A new study shows that humans can distinguish between two surfaces that are identical except for a single atomic substitution – one is coated with alkylsilanes, the other with aminosilanes.

World of chemistry reported a few years ago that humans can detect the difference in surface energy between a hydrophobic monolayer and a hydrophilic plasma-oxidized surface. Now Charles Dhong and his team at the University of Delaware, USA, have decided to find out more about the chemical changes humans can perceive on the surface. To this end, surfaces were made from silane monolayers before making minor changes to the monolayer chains, e.g. B. replacing a nitrogen atom with a carbon and changing the chain length. They created surfaces that were so smooth that humans couldn’t perceive slight changes in roughness between them, which meant that it was the surface chemistry that allowed them to distinguish the surfaces as different. The people taking part in the tests were asked to pick the odd sample from three samples. The test subjects were able to distinguish between surfaces containing silanes with an alkyl chain of four carbon atoms and surfaces containing silanes with the final carbon replaced by an amine group.

The subjects do not feel the chemical difference directly because the mechanoreceptors in the fingers are not sensitive to chemicals, like the receptors used for smell and taste. However, the differences in chemistry lead to differences in friction that humans can detect. Dhong’s team says the tactile contrast mechanism is likely due to a difference in the order of the monolayers. The researchers used a Hurst exponent, which in this case was used as a measure of the order or perturbation of the silane monolayer, to predict which surfaces could be distinguished. The surface with the amine group is more ordered due to hydrogen bonds, and this increased order reduces friction.

The test subjects were also able to distinguish a surface with five carbon chains from one with eight carbon chains because the longer chains were more ordered due to a phase transition. Using perturbation versus order to predict human performance was more reliable than using surface energy, and the scientists even found that two surfaces with nearly identical surface energies could be distinguished.

A note of caution: Samples that varied by single atom substitution were not always identifiable. Replacing carbon with an amine group on a longer chain was insufficient for the human subjects to distinguish between the surfaces. This substitution did not change the order and therefore did not change the friction enough to be noticeable.

A scheme showing the silane screening

The team also made a mechanical model finger from polydimethylsiloxane (PDMS), treated to have a hydrophilicity similar to that of human skin, to predict which surfaces can be distinguished from a real human finger. Inside the PDMS finger was an acrylic bone that provided stiffness and mimicked the distal phalange of a human finger. The scientists measured the friction as they slid the dummy finger across each surface as a function of speed and force. Comparing the friction of different surfaces gave a prediction of how easy it would be for a person to tell them apart. Surfaces predicted to be differentiable under a combination of force and speed must not be under any other combination of force and speed.

Cara Nunez, an expert in human haptic perception at Stanford University in the USA, says: “This is exciting preliminary work as it demonstrates the sharpness of human touch in relation to chemistry and presents the chemical composition of the surface as an additional degree of freedom Researchers have and engineers can manipulate to design haptic interfaces. ‘

“Haptics researchers mainly focus on physical components of touch, such as mechanical deformation, while this work examines how chemical changes at the atomic level also play a role in the identification and perception of interactions with objects,” added Nunez .

Dhong’s team says their work could help researchers discover new materials for haptic research and lead to better haptic tools. Fine-tuning the surface chemistry could be an easy way to create new tactile experiences for virtual reality users and when using surgical dummies in medical education.

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