2. BACKGROUND AND LITERATURE SUMMARY

2. BACKGROUND AND LITERATURE SUMMARY 2.1 Friction The friction parameter generally measured is the coefficient of friction. Different techniques that simultaneously measure the normal load W between the contacting surfaces and the force F necessary to initiate motion, can be used to determine the skin friction coefficient, defined as:

µ=

F W

2.1

This is called Amonton’s law [3] or Coulomb law of dry friction, where µ is a constant, independent of applied loads under which the frictional resistance is measured and of the size of the surface areas which are in contact with one another. The friction coefficient can be measured in two ways: (i)

the static friction coefficient µ s and

(ii)

the dynamic or kinetic friction coefficient µ k, where µ k < µ s.

The static friction coefficient is defined as the ratio of the force required to initiate relative movement to the normal force between the surfaces; the dynamic or kinetic friction coefficient is defined as the ratio of the friction force to the normal force when the two surfaces are moving relative to each other and it is supposed to be independent of the speed of movement. Much research has been focused on the dynamic friction coefficient wherein the two surfaces move at a relative constant velocity.

For manufacturers, both static and dynamic friction are interesting as many tools are used under both conditions. Skin and polymers appear to behave similarly with respect to friction and do not follow the basic laws of friction; µ can be greater than 1.0 and it is not unusual to find µ k > µ s, as Bullinger el al. [4] show for human palmar skin. According to this, in order to consider the highest values, we should study the dynamic coefficient of friction, though studying the static coefficient would also be interesting. According to Amonton' s law [3], the dynamic friction coefficient remains unchanged regardless of the probe velocity or applied normal load in making the

measurement. Amonton' s laws [3] hold true in the case of solids with limited elastic properties. Although Naylor [1] concluded Amonton' s law to be true, later studies by El-Shimi [2], Comaish and Bottoms [5] and Koudine et al. [6] found that skin deviates from Amonton' s law [3], because their studies found the friction coefficient to be inversely proportional to the load. El-Shimi [2] and Comaish and Bottoms [5] reasoned that the rise in friction coefficient with decreasing load resulted from the viscoelastic nature of the skin allowing for a non-linear deformation of the skin. According to Comaish and Bottoms [5], skin obeys Amonton’s law over a limited range of loads only. In a broader range, a logarithmic relationship exists between the friction force F and the normal force W [35] log F = log µ + n ⋅ log W → F = µ ⋅ W n , where the coefficient n

mg (see Table 1.4 for coefficient of friction, µ ) 2⋅µ

2.3

People usually exert larger normal forces than are necessary to prevent slippage on a tool or a handheld object. The difference between the produced force and the minimally necessary force is called a safety margin. Actually, the applied grip force is adjusted to satisfy two conflicting demands: to prevent slipping and not to produce unduly high forces.

Figure 2.6: The pinch force, Fp, required to support an object, which is related to both the weight and friction, µ, of the object [37]

Mathematical justification for equation 2.3: To prevent the object from slipping

F > 0 → 2 ⋅ F f = 2 ⋅ µ ⋅ Fp > mg → F p > Material

Dry

mg 2⋅µ

Moist

Combined

Sand Paper (#320) Smooth vinyl Textured Vinyl Adhesive Tape Suede Aluminium Paper

(n = 42) 0.41 (0.10 0.39 (0.06) 0.27 (0.09)

(n = 42) 0.66 (0.14) 0.66 (0.11) 0.42 (0.07)

(n = 84) 0.61 (0.10) 0.53 (0.18) 0.50 (0.11) 0.38 (0.13) -

Table 2.3: Coefficients of friction, µ, (average (standard deviation)) for human palmar skin against various materials, n=7 subjects [30] Note that the required pinch force increases as the skin dries. Johansson and Westling [26] and Armstrong [37] demonstrated that the force exerted to lift an object with the fingers is related to weight and friction, but there is significant inter-subject variability (Figure 2.7).

