Igniting Downstream Consumer Demand with “Goodness of Feel” for High-Touch Surfaces

Executive Summary

In conjunction with industry partners, Daikin has developed a unique way of characterizing surfaces with which consumers tend to have more positive and pleasing experiences, especially through the sense of touch. This new property has been designated “Goodness of Feel”, or GoF. By correlating this property with lab-measured surface characteristics, Daikin has also been able to develop Optool PFPE (perfluoropolyether) materials that use GoF to differentiate consumer products that are dependent on touch screens.

In the text that follows, Daikin aims to describe the process by which the idea of the GoF property was developed and how it can be correlated to other simple surface properties. By understanding the details of this story those familiar with hydrophobic coating technology can take advantage of this concept and utilize it to enhance the downstream value provided by their own products.

Key Terms Glossary

Keep-clean – A surface that is described as “keep-clean” has usually been treated with one of a few very specific brands of Daikin material. Optool PFPE nanocoatings are one such brand that generates a keep-clean surface. This means that the surface is extremely difficult to soil or smudge and thus it “keeps clean”. Think of this like the first line of defense against a dirty surface.

Easy-to-clean – An “easy-to-clean” surface is one that has also usually been treated with one of a few very specific brands of Daikin material. Optool PFPE coatings inherently generate an easy-to-clean surface. This means that a surface will allow for easy removal of contaminants that do happen to be deposited onto a surface. Think of this as an embedded second line of defense against a dirty surface.

PFPE – Perfluoropolyether polymer that forms the basis of Daikin’s Optool product line

Oleophobic – something that is resistant to oils

Hydrophobic – something that is resistant to water

Omniphobic – something that is resistant to both oil and water

GoF – Goodness of feel

CoF – Coefficient of friction

Omniphobic Coatings and Surfaces of Consumer Products

While anti-fingerprint coatings have been marketed by an assortment of companies for over a decade now, true 100% fingerprint repellency remains an elusive “holy grail”. That said, the journey towards fulfillment of the “anti-fingerprint” promise has led to useful advancements in coatings technologies over the years, particularly from fluoropolymers. Current state-of-the-art hydrophobic (“water fearing” or water repellent) and oleophobic (“oil fearing” or oil repellent) coatings – otherwise often referred to as omniphobic coatings when they possess both properties simultaneously – generate a “fingerprint mitigating” property on a surface from which it is significantly easier to clean fingerprints. Thus, these coatings have become indispensable to both the ophthalmic (eyewear) and the consumer electronics industries where smudge-free surfaces are required.

Omniphobic surfaces generated by one particular type of fluoropolymer, known as functional perfluoro polyethers (PFPEs, like Daikin’s Optool brand of materials), increase the fingerprint mitigation of coated surfaces by decreasing the adhesion strength between fingerprint sebum (a fancy way of saying fingerprint “smudges” or “gunk”) and a surface so that it is much less than the adhesion strength between the sebum and a human fingertip. This causes the sebum to preferentially stick to a human finger rather than an Optool-coated surface.

Because of the complex chemical nature of fingerprints, a surface designed to repel them must reduce the adhesion strength of both water-borne and oil-borne substances. Hydrophobic surfaces prevent the wetting of a surface by water. In other words, this means that water does not spread out on a hydrophobic surface, but rather forms little droplets or beads. A similar phenomenon is observed with oil and oleophobic surfaces. Combining both properties, omniphobic surfaces prevent water and oil from wetting and spreading by decreasing what is known as “surface free energy”.

Low surface energies coincide with a highly hydrophobic and oleophobic surface, which can be quantitatively defined as a surface presenting a high water contact angle (hydrophobic) and high oil contact angle (oleophobic). Cost-effective omniphobic coatings available today can decrease the surface energy enough to achieve a “WCA” (water contact angle) of up to 120 degrees and an “OCA” (oil contact angle) of up to 70 degrees. These high water and oil contact angles are the defining quantitative features of omniphobic surfaces that are effective at mitigating the transfer of fingerprint-sebum and similar smudges to a surface. This mitigating effect generates what Daikin refers to as keep-clean functionality. Figure 1 below shows an example of an Optool coated surface displaying this keep-clean functionality. Additionally, the right side of Figure 1 introduces keep-clean’s sister property, easy-to-clean, which is described in more detail in the next paragraph.

Figure 1 – Optool coated substrates possessing both keep-clean and easy-to-clean surface functionality

During the early adoption of keep-clean technology, companies in both the consumer electronics and ophthalmic industries were satisfied with the fact that the use of this technology made fingerprints much less likely to be transferred to the surfaces of their products. With this alone, they felt that they had improved the value of their products. However, another significant benefit to this technology that people quickly became aware of was the same technology’s facilitation of an easier smudge removal process should something be transferred in the first place. Enabling this easy removal process was the easy-to-clean functionality that people have now come to expect in conjunction with keep-clean. These critical surface properties for products with a need for optical clarity have contributed to the growth of a nearly $90 million market that is likely to continue to grow over the coming decade (Figure 2 below – Citation 1 from BIS Research Inc.).

