Easy ice removal
The accumulation of ice on a surface can lead to hazardous conditions, such as on the surface of an airplane wing or the side of a tall building. Ice adhesion, even to a surface treated to minimize the bonded force, will usually depend on the amount of surface coverage. Golovin et al. compared strength-limited deicing with toughness-limited deicing. Whereas normal deicing materials focus on minimizing the adhesion strength, the authors show that if a material is designed with low-adhesion toughness, deicing is no longer a function of the coverage area.
Science, this issue p. 371
Abstract
Ice accretion has adverse effects on a range of commercial and residential activities. The force required to remove ice from a surface is typically considered to scale with the iced area. This imparts a scalability limit to the use of icephobic coatings for structures with large surface areas, such as power lines or ship hulls. We describe a class of materials that exhibit a low interfacial toughness with ice, resulting in systems for which the forces required to remove large areas of ice (a few square centimeters or greater) are both low and independent of the iced area. We further demonstrate that coatings made of such materials allow ice to be shed readily from large areas (~1 square meter) merely by self-weight.
The accretion of ice on surfaces can have a severe detrimental impact on a range of commercial and residential activities (1). Consequently, there has been an effort to create coatings that protect against the buildup of ice. Typically, the efficacy of these coatings has been evaluated by measuring the force, F, to debond a specified area, A, of ice, and defining an ice adhesion strength τice = F/A as the characteristic property for the system (2). The term icephobic is generally used to describe surfaces for which τice < 100 kPa (3, 4), in comparison to structural materials such as aluminum and steel, for which τice > 1000 kPa (2, 5, 6).
Using τice to characterize an interface inevitably requires that the force necessary to remove ice scales with the iced area. Many engineering structures susceptible to icing, such as airplane wings, wind turbine blades (7), and boat hulls (8), have surface areas that can approach thousands of square meters. Consequently, even with extremely icephobic coatings, structures with large surface areas would require prohibitively high forces to detach entire sheets of ice from the surface. In this work, we developed low–interfacial toughness (LIT; interfacial toughness Γ < 1 J/m2) materials for which the force required to remove adhered ice from large areas (few cm2 or greater) is both low and independent of interfacial area.
An interfacial cohesive strength, as represented by the ice adhesion strength, is one way to describe the bonding across an interface (9). A countervailing perspective on fracture (10, 11) is that an interface should be described in terms of its bonding energy (or, more correctly, its toughness). Further, although the work of adhesion is often discussed in connection to ice adhesion, it is the strength that is generally used to describe failure (1, 2, 4). This is true whether the adhesion is viewed in terms of surface energy (3, 12, 13), interfacial cavitation (14), or lubrication (15–17).
The two competing perspectives for delamination, strength and toughness, can be rationalized by means of cohesive zone models of fracture (18–22). Simple analytical models (23) can be used to demonstrate that the shear strength of the interface,
We first verified the concept that the force required to remove an ice layer reaches an asymptotic value if the interface is long enough. This was done using substrates made from common plastics such as polyethylene, polypropylene, and polystyrene (substrate thickness t = 1.6 mm; see Table 1) without any additional modification. We used a setup similar to those reported previously (2, 14, 24) but instead of using relatively short lengths corresponding to a few millimeters of bonded ice (2, 14, 24–26), we designed our apparatus so that much longer interfaces could be evaluated (Fig. 1, C and D, insets) (23). Plots showing the force (per unit width) necessary to detach the ice,
A comparison is made between 1-mm-thick coatings of icephobic silicones on aluminum, 1.6-mm-thick plastic substrates, and 1- to 2-μm-thick LIT coatings on aluminum [see (23) for fabrication and composition descriptions].
(A) The force per unit width required to debond ice from four polymers (each 1.6 mm thick). Up to a critical length Lc, the shear strength of the interface,
We repeated this experiment for aluminum substrates coated with different icephobic coatings (t ≈ 1 mm; Fig. 1B). These coatings were all based on polydimethylsiloxane (PDMS) rubber, which has been studied for its low–ice adhesion properties enabled via lubrication (25), interfacial cavitation (14, 24), low surface energy (30, 31), and interfacial slippage (32, 33). In contrast to the plastic substrates, these materials did not exhibit a toughness-controlled regime of delamination within the range of bonded lengths studied.
