Ice Adhesion. Группа авторов
loss of the pure Cassie-Baxter wetting state necessitates that some cohesional failure occurs to remove the ice from the surface, and thus a higher ice adhesion strength is manifested (c) Superhydrophobic surfaces can become damaged with deicing cycles, leading to a loss in the anti-icing property. Adapted from [120].
Let us now consider a superhydrophobic surface with a different morphology than square pillars. This time the surface exhibits the Cassie-Baxter wetting state by suspending a droplet on micro-spikes. Upon lowering the temperature of the surface, the tips of the spikes become wetted and eventually embedded in the ice that forms, as shown in Figure 1.11(c). The failure mode here is cohesive in so much as the crack propagation occurs through the solid surface, with the ice leaving with the embedded spike tips. The measured adhesion strength to this surface will be very low for the first cycle, as only a small cross-sectional area of the solid surface’s nanospikes will have been sheared. However, as seen in Figure 1.11(c), subsequent icing cycles see the ice-air interface move lower and lower, and more and more micro-spike tips become embedded. Subsequent icing cycles will have larger and larger adhesion strengths as the superhydrophobic surface gets more and more damaged [120].
Figure 1.12 (a) Twill Dutch-weave and (b) plain Dutch-weave pattern stainless steel meshes as viewed under a scanning electron microscope, (c) The ice dislodged from the plain Dutch-weave pattern stainless steel mesh shows a near-perfect imprint of the solid surface, suggesting the ice dislodged interfacially. Adapted from [108].
In light of the previous discussion of how superhydrophobic surfaces often fail to possess an anti-icing property, we take this opportunity to summarize the features necessary to reduce the ice adhesion strength to a substrate. In short, a surface will exhibit a low ice adhesion strength if:
1 The surface promotes micro-cracking/air bubble trapping which increase the number of crack initiating points at the ice-solid interface.
2 The surface possesses a gently curving morphology to promote crack propagation along the ice-solid interface.
One such promising anti-icing surface has been reported by Ling et al. (2016) [108]. They measured the ice adhesion characteristics of stainless steel Dutch-weave filter meshes, shown in Figure 1.12(a)&(b). These hydrophilic materials, with many anchor points for the ice, paradoxically presented lower ice adhesion strengths than a polished stainless steel substrate. This phenomenon is explained as follows: (1) The stainless steel mesh was completely wetted by the liquid water, as shown by a low contact angle and low receding contact angle. (2) Upon freezing, the water expands by a greater degree than the metal. This volume expansion causes stress concentration leading to the formation of micro-cracks at the ice-solid interface. (3) The arched topography of the meshes, being absent in sharp corners, allows for easy interfacial crack propagation and thus adhesional failure of the ice-solid bond. The adhesional failure of the ice formed on the stainless steel meshes is well evidenced by the ice surface’s near-perfect imprint of the mesh structure, shown in Figure 1.12(c) [108].
1.4 The Sliding Friction of Ice
Section 1.3 considered the mechanical separation of ice from a solid substrate to which it had been frozen. Naturally this leads us to next consider the tribology of ice. That is, what occurs when a solid surface is put in relative motion to frozen water? As discussed in Section 1.1.3, human’s employment of the low frictional force of ice for transportation dates back to at least 7000 BCE in Scandinavia where skis allowed people to efficiently navigate and hunt [39]. Another historical use of a low friction surface for transportation comes from an Egyptian carving dated to 2400 BCE. This relief depicts humans pouring a lubricant in front of a sledge to facilitate sliding [127, 128].
Although it is evident that humans have devised methods to deal with friction for millenia, the first explicit mention of friction as a force was not made until Aristoteles’ Questiones Mechanicae circa 350 BCE [127, 128]. Further, the first quantitative studies of friction were not performed until circa 1500 CE by Leonardo da Vinci. Even then, da Vinci’s notes were not published until the end of the 19th century, leaving the opportunity for French physicist Guillaume Amonton to rediscover his two laws of friction in 1699 [127, 128]. These are
1 “Friction produces double the amount of effort if the weight be doubled” i.e. The force of friction is directly proportional to the applied load.
2 “The friction made by the same weight will be of equal resistance at the beginning of its movement although the contact may be of different breadth and length” i.e. The force of friction is independent of the apparent area of contact for a given load.
Amonton also made the important identification of surface roughness being the fundamental cause of friction, with friction being the force required to lift interlocking asperities over one another during their relative sliding motion [127]. In 1785, Charles Augustin Coulomb investigated the effect of various relevant factors on frictional resistance, such as: the material in contact, the surface area, the normal pressure, the length of time that the surfaces remain in contact, and the ambient conditions on frictional resistance. Coulomb’s work was also the first to formulate frictional force as an equation. where FT, FN, and μ are the frictional force, the normal force and the coefficient of friction, respectively. The coefficient of friction in Equation 1.42 is assumed to be independent of the relative sliding velocity, an assumption often referred to as the third law of friction. However, the third law only holds for moderate sliding velocities [127, 128].
As discussed in Section 1.1.3, the Industrial Revolutions ignited much scientific study into ice. In 1859, Michael Faraday showed that two ice cubes will fuse together when brought into contact. He concluded from this observation that the surface of ice is covered with a liquid-like layer [129]. That same year, James Thomson attributed the existence of the liquid-like layer to pressure melting [130]. Osborne Reynolds used Thomson’s pressure melting theory to explain the ease of skating on ice. That is, the pressure of the skater must lead to a thicker liquid-like layer which lubricates the surface of the ice [131]. This theory whence ice derives its low friction force prevailed until 1939 when Bowden, Hughes and Desch suggested the now generally accepted theory that frictional heating is the main contributor to the low coefficient of friction of ice [132].
In this section we will discuss the role that the liquid-like layer plays in determining the frictional force of ice.
1.4.1 Ice Friction Regimes
As alluded to in the introduction to this section, the low frictional force of ice is derived from the lubricating liquid-like layer that exists on its surface. As will be discussed, the thickness of the lubricating liquid layer plays an important role in the force of friction experienced by surfaces sliding relative to one another. First consider the sliding contact of two surfaces in the complete absence of a lubricating liquid. As shown in Figure 1.13(a), solid surfaces are never completely flat meaning that two surfaces are in contact at their asperities. When the asperities of different surfaces come into contact, chemical or physical bonds are formed between them. Thus, the (tangential) friction force, FT, is a measure of the force necessary to break the bonds between contacting asperities.