Tactile perception: Finger friction, surface roughness and perceived coarseness

Abstract

Finger friction measurements performed on a series of printing papers are evaluated to determine representativeness of a single individual. Results show occasionally large variations in friction coefficients. Noteworthy though is that the trends in friction coefficients are the same, where coated (smoother) papers display higher friction coefficients than uncoated (rougher) papers. The present study also examined the relationship between the measured friction coefficients and surface roughness to the perceived coarseness of the papers. It was found that both roughness and finger friction can be related to perceived coarseness, where group data show that perceived coarseness increases with increasing roughness.

Introduction

This study forms part of a larger project with the overall scope to interconnect human tactile perception of various materials with their physical properties. While it is not yet established which surface properties determine tactile feel, it seems likely that in addition to the (visco)elastic properties of the material, properties such as topography and friction are important [1], [2]. In order to investigate the importance of various surface properties in tactile perception, there is a need for relevant techniques to measure assorted physical properties of the objects under investigation. Since skin is one of the contacting surfaces in tactile perception, one important and relevant physical measure is friction between a real finger and the surfaces of interest. Ideally the counterface for such studies should permit systematic variation of a limited number of parameters, such as for example roughness, while as many other variables as possible are held constant. Paper is a useful model surface in this regard since large areas of identical sample can be manufactured, and different manufacturing approaches lead to subtle differences in surface properties. Furthermore, there is a great interest from the paper community in controlling the tactile response to paper.

In the literature, a number of studies have emerged during the last couple of years, where friction has been measured while sliding a finger over a stationary surface [2], [3], [4], [5], [6], [7], [8], [9], [10]. Gee et al. [4] developed a friction device with the aim of measuring friction while sliding a finger over the sample surface in order to more accurately mimic sensory perception. Instead of the traditional technique of a probe sliding over the skin, they let a finger slide over the sample surface. This apparatus discriminated friction coefficients between a range of different materials, for example rubber, steel and glass. Whereas it is the dynamic friction coefficient that is most often considered, Lewis et al. studied grip on packaging materials by measuring the static coefficient of friction. Much focus lies in studying interactions between skin and fabrics, both by sliding a finger [5], [9] and a forearm [11], [12] over a textile. For example, Darden and Schwartz [9] measured friction between a fingertip and fabrics, and also investigated possible tactile attributes of fabrics using human evaluators. However, measured friction coefficients were not correlated well with the tactile descriptors found. In addition, a couple of other studies have combined finger friction measurements and sensory evaluation [2], [3], [6]. Childs and Henson [3] measured friction between a finger and screen-printed surfaces with different patterns. The participants stroked their index finger over the surface, while reporting their tactile perceptions of given word pairs such as “smooth–coarse” and “happy–sad”. Both the sliding friction coefficient and the roughness (peak separation) could partially be related to the perceived feelings. In another study, friction measurements between an index finger and different car interior materials and aluminium samples with different roughness have been performed by Liu et al. [2] in order to investigate touch-feel perception. Both measured surface roughness and friction could be correlated with the “rough–smooth” and “grippy–slippery” perceptions.

In order to obtain a better understanding of what is affecting tactile perception, perceptual features, measured with subjects, need to be studied. Most studies on tactile perception have a one-dimensional approach, where roughness is the most extensively investigated perceptual feature [13], [14], [15], [16].

The aim of this paper was to investigate if and how perceived coarseness (“strävhet” in Swedish) is linked to physical roughness and friction. Free magnitude estimation was used to scale the perceived coarseness on 8 coated and uncoated printing papers. Magnitude estimation is the most common psychophysical ratio scaling method [17]. The method is based on the hypothesis that people can make direct numerical estimations of their intensity impression of a stimulus. To avoid biased numerical judgements, free magnitude estimation was used.

The authors have already managed to measure friction between a moving finger and a stationary paper surface, using a piezoelectric force sensor [18]. These results employing a single participant show that measurements with the device can discriminate a set of similar samples such as printing papers in terms of finger friction. A negative correlation was also found between the measured friction and the surface roughness. A goal of this study is to determine whether finger friction measurements performed by an experienced, trained experimenter can be considered representative of a larger population.

In this article, finger friction measurements are reported together with a tactile experiment on perceived coarseness performed by a group of undergraduate students.