A low interfacial shear strength does not necessarily imply a low toughness. Thus, a material that debonds from ice more readily if the interface is short does not necessarily debond more readily if the interface is long. This can be seen by comparing the results for polyvinyl chloride (PVC) and polyamide (Fig. 1A) or the results for polypropylene and silicone B (Fig. 1, A and B). The shear strength of the interface between ice and polypropylene can be calculated from the initial slope of the line in Fig. 1A as
These data can be reexpressed in terms of the apparent ice adhesion strengths for the two interfaces by dividing the force by the initial bonded area (Fig. 1D). As such, the apparent ice adhesion strength, for a length of 100 cm for polypropylene (τice ≈ 12 kPa), was less than half that of the icephobic PDMS (τice ≈ 29 kPa), although the true ice adhesion strength
There are several contributions to interfacial toughness. One is associated with the bonding energy between the ice and the coating. A lower bound on this energy would be ~0.1 J/m2, corresponding to van der Waals interactions (5, 34). An additional contribution could come from localized losses within the coating, associated with the high-stress region at the crack tip. These two effects would be classically considered to be contributing to interfacial toughness. However, if the process of delamination causes deformation of the coating, then the strain energy associated with this deformation must also be considered as a contribution to the effective toughness between the ice and substrate. From a cohesive zone perspective, one can consider the toughness of an interface to be given by the area under the force displacement curve of the entire interface, including the coating (18, 19). Therefore, assuming linear elasticity, this contribution to the toughness can be estimated as
To investigate this concept, we varied the thickness of PVC films and confirmed that Γ scaled with the coating thickness t (Fig. 2A). Lowering t from 150 μm to 2 μm (23) reduced Γ from ~3 J/m2 to 2 J/m2. Previous work (14, 35) has shown that for icephobic elastomers,
(A) The effect of coating thickness on the effective interfacial toughness between ice and an aluminum substrate coated with plasticized PVC for four different contents of the plasticizer MCT. (B) The force per unit width required to fracture ice from three different thicknesses of PVC plasticized with 50 wt % MCT. Note that for the thickest sample, strength controlled the fracture up to at least L = 20 cm. (C) The force per unit width required to fracture ice from thin (t ≈ 1 to 2 μm) PVC coatings with four different contents of MCT. A toughness-controlled regime of fracture was always observed for lengths less than 20 cm. (D) The effect of plasticizer content on Γ for three different thicknesses of plasticized PVC. All experimental results shown were obtained at –10°C. (E) The force required to fracture ice from the LIT PDMS and LIT PVC systems (thickness ≈ 1 to 2 μm). Even over an interfacial length of 1 m, the necessary force of fracture remained constant beyond Lc. The inset shows our experimental setup, performed in a walk-in freezer at –10°C. Error bars denote SD (N ≥ 5).
To further reduce Γ, we explored the effects of plasticizing PVC with medium-chain triglyceride oil (MCT) (23). Figure 2A shows the general drop in toughness observed with increased plasticization. As shown in Fig. 2B, the additional drop in shear strength associated with the addition of 50 weight percent (wt %) MCT was large enough for the transition length to become too long for the toughness to be measured for the thicker coatings. The general trends among strength, toughness, coating thickness, and level of plasticization can be seen in Fig. 2, C and D.
By optimizing the thickness and plasticizer content within the PVC, we fabricated LIT materials exhibiting Γ as low as 0.27 J/m2 (
We coated 1.2-m-long aluminum beams with these LIT PVC and LIT PDMS coatings (with a nominal thickness of t ≈ 1 to 2 μm) (23), and conducted large-scale testing inside a walk-in freezer at –10°C. Figure 2E shows that the force of detachment did not increase for L > Lc, even over 1 m of interfacial length (
For a given ice thickness, there will always be an interfacial length beyond which LIT materials require less force than icephobic materials to remove the adhered ice (fig. S19). As an example, to mimic the deicing of power line cables, we conducted off-center loaded beam tests by flexing 1.2-m-long uncoated and coated (with either icephobic or LIT coatings) aluminum beams with ice adhered on one side (23). The icephobic (silicone B; t ≈ 1 mm) and LIT (silicone B + 40 wt % silicone oil; t ≈ 1 to 2 μm) coatings were fabricated using the same polymer, PDMS, but the icephobic PDMS system exhibited low interfacial strength (Γ > 8.8 J/m2 and
(A) Comparison of uncoated, icephobic, and LIT aluminum beams adhered to a sheet of ice (100 cm × 2.5 cm × 0.8 cm) undergoing off-center load flex tests inside a walk-in freezer held at –20°C (23). Ice fractured from the LIT-coated specimen with a remarkably low apparent ice adhesion strength of 0.39 kPa, whereas ice remained adhered to the uncoated aluminum and icephobic specimens even at severe deflections (movie S2). (B), An aluminum sheet coated with LIT PDMS before, during, and after fracture from a large sheet of ice (0.95 m × 0.95 m × 0.01 m). The weight of the ice sheet alone was sufficient to cause fracture, displaying an exceedingly low apparent ice adhesion strength of 0.09 kPa. A comparison is also made to uncoated aluminum (movie S5).
The isothermal freezing conditions within a freezer (Fig. 1, C and D, and Figs. 2E and 3A) differ from those experienced in Peltier plate–based systems (data shown in Figs. 1 and 2), in which ice is formed via unidirectional cooling from the surface. Ice formation conditions, particularly ambient temperature, can substantially affect the structure and interfacial properties of ice (1, 36–38). The similitude of our data for L ≤ 20 cm (Peltier) and L > 20 cm (freezer; see Fig. 1, C and D, and Figs. 2E and 3A) lengths of ice indicated that LIT materials can be effective in shedding ice in different ice formation conditions. Additionally, we measured
To evaluate a third ice formation condition, we coated a 1 m × 1 m aluminum panel with our LIT PDMS and allowed ice to form outdoors at –7°C overnight (23) (Fig. 3B). We observed that the weight of the ice at a thickness of 1 cm, once fully frozen, was enough to completely and cleanly remove the attached ice (movie S5). This yielded τice = 0.09 kPa. Whereas varying icing conditions can lead to tensile cracking and subsequent fragmentation of the ice, as long as the fragmented length remains greater than Lc, LIT materials will remain an effective means of ice removal.
Supplementary Materials
science.sciencemag.org/content/364/6438/371/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S20
Table S1
Movies S1 to S5
Data S1
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