The human fingertip can distinguish between materials that have minuscule chemical differences – even a substitution as small as a single atom.
Generally, what we feel with our fingers are physical bumps in a material’s surface structure. Charles Dhong at the University of Delaware and his colleagues set out to find whether it would be possible to feel a chemical difference in which the internal molecular structures of two materials slightly vary but their surfaces are equally smooth.
They did this by taking a silicon wafer and attaching a layer of a simple compound that was just one molecule thick. They tested several compounds, each only slightly different from the others.
Out of six pairs of compounds, human testers could distinguish between three. With one pair, where the team only substituted a single carbon atom for a nitrogen one, the testers could tell the two apart with 68 per cent accuracy.
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“When we make our samples, physically they’re almost identical, the differences are on a sub-nanometre scale,” says Dhong. “But when test subjects felt them, some people said that some felt a little gritty and other ones were more pleasant and velvety.”
The chemical difference between the two compounds that the testers were best able to tell apart caused a slight change in how much friction they felt when running a finger over them. This alteration wasn’t due to bumps in each material, but rather the way their molecules fitted together.
Dhong says this could be useful for people with visual impairments or to make textures in virtual reality that feel real.
“If you wanted to create a texture that feels like running your hand across really nice paper or soft velvet or a wooden table, how can you do that with something like a screen? This gives us a lot more options to really expand this toolbox,” says Dhong.
Journal reference: Soft Matter, DOI: 10.1039/D1SM00451D