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Modus Operandi of Protective and Anti-Icing Mechanisms Underlying the Design of Longstanding Outdoor Icephobic Coatings Ludmila B. Boinovich, Alexandre M. Emelyanenko, Kirill A. Emelyanenko, and Evgeny B. Modin ACS Nano, Just Accepted Manuscript • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019
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Modus Operandi of Protective and Anti-Icing Mechanisms Underlying the Design of Longstanding Outdoor Icephobic Coatings Ludmila B. Boinovich1*, Alexandre M. Emelyanenko1, Kirill A. Emelyanenko1 and Evgeny B. Modin2 1A.
N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of
Sciences, 119071 Moscow, Russia. 2CIC
nanoGUNE, Donostia San Sebastian 20018, Spain.
*Corresponding author. E-mail:
[email protected] ABSTRACT: Atmospheric icing became a global concern due to hazardous consequences of ice accretion on air, land and sea transport and infrastructure. Icephobic surfaces due to their physicochemical properties facilitate a decrease in ice and snow accumulation under outdoor conditions. However, a serious problem of most superhydrophobic surfaces described in the literature is poor operational durability under harsh corrosive and abrasive loads characteristic of atmospheric operation. Here we elucidate main surface phenomena determining the anti-icing behavior and show experimentally how different mechanisms contribute to long-term durability. For comprehensive exploitation of those mechanisms, we have applied a recently proposed strategy based on fine tuning of both laser processing and protocols of deposition of the fluorooxysilanes onto the nanotextured surface. Prolonged outdoor tests evidence that a developed strategy for modification of materials on the nanolevel allows overcoming the main drawbacks of icephobic coatings reported so far and results in resistance to destroying atmospheric impacts. KEYWORDS: robust superhydrophobicity, nanotextured surfaces, nanosecond laser treatment, durability, wear resistance, aluminum alloy, atmospheric icing
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The economies of many countries especially those located in the northern latitudes bear enormous economic losses associated with atmospheric icing of ships and aircraft, transport and roads, meteorological instruments and antennas, power lines and pipelines. The methods routinely used currently for de-icing of structures generally consume considerable energy and/or chemicals and are not always efficient and environmentally safe. Suppression or reduction of icing by means of passive protection based on icephobic surfaces is a hot topic in material science. Here we consider the icephobic surfaces in the most general sense as the surfaces, which due to their physicochemical properties promote a decrease in the accumulation of ice and snow on surfaces exposed to outdoor conditions and regularly subjected to atmospheric precipitations. Superhydrophobic coatings on the surfaces of protected materials in many cases demonstrate considerable icephobicity associated with low ice/snow adhesion,1–5 water repelling properties,1–7 and enhanced stability of the supercooled state of aqueous droplets.6,8 The key reasons inhibiting so far the notable practical application of superhydrophobic coatings were discussed in the literature. These are: a drastic decrease in surface icephobicity, related to the weak mechanical stability of multimodal roughness and high wear sensitivity, as well as the fragility of surface texture;1,4,6,9,10 the removal of low surface energy layer by aqueous precipitations;11,12 and low chemical stability of underlying texture under corrosion damage.13 Besides, the icephobicity can be rendered ineffective at particular environmental conditions, for example, at enhanced water droplet evaporation.14 Taking aluminum alloys as one of the most commonly used structural materials, here we will show the stable anti-icing behavior of coating designed on aluminum. Although one may find a number of superhydrophobic coatings demonstrating various anti-icing properties in laboratory experiments1–8,15–20 or retaining the superhydrophobicity at several months of outdoor exposure,21–23 to the best of our knowledge there were no reports on prolonged outdoor tests of anti-icing performance of designed coatings in natural environmental conditions. At the same time, the behavior of the coatings in conditions of composite effect of several damaging factors might be critically different from that predicted from results of experiments where each damaging factor acts separately. In this study, we present a detailed description of the anti-icing behavior for the coating which showed resistance to destroying external atmospheric impacts during several harsh winter seasons and preservation of icephobicity in a wide variety of atmospheric conditions. To obtain this coating we have applied the strategy developed in our recent studies24,25 and based on tunable high-intensity nanosecond IR laser processing of the material.
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Our results evidence that laser chemical modification and nanotexturing coupled with the chemical binding of low surface energy component allow overcoming the main drawbacks of icephobic coatings. The key aspects determining the anti-icing behavior of the designed coatings will be elucidated, and the detailed data on different anti-icing properties of the coatings will be discussed in the context of relevant mechanisms of icephobicity. Overall, the additional insight on the performance and the functioning of icephobic water repelling coatings for aluminum structures gives the guidelines for the efficient design of such coatings to mitigate the negative role of hazardous atmospheric icing.
