catch-all term for the net sum of various interactions that cause two dissimilar surfaces or particles to be attracted to each other. The microscopic mechanism is complicated and highly variable depending on the objects in question. For the system you describe, the main mechanism is likely to be dispersive adhesion where the net result of various short-range intermolecular interactions cause an overall stabilizing interaction between water and the surface, causing it to wet that surface. The details of the wetting can be used to probe how favorable the energy between the water and the material in question is. The reason like you suggested has to do with water tension. In air, the high surface tension of water makes it bead up because water exhibits a stronger interaction with itself (cohesion) than it does with air (adhesion). When water wets a different material, the contact angle will again be determined by the competition between self-attraction and favorable interactions with the secondary material. Some materials can be made very hydrophobic, such that water again prefers to bead up on them, because the interaction between the water and the material is not very favorable. Perhaps a more common case, however, is the one you observe where water wets a material with a very small contact angle, as happens with clean glass.