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According to [[thermodynamics]], matter seeks to be in a low-energy state, and bonding reduces chemical energy. Water is electrically polarized, and is able to form [[hydrogen bond]]s internally, which gives it many of its unique physical properties. But, since hydrophobes are not electrically polarized, and because they are unable to form hydrogen bonds, water repels hydrophobes, in favour of bonding with itself. It is this effect that causes the [[hydrophobic effect|hydrophobic interaction]]—which in itself is misleadingly named as the energetic force comes from the [[hydrophilic]] molecules.<ref>{{cite journal|last1=Goss|first1=Kai-Uwe|last2=Schwarzenbach|first2=René P.|title=Rules of Thumb for Assessing Equilibrium Partitioning of Organic Compounds: Successes and Pitfalls|journal=Journal of Chemical Education|volume=80|pages=450|year=2003|doi=10.1021/ed080p450|issue=4|bibcode = 2003JChEd..80..450G }}</ref> Thus the two immiscible phases (hydrophilic vs. hydrophobic) will change so that their corresponding interfacial area will be minimal. This effect can be visualized in the phenomenon called [[phase (matter)|phase]] separation.
 
==Research and development==
The self-cleaning property of superhydrophobic micro-[[nanotechnology|nanostructured]] surfaces was reported in 1977,<ref name=Barthlott1977>{{cite journal|first = Wilhelm|last = Barthlott|coauthors = Ehler, N.|year = 1977|title = Raster-Elektronenmikroskopie der Epidermis-Oberflächen von Spermatophyten|journal = Tropische und subtropische Pflanzenwelt|volume = 19|publisher = Akad. Wiss. Lit. Mainz|pages = 110}}</ref> and perfluoroalkyl and perfluoropolyether [[superhydrophobic]] materials were developed in 1986 for handling chemical and biological fluids. Other biotechnical applications have emerged since the 1990s.<ref name=Barthlott1997>{{cite journal |last = Barthlott|first = Wilhelm|coauthors = C. Neinhuis|year = 1997|title = The purity of sacred lotus or escape from contamination in biological surfaces|journal = [[Planta]]| volume = 202|pages = 1–8 |doi = 10.1007/s004250050096}}</ref><ref name=Cheng2005>{{cite journal|title=Is the lotus leaf superhydrophobic?|journal = [[Appl. Phys. Lett.]]| year = 2005|volume = 86|issue = 14|pages = 144101|author = Cheng, Y. T., Rodak, D. E.|doi=10.1063/1.1895487|bibcode=2005ApPhL..86n4101C}}</ref><ref name=Narhe2006>{{cite journal|title = Water condensation on a super-hydrophobic spike surface|journal = [[Europhys. Lett.]]|year = 2006|volume = 75|issue = 1|pages = 98–104|author = Narhe, R. D., Beysens, D. A.|doi = 10.1209/epl/i2006-10069-9|bibcode=2006EL.....75...98N}}</ref><ref>{{Cite web|title = Mimicking nature: Physical basis and artificial synthesis of the Lotus effect|author = Lai, S.C.S.|url = http://members.ziggo.nl/scslai/lotus.pdf}}</ref><ref name=Koch2008>{{Cite journal|author = Koch, K.|coauthors = Bhushan, B. & Barthlott, W.|year = 2008|title = Diversity of structure, Morphology and Wetting of Plant Surfaces. Soft matter|doi = 10.1039/b804854a|journal = Soft Matter|volume = 4|pages = 1943|issue = 10}}</ref>
 
