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Charles Dhong1 Rachel Miller1 Darren Lipomi1

1, University of California, San Diego, La Jolla, California, United States

Human definitions of “softness” are closely related to the mechanical definition of compliance. The compliance is determined by the mechanical properties (Young’s modulus) of the material as well as the geometry of the object. The compliance determines how deep a finger pushes into an object (indentation depth) and contact area between the finger and an object. There is currently little information about whether humans are sensitive to either contact area or indentation depth, and by how much. To decouple the effect of contact area from indentation depth, we prepared a set of PDMS samples that have different indentation depths, at constant contact area and vice versa. Usually the contact area and indentation depth of a bulk sample is determined by the Young's modulus. We made samples that vary in contact area, but have a constant indentation depth by taking advantage of “substrate stiffening” which occurs when films are thin (< 3 mm). In addition, we also fabricated samples that vary in indentation depth, but have constant contact area through micropatterning. Human subjects were asked to pick which PDMS sample "felt softer" from nine different combinations of indentation depth and contact area. This allowed us to quantify the respective magnitudes of indentation depth and contact area as a cue in tactile perception. We also present applications and design criterions to modulate “softness” for human-machine interfaces.

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