Super-Hydrophobic/Icephobic Coatings Based on Silica ... - MDPI

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Dec 2, 2016 - Junpeng Liu, Zaid A. Janjua, Martin Roe, Fang Xu, Barbara Turnbull, .... photoelectron spectroscopy (XPS, ESCALAB Mark II, VG Scientific,.
nanomaterials Article

Super-Hydrophobic/Icephobic Coatings Based on Silica Nanoparticles Modified by Self-Assembled Monolayers Junpeng Liu, Zaid A. Janjua, Martin Roe, Fang Xu, Barbara Turnbull, Kwing-So Choi and Xianghui Hou * Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK; [email protected] (J.L.); [email protected] (Z.A.J.); [email protected] (M.R.); [email protected] (F.X.); [email protected] (B.T.); [email protected] (K.-S.C.) * Correspondence: [email protected]; Tel: +44-115-95-13920 Academic Editor: Thomas Nann Received: 1 October 2016; Accepted: 28 November 2016; Published: 2 December 2016

Abstract: A super-hydrophobic surface has been obtained from nanocomposite materials based on silica nanoparticles and self-assembled monolayers of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) using spin coating and chemical vapor deposition methods. Scanning electron microscope images reveal the porous structure of the silica nanoparticles, which can trap small-scale air pockets. An average water contact angle of 163◦ and bouncing off of incoming water droplets suggest that a super-hydrophobic surface has been obtained based on the silica nanoparticles and POTS coating. The monitored water droplet icing test results show that icing is significantly delayed by silica-based nano-coatings compared with bare substrates and commercial icephobic products. Ice adhesion test results show that the ice adhesion strength is reduced remarkably by silica-based nano-coatings. The bouncing phenomenon of water droplets, the icing delay performance and the lower ice adhesion strength suggest that the super-hydrophobic coatings based on a combination of silica and POTS also show icephobicity. An erosion test rig based on pressurized pneumatic water impinging impact was used to evaluate the durability of the super-hydrophobic/icephobic coatings. The results show that durable coatings have been obtained, although improvement will be needed in future work aiming for applications in aerospace. Keywords: super-hydrophobic; icephobic; silica nanoparticles; fluorosilane; self-assembled monolayers; durability

1. Introduction Ice formation and accretion may hinder the economic and environmentally friendly operation of aircraft [1] and pose a serious hazard that may cause accidents. For aircraft, it is necessary to have a de-icing and anti-icing system on the ground and during flight. However, current de-icing and anti-icing systems release chemicals into the environment, build up weight, increase fuel consumption and add complexity to the aircraft systems [2]. Aiming for an environmentally friendly and cost-effective way to solve the issue of ice formation and accretion, a durable icephobic coating on the surface of aircraft is potentially an ideal solution. A surface that exhibits a water contact angle of 150◦ or greater with very little flow resistance is considered to be super-hydrophobic [3]. Super-hydrophobic surfaces are effective in allowing the incoming water droplets to bounce off, delaying ice formation and reducing the ice adhesion strength [4]. In order to fabricate super-hydrophobic surfaces, both the surface chemical composition and morphology need to be tuned to achieve a low surface energy and desirable surface roughness [5]. Nanomaterials 2016, 6, 232; doi:10.3390/nano6120232

