“If your finger was the size of the earth, you could feel the difference between houses and cars”: This is how Swedish scientists describe the results of a survey on the human sense of touch, which was published in September 2013. The survey carried out as a collaboration between the Royal Institute of Technology in Stockholm (KTH), Stockholm University and the National Institute of Standards and Technology (NIST) provided new insights: People are capable of feeling tiny wrinkles in surfaces of 13 nm – up until now researchers were of the opinion that the limit of human perception was 1,000 nm. Mark Rutland, Professor for Surface Chemistry at the KTH and one of the conductors of the project, on nanometre fine surfaces, tactile spaces and movies for the fingers.
Mr. Rutland, how significant are the findings of your survey?
Mark Rutland: Let me give you a comparison using visual perception: It is as significant as if we had just found out how humans perceive colours. First of all, we measured and described how people feel things – in other words how the sense of touch physically works. Furthermore – and this is perhaps the most exciting part – we discovered that that we are capable of sensing a nanometre scale pattern with our almost millimetre structured finger. 13 nm correspond to the size of a very large molecule.
Could you briefly explain for non-scientists how the survey was carried out?
Mark Rutland: Scientists from two disciplines – surface chemistry and psychology – were involved in the project, so we carried out a “psycho-physical” survey. The test persons – 20 ladies aged between 21 and 32 – had to try and differentiate between different surfaces, the results were measured and compared. For this purpose, we initially produced 16 chemically identical surfaces, however the corrugation of the surfaces varied: The length of the waves ranged from 300 nm to 90 μm, the height of the waves from 7 nm to 4.5 μm. In addition, two non-corrugated, i.e. completely flat surfaces were produced.
The challenge was to make a family of surfaces that were identical in every respect except for their wrinkled structure – where the wavelength and amplitude were the parameters varied. Thus we could be sure that the participants were only sensitive to the topographical differences when they touched the surfaces, and no other differences – for example regarding the hardness or temperature.
How are such surfaces produced?
Mark Rutland: We stretched a polymer and stiffened its surface – as it relaxed, the difference in elasticity between the surface and the bulk led to a wavy pattern. The polymer was then used as a mould for an epoxy resin that was much more robust than the original polymer.
What was the next step?
Mark Rutland: The psychologists in the team organised a study where a number of participants were presented with a series of pairs of surfaces which they touched while blindfolded and were asked to put a number on the comparisons saying how similar they were. A large number of comparisons, involving repeats, and identical pairs were made and the results were fed into a statistical model. The smallest waves that could be differentiated between from a totally flat surface were 760 nm wide and 13 nm high.
The next job was to try to work out if we could explain the perceptual dimensions in terms of physical measurements – which we could and this was one of the reasons why the study was so important. For the first time ever we have been able to relate the perception or feel directly to physical properties
How do people feel things?
Mark Rutland: Friction determines what an object feels like: When a finger is moved over a surface, it senses vibrations. These vibrations are caused by the finger texture as it bounces over features on the surface. Depending on the particular surface, the vibrations are perceived in different ways. At the same time, the structure of the surface determines how much pressure we have to assert with our finger in order to achieve the optimum friction load. In the case of high friction surfaces, we have to assert little load and for low friction surfaces we have to assert a higher load.
And this correlation between the structure of the surface and the pressure asserted by the finger is measurable?
Mark Rutland: Precisely. We measured the pressure that the test persons asserted during the various tests using special devices and statistically evaluated the data obtained. Thus, in the end we were able to illustrate the correlation between the surface and the interaction in an accurate numerical model. These findings can in turn be used for the production of surfaces.
What impact will your findings have in practice? Can you state some concrete examples?
Mark Rutland: Of course, there are countless possibilities. For example, they are useful for the textile, furniture and entire cosmetics sectors – nappies, tissues, skincare products or shampoos that make the hair feel different. Surface structures and haptic aspects also play an important role in the paper and packaging industry. Furthermore, our findings are highly interesting for the robotic and artificial intelligence sectors, because they will help to further develop the “artificial sense of touch”. Last, but not least they will assist the development of smartphones or other touchscreen products. We will soon be able to produce touchscreens, which can be operated in a completely different manner. We would vibrate the screen to mimic the vibrations associated with texture which would give the impression of tactile differences. Thus, one could design part of a touchscreen so that it feels different to the rest of the surface when the device vibrates. This is extremely useful among others when it comes down to developing products for people who have impaired sight or hearing.
Will it soon be possible to manufacture imitation materials that have exactly the same tactile properties as other materials, i.e. wood, velvet or terracotta?
Mark Rutland: In the first instance, we will be able to create surfaces that don’t feel like any other known surface – we can identify regions in the “tactile space” that are currently not occupied and create new sensations. But ultimately it will be possible to manufacture surfaces to give the illusion of something else. However, one mustn’t forget one thing: It is impossible to separate the senses from each other – we find it pleasurable to touch velvet, but are disgusted by a spider, although objectively speaking they feel very similar. Therefore, in order to create the perfect illusion, the visual and the audial response of a surface also have to be considered. If something doesn’t look like terracotta, we are unlikely to think it feels like terracotta.
Furthermore, generally speaking it will cost a great deal more effort – and also money – to fake a surface rather than use the original.
Nevertheless, it would be interesting for the entertainment industry to imagine that one day there may be a kind of “audio/visual cinema”, which enables us to feel coarse stones, cuddly dogs or soft skin on the screen?
Mark Rutland: I believe that it will indeed be possible to convey the tactile signature of a material onto a touch screen one day.
Back to the present: Haptic and tactile aesthetics also play an important role in marketing – consumer brands invest a lot of money in the haptic design of their products, especially in the packaging sector. What does your survey imply in this respect?
Mark Rutland: A great deal. We can implement the newly acquired knowledge for the field of tactile aesthetics in the same way that colours and their intensity are implemented in the field of visual aesthetics.
The field of visual aesthetics has been thoroughly researched: We perceive certain colours as being “cold”, others as “warm”. It is a proven fact that some colours have a calming effect, whereas others have an agitating impact, etc. Will we be able to categorise the tactile quality and the impact of surfaces in a similar way someday?
Mark Rutland: Definitely. For example, there is currently a lot of sensory research aimed at categorising “softness” and what parameters are responsible for it – for example cotton vs. satin. We will have taken an important step forward here soon. Once we have comprehended the correlation between the different tactile, physical and psychological factors, one day it will be possible to describe what “ecofriendly” or “luxurious” feel like – and ultimately implement this information accordingly for product or packaging designs. The result will be a powerful, new marketing instrument that will allow consumers, responses and target groups to be influenced and addressed in a new way.
Is it conceivable that we will be able to develop surfaces one day that evoke certain feelings and connotations?
Yes and some consumer brands have already made good progress here. I even think it is possible that at some time in the future there will be patents for certain surfaces and thus also “feelings”.
How long will it take before research into the sense of touch has reached the same level as that of visual perception?
Mark Rutland: Although our findings have without doubt brought us a huge step forward, it will definitely take at least another decade, if not two, until we know as much about touch as we do about sight.
And when will your findings be put to concrete use in areas such as product design and marketing?
Mark Rutland: A lot still has to be done here. We have demonstrated that nanostructure can be used as part of the tactile architecture. Now, it is down to the product designers and marketing professionals to find out exactly what is actually “attractive” and “less attractive”, before the corresponding products can be designed and manufactured.
To use the colour analogy again: We have helped establish what wavelengths are visible. However, mixing the colours, their intensity and combination – all of that belongs to the realm of art!
A detailed report about haptic advertising can be found here.