Rough surface contact

R7.4.1 (2005). Multiscale Modeling of Two Dimensional Rough Surface Contacts. 

We describe a model that combines rough surface contact mechanics with elastic pad mechanics. The theory is capable of describing planarization in both the classical contact and roughness-dominated regimes (see the solid curves in Fig. 6.7). We then introduce a simple approximation for the asperity filtering regime. 

The surface tension of a liquid is mainly controlled by the intermolecular interaction among the molecules. Thus, it can be determined directly from the properties of the molecule. However, the surface tension of the solid surface is affected by its structure and roughness. When the surface is rough enough to keep gases or vacuum on it, the mean polarizability becomes very low. The water droplets on such a rough surface contact the solid... 

Dettre, R.H., Johnson, R.E.J., 1964. Contact angle hysteresis n. Contact angle measurements on rough surfaces Contact SI32. The dream of sta3ing clean lotus and biomi-metic surfaces. In Fowkes, F.M. (Ed.), Angle, Wettability, and Adhesion. American Chemical Society, Washington, DC, pp. 136—144. 

Consider now a bacterium which is 1 jim in diameter. This now behaves in a completely different way to the truck tire, as seen in Fig. 4.14. Because of the rapid decline in gravitational force with diameter, the weight of a bacterium is now extremely small, less than 1 pN, or less than a single weak chemical bond force. But adhesion, according to Bradley s rule, has not declined so fast and is around 1 pN for a smooth bacterium or 1 nN for a rough surface contact. Thus a bacterium in dry conditions will always stick to a surface and cannot behave like a track tire which exhibits zero adhesion. A key problem with dry bacteria is releasing them from surfaces. When wet, the adhesion is deaeased substantially, as described in Chapter 6, but bacteria can still show strong adhesive behavior even under that condition (see Chapter 12). 

Measuring the contact between rough surfaces is difficult because the black contact spot then becomes broken up, and contact is then only made at a few localised contact spots. This difference between smooth surface and rough surface contact is illustrated in Fig. 9.8. 

A press-fit interface is essentially a rough surface contact and consequently behaves as a spring. If the pressure is high there are few air gaps so it is very stiff and allows transmission of an ultrasonic wave. If the pressure is low then interface stiffness is lower and most ultrasound is reflected. 

It has been demonstrated that typical engineering rough surface contacts were sensitive to ultrasound frequencies in the regime 1 — 50MHz and that the reflection of ultrasound could be used to determine information about the natime of the contact at an interface [5]. 

Figure 2. Partial Reflection of Ultrasound at a Rough Surface Contact...

This phenomenon has been widely used to study the rough surface contacts [1, 2, 3] where the wave passes through regions of contact but is reflected at regions of non-contact. Interpretation of the reflected wave can give information about the extent of the contact region, the real area of contact, and the contact pressure. 

The relationship between the real contact area and applied load is shown in Fig. 6. For a comparison, the ideal smooth case is also plotted. It can be seen that the rough surface contact area is linearly proportional to the applied load and represents only a small percentage of the smooth contact result. It is consistent with Greenwood and Trip [18] extension of the Hertzian theory for the case of the elastic contact of rough spheres. Figure 7 shows the variation of real contact area with applied load for four different spatial resolutions. For each spatial resolution, a nearly linear behaviour is observed. The slope of the lines decreases with increasing spatial resolution. Therefore at the constant load the contact pressure increases and real contact area decreases with spatial resolution. This is in accordance with previously published works [19], [20]. 

In the context of the structural perturbations at fluid-solid interfaces, it is interesting to investigate the viscosity of thin liquid films. Eaily work on thin-film viscosity by Deijaguin and co-workers used a blow off technique to cause a liquid film to thin. This work showed elevated viscosities for some materials [98] and thin film viscosities lower than the bulk for others [99, 100]. Some controversial issues were raised particularly regarding surface roughness and contact angles in the experiments [101-103]. Entirely different types of data on clays caused Low [104] to conclude that the viscosity of interlayer water in clays is greater than that of bulk water. 

The effect of surface roughness on contact angle was modeled by several authors about 50 years ago (42, 45, 63, 64]. The basic idea was to account for roughness through r, the ratio of the actual to projected area. Thus = rA. lj apparent and similarly for such that the Young equation (Eq.-X-18) becomes... 

Greenwood J A 1967 On the area of contact between rough surfaces and flats J. Lub. Tech. (ASME) 1 81... 

Detennining the contact area between two rough surfaces is much more difficult than the sphere-on-flat problem and depends upon the moriDhology of the surfaces [9]. One can show, for instance, that for certain distributions of asperity heights the contact can be completely elastic. However, for realistic moriDhologies and macroscopic nonnal forces, the contact region includes areas of both plastic and elastic contact with plastic contact dominating. 

Greenwood J A 1992 Contact of rough surfaces Fundamentals of Friotion Maorosoopio and Miorosoopio Processes (NATO ASI Series E220) eds I L Singer and FI M Pollock (Dordrecht Kluwer) pp 37-56... 

Clean the surface with a wire brush to loosen the oxide film and then wipe it off with a soft cloth. The use of a wire brush serves a dual purpose first, scraping and removing the oxide film, and secondly, providing the surface with a moderate knurling (roughness), which helps to make a better surface-to-surface contact and, in turn, a better joint. 

Fig. 5. Sessile drop on a rough surface true contact angle BTA and apparent contact angle BTH. Thick curve = surface of solid (s) thin curve = surface of liquid (1) v = vapour. T is the triple point HTR a horizontal AT a tangent to the solid surface BT a tangent to the liquid surface.

As long as the liquid actually wets the rough surface, a less contentious approach linking the roughness factor to the extent of contact would seem to be via the spreading coefficient as shown in Eq. 20 and summarised in Table 1. If air is trapped within pits by the liquid, a composite surface is produced. 

If the contact angle is known, insight into the extent of wetting to be expected at equilibrium can be obtained from calculations for idealised rough surfaces. The conclusions may require modification when kinetic effects, such as setting of the adhesive, are taken into account. 

If contact with a rough surface is poor, whether as a result of thermodynamic or kinetic factors, voids at the interface are likely to mean that practical adhesion is low. Voids can act as stress concentrators which, especially with a brittle adhesive, lead to low energy dissipation, i/f, and low fracture energy, F. However, it must be recognised that there are circumstances where the stress concentrations resulting from interfacial voids can lead to enhanced plastic deformation of a ductile adhesive and increase fracture energy by an increase in [44]. 

The line which defines the crystal surface can have straight pieces ( facets ) as well as curved ones. The latter correspond to a rough surface, as explained in the next section. The point at which straight and curved pieces meet can be either a sharp corner or a smooth tangential connection like z [21], where x is the deviation from the contact point in the direction... 

The oxidation rate of pure Fe in Oj has been shown to be affected by specimen shape and the original surface profile . For iron oxidising in Oj, 50 50 O2 + HjO and O2 + COj rough surfaces have been found to oxidise more slowly than smooth surfaces, since surface irregularities hinder the oxide flow . The oxide is unable to deform sufficiently to maintain intimate contact with the metal surface such that porous scales are formed . 

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