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Various methods have been developed to prepare a rough surface from a low-surface-energy material or to modify a rough surface with a low-surface-energy material, such as electrochemistry, mechanical machining, chemical etching, spin coating and chemical vapor deposition [6–12]. Among them, a combination of spin coating of a rough material and chemical vapor deposition of a low-surface-energy material is straightforward and inexpensive. Coatings incorporating silica nanoparticles have been attracting significant interest due to high thermal and mechanical stability and high surface roughness [13]. Among low-surface-energy materials, fluoroalkyl silanes are promising for practical applications because of their high mechanical and chemical stability resulting from strong immobilization through siloxane bonding [14]. In previous research, hydrophobic coatings based on silica were widely reported. However, the icephobicity, icing behavior and durability of coatings based on silica nanoparticles were less investigated. In addition, the durability of hydrophobic/icephobic coatings is very important for practical applications, especially in aircraft applications, and has remained challenging. Xu et al. [15] reported an erosion test method based on the impingement of water droplets released from a higher stage using gravity. In this experiment, an erosion test rig with the impact of impinging by high-velocity pneumatic water was set up and used to evaluate the durability. In the current work, silica nanoparticles were deposited by the spin-coating method to form a nanostructured rough surface to trap small-scale air pockets. Self-assembled monolayers (SAMs) of fluoroalkyl silane, 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS), were grafted onto the silica nanoparticle surface by the chemical vapor deposition method to obtain a low surface energy. The hydrophobicity, icephobicity and durability of the coatings were investigated. 2. Experimental Section 2.1. Fabrication of Silica-Based Nano-Coatings with Self-Assembled Monolayers Silica nanoparticles, polystyrene and POTS (98%) were purchased from Sigma-Aldrich Company (Dorset, UK). Chloroform was purchased from Fisher Scientific Company (Loughborough, UK). All chemicals were used as received. 0.5 g silica nanoparticles and 0.019 g polystyrene were dissolved into 30 mL chloroform by continuous stirring for about 1 h. The mixture was deposited onto substrates at a speed of 1500 rad/min for 1 min using a spin coater (KW-4A, Chemat Group, Northridge, CA, USA). For ice adhesion test, the Al substrates with roughness average (Ra ) of 2.64 nm in area of 5 µm × 5 µm are alloy (2024-T4). For all other tests, the substrates are glasses with Ra of 1.66 nm in area of 5 µm × 5 µm. Then the samples were transferred into a furnace for heat treatment at 550 ◦ C for 2 h to remove the organic components and fuse the silica nanoparticles together. Then the silica based coatings with thickness of about 30 µm were formed. To reduce the surface energy and obtain super-hydrophobic surfaces, the samples were grafted by self-assembled monolayers of POTS using chemical vapor deposition method in a sealed vessel with 0.3 mL POTS at 180 ◦ C for 3 h. Coatings based on commercial super-hydrophobic and icephobic silicone were also fabricated for comparison. 2.2. Characterization of Morphology, Composition and Hydrophobicity The surface morphology was investigated by a scanning electron microscope (SEM, XL30, Philips FEI, Eindhoven, Netherlands) under an acceleration voltage of 20 kV after Pt was deposited on the samples to prevent charging by electron beam. The composition was measured by energy dispersive X-ray spectroscopy (EDS, Oxford Instruments plc., Oxfordshire, UK) with an electron accelerating voltage of 20 kV by accumulating the counts for 60 s. The binding energies of elements were characterized by an X-ray photoelectron spectroscopy (XPS, ESCALAB Mark II, VG Scientific, Waltham, MA, USA) using Al Kα X-ray as the radiation source with wavelength of 1486.6 eV. The Fourier transform infrared (FTIR) spectra were recorded by a spectrometer (Spectrum One, PerKin Elmer, Akron, OH, USA) using attenuated total reflection mode in the range between 650 cm−1

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and 1300 cm−1 . Hydrophobicity of the surfaces was characterized using a contact angle goniometer 1300 cm−1First . Hydrophobicity ofInc., the Portsmouth, surfaces wasVA, characterized using a out contact angle goniometer (FTA200, Ten Angstroms, USA) with pumping rate of 1 µL/s. (FTA200, First Ten Angstroms, Inc., Portsmouth, VA, USA) with pumping out rate of 1 µ L/s. 2.3. Icephobicity Test 2.3. Icephobicity Test Ice adhesion tests were performed using a centrifuge method with a glaze ice block (mass of 1.3 g) in a low temperature with temperature of −5 ◦ C.method Using the rotation at the detachment Ice adhesion testschamber were performed using a centrifuge with a glazespeed ice block (mass of 1.3 g) of glaze ice block, the ice with adhesion strengthofis−5calculated the icespeed blockatmass and beam in athe low temperature chamber temperature °C. Using using the rotation the detachment length [16]: ice block, the ice adhesion strength is calculated using the ice block mass and beam length of the glaze F = mrω 2 (1) [16]:

(1) where F is the centrifugal force (N), m is the mass ice 2block (kg), r is the radius of the beam (m) and 𝐹 =of 𝑚𝑟ɷ ω is the speed of rotation (rad/s). From the centrifugal force, the shear stress is determined: where F is the centrifugal force (N), m is the mass of ice block (kg), r is the radius of the beam (m) and ɷ is the speed of rotation (rad/s). From the centrifugal F force, the shear stress is determined: (2) τ= A 𝐹 (2) τ= 𝐴 where A is the Area iced (m2 ), τ is the shear stress (Pa). Six silica-based coating samples were measured where A accuracy. is the Area iced (m2), τ is the shear stress (Pa). Six silica-based coating samples were for better measured for better accuracy. The water droplet icing tests were performed by monitoring the water droplets on three spots of ◦ C.water The water droplet icing tests were monitoring dropletsthe onvideo three of spots coated samples and uncoated samples onperformed a cold platebysetting at −10the By observing the of coated samples and uncoated samples on a cold plate setting at −10 °C. By observing the video of water droplets, icing duration can be obtained. the water droplets, icing duration can be obtained. 2.4. Durability Test 2.4. Durability Test To evaluate the durability, erosion test rig (as shown in Figure 1) under pressurized pneumatic evaluate with the durability, erosion test velocity rig (as shown in Figure 1) under pneumatic waterTo impinging gas pressure of 15 psi, of 22 m/s and liquid flow pressurized rate of 22 mL/min was water with gasdroplets pressurewere of 15spray psi, velocity 22 m/ssamples and liquid ratedurations of 22 mL/min was set up.impinging Pressurized water onto theofcoated forflow various between setand up. 60 Pressurized were was spray onto the on coated for various durations between 30 min. The water droplets contact angle measured threesamples spots before and after the erosion test. 30 and 60 min. The water contact angle was measured on three spots before and after the erosion test.

