Establishing direct relationships between soft material perception and rheology

In soft materials, a clear relationship between material properties and human sensory perception has long been desired for design of consumer products, but the link has remained evasive. Favorable perception indicates that customers enjoy a product and are likely to continue using it or purchase it again. Perception is frequently measured subjectively by consumer test panels in terms of descriptive sensory words such as softness, smoothness, thickness, etc. that lack established scientific definitions. In this work, we move beyond ambiguous definitions and detail a method to objectively measure and quantify human-material interactions using a representative series of viscoelastic putties. We show that human behaviors have direct rheological meaning with features that are characterized using transient recovery rheology. The rheology scales logarithmically at perception-relevant timescales, akin to Fechner's law. Our work explains variability in user-reported perception and demonstrates a way to construct direct relationships between user behavior and measurable rheology.