This paper discusses coefficient of dynamic friction mu(k), and perceived discomfort for differently textured surfaces when touched by a normal hand, a 'sweaty' hand and a hand contaminated with glycerol, paraffin oil or lard. Fourteen male subjects applied forces of 1, 10 or 20 N using the index finger of the dominant hand while striking across a specimen of the same polycarbonate material with 100, 75, 50 or 25% skin-contact area (by ridges). Surface pressures in the range 6-288 kPa were recorded. Generally there was a very low correlation between mu(k) and perceived discomfort. Wider grooves and increased normal forces were associated with discomfort but did not necessarily provide more friction. Unlike most materials, palmar skin showed sticky characteristics and mu(k) frequently exceeded 1.0 when the surface loads were low. The coefficient of friction on palmar skin decreased with increasing surface loads. A considerable reduction Of mu(k) was noticed when the normal force increased. A 20-fold increase in normal force from 1 to 20 N (an increase in surface load from 6.3 to 81.4 kPa) resulted in only an eight-fold increase in frictional force while mu(k) dropped from 2.22 to 0.85. A decrease was noticed for all tested surfaces and during all tested environmental conditions; however, increased normal force increased the perceived discomfort. A 25-50% increase in coefficient of friction could be gained under contaminated conditions using two out of four fairly similar textures. Grooves in surfaces reduced /mu(k) under normal and sweaty conditions, but improved mu(k) when contaminants were present. Patterns with a large skin-contact area showed either a high or a low coefficient of friction depending on the environmental conditions. A plain, glossy, non-textured surface of polycarbonate under normal skin conditions yielded mu(k) = 2.22 when the surface pressure was low (6 kPa). When the skin was exposed to paraffin oil, mu(k) dropped drastically to 0.28. Oil and lard reduced mu(k), but sweat increased it in relation to a normal hand. Wider grooves in the texture provide friction during contaminated conditions at the price of discomfort.