Figure 1. A schematic diagram of the water impinging test.

3. Results Discussion 3. Results and and Discussion Treatment and and Morphology Morphology 3.1. Surface Treatment Figure 2 shows the schematic of the surface modification process and the conversion from hydrophilic toto super-hydrophobic. Aiming to aobtain a super-hydrophobic silica hydrophilic super-hydrophobic. Aiming to obtain super-hydrophobic surface, silicasurface, nanoparticles nanoparticles were spin-coated onto the glass substrates, followed by the chemical vapor deposition were spin-coated onto the glass substrates, followed by the chemical vapor deposition of self-assembled of self-assembled monolayers POTS to form a low energy on surfaces and between covalent monolayers of POTS to form aof low surface energy on surface rough surfaces andrough covalent bonding bonding between self-assembled monolayers POTS and silica nanoparticles. self-assembled monolayers of POTS and silicaof nanoparticles.

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Figure 2. The schematic of the surface modification process by self-assembled monolayers and Figure 2. The schematic of the surface modification process by self-assembled monolayers and conversion hydrophilic (a) to surface super-hydrophobic (b). Figure from 2.from The schematic of modification (b). process by self-assembled monolayers and conversion hydrophilic (a)the to super-hydrophobic conversion from hydrophilic (a) to super-hydrophobic (b).

The uniformity and morphology of modification the coatings before after surface treatment Figure 2. The schematic of the surface process by and self-assembled monolayers and were The The uniformity and morphology of before andafter surface treatment were uniformity and morphology of the the coatings coatings and surface treatment characterized by from SEM techniques the SEM images are shown inafter Figure 3. The imageswere show conversion hydrophilic (a) and to super-hydrophobic (b).before characterized by SEM techniques and the areshown shown inFigure Figure 3.ofThe images characterized by SEM techniques andstructures the SEM SEM images images are 3. The images showshow distinguishable particles and porous which will allow in the trapping small-scale air The and morphology of at the coatings before andthe after were distinguishable particles and porous structures which allow trapping oftreatment small-scale airthe distinguishable particles porous structures which willwill allow the trapping of also small-scale pockets pockets anduniformity reduce theand fractional coverage the solid-liquid interface. It surface can be seenair that characterized by SEM techniques andand the after SEM are shown 3. The show pockets andfractional the fractional at theimages solid-liquid interface. It can also be images seen that and reduce the coverage atcoverage the solid-liquid interface. It can in also seen that the morphology morphology isreduce quite similar before treatment. There is Figure nobe obvious change inthethe distinguishable particles and porous structures which will allow the trapping of small-scale air morphology is quite similar before and after treatment. There is no obvious change in the morphology silica and particles the surface as the POTS in tends be very thinof selfis quite similarofbefore afterduring treatment. There treatment is no obvious change the to morphology silica pockets and reduce the fractional coverage at the treatment solid-liquid interface. Ittends can also bevery seenthin thatselfthe morphology of silica particles during the surface as the POTS to be assembled monolayers. particles during the surface treatment as the POTS tends to be very thin self-assembled monolayers. morphology is quite similar before and after treatment. There is no obvious change in the assembled monolayers. morphology of silica particles during the surface treatment as the POTS tends to be very thin selfassembled monolayers.

Figure 3. 3. Scanning electron microscope ofsilica silicananoparticles nanoparticles coating before (a) and Figure 3. Scanning electron microscope(SEM) (SEM) images coating before (a)(a) and Figure Scanning electron microscope (SEM) images of of silica nanoparticles coating before and after surface treatment (b). after surface treatment (b). after surface treatment (b). Figure 3. Scanning electron microscope (SEM) images of silica nanoparticles coating before (a) and

