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May 2019

A Diversity of Applications for Contact Angle - Part Two

Last month I introduced the first half of a chapter in a book that I've been asked to write. Below is the second half of the chapter. Thank you for reading. If you have any feedback you feel prompted to share, you can contact me here.

You can read Part One here. Part Two continues with a detailed analysis of applications that benefit from contact angle.

Chapter Title: A Diversity of Applications for Contact Angle (Part Two)

11. A prominent player in the $88 billion global hair care industry measures wetting properties on hair to determine the optimal formulations of hair products. PDMS (polydimethylsiloxane) is a popular silicone polymer used to increase hydrophobicity. Conditioners that provide long-lasting hydrophobicity will reduce breakage, improve sheen, and permit hair to stay in a natural beautiful state for longer periods than products that provide short-term benefits. On your next night out, tell your date that their hair looks hydrophobically beautiful and see what their reaction is.

12. There are few products that could benefit in more ways from an increased scrutiny of and attention to wetting properties than the automobile. Nearly 10% of the $100 billion global coatings market is in automotive. Nanocoatings and other coatings that directly control wetting properties have and are becoming increasingly important to automotive engineers. The variety of applications for surface treatments in automotive are so varied, we will detail some of these separately below:

a. Hydrophobic glass coatings on windows improve visibility in the rain and improve wiper performance on windshields.

b. Hybrid coatings that are both hydrophobic and hydrophilic can reduce fogging on windows.

c. Advanced glass coatings that produce a thin film of nanoparticles can convert a window surface from one that behaves in the Wenzel state to one that behaves in the Cassie state – thus imbuing self-cleaning anti-fouling properties. A superhydrophobic clear nanocoating on a windshield, for example, will greatly improve visibility by allowing rain drops to simply blow off without the need to be wiped off. During heavy rain, the coating improves wiper performance.

d. Nanocoatings that provide icephobic properties to painted and treated surfaces such as body panels and wheels, improve a car’s performance, safety, and attractiveness.

e. Coatings based on Nano-Tio2 and nanosilica are used to promote self-cleaning properties on exterior car parts.

f. As referenced above in the section on oleophobic coatings for phones and tablets, automotive engineers are introducing larger and more critical displays that use touch-based HMI (human machine interfaces). These surfaces are treated to be anti-fingerprint and anti-microbial using coatings similar or the same as those used on smartphones and tablets.

g. Additional anti-fingerprint nanocoatings are being developed for door handles, knobs, controls, and other high traffic plastic, metal, and painted components in and on an automobile.

h. Optical components such as rearview camera lenses and review mirrors are treated with coatings optimized to be anti-reflective, hydrophobic, and self-cleaning.

i. The clearcoat, or final coat of a multilayer automotive coating system, is optimized to provide anti-scratch properties. Hard nanoparticles (e.g., TiO2, ZrO2, and SiO2) are used to improve performance in harsh environments where mechanical abrasions and severe weather can tax traditional topcoats.

j. Nanostructured ceramic coatings are used on engine components to reduce friction, improve wear, and reduce energy consumption.

k. Nanocoatings are also being used and developed to provide enhanced UV protection. ZnO, TiO2, and CeO2 are added to paint to produce coatings with greater UV resistance as well as add metallic luster.

l. If your kids eat ice cream in the back seat of your new car, you may appreciate new omniphobic coatings designed to extend the life and appearance of your car interior.

13. A prominent leader in the US paint market wanted to better understand the wetting properties of bricks and other masonry products – both before and after surface treatments are applied. Since the materials are absorbent, it’s not possible to use conventional sessile drop contact angle measuring techniques. A captive bubble technique was used to measure contact angle. The small coupon of the material is submerged in a large cuvette filled with water and allowed to become fully saturated. Then, using an inverted needle, a bubble of air is released on the underside of the substrate. The resulting contact angle is then measured. The captive bubble technique is also useful for measuring wetting on surfaces that exhibit extremely low contact angle using the traditional method – e.g., soft contact lenses made of hydrogels.

14. Aviation presents engineers with special challenges. It’s enough to get a big piece of metal off the ground and flying safely at 500 mph (800 km/h). It’s another challenge, however, to fly these big birds in the winter. Traditionally, de-icing liquids are used prior to take off. But these can present environmental problems when the liquids end up in the ground water. Current research encourages the exploration of multi-scale roughness – that is, features at both the micro and nano scale. Such superhydrophobic surfaces are also excellent at repelling ice buildup which can affect performance and safety of aircraft. Observation of penguins have led some researchers to consider biomimicry. You don’t see penguins walking around with chunks of ice stuck to their wings. If we can make airplane wings as ice-free as penguin wings, now that’s good engineering (even if penguins can’t fly).

