From Wikipedia, the freeencyclopedia Forother uses, see Hydrophobia (disambiguation).

  150 deg water contact angle on a surface modified using plasma technologysystem surface chemistryDew drop on a hydrophobic leafsurfac Cutting a water droplet using asuperhydrophobic knife on superhydrophobic surfaces Water drops on the hydrophobic surface of grass In chemistry, hydrophobicity (from the Attic Greek hydro,meaning water, and phobos,meaning fear) isthe physical property of amolecule (known as a hydrophobe) that is repelled from a mass of water.^[1]

  Hydrophobicmolecules tend to be non-polar and, thus, prefer other neutral molecules andnon-polar solvents.Hydrophobic molecules in water often cluster together, forming micelles.Water on hydrophobic surfaces will exhibit a high contactangle.

  Examples ofhydrophobic molecules include the alkanes, oils, fats, and greasysubstances in general. Hydrophobic materials are used for oil removal fromwater, the management of oil spills, and chemical separation processes toremove non-polar substances from polar compounds.

  Hydrophobic isoften used interchangeably with lipophilic,"fat-loving." However, the two terms are not synonymous. Whilehydrophobic substances are usually lipophilic, there are exceptions—such as the silicones and fluorocarbons.

  The hydrophobicinteraction is mostly an entropic effect originating from the disruption of highlydynamic hydrogenbonds between molecules ofliquid water by the nonpolar solute. By aggregating together, nonpolarmolecules reduce the surface area exposed to waterand minimize theirdisruptive effect.^[2] Thus, the two immiscible phases (hydrophilic vs.hydrophobic) will change so that their corresponding interfacial area will beminimal. This effect can be visualized in the phenomenon called phase separation.

  Mainarticle: Superhydrophobe A water drop on a Lotus plant leaf.

Superhydrophobic surfaces, such as the leaves of the lotus plant, arethose that are extremely difficult to wet. The contactanglesof a water droplet exceeds 150° and the roll-off angle is lessthan 10°.^[3] This is referred to as the Lotuseffect.

Theory

  In 1805, ThomasYoung defined the contact angle θ by analyzing the forces acting on a fluid dropletresting on a solid surface surrounded by a gas.

   A liquid droplet rests on a solid surface and is surrounded by gas. Thecontact angle, θ_C, isthe angle formed by a liquid at the three-phase boundary where the liquid, gas,and solid intersect.

  A droplet resting on a solid surface and surrounded by a gas forms acharacteristic contact angle θ. If the solid surface is rough, andthe liquid is in intimate contact with the solid asperities, the droplet is inthe Wenzel state. If the liquid rests on the tops of the asperities, it is inthe Cassie–Baxter state.

where

 = Interfacial tension between the solid and gas

 = Interfacial tension between the solid and liquid

 = Interfacial tension between the liquid and gas

θ can be measured using a contactangle goniometer.

  Wenzel determinedthat when the liquid is in intimate contact with a microstructured surface, θ willchange to θ_W* where r is theratio of the actual area to the projected area. Wenzel's equation shows that microstructuring asurface amplifies the natural tendency of the surface. A hydrophobic surface(one that has an original contact angle greater than 90°) becomes morehydrophobic when microstructured – its new contact angle becomes greater thanthe original. However, a hydrophilic surface (one that has an original contactangle less than 90°) becomes more hydrophilic when microstructured – itsnew contact angle becomes less than the original. Cassie and Baxter found that if the liquid issuspended on the tops of microstructures, θ will change to θ_CB*: where φ is thearea fraction of the solid that touches the liquid. Liquid in the Cassie–Baxter state is more mobile thanin the Wenzel state.

  We can predictwhether the Wenzel or Cassie–Baxter state should exist by calculating the newcontact angle with both equations. By a minimization of free energy argument,the relation that predicted the smaller new contact angle is the state mostlikely to exist. Stated in mathematical terms, for the Cassie–Baxter state toexist, the following inequality must be true.

  A recentalternative criterion for the Cassie–Baxter state asserts that theCassie–Baxter state exists when the following 2 criteria are met: 1) Contactline forces overcome body forces of unsupported droplet weight and 2) Themicrostructures are tall enough to prevent the liquid that bridgesmicrostructures from touching the base of the microstructures.