3.2. Confirmation of Self-Assembled Monolayers after surface treatment (b). Monolayers Confirmation Self-Assembled Monolayers 3.2.3.2. Confirmation ofofSelf-Assembled To confirmwhether whetherthe thePOTS POTS had had been been successfully successfully deposited onto the silica nanoparticles, To confirm To confirm whether the POTS had been successfully deposited depositedonto ontothe thesilica silicananoparticles, nanoparticles, 3.2. Confirmation of Self-Assembled Monolayers elemental analysis was performed using EDS. There are five elements including H, C, F, O and Si in elemental analysis was performed using EDS. There are five elements including H, C, F, OO and Si in elemental analysis was performed using There are five elements including H, C, F, and Si in To confirm whether had most been unique successfully deposited silica nanoparticles, the structure of POTS. F isthe thePOTS best and element to proveonto the the existence of such POTS the structure of POTS. F is the best and most unique element to prove the existence of such POTS the structure of POTS. F is the best and most unique element to prove the existence of such POTS elemental analysis performed using areCfive elements H,ofC,contamination. F, O and Si in coatings because H was is not easy to detect byEDS. EDSThere and any detected mayincluding be a result coatings because H isnot noteasy easyto todetect detect by by EDS EDS and CCdetected may be a result of of contamination. coatings because H andFany any detected may a result contamination. the structure ofitis POTS. F seen is thethat bestthere and is most unique element to prove thebeexistence such From Figure 4, can be a clear peak after POTS treatment, whileof there isPOTS no F From Figure 4, it can be seen that there is a clear F peak after POTS treatment, while there is no F Frompeak Figure 4, it can beis seen that to there is by a results clear peak POTSmay treatment, while there is no F peak coatings because H not easy detect EDS F and anyafter C detected result of contamination. before POTS treatment. The EDS therefore suggest that be a aPOTS coating had been peak before POTS treatment. The EDS results therefore suggest that a POTS coating had been From Figure 4, the it can be nanoparticles. seen theretherefore is a clear Fsuggest peak after treatment, while no F before POTS treatment. The EDSthat results thatPOTS a POTS coating hadthere beenisdeposited deposited onto silica deposited onto POTS the silica nanoparticles. peak before treatment. The EDS results therefore suggest that a POTS coating had been onto the silica nanoparticles. deposited onto the silica nanoparticles.

Figure 4. Energy dispersive X-ray spectroscopy (EDS) results for silica nanoparticles with fluoroalkyl silane, (POTS) treatment and without treatment. Figure 4. 1H,1H,2H,2H-perfluorooctyltriethoxysilane Energy dispersive X-ray spectroscopy (EDS) results for silica nanoparticles with fluoroalkyl Figure 4. Energy dispersive X-ray spectroscopy (EDS) (EDS) results for nanoparticles withwith fluoroalkyl Figure 4. 1H,1H,2H,2H-perfluorooctyltriethoxysilane Energy dispersive X-ray spectroscopy results forsilica silica nanoparticles fluoroalkyl silane, (POTS) treatment and without treatment. silane, 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) treatment and without treatment. silane, 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) treatment and without treatment.

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To further verify Nanomaterials 2016, 6, 232 the surface status and absorption of POTS of the treated silica particles 5and of 9 further verify the and absorption of 5POTS theXPS treated silica particles and thoseTo before treatment, XPSsurface analysisstatus was carried out. Figure showsofthe results of the F1s, F KLL thoseenergy before treatment, XPSsurface analysis was carried out.due Figure 5POTS shows results of(K the F1s, FbyKLL (the of the electrons ejectedstatus from the atoms toofthe filling of XPS the F1s state shell) an To further verify the and absorption ofthe the treated silica particles and (the energy theL electrons ejected the atoms due to the fillingan ofLXPS the F1s state an electron fromof the shellXPS coupled withfrom the carried ejection of an electron from shell), C1s and Cshell) − F regions those before treatment, analysis was out. Figure 5 shows the results of(K the F1s, Fby KLL electron from the L shell coupled with the ejection of an electron from an L shell), C1s and C−F regions of the spectra of the samples before and after treatment. From Figure 5a, it can be clearly seen that (the energy of the electrons ejected from the atoms due to the filling of the F1s state (K shell) by an of theisspectra of the samples From Figure it can be861.08 clearly seen there afrom F1s the peak centered atbefore 688.08 eV after and Ftreatment. KLL peaks centered at 5a, 834.08 and eV forthat the electron L shell coupled withand the ejection of an electron from an L shell), C1s and C−F regions there a F1s on peak atbefore 688.08 eV after and Ftreatment. KLL peaks centered 834.08 and 861.08 eV for the coating based nanoparticles after POTS coating, while there is at no F peak for coating based of the isspectra of silica thecentered samples and From Figure 5a, it can be the clearly seen that coating based on silica nanoparticles after POTS coating, while there is no F peak for the coating based on untreated In the scan for the at C− peakand shown in eV Figure 5b, there is a F1s silica peak nanoparticles. centered at 688.08 eVhigh-resolution and F KLL peaks centered 834.08 861.08 for the on untreated nanoparticles. In appears the high-resolution thewhile C−F shown the the C−Fbased peaksilica centered at 291.08 eV after POTS scan treatment there is for no the Cin −coating FFigure peak 5b, before coating on silica nanoparticles after POTS coating, whilefor there is nopeak F peak based C−Funtreated peak The centered at 291.08 eVofIn appears after POTS treatment while is no C−F before treatment. combined results EDS XPS confirm that self-assembled monolayers ofpeak POTS have on silica nanoparticles. theand high-resolution scan for the C−Fthere peak shown in Figure 5b, the treatment. The combined results ofsilica EDSnanoparticles. and confirm ofbefore POTS been successfully grafted onto theappears Thisthat is inself-assembled good agreement previous C−F peak centered at 291.08 eV afterXPS POTS treatment while there is monolayers no with C−F the peak have been successfully onto theand silica nanoparticles. is in good monolayers agreement with the results by Lai and Zhanggrafted etresults al. [14,17]. treatment. The combined of EDS XPS confirm that This self-assembled of POTS previous results by Lai and Zhang et al. [14,17]. have been successfully grafted onto the silica nanoparticles. This is in good agreement with the

