Charles B. Dhong | Searcharxiv

arXiv2026

Role of Single Chemical Heterogeneities in Generating Anisotropic Tactile Sensitivity and Soft Sliding Friction Phenomena

Physical heterogeneities in the context of sliding friction, such as a human finger exploring an object, have been well studied, yet the behavior of chemical heterogeneities in mesoscale soft sliding remains underexplored, despite the similar prevalence of chemical and physical variations in real systems. Here, we experimentally characterized the friction of a planar soft elastic probe sliding across a single chemical heterogeneity that was formed at the interface of two silanes on silicon wafers. By constructing phase maps across multiple loads and velocities, we quantified the occurrence of several frictional phenomena at and around the chemical edge, including stiction spike formation, edge slope direction, baseline shifts, and baseline drift, and quantified their sliding direction-dependent formation. We found that chemical heterogeneities made by more disparate materials (butyl- and aminopropyl-terminated) exhibited several phenomena that were more often direction-independent compared to chemical heterogeneities formed from more similar materials (butyl- and hexyl-terminated). We attributed this directional asymmetry to elastic body effects. In subsequent human testing (n=36), we observed that humans also exhibited directional-dependent accuracy (66.7% versus 38.9%) on one pair (butyl- and hexyl-terminated) but not the other (77.8% versus 75%), which in the context of our phase maps, suggests that the slope of the friction force when sliding over a chemical edge is important for generating a clear edge of a tactile feature, rather than the differences in simple material properties or other friction phenomena.

cond-mat.soft↗

arXiv2024

Transparent and Electrically Switchable Thin Film Tactile Actuators Based on Molecular Orientation

Most tactile actuators create tactile sensations through vibrations or the mechanical and electrochemical formation of bumps. However, tactile sensations of real objects arise from friction which is derived not only from physical topography, but also surface chemistry. Here, we show that molecular rearrangement can be leveraged to create new classes of tactile actuators based on the phases of liquid crystals embedded in a solid and transparent polymer film. We found that humans can feel differences by touch, especially between planar alignment and its disrupted phase, as actuated by a DC electrical field. In subjective terms, the sensation was described as a tacky to polished-like feeling. 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. This enables a new class of tactile actuators based on molecular orientation (TAMO) for haptic interfaces.

cond-mat.soft↗