  A new criterionfor the switch between Wenzel and Cassie-Baxter states has been developedrecently based on surface roughness and surface energy. The criterion focuses on the air-trapping capabilityunder liquid droplets on rough surfaces, which could tell whether Wenzel'smodel or Cassie-Baxter's model should be used for certain combination ofsurface roughness and energy.

  Contact angle is ameasure of static hydrophobicity, and contact angle hysteresis and slide angleare dynamic measures. Contact angle hysteresis is a phenomenon thatcharacterizes surface heterogeneity. When a pipette injects a liquid onto a solid, theliquid will form some contact angle. As the pipette injects more liquid, thedroplet will increase in volume, the contact angle will increase, but itsthree-phase boundary will remain stationary until it suddenly advances outward.The contact angle the droplet had immediately before advancing outward istermed the advancing contact angle. The receding contact angle is now measuredby pumping the liquid back out of the droplet. The droplet will decrease involume, the contact angle will decrease, but its three-phase boundary willremain stationary until it suddenly recedes inward. The contact angle thedroplet had immediately before receding inward is termed the receding contactangle. The difference between advancing and receding contact angles is termedcontact angle hysteresis and can be used to characterize surface heterogeneity,roughness, and mobility. Surfaces that are not homogeneous will have domainsthat impede motion of the contact line. The slide angle is another dynamicmeasure of hydrophobicity and is measured by depositing a droplet on a surfaceand tilting the surface until the droplet begins to slide. In general, liquidsin the Cassie–Baxter state exhibit lower slide angles and contact anglehysteresis than those in the Wenzel state.

Research and development

  The self-cleaningproperty of superhydrophobic micro-nanostructured surfaces was reported in 1977, and perfluoroalkyl and perfluoropolyether superhydrophobic materials were developed in 1986 for handlingchemical and biological fluids. Other biotechnical applications have emergedsince the 1990s.In recentresearch, superhydrophobicity has been reported by allowing alkylketene dimer (AKD) to solidify into a nanostructured fractalsurface. Many papers have since presented fabrication methodsfor producing superhydrophobic surfaces including particle deposition,sol-gel techniques,plasma treatments,vapor deposition, and casting techniques. Current opportunity for research impact lies mainlyin fundamental research and practical manufacturing.Debates have recently emerged concerning theapplicability of the Wenzel and Cassie–Baxter models. In an experiment designedto challenge the surface energy perspective of the Wenzel and Cassie–Baxtermodel and promote a contact line perspective, water drops were placed on asmooth hydrophobic spot in a rough hydrophobic field, a rough hydrophobic spotin a smooth hydrophobic field, and a hydrophilic spot in a hydrophobic field.Experiments showed that the surface chemistry andgeometry at the contact line affected the contact angle and contact anglehysteresis, but the surface area inside the contact line had no effect. Anargument that increased jaggedness in the contact line enhances dropletmobility has also been proposed.

  Many veryhydrophobic materials found in nature rely on Cassie'slaw and are biphasic on the submicrometer level with one component air.The Lotuseffect is based on thisprinciple. Inspired by it, a lot of functional superhydrophobic surfaces wereprepared.

  An example of a biomimetic superhydrophobic material in nanotechnology is nanopin film. In onestudy, a vanadiumpentoxide surface that can switchreversibly between superhydrophobicity and superhydrophilicity under the influence of UV radiation is presented. According to the study, any surface can be modifiedto this effect by application of a suspension ofrose-like V₂O₅ particles,for instance with an inkjet printer. Onceagain hydrophobicity is induced by interlaminar air pockets (separated by 2.1 nm distances). The UV effect is also explained. UV lightcreates electron-holepairs, with the holes reacting with lattice oxygen, creating surfaceoxygen vacancies, while the electrons reduce V⁵⁺ to V³⁺. The oxygen vacancies are met bywater, and it is this water absorbency by the vanadium surface that makes ithydrophilic. By extended storage in the dark, water is replaced by oxygen and hydrophilicity is once again lost.

Potential applications

  Active recentresearch on superhydrophobic materials might eventually lead to industrialapplications. For example, a simple routine of coating cotton fabric withsilica or titania particles by sol-gel technique has been reported, which protects thefabric from UV light and makes it superhydrophobic. Also, an efficient routinehas been reported for making polyethylene superhydrophobic and thus self-cleaning—99% ofdirt adsorbed on such surface is easily washed away. Patterned superhydrophobicsurfaces also have the promises for the lab-on-a-chip, microfluidic devices andcan drastically improve the surface based bioanalysis.^[