previous results by Lai and Zhang et al. [14,17].

Figure 5. X-ray photoelectron spectroscopy (XPS) results for F (a) and C−F (b) of silica nanoparticles Figure 5. X-ray photoelectron spectroscopy (XPS) results for F (a) and C−F (b) of silica nanoparticles with treatment treatment and without treatment treatment byself-assembled self-assembled monolayers ofC−F POTS. Figure 5. X-rayand photoelectron spectroscopy (XPS) resultsmonolayers for F (a) andof (b) of silica nanoparticles with without by POTS. with treatment and without treatment by self-assembled monolayers of POTS.

Understanding the formation mechanism of the SAMs is important for further optimization. In Understanding the formation mechanism of the SAMs is important for further optimization. a previous report, itthe is inferred that the reaction starts from hydrolysis of the optimization. POTS precursor Understanding formation mechanism of the SAMs is the important for further In In a previous report, it is inferred that the reaction starts from the hydrolysis of the POTS precursor forms Si–OH bonds fromthat thethe Si–OCH 2CH 3 bonds. covalent linkage occursprecursor through awhich previous report, it is inferred reaction starts fromThen, the hydrolysis of the POTS which forms Si–OH bonds from the Si–OCH2 CH3 bonds. Then, covalent linkage occurs through interfacial condensation andfrom polymerization reactions between thecovalent hydroxyllinkage groupsoccurs and thethrough silanol which forms Si–OH bonds the Si–OCH 2CH3 bonds. Then, interfacial condensation and polymerization reactions between the hydroxyl groups and the silanol groups [14]. interfacial condensation and polymerization reactions between the hydroxyl groups and the silanol groups [14]. In [14]. the FTIR spectra shown in Figure 6, besides silica absorption peaks at about 810 cm−−11 and groups In the spectra shown in Figure 6, besides−1silica absorption peaks at about 810 cm −1 and −1, aFTIR 1086 In cmthe Si–OH absorption around cm−1silica is observed from peaks the samples before and after FTIR spectra shownpeak in Figure 6, 965 besides absorption at about 810 cm and −1 , a 1086 cm Si–OH absorption peaksuggest around that 965 cm is observed from the samples before and after −1 −1 treatment [18–20]. The FTIR results the surface of the silica nanoparticles is terminated 1086 cm , a Si–OH absorption peak around 965 cm is observed from the samples before and after treatment The FTIR results that the surface of the silica nanoparticles is terminated with –OH [18–20]. groups [21–22] which act suggest as anchoring points for the formation of covalent bonds with the treatment [18–20]. The FTIR results suggest that the surface of the silica nanoparticles is terminated with –OH groups [21,22] which act as anchoring points for the formation of covalent bonds with the hydrolyzed POTS [6]. with –OH groups [21–22] which act as anchoring points for the formation of covalent bonds with the hydrolyzed POTS [6]. hydrolyzed POTS [6].

Figure 6. Fourier transform infrared (FTIR) absorption spectra of silica nanoparticles before and after Figure 6. Fourier Fouriertransform transform infrared (FTIR) absorption spectra of silica nanoparticles before and treatment. Figure 6. infrared (FTIR) absorption spectra of silica nanoparticles before and after after treatment. treatment.

3.3. Surface Hydrophobicity 3.3. Surface Hydrophobicitymonolayer of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) will form The self-assembled low-surface-energy surfaces which will contribute to the super-hydrophobicity. Figure 7 shows the The self-assembled monolayer of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) will form low-surface-energy surfaces which will contribute to the super-hydrophobicity. Figure 7 shows the

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3.3. Surface Hydrophobicity Nanomaterials 2016, 6, 232 The self-assembled

6 of 9 monolayer of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) will form low-surface-energy surfaces which will contribute to the super-hydrophobicity. Figure 7 shows the water contact contact angle angleof ofthe thewater waterdroplets dropletsononthe the silica coating without (Figure with (Figure water silica coating without (Figure 7a)7a) andand with (Figure 7b) 7b) POTS treatment. water contact angle changes from ± 0.9° withouttreatment treatmenttoto163 163° 7.4° ◦ without ◦ ±± 7.4 ◦ ± 0.9 POTS treatment. TheThe water contact angle changes from 13◦13° measured for six samples after treatment with the same processing conditions, indicating a transition measured for six samples after treatment with the same processing conditions, indicating a transition from hydrophilic as aa result from hydrophilic to to super-hydrophobic super-hydrophobic as result of of POTS POTS treatment. treatment. The The water water droplets droplets will will bounce bounce off from the surface in the case of a very small angle inclination of the sample surface, even if the off from the surface in the case of a very small angle inclination of the sample surface, even if the angle angle of inclination is invisible. The bouncing off phenomenon of the water droplets is shown in of inclination is invisible. The bouncing off phenomenon of the water droplets is shown in Video S1. Video S1.