15. In the past 30 years, significant advances have been made in controlled drug delivery systems. Increasingly, pharmaceutical scientists are interested in surface energy and wetting properties. One US-based drug developer controls the surface characteristics of compressed tablets by controlling the concentration of potassium salt such as poly(maleic acid-alt-octadecene). Dissolution rates can be tightly controlled by precisely tweaking and regulating the proportion of polymeric material. There is also a relationship between dissolution release rates and wetting properties. As the surface of a tablet is made increasingly hydrophobic, the release time increases. Drug developers are tightly controlling the concentration of polymers in drug capsules in order to control the drugs release mechanisms – anywhere from immediate release to extended long-term release. Future drug delivery development will require not only a greater understanding of wetting properties, but also overcome limits set by human physiology.

16. A supplier for one of Detroit’s big three automakers is tasked with making plastic trim parts that will adhere to various surfaces – including other plastic surfaces. Polyolefins, in particular, are naturally hydrophobic. By clearly understanding the surface energy and wetting properties of the materials before and after treatment, engineers can better develop processes that improve wetting and adhesiveness. Corona discharge is a popular method for improving adhesion in packaging and printing applications. Electrons are shot onto the surface which disrupt the long homogenous molecular chains which greatly improves wetting behavior and adhesion. Flame treatment is also used to increase surface energy on polyolefin surfaces. However, due to the large number of parameters that must be precisely controlled, it can be challenging to obtain optimal settings. Contact angle can be used to monitor and tweak the setup values to ensure excellent bonding.

17. A federal government contractor specializing in military apparel received a bid from the US Army to develop fabric that can be used to fabricate army fatigues that are water repellent. For some years now, coatings like NeverWet have been commercially available that can be used to make many types of surfaces superhydrophobic. However, NeverWet can discolor the material and after some wear and washings, the coating loses its effectiveness. The army is looking for a more permanent and durable solution. Imagine you’re a soldier who gets stuck in a downpour and then needs to spend the next 12 hours in a manhole in freezing temperatures. If you can imagine, it’s a lot more endurable if you can keep dry. Researchers at MIT and Luna Innovations are developing technologies for making army fatigues omniphobic by creating a coating that has both micro-level as well as nano-level surface topology. In addition to being super water repellant, efforts are underway to tweak the coating to also make the fabrics antimicrobial, flame resistant, odor-reducing, and able to withstand multiple washings.

18. The Atacama Desert in northern Chile is the driest place on earth. Some parts receive less than one tenth of an inch of rain per year. Researchers have built fog nets that harvest water from the fog of ocean winds. These nets have both hydrophobic and hydrophilic materials. The operation of these nets mimics the behavior of the Namid Desert Beetle which captures droplets of fog on hydrophilic bumps on its back. These small drops then travel down hydrophobic troughs on its back and into the beetle’s mouth. In the spirit of biomimicry, these fog catchers are like super large Namid beetles. The water that runs off can be used for human use or to water crops that otherwise would not grow in such an inhospitable environment.

19. Biomaterials consist of manmade materials that are optimized to be compatible with and friendly to living cells – typically in humans. Contact lenses, dental implants, breast implants, and medical devices are all examples of products that need to be biocompatible. Researchers have discovered that attention to wetting properties is important in developing and enhancing biomaterials. In general, if a material’s wettability can be improved (and thus result in a lower contact angle), then biocompatibility improves. However, there are exceptions. Devices that come in contact with blood, for instance, if they are excessively hydrophilic, can cripple cell-cell interactions. In the case of tissue engineering, a variety of hydrophobic and hydrophilic surfaces are necessary in order to maximize biocompatibility. In many cases, contact angle can be used to predict the long-term performance of biomaterials. However, this is one area where additional research is necessary to better understand the relationship between wetting properties and biocompatibility.

20. Commercial office buildings and solar farms are both covered in large sheets of glass. These surfaces are prime candidates for wetting optimization. By making solar panels highly hydrophobic, they can be more self-cleaning and thus are less expensive to maintain and more efficient after rain storms. In the case of commercial buildings, the self-cleaning aspect is also important. However, additional technologies are being developed using chromogenic materials that will adjust according to temperature (thermochromic), radiation (photochromic), or by an electrical stimulation (electrochromic). These so-called smart windows can greatly improve the comfort of the occupants, lower heating and cooling costs, and lower cleaning costs as well. Developments in thin film technology increasingly rely on contact angle to enhance their self-cleaning and anti-pollution benefits.

The above list represents only 20 applications for contact angle. The complete list is much longer and impossible to compile as many wetting research projects are being conducted in private labs for products that are being developed under the cloak of secrecy. Nonetheless, it’s obvious from the small sampling in this chapter, that the impact of and appreciation for a better understanding of wetting phenomena drives researchers to better understand how contact angle can measure improvements to a vast array of products in the physical world.

The measurement of contact angle extends beyond a single industry, discipline, or application. In fact, it could be argued that every business, every product, and every innovation that has relevance to the physical world depends on – either directly or tangentially – the wetting properties of materials. Curiously and despite its omnipresence in the world, contact angle is not much of a household term. In fact, most people have never heard of it, don’t understand why it’s important and have little grasp for why people would want to measure it so precisely.


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