Human finger contact with small, triangular ridged surfaces
Highlights
► We developed an analytical model to analyse friction between a fingerpad and ridged surfaces. ► The model compared well with experiments using aluminium, brass and steel surfaces. ► For low ridge height and width the friction was dominated by adhesion. ► At higher ridge heights (above 43 μm) interlocking friction contributes greatly. ► At a ridge height of above 250 μm interlocking friction also contributes.
Introduction
Ridges are often added to surfaces to improve grip of objects such as; sports equipment, kitchen utensils, assistive technology, aids, etc. Although considerable work has been carried out to study skin friction, there is very little work in the literature to suggest how these ridged patterns affect friction or how any effects can be modelled (for an overview see for example [1], [2]). A survey as part of a previous study [3] examined a wide range of textures found on 69 typical handheld objects including, amongst other things, food packaging and household utensils. Texture designs fell into four main categories; criss-cross patterns, dimples, pimples and ridges. The most common category was a ridge pattern, either triangular or rectangular in cross-section and between 0.1 to 5.0 mm in height. This paper is concerned with fine surface textures that have triangular ridges at the small end of this spectrum, ranging from 0.003 to 0.26 mm in height.
Many important aspects of finger friction have been well investigated. Studies examining the effect of load [4], [5], have shown that above a normal load of around 1 N, contact area plateaus and the adhesion mechanism dominates finger friction on smooth surfaces made from various metals, polymers and glass. Other studies concerned with the presence of moisture, have postulated that water absorption, possibly together with capillary adhesion can cause increased adhesion friction due to an increase in contact area [6] and that moisture can also cause a “stick-slip” feeling for a rubbing contact between a finger and artificial skin [7]. A study on the effect of contact pressure, showed that for contact situations where the adhesion mechanism dominates, friction coefficients decreased with increasing contact pressure, but if deformation played an important role, contact pressure had less of a measureable effect [8].
Previous studies on the effect of surface roughness on friction include one that measured friction between the finger and 21 different grades of paper (Ra values ranging from 1.2 to around 4.0 μm) and found that the rougher papers had a lower friction coefficient than the smoother papers [9]. Hendricks & Franklin studied Ra values from 0.1 to around 10 μm for metals and polymers and showed that at these levels of roughness, friction when in contact with forearm skin decreased as Ra went up (see Fig. 1) [10]. This was thought to be due to the decrease in contact area that would be seen at higher roughness. Clearly the situation for the finger will be different to other areas of the body at higher roughness values due to the higher roughness brought about by the ridge pattern (values of Rq have been reported between 7 and 17 μm [3]). Another study by Derler et al. examined index fingerpad (mean Rz values between 62 and 99 μm) and edge of hand contacts (mean Rz values between 33 and 73 μm) with smooth glass (Rz = 0.05 ± 0.01 μm) and rough glass (Rz = 45.0 ± 5.6 μm) [8]. As with the study by Hendricks & Franklin [10], they found that under dry conditions, friction coefficient decreased with surface roughness (for both fingerpad and hand).
One study involving larger scale texture [11] investigated the effect of rectangular cross-section ridges (made from polycarbonate) on friction. The ridge height was 0.5 mm, and the ridge width and groove width ranged from 0.5 mm to 1.5 mm. The tests were done on 14 male volunteers in ambient conditions, and it was also found that the friction decreased when ridges were added to the surface. This was attributed to a lower area of contact, and therefore less adhesion.
Tomlinson et al. [4], however, showed that for surfaces with triangular ridges (giving roughness values up to 100 μm–see Fig. 2 for examples of 2D profiles) a threshold existed above which friction increased. This was thought to be due to the initiation of interlocking of the ridges on the finger pad. The friction data is shown in Fig. 3. Interestingly, closer examination of the data at lower roughness indicates that friction remained relatively stable rather than decreasing as seen in work with surface textures without “directionality”. It may be that for this type of surface texture the interlocking actually initiates at relatively low values of roughness and then has an increasing effect as roughness rises.
Tests were carried out on aluminium (HE 30), brass (CZ 121) and steel (S 275). A shaping machine was used to put thin horizontal grooves in the metal. The tool used was a 60° point tool, and the grooves were machined at 0.5 mm feed, 0.3 mm deep cut; 0.4 mm feed, 0.2 mm deep cut; 0.3 mm feed, 0.15 mm deep cut; 0.2 mm feed, 0.1 mm deep cut; and 0.1 mm feed, 0.05 mm deep cut.
Given the limited work in the literature on this subject there is clearly a need for more to be done in this area, particularly modelling, to provide a basis for improved grip design.
One major driver for this is safety. In 2007/2008 there were 43,518 reported accidents at work due to handling, lifting or carrying [12]. If the grip on these products can be optimised, it is hoped that some of these accidents can be prevented. In addition to safety aspects, improved grip can also enhance the performance of products, such as in sports equipment, or enable a larger group of people to use a product. For example; if screw top bottles are easier to open, more independence could be given back to the elderly, who often struggle to open them.
The objective of this work was to advance the findings of the previous work on triangular ridges by Tomlinson et al. [4] and develop an analytical model for friction that could provide a basis for designing more effective surface textures for good grip.
Section snippets
Analytical model development
Previous work analysing skin friction has suggested that there are two principle mechanisms; adhesion and hysteresis friction [13]. For a finger contacting a flat surface, hysteresis is said to be negligible [4], [14] and this was also found for a sphere contacting a forearm [15]. However, in the tests carried out with triangular ridges [4], the presence of ridges introduced hysteresis as an additional component of friction, so both mechanisms must be analysed as well as the interlocking
Modelling results
Assuming a loss fraction of 0.45 [19], and using Equations 1, 14 and 15, the friction force for a finger contacting a triangular ridged surface can be predicted. Fig. 6 shows the predicted and measured values of frictional force for the materials tested. Data is presented for three materials, each having been used to create five different fine-ridged surfaces and each surface having been tested at five different normal loads (giving 75 data points in all). Tests were carried out using one
Mechanisms of friction
The experimental data in Fig. 3 indicates that the coefficient of friction plateaus with roughness, however, prediction made from the analysis of the different friction mechanisms do not support this (see Fig. 8). This was investigated further by analysing the different friction mechanisms as the finger moves over a fine ridged surface.
Fig. 7 shows that at low ridge height, adhesion is the most dominant friction mechanism. This is because the surface profile is such that the ridges do not
Conclusions
In this study a model was developed for addressing finger-texture contact and predictions from the model were compared to experimental values.
In the cases modelled, adhesion was the predominant mechanism (responsible for more than 50% of the total friction force, and in some cases, up to 100%) for samples with shallow ridges of a height lower than 42.5 μm (note: the spacing of the ridges also has an effect, but for the samples used in this study, ridge height was the main parameter that dictated
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