An apparatus for sound, vibration and friction measurements of soft materials in aqueous environments

The clean rinse feel of personal wash products is one of the major technical drivers of consumer preference and usually is measured by in-vivo consumer studies in the consumer goods industry. We report here a custom-made apparatus based on friction and vibration measurements that can be correlated to consumer perceptions of clean rinse feel. The apparatus consists of a rotary stage powered by a motor which is controlled by a programmable controller, a long swiveling arm, an artificial finger, and an underwater sample stage. The artificial finger can adjust the applied normal force on the substrate. The sliding speed of the artificial finger is adjusted and monitored through a computer. Data acquisition software is triggered by the combination of the software and hardware. Four sensors including two normally mounted load cells, an underwater hydrophone and an accelerometer attached to the surface of the artificial finger are used to detect the normal forces, vibration and underwater sound, while the artificial finger slides over the underwater substrate surface and washes the products off from substrate. A friction coefficient can be derived from the data of two load cells and rinse profiles constructed as a function of sliding time. Rinse profiles are shown to be different with application of different cleansers and are correlated to consumer perceptions of slimy or squeaky-clean for different products.