View source for Hydrophobic / Hydrophilic

== Definition== 
Hydrophobic describes materials, molecules, or even parts of molecules which do not readily make contact with water. In contrast, Hydrophilic materials have an affinity for water.

A simple way to tell whether a certain solid is hydrophobic or hydrophilic is to place a drop of water on a flat piece of the solid. If the water beads up and has a contact angle <math>>90 ^\circ </math>, the solid is hydrophobic. If the water drop spreads out and touches the solid with a contact angle <math><90 ^\circ </math>, the solid is hydrophilic [1].

== Applications ==

*The lotus leaf is a naturally occurring hydrophobic surface. In the image below [ [http://commons.wikimedia.org/wiki/File:Indian_Lotus_%28Nelumbo_nucifera%29_leaf_I2_IMG_6266.jpg 2] ], the surfaces of the drops of water curve back in underneath the drops forming very high contact angles with the lotus leaf.

[[image:LotusLeaf2.jpg]]

*An example of utilizing hydrophobic and hydrophilic properties in academic research appears in the 1999 article [[Mesoscale Self-Assembly of Hexagonal Plates Using Lateral Capillary Forces: Synthesis Using the “Capillary Bond”| ''Mesoscale Self-Assembly of Hexagonal Plates Using Lateral Capillary Forces: Synthesis Using the "Capillary Bond" '']]. The researchers, Ned Bowden, Insung S. Choi, Bartosz A. Grzybowski, and George Whitesides, construct hydrophobic and hydrophilic surfaces to achieve self-assembly through capillary forces.

*Surfactant molecules found in soap have hydrophilic heads and hydrophobic tails. The combination of hydrophilic and hydrophobic parts makes surfactants useful for cleaning [1].

*Innovative materials make use of hydrophilicity and hydrophobicity for applications such as water-repellent coatings and self-cleaning textiles and glass. This [http://news.nationalgeographic.com/news/2003/02/0227_030227_lotusmaterial.html National Geographic article] describes the development of one such textile [4].

== References ==
[1] de Gennes, P., Brochard-Wyart, F. & Quere, D. "Capillarity and Wetting Phenomena," Springer Science+Business Media, LLC, New York (2004).

[2] Garg, J.M. http://commons.wikimedia.org/wiki/File:Indian_Lotus_%28Nelumbo_nucifera%29_leaf_I2_IMG_6266.jpg.

[3] Bowden, N., Choi, I.S., Grzybowski, B.A., & Whitesides, G.M. ''J. Am Chem. Soc.'' '''121''', 5373-5391 (1999).

[4] Roach, J. "New Water-Repellent Material Mimics Lotus Leaves," National Geographic (23 Feb 2003).  http://news.nationalgeographic.com/news/2003/02/0227_030227_lotusmaterial.html