Figure 7. Water contact angle of water droplets on silica nanoparticles–based coating without (a) and with (b) POTS treatment. treatment.

The Wenzel Wenzelmodel model the Cassie-Baxter model are used generally used explain the The and and the Cassie-Baxter model are generally to explain theto hydrophobicity hydrophobicity of coatings with high roughness. In the Wenzel model, water droplets follow the of coatings with high roughness. In the Wenzel model, water droplets follow the profile of a rough profile of a rough surface and are pinned to the surface, which results in them being unable to slide surface and are pinned to the surface, which results in them being unable to slide on the surface [18]. on the surface [18]. However, in the silica nanoparticles–based samples,tend water to slide However, in the silica nanoparticles–based samples, water droplets to droplets slide ontend the surface on the surface very easily, suggesting the Cassie-Baxter model is more suitable to explain our very easily, suggesting the Cassie-Baxter model is more suitable to explain our experimental results. experimental results. In Cassie’s equation: In Cassie’s equation: cosθ A A==f 1f1cosθ cosθ cosθ− − ff22, ,

(3) (3)

where θ θAA isisthe theapparent apparentcontact contact angle measured substrate surface; θ iswater the water contact angle measured on on the the substrate surface; θ is the contact angle ◦ [19]; f and f are the fractions of the solid angle on the fluoridated smooth surface and it was 100 on the fluoridated smooth surface and it was 100° [19]; f1 and 21are the2 fractions of the solid surface surface air in contact with droplets; water droplets; f 2 = The 1 [20]. The f 1 calculated using the average and air and in contact with water and f1 and + f2 =f 11+[20]. f1 calculated using the average water ◦ is 5.3% and it indicates that 94.7% of the surface is occupied by air, which water of 163 contactcontact angle angle of 163° is 5.3% and it indicates that 94.7% of the surface is occupied by air, indicates that a combination of silica nanoparticles and POTS allows air to be be trapped trapped easily, easily, resulting in a super-hydrophobic surface. surface. 3.4. Water Droplet Droplet Icing 3.4. Water Icing Behavior Behavior According nucleation rate According to to classical classical nucleation nucleation theory theory and and observation, observation, it it was was reported reported that that the the nucleation rate and macroscopical growth velocity of ice can be greatly reduced by a super-hydrophobic surface owing and macroscopical growth velocity of ice can be greatly reduced by a super-hydrophobic surface to an extremely low, actual solid-liquid contact area caused bycaused the trapped airtrapped pocketsair [4].pockets As previously owing to an extremely low, actual solid-liquid contact area by the [4]. As discussed, reducedthe solid-liquid interface fraction of 5.3% will of contribute an icing delay to previously the discussed, reduced solid-liquid interface fraction 5.3% willtocontribute to andue icing the limited thermal exchange between the solid-liquid surface. The water droplet icing test results in delay due to the limited thermal exchange between the solid-liquid surface. The water droplet icing Figure 8 show that 289 s were needed for the formation of ice on the super-hydrophobic surface of the test results in Figure 8 show that 289 s were needed for the formation of ice on the super-hydrophobic silica-based nano-coating and 24 s were needed bare substrates. Forbare the commercial surface of the silica-based nano-coating and 24fors the were needed for the substrates. silicone For the icephobic samples, 204 s were needed for ice formation. The water droplet icing test results of coated commercial silicone icephobic samples, 204 s were needed for ice formation. The water droplet icing samples show a significant compared thecompared bare substrates and an substrates improvement in test results of coated samplesdelay showina icing significant delay with in icing with the bare and an icephobicity with compared the commercial icephobic products. improvementcompared in icephobicity with the commercial icephobic products.

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droplet icing icing test test results. results. Figure 8. Water droplet Figure 8. Water droplet icing test results.

