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Below are some of the applications and industries that benefit from a better understanding of contact angle, surface energy, and surface tension:
Material scientists are interested in designing nanosurfaces with unique and controllable properties. In effort to mimic the Lotus leaf, superhydrophobic surfaces are being produced with very low wetting in order to produce high contact angle and provide a self-cleaning attribute. Lotusan paint is an example of a commercial product which mimics the Lotus leaf. Others are working on coatings for fabrics and glass which will reduce wetting and exhibit super-repellency. In addition to surfaces, scientists are interested in the building nanomaterials and metamaterials for use in colloidal and microfluidic systems.
Contact angle plays a critical role in determining wafer cleanliness and assessing the efficacy of HMDS, plasma cleaning, and other cleaning, coating, etching, and bonding processes used in the production of semiconductor wafers. The ramé-hart Model 400 was designed specifically for the semiconductor industry.
Textile & Fiber
The importance of contact angle measurement in the textile and fiber industry need not be overstressed. Cotton yarn is usually wetted by water, but synthetic fabrics have definite contact angle for water. Nylon, for example, gives a contact angle of about 40 degrees. Fabrics must be coated, therefore, with suitable wetting agents. Otherwise, it will be difficult to remove dirt and soil while washing the fabric with water. But it is advantageous to use a non-wetting surface for a raincoat or umbrella cloth. Coating by a silicon polymer gives a highly non-wetting system. Waterproofing or water-repellency is an important industrial process, which depends on contact angle values.
Polymers and Plastics
Most of the polymers like polyethylene, polypropylene, Teflon, etc. show high contact angle behavior with many liquids. A number of applications of polymers have taken advantage of this situation. An example is the Teflon coated frying pan. Contact angle measurement has shown that the contact angle is about 35 degrees for cooking oil on Teflon; thus oil must not stick to the surface making it easier for cleaning. In many cases, however, the contact angle of polymers is too high for the application of inks and coatings. Thus surface modifications are performed to lower the contact angle and increase the surface energy in order to promote bonding and adhesion.
The efficiency of insecticide sprays also depends on their wetting behavior on the surface of insects. Usually with most insecticides, an organic liquid having a low surface tension is used as a spray so that it spreads completely. Contact angle is therefore an essential parameter to be considered in any pesticide or insecticide spray formulation.
Oil and Petroleum
An important technological application that emerged out of contact angle studies is in the enhanced oil recovery from sand beds. Laboratory experiments on displacing petroleum by water in glass capillaries have demonstrated that a considerable fraction of the oil remains attached to the wall when the central space of the capillary is already filled with water. In a sand column, the amount of oil remaining in the sand when water appeared at the downstream end of the column is directionally proportional to the contact angle. Flooding the oil wells with surfactants along with water or steam reduces the pressure drop across each oil-water meniscus, reduces the oil-water interfacial tension and changes the contact angle so that water displaces oil at the liquid-solid interface. The process is called ‘tertiary oil recovery’ and it is now possible by this method to recover more than 90% of oil from an oil well.
Hard Disk Drives
The processes involved in manufacturing a magnetic hard disk and the magnetic head slider are sensitive to surface cleanliness as well as surface wettability as it applies to loading the surface with a lubricant. Contact angle and surface energy play a critical role in helping engineers design and inspect hard drive products that overcome stiction, reduce flying height, and increase product reliability.
Understanding of contact angle is also important in the mining industry. An example is froth flotation, the process of concentrating minerals of heavy metals (e.g., zincblende, galena, chalcopyrite, etc.). This is achieved by agitating finely divided minerals in froth of water and air, so that some float and others sink. The useful minerals become attached to the air bubbles, rise with them into the froth layer and are collected. If the contact angle between the particle-water-air interface is smaller, the particle does not float easily. Therefore this contact angle is increased to as much as 60 degrees by adding ‘collectors’ like alkyl xanthanates (e.g., CH3.O.CS.Sna).
In heat exchangers and condensers used in chemical industries, maximum efficiency is achieved if the metal surface is coated by a non-wetting agent like calcium stearate or oleic acid. Liquids condensed on such coated walls form droplets because of high contact angle and fall down easily. This technique is called 'dropwise’ condensation. Contact angle measurement is important in controlling droplet surface area to increase or decrease evaporation and to optimize wetting of surfaces in heat exchangers.
Another area where contact angle plays an important role is in detergency, the process of cleaning clothes, etc., by a surface-active agent. If the soiled cloth is to get rid of dirt, grease, oil, etc., it is necessary that water must spread and penetrate the dirt particles. If spreading is to occur, the interfacial tension between solid-liquid and liquid vapor must be as small as possible. What the detergent does is lower these interfacial tensions by the process of adsorption. Contact angle measurements of lotions, oils, soaps and other toilet preparations on human skin and hair strands provide valuable information for their correct formulation and are now routine.
Knowledge of contact angle behavior of liquid metals on metal and oxide surfaces is essential in understanding the process of soldering, brazing, tinning and heat transfer. Contact angle is used to characterize surface roughness, oxidation, and wetting behavior for all types of metals and metal finishing.
Plating and Printing and Inks
Successful plating requires clean surfaces. Contact angle predicts adhesion and is used for quality assurance testing on surfaces that are be plated. New ASTM Standards (e.g., D7490 and D7334) use contact angle to determine wettability. Wetting is important to adhesion and the prevention of surface problems such as cratering, dewetting, and crawling. While wetting is important to adhesion, excessive wetting can cause inks to bleed and fail to perform properly. In the manufacture of inks, the contact angle formed by a drop of ink on paper determines the printing quality. It has been practically observed that it must be ideally between 90 degrees and 110 degrees. If it’s less than 90 degrees, the ink will spread on paper. If it’s more than 110 degrees, breaks will occur while printing.
Studies and measurements of contact angles of liquids on solids have great technological importance. This is especially true with water; every action of water on earth is controlled by its wetting behavior with the solid with which it comes into contact. For example, contact angle of water on our skin is about 90 degrees. If it were zero, water could have penetrated the pores of the skin and possibly been absorbed by blood. The structure of a bird’s feather is constructed such that the contact angle of water on a typical feather is as high as 150 degrees. Machines such as Lathes are usually coated with a thin layer of grease or oil especially during transportation to prevent corrosion by water. This is achieved by the fact that a grease or oil layer forms a highly non-wetting system with water.
Other applications of contact angle measurements are in the field of adhesives, lubricants, surface treatments, polymers, biomedicine, etc. Many more specific examples can be cited, but from those discussed already, it is clear that contact angle plays a significant role in a number of manufacturing industries. For further reading:
Chemistry of Surfaces, 3rd Ed, Arthur W. Adamson, Wiley-Interscience, New