Figure 2 – Consumer goods market size and surface area requiring omniphobic technology –BIS 2019 (data in reference 1 of the sources list was used to generated this graphic)

Smooth Tactile Sensations Generating “Goodness of Feel” – An Emergent Property

Today, keep-clean and easy-to-clean touch screens are the ubiquitous standard for gaming, drawing, and scrolling through endless social media feeds. Other tangential properties, however, have also begun to emerge as important alongside the traditional keep-clean and easy-to-clean functionalities. A novel, yet extremely important, surface property began undefined and ambiguous, but has since been termed “Goodness of Feel” or “GoF” for short. It can be described as the pleasant and smooth tactile sensation one experiences when running their finger over a surface. It turns out that this surface characteristic is actually imparted by the same material that has been used for easy-to-clean and keep-clean. Developing an understanding of why this happens to be the case is critical for taking advantage of it, and is the subject of much of the rest of this paper.

While not generally relevant to the ophthalmic (eyewear) industry – because people tend not to touch their lenses intentionally – GoF has a profound impact on a user’s perception of the quality of his or her experience while interacting with consumer electronic devices via the touch screens discussed in the previous paragraph. Tactile sensations are regularly experienced by smartphone and tablet users as they communicate with their devices using their fingertips. Eventually, electronics companies using Optool PFPE coatings on their products began to receive feedback from customers suggesting that they valued more than just durable keep-clean and easy-to-clean functionality on their touch screen surfaces. Users also shared an overwhelming preference for smooth and silky touch sensations from their screens.

Connecting GoF, CoF and Optool PFPE

Since the newly identified property of interest was surface-functional, Daikin was asked to get involved and lend expertise on phenomena generated by keep-clean and easy-to-clean surface treatments. Through value-chain collaboration between Daikin and the CE (consumer electronics) industry, the additional and yet previously undefined surface property imparted by omniphobic PFPE coatings began to take shape. In as early as 2012, initial studies of bare glass vs. OPTOOL PFPE coated glass made evident that in addition to generating omniphobic surfaces, Daikin’s Optool PFPE coatings were also producing a significantly lower Coefficient of Friction (CoF, not to be confused with GoF) on coated glass compared with bare glass. Figure 3 below summarizes the results of those early CoF studies.

Figure 3 – Coefficient of Friction of bare glass vs. Optool-coated glass

Although measuring CoF between a glass surface and an actual human finger proved difficult, measurements of CoF between common materials like paper, bare glass, and Optool-coated glass indicated CoF differences between these surfaces with a high degree of reproducibility. By mid-2012, the Daikin team was ready to put together a study to determine if the laboratory-measured property (CoF) could be used to predict the perceived GoF for a surface. Thereby, the team could create a quantitative method for predicting which surfaces consumers would be more likely to have positive experiences with.

Coefficient of Friction and “Goodness of Feel” – A New Way to Analyze Surface Properties

As the first study to link consumer preference for touch-screen surfaces (GoF) with the CoF of those surfaces, test material selection was critical for successfully understanding differences. The set of substrate materials not only needed to have relevance to the consumer electronics industry, but also had to span a wide range of CoFs. Several sample-types were prepared by applying hydrophobic coatings to chemically strengthened glass via methods typically employed by commercial applicators. In addition, aluminum foil and polymer-based cell phone screen protectors known to have significantly different CoFs were collected. To round out the sample set, commercially available glass replacement screens for consumer electronic devices were also included. Finally, participants from the Daikin family – people of different cultures, countries, genders, who may have different biases and perspectives – were recruited for the study.

Noted earlier, the hypothesis to be evaluated was whether CoF measured in the laboratory could be used to predict GoF (how good the surface would feel to human touch). More generally speaking, the team sought an understanding of how good or bad a surface might feel when making gestures and motions typical of those made while interacting with a smart electronic device. To make the connection between CoF and GoF it was necessary to measure the CoF values of the various sample types beforehand to first confirm that the substrates could be differentiated by CoF measurements. Human tactile preferences versus laboratory CoF measurements would then be compared once human judgements of the surfaces were aggregated and analyzed.

GoF from CoF – A Discussion of Results

Participants were recruited internally from Daikin America to build a pool of twenty-five “consumer participants” that would perform a blind “feel-test” on the collected surfaces. They were first shown examples of typical smart device interactions: swiping, scrolling, pinching, and zooming. Next, participants were asked to use the demonstrated physical interactions to touch and compare the different surfaces directly. Prior to final judgement, participants were told to consider each sample as if it were a smart device, but they were not told what any of the surfaces actually were at any point during their analysis.

After ceasing physical contact with the specimens the participants ranked, on a scale of 1 to 10, how pleasing each sample felt (Daikin had not yet come up with the GoF term). A score of “1” indicated that the surface felt awful, whereas a score of “10” meant the surface felt as pleasant as imaginable. Finally, and irrespective of their 1 to 10 ratings, the participants were asked general questions about how they would describe a surface that they would prefer to interact with.

