CONNECTING MATERIALS CHEMISTRY TO HUMAN FINE TOUCH THROUGH STRUCTURE-FRICTION-PROPERTY RELATIONSHIPS

Abstract

The friction generated between a finger and an object forms the mechanical stimuli behind fine touch perception. Devices which create touch perception, like haptic devices and assistive tactile aids, generate touch sensations on a user’s finger through several methods. However, current haptic technologies are limited in reproducing the feel of everyday objects or relaying complex spatial information (e.g. mathematical plots or scientific diagrams). To control friction, and therefore tactile perception, current static tactile aids and dynamic haptic devices primarily focus on creating physical features like either bumps or pins in static devices, or generating vibrations through mechanical or electrochemical formation of bumps. However, the tactile sensations generated in everyday objects arise from friction, which is derived from not only physical topography, but also surface chemistry. Therefore, instead of relying on tactile stimuli generated through physical features, we sought to harness the chemical aspects and molecular properties of surfaces to create richer tactile interfaces and explore mechanistic origins in tactile perception.

To connect tactile perception with molecular structure, this work systematically altered the molecular-scale properties of surfaces such as surface chemistry, degree of crystallinity, and molecular ordering and orientation—while minimizing variations in physical roughness. Then, through mechanical friction testing with a custom set-up mimicking the properties of a human finger along with human psychophysical testing, we used friction measurements to guide predictions of human responses and to develop mechanisms driving tactile perception to understand how chemical structure can be tailored for distinct sensations.

First, we varied silane-derived monolayers deposited onto surfaces smoother than the limits of human perception by physical roughness. Through mechanical friction testing and cross-correlation analysis, we made predictions of which pairs of silanes might be distinguishable by humans. We predicted, and demonstrated, that humans can distinguish between two isosteric silanes which differ only by a single nitrogen-for-carbon substitution. The mechanism of tactile contrast originates from a difference in monolayer ordering, as quantified by atomic force microscopy, which was replicated in two alkylsilanes with a three-carbon difference in length. This approach may be generalizable to other materials and lead to new tactile sensations derived from materials chemistry.

Second, we sought to develop a new way to create tactile sensations by relying on differences in microstructure as quantified by the degree of crystallinity in polymer films. To isolate crystallinity, we used polystyrene films which differed in tacticity and annealing conditions. These films were sufficiently thin as to be rigid, removing any effects from bulk stiffness and furthermore, had variations in roughness lower than detectable by humans. Psychophysical testing verified that humans could discriminate the microstructure of chemically similar polystyrene films due to its influence on mesoscale friction. Although related, human performance was not strongly correlated with a straightforward difference in the degree of crystallinity. Rather, human performance was best predicted by quantifying transitions in stick-slip friction phenomenon, specifically steady to unsteady sliding and the generation of slow frictional waves.

Lastly, to determine if we could create a new class of tactile actuators based on changes in molecular orientation and ordering, we investigated liquid crystals embedded in a solid and transparent polymer film. Here, we showed that molecular rearrangement can be leveraged to create new classes of tactile actuators based on the inherent switchable molecular ordering of liquid crystals. We found that humans can feel differences by touch, especially between planar alignment and its disrupted phase, as actuated by a DC electrical field. We attribute the mechanism of tactile contrast to microscale phase separation and changes in molecular orientation, as the nanoscale differences in topography are too small to be detected on their own by humans. This molecular rearrangement occurs quicker (<17 ms) than actuation through ionic or fluid movement, enabling a new class of tactile actuators based on molecular orientation (TAMO) for haptic interfaces.

These findings could be integral to engineering and adoption of more effective technologies in virtual reality, remote surgery, and assistive aids for the visually impaired, ranging from static tactile aids to dynamic screens and haptic devices.