RESULTS AND DISCUSSION Durability of Superhydrophobic State and Resistance to Prolonged Contact with an Aqueous Medium. Combination of laser chemical modification and laser micro- and nanotexturing of the material surface allows fine tuning of the chemical composition, the morphology of the processed surface and binding of chemisorbed low surface energy hydrophobic agent.24,25 This strategy was applied in our study. For fabrication of icephobic coatings on aluminum, high-intensity nanosecond IR laser processing in ambient atmospheric conditions was used. Chosen parameters of laser treatment allowed obtaining hierarchical surface roughness due to a combination of laser melting, material ablation and deposition of nanoparticles onto the surface from the laser plume. Simultaneously, laser-induced chemical processes enriched the surface layer by oxynitrides and γoxides24,25 (Figure 1a,b) chemically stable in contact with the aqueous medium.24–27 The designed superhydrophobic coatings were characterized by extreme water repelling properties with contact angles of 171.8±1.5° and roll-off angles of less than 1.5°. It is worth noting that obtained surface texture is very stable against water film condensation inside the grooves even being subjected to supersaturated vapors or water aerosol flow. The vapor pressure, p, above the liquid condensed inside the pore is defined by the Kelvin equation: 𝑝 = 𝑝𝑠𝑎𝑡 ∙ exp (
― 𝜎𝑤𝑣 ∙ υ ∙ K 𝑅𝑇
)
(1)
where psat corresponds to the saturated vapor pressure at given temperature; σwv is the water/vapor surface tension; υ is the water molar volume; K is the meniscus curvature of the capillary-condensed water; R is the gas constant; and T is the temperature.
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Figure 1. (a,b) Top views of the designed icephobic coating at different magnifications. (c) ESEM visualization of dropwise condensation taking place onto the surface of coating at −15.0 °C and relative humidity of ~ 120%. (d) The water aerosol flow (droplet sizes 200 – 1200 nm, temperature ~ +15.0 °C) directed on the horizontal aluminum plate with the designed coating (see also the Video 1 in the Supporting Information). (e) Water droplet on the coating subjected to the water aerosol flux preserves high contact angle of ~ 167° despite of supersaturation and the presence of micro/nano droplets. (f) Frost and ice formation on the hydrophobic part of the sample (right-hand side on the image) and separate ice crystals on superhydrophobic part (left-hand side) at temperature of −19.5 °C and relative humidity of ~130%. (g) Potentiodynamic polarization curve for the designed coating after 100 h of immersion in 0.5 M NaCl aqueous solution (red curve).The corresponding curve for bare alloy, immersed for 100 h in the same solution is shown for the comparison (blue curve). The curvature of a liquid meniscus formed inside the surface groove or pore is determined by a contact angle θ formed by the meniscus with the pore wall, and by effective radii of curvature along 4 ACS Paragon Plus Environment
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the three-phase contact line. For the cylindrical pore of radius r with nonwettable walls, the meniscus curvature, K=2cos(θ)/r, is negative. Therefore, analysis based on Equation (1) shows that the supersaturation required to induce a spontaneous condensation on the superhydrophobic surface is higher, the higher the contact angle and smaller the characteristic pore sizes of the textured surface are. Increase in a vapor supersaturation may be accompanied by filling the surface grooves and transition of wetting from heterogeneous Cassie-Baxter to homogeneous Wenzel wetting mode. The experiments with the coating designed in this study have shown that dropwise condensation (Figure 1c-e) taking place in supersaturated vapors at both positive and negative temperatures does not lead to a notable decrease in the contact angle. The maintenance of vapor supersaturation leads to droplets growth inside the grooves followed by their departure and transition upside onto the outer surface (Figure 1c), governed by capillary pressure gradients inside the droplet. Even at high supersaturation there exist such narrow pores that capillary condensation in them is thermodynamically unfavorable, and the air remains captured within the narrow part of the pore. For the pores expanding outwards the texture, the gradient of capillary pressure within the liquid condensed in the pore pushes the liquid off the texture. The similar phenomena were detected earlier28 for the droplets formed on the surface with boehmite nanostructures. It should be stressed that for the coating fabricated in this study, the nanoliter droplets formed on the internal walls of the grooves have very high contact angles due to the hierarchical roughness at the nanolevel. The study of the resistance of our superhydrophobic coatings to interaction with the flows enriched with the water aerosol at T=18 °C showed the preservation of extremely high contact angles for the water droplets formed in the course of aerosol droplets deposition, growth and coalescence atop of the fabricated coating. Figure 1d shows the horizontally