Figure 2.7: Pinch force versus object weight for two different materials [32] According to the National Institute of Technology and Evaluation (Japan, 2002), the grip strength decreases with the age, therefore the pitch force that can be applied is lower.

2.4 Sports materials The grip is very important in most sports, particularly in Rugby. Manufacturers put much effort in developing new textures to improve the grip and general performing of the ball during the match. In the old days, rugby balls were made of hand-stitched leather. But it would often get very heavy when it rained, making it very difficult to handle in slippery conditions. The leather casing has now been replaced by hi-tech materials designed to help the balls keep their shape and withstand the weather.

Gilbert is a trademark for rugby balls, their balls are exported to all the world’s major rugby playing countries. Some big panels to test were provided by Gilbert on four different textures: Smooth, Revolution, Xact and Xact-7. The same rubber compound is used for the smooth and Revolution balls. The pimples on the Xact-7 balls are square while the pimples on the Revolution and Xact balls are round. Revolution, Xact and Xact-7 are commercialized but not Smooth, that has been exclusively manufactured to test and compare with the other balls. From the other three balls, the more expensive one is Xact-7. To match the extreme performance of a Rugby World Cup Sevens, Gilbert launched the Xact-7 ball in February 2005, with a grip developed especially for the abbreviated form of the game. The new Xact-7 grip allows for long, accurate passing, one-handed off-loads and increased ball security. The patented Xact-7 dimple formation also allows for controlled and accurate drop kicks, a speciality of the short format game.

2.5 Conclusions Although there have been limited studies dealing with the measurement of the skin friction coefficient, past studies show that differences in skin, because of various factors - such as age and hydration - can be correlated with the friction coefficient. Friction coefficient studies can serve as a quantitative method to investigate how skin differs on various parts of the body and how it differs between different people. It is also a useful method for tracking the changes resulting from the environmental and chemical treatments - such as sunlight - and when various chemicals are applied to the skin - such as soaps, lubricants and skin creams. The reviewed studies show that friction is an important parameter for understanding the skin' s mechanical state. The reviewed studies also indicate that the design of the test apparatus is an extremely important factor, because test design parameters can also have an influence on friction measurements [14]. On the whole, few studies address the effects of anatomic region, age, gender, or race as they pertain to the friction coefficient. To date, no significant differences have been found with regard to gender or race. Age-related studies have been contradictory where some authors found no difference and others found differences [14]. If these differences in friction coefficient exist between young

and elderly people and, as Cole [27] found, friction in elderly individuals is nearly twice the average for young people, it may enable them to perform a task that would otherwise be impossible, due to their reduced grip strength. From what have been said, we can conclude that the most important parameters to be taken into account are: - State of the hands (wet, dry, soapy or with any type of moisturizing cream) - Age (to develop things that can easily be used by both young and elderly people) - Pinch force people exert depending on what they are grasping, - Roughness of the materials Some of these parameters have to do with the material and, therefore can be controlled by the manufacturer and others have nothing to do with it, but can affect during the measuring procedure and cause confusion if we do not measure under the same conditions. Besides, the reliability of any study of skin friction depends upon standardized test procedures [19]. Skin preparation and environmental conditions in many of the early skin friction studies have been neglected [5, 7]. The study by Highley et al. [7] incorporated a standardized procedure which detailed skin conditions prior to testing. This study determined that, after washing, skin required a minimum of 30 minutes to regain normal surface lipids and perspiration levels. The best way to measure friction would be while the subject is dealing with a certain task, for example gripping something or opening a bottle. The majority of studies that have observed friction properties of skin have incorporated measuring instruments that determine friction while the subject remains sedentary. Therefore, the first thing to do would be to measure coefficients of friction for some materials with a relatively easy apparatus and study how all the variables affect this measurement. Then, a further research could be to develop a more practical and realistic device and compare these data with others measured with it, to see how friction is influenced by external factors while people are using their hands in a normal way.

This work aims to do a preliminary study of skin friction, in order to have a basic knowledge of such a complicated phenomenon and from which further investigations could be carried out.