Researchers have developed a formula that enables them to recreate different levels of perceived softness. Based on the results from their experiments, they created equations that can calculate how soft or hard a material will feel based on material thickness, Young’s modulus (a measure of a material’s stiffness), and micropatterned areas. The equations can also do the reverse and calculate, for example, how thick or micropatterned a material needs to be to feel a certain level of softness.
The researchers began by examining two parameters used to measure a material’s perceived softness: indentation depth (how deep a fingertip presses into a material) and contact area between the fingertip and the material. Normally, these parameters change simultaneously as a fingertip presses into an object. Specially engineered materials were developed that decoupled the two parameters and then tested them on human subjects. The researchers created nine different elastomeric slabs, each with its own unique ratio of indentation depth to contact area. The slabs differed in amount of micropatterning on the surface, thickness, and Young’s modulus.
Micropatterning consists of arrays of raised microscopic pillars dotted on the surface of the slabs. The tiny pillars allow a fingertip to press deeper without changing the contact area. The slabs were tested on 15 subjects who were instructed to perform two tasks. In the first task, they presented subjects with multiple pairs of slabs and asked them to identify the softer one in each pair. In the second task, the researchers had subjects rank the nine slabs from softest to hardest.
Overall, the slabs that subjects perceived as softer were thicker, had little to no micropatterning on the surface, and had a low Young’s modulus. Meanwhile, slabs that felt harder were thinner, had more micropatterning, and a high Young’s modulus.