Your finger can feel the change of a single atom in a material

Our fingers have a very large number of nerve endings, giving us a sensitive, precise and effective touch. It is for this reason that the slightest cut on the fingertips (the distal phalanges) is painful. But how sensitive are we? A recent study reveals that our fingers allow us to differentiate materials based on minute chemical differences, even when it comes to the substitution of a single atom.

You already know that your fingers let you feel the slightest change in surface and temperature, but what about differentiating identical materials on the surface? How much can our sense of touch surprise us? Charles Dhong of the University of Delaware and his colleagues investigated whether it was possible to sense a chemical difference in which the internal molecular structures of two materials vary slightly, without their surfaces differing. To do this, they carried out tests with combinations of materials/compounds whose surface was designed to be identical to the touch. The results were published in the journal Soft Matter. 

The researchers chose a silicon wafer as a base and attached to it a layer of a simple compound, only one molecule thick. They tested several compounds, all slightly different from each other. Out of six pairs of compounds, testers (humans) were able to distinguish three. In the case of a pair, where the team replaced only a single carbon atom with a nitrogen atom, the testers were able to distinguish the two pairs with an accuracy of 68%.

Sense of touch: sub-nanometric precision
“When we make our samples, physically they are almost identical, the differences are on a sub-nanoscale,” Dhong explains. "But when test subjects smelled them, some said some felt a little grainy and others were more pleasant and velvety."

The chemical difference between the two compounds that the testers were best able to distinguish was a slight change in the degree of friction they felt when running a finger over them. This alteration was not due to differences in surface area, but rather to the way their molecules fit together.

According to Dhong, these results could be useful for creating realistic feel textures in virtual reality environments. "If you want to create a texture that gives the impression of passing your hand over a beautiful paper, a soft velvet or a wooden table, how do you do it with a screen? It gives us a lot more options to really expand that toolbox,” Dhong explains.


Predicting human touch sensitivity to single atom substitutions in surface monolayers for molecular control in tactile interfaces

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