In recent research, superhydrophobicity has been reported by allowing alkylketene [[Dimer (chemistry)|dimer]] (AKD) to solidify into a nanostructured fractal surface.<ref>{{cite journal |first=T. |last=Onda |last2=Shibuichi |first2=S. |last3=Satoh |first3=N. |last4=Tsujii |first4=K. |title=Super-Water-Repellent Fractal Surfaces |journal=[[Langmuir (journal)|Langmuir]] |volume=12 |pages=2125–2127 |year=1996 |doi=10.1021/la950418o |issue=9}}</ref> Many papers have since presented fabrication methods for producing superhydrophobic surfaces including particle deposition,<ref>{{cite journal |first=M |last2=Nakajima |last=Miwa |first2=Akira |last3=Fujishima |first3=Akira |last4=Hashimoto |first4=Kazuhito |last5=Watanabe |first5=Toshiya |title=Effects of the Surface Roughness on Sliding Angles of Water Droplets on Superhydrophobic Surfaces |journal=[[Langmuir (journal)|Langmuir]] |volume=16 |pages=5754–60 |year=2000 |doi=10.1021/la991660o |issue=13}}</ref> sol-gel techniques,<ref>{{cite journal |first=NJ |last=Shirtcliffe |last2=McHale |first2=G. |last3=Newton |first3=M. I. |last4=Perry |first4=C. C. |title=Intrinsically superhydrophobic organosilica sol-gel foams |journal=[[Langmuir (journal)|Langmuir]] |volume=19 |pages=5626–5631 |year=2003 |doi=10.1021/la034204f |issue=14}}</ref> plasma treatments,<ref>{{cite journal |first=DOH |last=Teare |last2=Spanos |first2=C. G. |last3=Ridley |first3=P. |last4=Kinmond |first4=E. J. |last5=Roucoules |first5=V. |last6=Badyal |first6=J. P. S. |last7=Brewer |first7=S. A. |last8=Coulson |first8=S. |last9=Willis |first9=C. |title=Pulsed plasma deposition of super-hydrophobic nanospheres |journal=[[Chemistry of Materials]] |volume=14 |pages=4566–4571 |year=2002 |doi=10.1021/cm011600f |issue=11}}</ref> vapor deposition,<ref>{{cite journal |first=M |last2=Nakajima |last=Miwa |first2=Akira |last3=Fujishima |first3=Akira |last4=Hashimoto |first4=Kazuhito |last5=Watanabe |first5=Toshiya |title=Effects of the surface roughness on sliding angles of water droplets on superhydrophobic surfaces |journal=[[Langmuir (journal)|Langmuir]] |volume=16 |pages=5754–5760 |year=2000 |doi=10.1021/la991660o |issue=13}}</ref> and casting techniques.<ref>{{cite journal |first=J |last2=Marzolin |last=Bico |first2=C |last3=Quéré |first3=D |title=Pearl drops |journal=[[Europhysics Letters]] |volume=47 |pages=743–744 |year=1999 |doi=10.1209/epl/i1999-00453-y |issue=6 |bibcode=1999EL.....47..743B}}</ref> Current opportunity for research impact lies mainly in fundamental research and practical manufacturing.<ref>{{cite journal |first=C |last=Extrand |title=Self-Cleaning Surfaces: An Industrial Perspective |journal=[[MRS Bulletin]] |pages=733 |year=2008}}</ref> Debates have recently emerged concerning the applicability of the Wenzel and Cassie–Baxter models. In an experiment designed to challenge the surface energy perspective of the Wenzel and Cassie–Baxter model and promote a contact line perspective, water drops were placed on a smooth hydrophobic spot in a rough hydrophobic field, a rough hydrophobic spot in a smooth hydrophobic field, and a hydrophilic spot in a hydrophobic field.<ref>{{cite journal |first=LC |last=Gao |title=How Wenzel and Cassie Were Wrong |journal=[[Langmuir (journal)|Langmuir]] |volume=23 |pages=3762–3765 |year=2007 |doi=10.1021/la062634a |pmid=17315893 |last2=McCarthy |first2=TJ |issue=7}}</ref> Experiments showed that the surface chemistry and geometry at the contact line affected the contact angle and contact angle hysteresis, but the surface area inside the contact line had no effect. An argument that increased jaggedness in the contact line enhances droplet mobility has also been proposed.<ref>{{cite journal |first=W |last2=Fadeev |last=Chen |first2=Alexander Y. |last3=Hsieh |first3=Meng Che |last4=Öner |first4=Didem |last5=Youngblood |first5=Jeffrey |last6=McCarthy |first6=Thomas J. |title=Ultrahydrophobic and ultralyophobic surfaces: Some comments and examples |journal=[[Langmuir (journal)|Langmuir]] |volume=15 |pages=3395–3399 |year=1999 |doi=10.1021/la990074s |issue=10}}</ref>
 
