ICOPOR y Bioingeniería

To initially characterize the durability of our icephobic coatings, we evaluated force vs. time curves, and thereby τice, for our surfaces over repeated icing/de-icing cycles. For surfaces damaged during the icing/de-icing process, the shape of the force vs time curves changed, and τiceincreased, with increasing icing/de-icing cycles. Both lubricated surfaces, as well as surfaces too soft to prevent physical damage, displayed such behavior within 10 icing/de-icing cycles (Fig. 2.11a,b and2.3a). However, these soft surfaces often displayed almost immeasurably low τicevalues. We measured τice = 0.15 ± 0.05 kPa for our most icephobic surface (Fig. 2.16). This is one of the lowest τicereported thus far, and over five orders of magnitude below τicefor aluminum. Ice slides off such surfaces solely under its own weight. However, additional icing/de-icing cycles begin to degrade the surface, raising τice(Fig. 2.16). Durable surfaces with interfacial slippage, typically possessing higher ρCL, maintain their low ice adhesion values (τice = 3.6 ± 1.0 kPa) over repeated

Time (s) 0 5 10 15 20 25 Force (N) 0 0.5 1 1.5 2 2.5 3 3.5 1 2 3 4 5 6 7 9 8 10

Figure 2.16:The force vs. time curves for coating Q, comprised of Sylgard 184 PDMS and 75 wt% silicone oil (Table2.1), which displayed an initial ice adhesion strength of τice = 0.15 ± 0.05 kPa. The ×’ symbol

denotes the time when ice first un-adhered from the coating.

To illustrate the significant advantage of coatings that repel ice through low ρCL in conjunc- tion with interfacial slippage, we conducted two simple tests for durability: repeated icing/de- icing and relatively mild abrasion (Fig. 2.17). We compare our coatings’ performance to other state-of-the-art icephobic coatings, such as commercial superhydrophobic surfaces (NeverWet©), lubricant-infused surfaces[3], extremely low surface energy fluorodecyl POSS coatings[56], and commercially available icephobic coatings (Nusil R-2180). As fabricated, our PU coating (coating CB; ρCL_{= 33 mol/m}3_{, 15 wt% safflower oil, θ}

adv/θrec = 67◦/29◦, ∆θ = 38◦), shows an order of magnitude reduction in τiceover the other state-of-the-art coatings considered here. Further, after just 10 icing/de-icing cycles, all other coatings, except those fabricated here, exhibit τice > 200 kPa (with the exception of the commercial coating Nusil R-2180, which is a low ρCLPDMS). Addi- tionally, after mild abrasion, only our PU coating remains icephobic, with an ice adhesion strength 2,500% lower than any other coating relying on lubrication or low surface energy.

We additionally tested our PDMS-based coating (coating OO), which can be repeatedly iced but is mechanically very poor, and a PU-based coating, where we intentionally added excess safflower oil (20 wt%) to form a lubricating, free-oil layer (coating CC). There is statistically no difference in τice values between the lubricated and interfacial slippage PU-based coatings initially, or after 10 icing/de-icing cycles (see Table2.1, Fig. 2.18b, Fig. 2.17). However, the lubricated PU coating easily delaminates from essentially all coated substrates (left inset, Fig. 2.18a) due to the presence of the free-oil layer. Similarly, SLIPS-based surfaces utilizing costly, fluorinated lubricants suffer a 10-fold increase in ice adhesion after just a few icing/de-icing cycles (Fig. 2.1). Thus, there is a marked advantage to producing interfacial-slippage-based icephobic coatings. Finally, note

Figure 2.17: Comparison of coatings in this work with other state-of-the-art icephobic surfaces. Also, additional durability characterizations are presented for the PU coating with interfacial slippage (coating CB in Table2.1). For details on each coating and test configuration, see Sec. 2.2.

that a silicon wafer treated with a PDMS-silane, a surface exhibiting interfacial slippage[52] due to pendent chains[89], also exhibited very low ice adhesion (τice = 11 ± 4 kPa, see Fig. 2.8, Fig. 2.17). In comparison, a Si wafer coated with a low surface energy fluorinated silane exhibits relative high ice adhesion (τice = 248 ± 57 kPa, Fig.2.8). However, these thin silane coatings can be abraded away relatively easily (Fig.2.17).