3.5. Ice Ice Adhesion Adhesion Strength Strength 3.5. 3.5. Ice Adhesion Strength Besides the the icing icing delay delay performance, performance, the the ice ice adhesion adhesion strength strength is is another another important important parameter parameter Besides for icephobicity. With low ice adhesion strength, the ice can be removed easily which is desirable for Besides the icing delay performance, the ice adhesion strength is another important parameter for icephobicity. With low ice adhesion strength, the ice can be removed easily which is desirable for de-icing. It was was revealed revealed that the average ice adhesion adhesion strength is linearly linearly correlated with + cosθ cosθ e, for icephobicity. With lowthat ice the adhesion strength, the icestrength can be removed easily which with is desirable for de-icing. It average ice is correlated 11 + e, with θ e standing for the estimated equilibrium contact angle which implies that a low ice adhesion de-icing. It was revealed that the average ice adhesion strength is linearly correlated with 1 + cosθ with θe standing for the estimated equilibrium contact angle which implies that a low ice adhesione, strength can be be obtained obtained from super-hydrophobic super-hydrophobic surfaces [1]. In this this experiment, the centrifuge with θe standing for the estimated equilibrium contact angle[1]. which implies that a low icecentrifuge adhesion strength can from surfaces In experiment, the adhesion test be method wasfrom used super-hydrophobic to evaluate evaluate the the ice ice adhesion adhesion strength of silica-based silica-based nano-coatings nano-coatings strength can obtained surfaces strength [1]. In this experiment, the centrifuge adhesion test method was used to of and aluminium substrates for comparison [16]. From Figure 9, it can be seen that all the measured measured adhesion test method was used to evaluate the ice adhesion strength of silica-based nano-coatings and aluminium substrates for comparison [16]. From Figure 9, it can be seen that all the shear stressesbetween between coated samples/glaze ice Figure block remarkably lessthe the shear and aluminium substrates for comparison [16]. ice From 9, it can be seen that allthan the measured shear stresses thethe coated samples/glaze block werewere remarkably less than shear stresses stresses between the Al substrates/glaze ice block. There are some variations in the ice adhesion shear stresses between the coated samples/glaze ice block were remarkably less than the shear between the Al substrates/glaze ice block. There are some variations in the ice adhesion results. results. For better accuracy, we tested six silica-based coating samples fabricated by the same stresses between the Al substrates/glaze ice block. There are some variations in the ice adhesion For better accuracy, we tested six silica-based coating samples fabricated by the same formulation. formulation. The difference between each especially between andby 7,by might be results. For better accuracy, we tested sixsample, silica-based coating samples fabricated the same The difference between each sample, especially between samples 5 and 7, samples might be5caused natural caused by natural variability in the shapes of ice blocks. The shear stresses between the ice and the formulation. The difference between each sample, especially between samples 5 and 7, might be variability in the shapes of ice blocks. The shear stresses between the ice and the silica-coated samples silica-coated samples are all lower than 100 kPa which is the threshold for icephobicity [21]. It is worth caused by natural variability in the shapes of ice blocks. The shear stresses between the ice and the are all lower than 100 kPa which is the threshold for icephobicity [21]. It is worth mentioning that mentioning that somewere glaze ice blocks were dropped the silica-coated sample before the rotation silica-coated samples are alldropped lower than 100 kPa whichfrom is the threshold forthe icephobicity [21]. Itshowing is worth some glaze ice blocks from the silica-coated sample before rotation started started showing extremely low ice adhesion strength. mentioning that some glaze ice blocks were dropped from the silica-coated sample before the rotation extremely low ice adhesion strength. started showing extremely low ice adhesion strength.

Figure 9. Ice adhesion results of silica-based nano-coatings on Al substrates (samples 2–7) and untreated Al surface (sample Figure9.9.Ice Iceadhesion adhesion results of silica-based nano-coatings on Al substrates 2–7) and Figure results of1).silica-based nano-coatings on Al substrates (samples(samples 2–7) and untreated untreated Al surface (sample 1). Al surface (sample 1).

A strict definition of icephobicity remains unclear. It was suggested that a surface should be calledAicephobic if it delays ice formationremains at temperatures below freezingthat point of watershould and/orbe if strict definition of icephobicity unclear. It was the suggested a surface A strict definition of icephobicity remains unclear. It was suggested that a surface should it has aicephobic weak adhesion strength to ice of less than 100 kPabelow [21]. The bouncingpoint off ofofincoming water called if it delays ice formation at temperatures the freezing water and/or if be called icephobic if it delays ice formation at temperatures below the freezing point of water droplets, the adhesion icing delay performance strength show the superit has a weak strength to ice ofand less the thanlow 100 ice kPaadhesion [21]. The bouncing off of that incoming water and/or if it has a weak adhesion strength to ice of less than 100 kPa [21]. The bouncing off of hydrophobic based on silica and nanoparticles areadhesion suitable strength for use as icephobic droplets, the coatings icing delay performance the low ice show that thecoatings superincoming water droplets, the icing delay performance and the low ice adhesion strength show that the regarding theircoatings icephobicity. hydrophobic based on silica nanoparticles are suitable for use as icephobic coatings regarding their icephobicity. 3.6. Durability under Impact of Pneumatic Water Impinging 3.6. Durability under Impact of Pneumatic Water Impinging