As indicated in Figure 4 below, by using the combination of GoF scoring and CoF measurements the team’s hypothesis was confirmed: “Goodness of Feel” could be generally predicted by CoF measurements. Regardless of the exact substrate, a lower coefficient of friction tended to correlate with a higher goodness of feel. Relatively confident design decisions about the future direction of consumer electronics surfaces could and would start to be made on this basis of this relationship.

Figure 4 – Comparing GoF as indicated by consumers vs. CoF for various substrates (one shouldnot worry much about the specific numbers for substrates, but should pay attention to the fact that lower CoF basically always predicts a higher GoF)

In addition, survey questions answered by participants after assigning GoF values have helped to zeroin on the most critical components of CoF to consider in order to generate the greatest GoF. Figures 5 through 8 below corroborate the idea that a low CoF surface is clearly desirable, but more so in the kinetic rather than the static sense. All participants agreed that a smooth, almost slippery, surface was critical once their fingers were in motion, however, not as much importance was indicated for the smoothness felt when beginning fingertip motion on a surface (static coefficient of friction). Quantitative and qualitative ratings indicated consistent preference for “smooth” feeling surfaces with a significantly lower CoF, however, it was also pointed out that an overly “greasy” feeling surface would not be acceptable. Humans like a silky smooth interaction with their surfaces, but not if that smoothness occurs due to a film of slimy grease or oil – it does seem that there can be too much of a good thing in this case.

Further probing in the time since this initial work has revealed that “smoothness” is really akin to the elimination of the “slip-stick” phenomenon commonly experienced when rubbing fingers across a squeaky clean glass surface. The squeaks produced by “slip-stick” on a surface are due to mechanical vibrations in the glass created by the pulsation of fingers quickly catching onto and then releasing from the glass surface. A low enough CoF to eliminate “slip-stick”, but high enough to not be considered overly slippery or slimy, is really what end users are looking for from their touch surfaces.

Figure 5 – Surface Haptics Survey Questions Summary: Summarizes the study participants’ responses to survey questions about consumer electronics surfaces. The orange slice representsthe percentage of the population answering “yes” while the blue represents the percentage answering “no.”

A) Is a greasy feeling acceptable?

B) Do you care about the kinetic coefficient of friction (friction when your finger is already in
motion) when judging the GoF of a surface?

C) Do you care about the static coefficient of friction (friction felt when starting your finger
in motion from a full stop) when judging the GoF of a surface?

D) Do you care about the static coefficient of friction AND the kinetic coefficient of friction
when judging the GoF of a surface?

Future Direction

With the “Goodness of Feel” study in hand, Daikin has since worked to help consumer electronics companies generate more business by using the implications of what was learned. Touch screens are now specifically designed with GoF in mind, and Optool surface modifications for keep-clean and easyto-clean target maximizing GoF by their application as well. Daikin’s consumer electronics partners have been successful in subsequent years by taking advantage of this insight and improving preference for their products. Since the 2015 Daikin GoF evaluation, other organizations have corroborated its conclusions and have even made headlines at conferences like those put on by the Society for Information Display with similar reports.

Today, opportunities abound for using the CoF-GoF connection in other markets beyond consumer electronics, both in industries that already use Optool for keep-clean and easy-to-clean, and those that currently don’t. Markets heavily featuring metal and glass products might be the most obvious place for the extension of this technology, yet further studies are needed to elucidate how this could be of value for consumers of products in those markets. Touch screens are being used more frequently in the technological evolution of automotive, aerospace, and industrial applications, but will those screens have the same requirements for “Goodness of Feel” as the consumer electronics industry? An answer to this question will have to wait until those markets are ready to tackle this idea, but Daikin stands ready to provide guidance to applicators and end-users wherever and whenever they want to make these sorts of improvements to their products. If you are reading this and believe that your organization is ready to make that jump, please reach out to Daikin America, Inc. via our “Contact Us” page today!

Summary of Key Learnings

CoF (coefficient of friction) and GoF (goodness of feel) seem to be correlated and thus CoF targeting can be used to generate surfaces that have a higher GoF.

Omniphobic coatings that reduce the likelihood of having both water-borne and oil-borne materials stick to a surface are crucial to creating a fingerprint mitigating surface, they can also be designed for high GoF.

A 100% antifingerprint coating that is robustly durable does not exist today.

Daikin’s Optool materials not only create fingerprint mitigating surfaces via their keep-clean and easy-toclean functionalities, but can also increase a coated surface’s GoF.

Durable omniphobicity, easy-to-clean, keep-clean, and high GoF are possible thanks to the molecular structure of Optool coating materials.

Citations

BIS Research Inc. Anti-Fingerprint Coatings Report: https://bisresearch.com/industry-report/anti-fingerprint-coatings-market.html