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|A Diversity of Applications for Contact Angle - Part One|
Recently I was solicited to write a chapter for a new book that will be coming out on contact angle. Below is my unedited draft copy of the first half of the chapter for you to enjoy and provide feedback on if you so feel prompted. The second half will follow next month. Thanks for reading.
Chapter Title: A Diversity of Applications for Contact Angle (Part One)
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.
In this chapter, we will attempt to explore some of the common and perhaps some less common applications that call for a better understanding of wetting properties. In most cases, company and product names will be omitted in an effort to comply with non-disclosure agreements protecting proprietary and confidential information. In short, all of the information detailed in this chapter is available from public sources.
Here is our list of amazing applications for contact angle:
1. A manufacturer of patio umbrellas and awnings use a contact angle meter to measure the wetting properties of the fabrics and textiles used in their products after they have been treated with a coating that inhibits wetting. The desire is to achieve a contact angle as high as possible – typically in the hydrophobic range, but superhydrophobic is even better. The goal is to produce patio gear that repels rain rather than absorb it. This keeps the fabric from getting soiled; it keeps the product lighter (less stress on supports); and it adds a self-cleaning capability when the drops that roll off pick up contaminants on the surface and take them with them for a ride to the ground.
2. A large beer company purchased a contact angle goniometer in order to measure the cleanliness of beer bottles after they were filled and washed but prior to the application of the label. The effort was to ensure good adhesion between the bottle and the label. By strict attention to the contact angle, process engineers were able to develop the most cost-effective wash and rinse cycles in an effort to use the least amount of water resources and detergent and still clean the bottle sufficiently for good label adhesion. Adhesion is a popular application for contact angle.
3. Advances made in solar panel technology have been strident in the past decade. By developing coatings for the panels that are hydrophobic, engineers have been improving the self-cleaning capabilities. As rain hits the panels, the drops roll off taking with them dust and other contaminants which lower the efficiency of the panel. Lowering the surface energy increases efficiency while also lowering the maintenance costs.
4. Dental bonding is required for a strong micromechanical and chemical coupling between man-made restorative materials (like fillings) and the bony tissue below tooth enamel called dentin. Low contact angle, that is, excellent wetting, is one of the requirements for good bonding in these cases. Etching methods are used to ensure cleanliness and high surface energy of the dentin surfaces to be bonded. Developers of bonding agents and methods are sensitive to the wetting requirements while balancing additional requirements such as biocompatibility, ease of use, cost, curing time, long-term durability, debonding, and microleakage reduction. The next time you find yourself in a dental chair, rest assured that the developers of the adhesions and methods that your dentist are using have been tested for optimal wetting performance.
5. Recently, the Bill & Melinda Gates Foundation issued a challenge to reinvent the toilet – offering grants and seeking partners in finding a better flush. Today nearly 5 billion people on our planet do not enjoy safe sanitation. The goal of this challenge is to invent a toilet that can be operated at low cost in areas of the world that do not have sewer infrastructure. More than one of the selected 16 recipients of these research grants have been exploring durable superhydrophobic coatings that will allow waste to funnel and travel through a system with little or no flush water. At the 2018 Toilet Expo, for example, Dr. Alison Parker of Cranfield University (UK), showcased a Nanomembrane Toilet which utilizes a novel waterless flushing method. After the solids are extracted, the residual liquid is heated and purified via a hydrophobic membrane.
6. Pesticides work best if they hit the plant and don’t roll or run off and enter the water supply. Scientist are working on formulations that produce small drops which wet the plant leaves with minimal runoff. Contact angle is used to characterize plant wetting properties. Then formulations are developed using surfactants to optimize the surface tension of the pesticides in order to achieve best results for a particular plant/pesticide combination. In addition, metrics like roll-off angle are important to understanding wetting and product application requirements.
7. Chipmaking is the most involved and complex manufacturing process in the world today. Current immersion lithography technology prints lines under 40 nanometers wide and on up to 40 or 50 layers in material that is only 1 micron thick. A basic processor for a lowly PC can have over 50 million transistors. This accomplishment is short of miraculous. But it could not be done without contact angle. At various stages of the chipmaking process, wetting tests are performed on the wafers using pure deionized water in order to judge the efficacy of the prior cleaning step in the process. Only through meticulous testing and cleaning can wafers make it through the myriad processes with zero or near zero defects.
8. Hydrogel contact lenses are fabricated from complex polymeric matrices that contain hydrophobic and hydrophilic ends. If a contact lens dries out, the hydrophilic ends migrate toward the middle of the lens in search of water while the hydrophobic ends turn away from the center toward the dry periphery. A surfactant is necessary to rewet a dried-out lens while biocides are also added in order to provide antimicrobial protection. Makers of contact lenses and solutions are keen on understanding wetting properties of their products in an effort to improve performance, comfort, safety, and biocompatibility.
9. Paint manufacturers use contact angle to develop new and improve existing formulations that greatly increase hydrophobicity. The aim is to develop coatings so water repellant that rain not only rolls freely off the painted surface, but that the drops take with them any dust or contaminants that are on the surface. This self-cleaning property can more than offset the higher cost of the paint or coating. A product called Lotusan has been on the market for nearly two decades. In addition to providing a highly hydrophobic surface, such coatings need to be durable, easy to apply, weather resistant, and resistant to cracks. A suburb of Hamburg, Germany called St. Pauli and the home of Hamburg’s red light district has successfully used hydrophobic paint on lower exterior walls to counter the wildpinklers, or public urinators. When one pees on a wall, it deflects back onto their shoes. Now that’s karma.
10. Hydrophobic means afraid (phobia) of water (hydro), or water repelling. A surface that repels oils, like those found on your hands, is said to be oleophobic. In a world that has become increasingly dependent on devices with touch screens such as tablets, smartphones, and control screens in cars, and now kiosks at fast food restaurants and airports, it has become increasingly important to keep these screens clean and clear. The screens on early devices were treated using a vacuum and vapor thin film deposition process. This thin layer of vaporized material is vacuum sealed to the glass. An evaporative oleophobic layer is then applied and vaporized. This final layer forms a chemical bond with the first. The downside is that over time, the surface can wear off. This is particularly evident on earlier iPhones. Today a variety of layer-by-layer deposition methods are being used and developed that create a rugged superoleophobic surface while remaining hydrophilic. One such method uses a polyelectrolyte-fluorosurfactant complex which promotes a superoleophobic condition in a Cassie state while the water contact angle, in a Wenzel state, is under 5°. Look for more novel methods to be introduced in the near future that tackle this challenge.