The human fingertip can distinguish between materials that have minuscule chemical differences聽鈥� even a substitution as small as a聽single atom.
Generally, what we feel with our聽fingers are physical bumps in a material鈥檚 surface structure. Charles Dhong at the University of Delaware and his colleagues set out to find whether it would be possible to feel聽a chemical difference in which the internal molecular structures of聽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聽molecule thick. They tested several compounds, each only slightly different from the others.
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Out of six pairs of compounds, human testers could distinguish between three. With one pair, where聽the team only substituted a聽single carbon atom for a nitrogen one, the testers could tell the two apart with 68聽per cent accuracy.
鈥淲hen we make our samples, physically they鈥檙e almost identical,聽the differences are on a sub-nanometre scale,鈥� says Dhong. 鈥淏ut when test subjects felt them, some people said that some felt a聽little gritty and other ones were more pleasant and velvety.鈥�
The chemical difference between the two compounds that the testers were best able to tell apart caused a聽slight change in how much friction they felt when running a finger over聽them. This alteration wasn鈥檛 due to bumps in each material, but聽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.
鈥淚f 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,鈥� says Dhong.
Soft Matter
