Ice Adhesion. Группа авторов
of ice can only be lowered by 0.1 to 1 K, depending on the slider [133].
Rather than pressure melting, Bowden, Hughes and Desch [132] suggested that frictional heating is the most important contributor to the low friction coefficient of ice. When a slider is translated relative to an ice surface with which it is mated, heat is generated. A portion of the generated heat will raise the temperature at the contacting asperities up to the melting temperature of ice. Once the melting temperature is attained, the ice surface will begin to locally melt at the contacting asperities, forming a non-continuous film of water. As discussed in Section 1.4.1, the liquid film will contribute to the lubrication of the slider on ice. Bowden, Hughes and Desch note that as the ambient temperature is raised, the lubricating melt layer can become continuous [132].
1.4.3 Parameters Affecting the Friction Coefficient of Ice In the following subsection the effect of each important parameter on the coefficient of friction of ice is discussed. Particularly, the important parameters will be presented through their relationship to the thickness of the pertinent liquid-like lubricating layer.
Let us first consider the effect of temperature. After having analyzed the friction regime diagram of Figure 1.13(b), it is not surprising that some studies have found a decrease in the coefficient of friction with increasing temperature, while others have observed the opposite [133]. When the ice-solid translation is occuring within the boundary friction regime the coefficient of friction will indeed first decrease with increasing temperature, but will begin to rise again as the temperature approaches 0°C. The minimum coefficient of friction will be obtained at temperatures between -2°C and -7°C, depending on the measurement method, the normal load on the slider, the linear sliding velocity, and the material of the slider [133]. At low temperatures, the lubricating film will be thin and the friction will be dominated by solid-solid interactions. This type of friction is typical of the regimes to the left of the minimum in Figure 1.13(b). As the temperature is increased, and approaches the melting point temperature, the lubricating liquid-like layer will thicken. As the liquid-like layer becomes thick enough to support the slider, friction will decrease with lessened solid-solid interactions, then increase with the buildup of capillary bridges. This type of friction is typical of the region to the right of the minimum in Figure 1.13(b), that is, the mixed friction regime, with a further increase in film thickness leading to the hydrodynamic regime.
Next, let us consider the effect of the sliding velocity, v. Numerous reports have been published on the coefficient of friction decreasing with increasing sliding velocity [133]. Naturally, higher velocities will lead to more frictional heat than lower velocities which results in greater melt water production. Within the boundary friction regime and into the mixed friction regime, the increased thickness of the liquid-like layer results in more lubrication.
Eventually, as shown in Figure 1.13(b), drag forces will begin to outweigh the benefits of decreased solid-solid contact as the sliding surfaces enter the hydrodynamic friction regime. Within the boundary regime, the coefficient of friction has been shown to be dependant on the velocity of the slider with
The effects of the applied normal force and the actual area of contact between the slider and the ice are inextricably linked. Under dry friction conditions, as shown schematically in Figure 1.13(a), an increase in the normal force will lead to more asperities coming into contact, and thus an increase in the friction force of the system. However, in the case of ice friction, it is generally accepted in literature that the coefficient of friction of a slider mated with ice decreases with increased applied normal force at a given temperature and slider velocity [133]. The increased normal force leads to increased actual area of contact between the asperities of the ice and the slider. The greater amount of contact points contribute to frictional heating which, in turn, will result in a thicker lubricating liquid-like layer. The coefficient of friction has been reported to become independent of the applied force with larger contact areas [133]. Under these conditions (and when the temperature approaches the melting point), the thickness of the lubricating liquid-like layer is such that complete wetting of the slider occurs. The slider-ice system is now in the hydrodynamic regime, where shearing of the liquid-like layer dominates in determining the force of friction [133]. Amonton correctly attributed friction to the roughness of surfaces in 1699 [127]. Generally, in the dry friction and boundary friction regimes, increased surface roughness leads to more contacting asperities between mated surfaces and thus requires the cleaving of more solid-solid bonds to slide the surfaces relative to one another. Various reports have been published which conclude that roughening the slider surface indeed increases the coefficient of friction on ice [133]. However, as discussed in Section 1.3.1, surface roughness and wettability are intimately linked. Surfaces of high roughness can exhibit high hydrophobicity through the trapping of air in their asperities. As such, Kietzig et al. (2009) have shown that laser irradiated stainless steel sliders which possess hierarchical hydrophobic surface roughness demonstrate lower coefficients of friction throughout the mixed friction and hydrodynamic friction regimes [152]. In the mixed and hydrodynamic friction regimes, a surface which is less easily wetted will result in less capillary bridges between the slider and the ice. However, the effect of surface hydrophobicity cannot be investigated independantly of other inherent material properties such as thermal conductivity and material hardness without affecting properties such as surface roughness.
Finally, the relative humidity of the surrounding environment has been shown to have a strong effect on the onset of the sliding movement. That is, the higher the humidity, the more lubricated is the interface between the slider and the ice, and thus the lower the frictional resistance [153].
1.5 Summary
In this chapter we have taken a journey through the history of ice on our planet, as well as early human’s symbiotic relationship with this seemingly inconsequential solid. In doing so, we see that rather than being incidental, ice has actually had a lasting effect on Earth’s lifeforms (and as a corollary, Earth’s lifeforms have had a lasting effect on ice). Our study of man’s relationship with surface icing has revealed that the conditions for the formation of ice were those that also led to the evolution of the human species. Further, history shows us that the scientific study of surface icing and the goal of engineering anti-icing surfaces is intimately linked to human modernity. In sum, the study of surface icing is perhaps, in a sense, the study of man’s role in the history of our planet.
This chapter’s review of the current state of scientific literature has aimed to offer the reader a method to rationally engineer surfaces which optimize: (1) the formation of ice; (2) the adhesion of ice; and/or (3) the sliding friction of ice. In Section 1.2, we used the thermodynamic classical nucleation theory to reveal a method to rationally design surfaces which inhibit the nucleation of ice. Following this, in Section 1.3, we discussed the pitfalls in the current trend of employing anti-wetting surfaces for anti-icing applications. By contrasting the ideal cases with reality, we arrive at the desired surface characteristics for the easy shedding of surface ice. Finally, in Section 1.4 we looked at the friction of solid surfaces sliding on ice from the point of view of the important liquid-like lubricating layer. In doing so, we have identified the important parameters which affect the thickness of ice’s liquid-like layer, and thus the force of friction.
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