Many very hydrophobic materials found in nature rely on [[Cassie's law]] and are [[Phase (matter)|biphasic]] on the submicrometer level with one component air. The [[Lotus effect]] is based on this principle. Inspired by it, a lot of functional superhydrophobic surfaces were prepared.<ref>{{cite journal |doi=10.1142/9789812772374_0013 |first=S.T. |last2=Liu |last=Wang |first2=Huan |last3=Jiang |first3=Lei |title=Recent process on bio-inspired surface with special wettability |journal=[[Annual Review of Nano Research]] |volume=1 |pages=573–628 |year=2006}}</ref>
 
An example of a [[bionics|biomimetic]] superhydrophobic material in [[nanotechnology]] is [[nanopin film]]. In one study a [[vanadium pentoxide]] surface is presented that can switch reversibly between superhydrophobicity and [[superhydrophilicity]] under the influence of UV radiation.<ref>''UV-Driven Reversible Switching of a Roselike Vanadium Oxide Film between Superhydrophobicity and Superhydrophilicity''Ho Sun Lim, Donghoon Kwak, Dong Yun Lee, Seung Goo Lee, and Kilwon Cho [[J. Am. Chem. Soc.]]; '''2007'''; 129(14) pp. 4128–4129; (Communication) {{DOI|10.1021/ja0692579}}
</ref> According to the study any surface can be modified to this effect by application of a [[suspension (chemistry)|suspension]] of rose-like V<sub>2</sub>O<sub>5</sub> particles for instance with an [[inkjet printer]]. Once again hydrophobicity is induced by interlaminar air pockets (separated by 2.1 [[nanometer|nm]] distances). The UV effect is also explained. UV light creates [[electron-hole pair]]s, with the holes reacting with lattice oxygen creating surface oxygen vacancies while the electrons reduce V<sup>5+</sup> to V<sup>3+</sup>. The oxygen vacancies are met by water and this water absorbency by the vanadium surface makes it hydrophilic. By extended storage in the dark, water is replaced by oxygen and [[hydrophilicity]] is once again lost.
 
===Potential applications===
 
Active recent research on superhydrophobic materials might eventually lead to industrial applications. For example, a simple routine of coating cotton fabric with [[silica]]<ref>C-H Xue et al. "Preparation of superhydrophobic surfaces on cotton textiles" Sci. Technol. Adv. Mater. 9 (2008) 035008 [http://dx.doi.org/10.1088/1468-6996/9/3/035008 free download]</ref> or [[titanium dioxide|titania]]<ref>C-H Xue et al. "Superhydrophobic cotton fabrics prepared by sol–gel coating of TiO2 and surface hydrophobization" Sci. Technol. Adv. Mater. 9 (2008) 035001 [http://dx.doi.org/10.1088/1468-6996/9/3/035001 free download]</ref> particles by [[sol-gel]] technique has been reported, which protects the fabric from UV light and makes it superhydrophobic. Also, an efficient routine has been reported for making [[polyethylene]] superhydrophobic and thus self-cleaning<ref>Z. Yuan et al. "Preparation and characterization of
self-cleaning stable superhydrophobic linear low-density polyethylene" Sci. Technol. Adv. Mater. 9 (2008) 045007 [http://dx.doi.org/10.1088/1468-6996/9/4/045007 free download]</ref>—99% of dirt adsorbed on such surface is easily washed away. Patterned superhydrophobic surfaces also have the promises for the lab-on-a-chip, microfluidic devices and can drastically improve the surface based bioanalysis.<ref name=Ressine2007>{{cite journal|author = Ressine, A.|coauthors = Marko-Varga, G., Laurell, T.|year = 2007|title = Porous silicon protein microarray technology and ultra-/superhydrophobic states for improved bioanalytical readout|journal = Biotechnology Annual Review|volume = 13|pages = 149–200|doi = 10.1016/S1387-2656(07)13007-6|pmid = 17875477}}</ref>
 
 
==Notas==