To demonstrate the real-world potential of our durable icephobic surfaces, we conducted out- door testing during the winter months of 2013 and 2014 in Ann Arbor, Michigan. Over the four months of exposure, both snow and ice accreted severely on an uncoated glass panel. The coated panel often had snow settle on it, but all ice that formed was quickly sheared off even from mild winds[75] (Fig. 2.19). After four months of exposure, the contact angles and τice for the coated surface were the same as before testing, highlighting the coating’s durability.

a b

Number of Taber Abrasion Cycles

0 1000 2000 3000 4000 5000 τ ice (kPa) 0 100 200 300 400 500 µ PU - Interfacial Slippage PDMS - Interfacial Slippage PU - Lubricated wt% Safflower Oil 0 10 20 30 τ ic e /ρ C L (kPa m 3 /mol) 0 0.5 1 1.5

Interfacial Slippage Lubricated

µ

Figure 2.18: a, Mechanical abrasion of three different icephobic coatings. The PDMS (coating NN) and lubricated PU (coating CC) were easily damaged or delaminated within 20 abrasion cycles, whereas the PU with interfacial slippage (coating CB) survived over 5,000 cycles while maintaining low ice adhesion. b, The effect of oil content in our PU on τiceafter normalizing by ρCL. The miscibility limit of safflower oil

is ≈ 16 wt%. It was clear that once the oil started to phase separate from the PU elastomer, the mechanism for reduced ice adhesion transitioned from interfacial slippage to lubrication.

Figure 2.19: Outdoor testing of a PDMS-based coating (coating NN; see Table2.1) for 4 months during winter 2014. On Februar 12, the un-coated panel was covered with a ≈ 7 mm layer of glaze, the type of ice with the strongest adhesion[8]. No ice had accreted on the coated panel. On March 4, 2014, snow followed a night of freezing rain, which completely covered the un-coated panel. The coated panel only had a small amount of accreted ice remaining.

(coating CB) including Taber abrasion (ASTM D4060), acid/base exposure, accelerated corrosion (ASTM B117), thermal cycling, and peel testing (ASTM D3359). We also measured τice over 100 icing/de-icing cycles, and evaluated the coating in a temperature range from -5◦C to -35 ◦C (Fig. 2.20b). After 5,000 abrasion cycles, causing over 600 µm of thickness loss, the coating re- mains icephobic because icephobicity is an inherent property of the coating. PDMS-based coatings (coating NN) or lubricated PU-based coatings (coating CC), though equally icephobic initially, are completely abraded away (and/or delaminated) after < 20 cycles (Fig. 2.18a). The use of high surface energy elastomers, and the lack of a free oil layer, allows us to create coatings that adhere very well to any underlying substrate. We observed no increase in τiceeven after 10 successive peel tests on steel, copper, aluminum, and glass, or after thermal cycling between -10◦C and 70◦C. The average ice adhesion strength for this coating after all durability testing is τice = 9 ± 2 kPa. We additionally subjected our icephobic polyurethane to a tensile stress of 2.5 MPa, causing the elas- tomer to elongate by 350% without breaking or losing its icephobic properties (Fig. 2.18a, right inset). Additional tensile testing showed strains in excess of 1,000% (Fig. 2.6). The developed, ex- tremely durable coatings can be spun, dipped, sprayed, or painted onto essentially any underlying substrate, of any size. Finally, we had the extremely low ice-adhesion strengths for multiple sur- faces independently verified by Mode-I type (peel test) and Mode-II (zero-degree-cone) adhesion testing at the US Army’s Cold Regions Research and Engineering Laboratory[9] (see Fig.2.20a).