leveled superhydrophobic substrate with macro and microscopic water droplets formed from the water aerosol flux with the droplets diameter in the range from 200 to 1200 nm.29 Large sizes of the substrate (150×150 mm2) allow monitoring the preservation of the heterogeneous wetting mode, jumping and free movement of deposited droplets along the surface under the flow pressure (Supporting information Video 1). Self-propelled jumping motion at high humidity which is evident from the abovementioned video was recently observed in the literature.30–33 Several papers stressed drastic decrease in the value of contact angle and the loss of the heterogeneous wetting mode during the temperature decrease below the dew point.34–37 In contrast, the coating designed in this study shows very different wettability behavior preserving high contact 5 ACS Paragon Plus Environment
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angles even for supersaturated conditions (Figure 1e). We attribute such attractive stability of superhydrophobic state of the coating fabricated using the nanosecond laser processing to the finely adjusted surface morphology with low fraction of micrometer pores/grooves and to the high chemical resistance of the coating. As discussed in the earlier literature, high relative humidity (RH) at negative temperatures may result in the formation of continuous frost film along the texture.38 Thorough analysis of our samples on the basis of ESEM images (Figure 1f) in the temperature range from −13 to −20 °C and moderate supersaturation (less than 140%) indicated the formation of frost film on the hydrophobic surface, but the absence of frost on the superhydrophobic coating designed in this work. However, further temperature decrease with simultaneous growth of a water vapor supersaturation caused the formation of frost layer atop of the superhydrophobic coating. To compare the frost formation peculiarities at low negative temperatures on the surface of different superhydrophobic coatings, we have monitored the behavior of water droplets deposited on top of superhydrophobic surfaces and cooled to −40 °C. Three superhydrophobic aluminum samples were used for such comparison: the sample fabricated in this study (sample 1), a sample textured by boiling in water (boehmitage process) followed by chemisorption of trichloro-1H,1H,2H,2H-perfluorodecylsilane (sample 2), and a sample textured using the two-stage process including chemical etching in FeCl3 aqueous solution and boiling in water with the following fluorosilane chemisorption (sample 3). The samples 2 and 3 prepared for the comparison were fabricated following exactly the protocols described in recent study.28 The contact of the superhydrophobic samples cooled to −40 °C using the Peltier plate with the laboratory air having RH=20% and the temperature +20 °C led to the surface frosting for all tested samples. At the same time, the frost formation for the sample 1 was delayed more than twice (~3 min) in comparison to the samples 2 (~1.5 min) and 3 (~1 min). On the bare aluminum plate the frost begun to form immediately after exposing the plate to the laboratory air. The notable delaying in frost formation for the superhydrophobic surface with respect to the frosting of untreated surfaces was earlier discussed in the literature,30 whereas here we have detected the notable difference in the delaying of frost formation for the superhydrophobic coatings, obtained using different fabrication protocols. The deposition of water droplet having a room temperature on the frost-coated sample 1 resulted in droplet crystallization with the contact angle around 154° and the preservation of an axial symmetry 6 ACS Paragon Plus Environment
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of a droplet (Figure S1a in the Supporting Information). Sample 2 showed the spreading of a water droplet upon crystallization (contact angle less than 10°). The contact angle of solidified water droplet atop of the sample 3 was around 45 degrees. However, due to breaking of the droplet axial symmetry on the surface of sample 3, the accuracy of contact angle measurement was low. Samples heating till frost and droplets thawing led to a slight increase in the values of contact angles. As shown in Figure S1b-c, the value of a contact angle on the sample 3 after droplet thawing was close to 90°, for the sample 2 the droplet remained flat indicating the wettability of the substrate, whereas the droplet for the superhydrophobic coating prepared in this work (sample 1) after thawing had the contact angle of 165°. One-year laboratory storage of the coating designed in this study (sample 1) showed negligible change in the contact angle. Long-term outdoor exposure in conditions of dust abrasive wear and periodic contact of coating with atmospheric precipitations such as rain, snow, and ice resulted in a moderate decrease in the contact angle and a slight increase in the average roll-off angle (Table 1).
Table 1. Variation of contact and roll-off angles for the designed superhydrophobic coating during long-term outdoor exposure. Time of exposure
Contact angle, degrees
Roll-off angle, degrees
As prepared sample
171.8±1.5°