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super-hydrophobic coatings based on silica nanoparticles are suitable for use as icephobic coatings regarding their icephobicity. 3.6. Durability under Nanomaterials 2016, 6, 232Impact of Pneumatic Water Impinging

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When an aircraft flies through the atmosphere, its surfaces may undergo impact from When an aircraft flies through the atmosphere, its surfaces may undergo impact from hydrometeors such as rain, which can adversely affect the structure of the aircraft and reduce the hydrometeors such as rain, which can adversely affect the structure of the aircraft and reduce the lifecycle of the components [22]. Therefore, the durability performance of the hydrophobic/icephobic lifecycle of the components [22]. Therefore, the durability performance of the hydrophobic/icephobic coatings is a critical factor for practical applications in aircrafts. In this experiment, pneumatic water coatings is a critical factor for practical applications in aircrafts. In this experiment, pneumatic water impinging was used in the erosion test rig to evaluate the durability. Figure 10 shows the water impinging was used in the erosion test rig to evaluate the durability. Figure 10 shows the water impinging test results for silica-based coatings for the as-prepared sample, after a 30 min test and after impinging test results for silica-based coatings for the as-prepared sample, after a 30 min test and a 60 min test. The super-hydrophobicity remained after the erosion test for 60 min. Although the water after a 60 min test. The super-hydrophobicity remained after the erosion test for 60 min. Although contact angle dropped from 163◦ ± 7.4◦ to 161◦ ± 4.9◦ after the 30 min erosion test and to 153◦ ± 2.6◦ the water contact angle dropped from 163° ± 7.4° to 161° ± 4.9° after the 30 min erosion test and to after the 60 min test, the degeneration of hydrophobicity is at a reasonable value, indicating a certain 153° ± 2.6° after the 60 min test, the degeneration of hydrophobicity is at a reasonable value, indicating durability. However, aiming for applications in aerospace, further optimization will be performed to a certain durability. However, aiming for applications in aerospace, further optimization will be improve the durability. performed to improve the durability.

Figure Figure 10. 10. Water contact contact angle angle before before and and after after erosion erosion test test from from water water impinging impinging for for silica-based silica-based nano-coatings nano-coatings for for as-prepared as-prepared sample, sample, after after 30 30 min min test testand andafter after60 60min mintest. test.

4. Conclusions 4. onto glass substrates to form nanostructured rough surface Silica nanoparticles nanoparticleswere weredeposited deposited onto glass substrates to a form a nanostructured rough with the function of trapping small-scale air pockets. Self-assembled monolayers (SAMs) of surface with the function of trapping small-scale air pockets. Self-assembled monolayers (SAMs) 1H,1H,2H,2H-perfluorooctyltriethoxysilane were grafted onto the the silica nanoparticle surface by the of 1H,1H,2H,2H-perfluorooctyltriethoxysilane were grafted onto silica nanoparticle surface by chemical vapor deposition method to reduce the surface energy. The The morphology, composition, and the chemical vapor deposition method to reduce the surface energy. morphology, composition, functional groups were characterized to to reveal the the and functional groups were characterized reveal therelationship relationshipbetween betweenthe the characteristics characteristics of of the ◦ nanocomposite material and the hydrophobicity. An average water contact angle of 163° suggests nanocomposite material and the 163 aa super-hydrophobicsurface surfacewas wasobtained obtainedon onsilica silicananoparticles nanoparticles with surface modification SAMs super-hydrophobic with surface modification byby SAMs of of POTS. water droplet test results that theformation icing formation of silica-based nanoPOTS. TheThe water droplet icingicing test results show show that the icing of silica-based nano-coatings coatings was significantly delayed compared to bare substrates and commercial icephobic products was significantly delayed compared to bare substrates and commercial icephobic products due to dueexistence to the existence ofsurface the lowenergy surface energy and air on the The ice adhesion the of the low and air pockets on pockets the surface. Thesurface. ice adhesion strength test strength test results the shear stresses the treated block surface/ice blocklower are much results show that theshow shearthat stresses between the between treated surface/ice are much than lower those than those the bare block. substrate/ice block. delay and low ice adhesion strength between the between bare substrate/ice The icing delayThe andicing low ice adhesion strength suggest icephobic suggest icephobic obtained from the super-hydrophobic silica-based coatings. To surfaces have beensurfaces obtainedhave frombeen the super-hydrophobic silica-based coatings. To evaluate durability, evaluate durability, a test rig of erosion from pneumatic water impinging was designed and set up. a test rig of erosion from pneumatic water impinging was designed and set up. The erosion test results The erosion test results show that super-hydrophobicity remained after testing for 60 min. Further show that super-hydrophobicity remained after testing for 60 min. Further optimization aiming for optimization aimingisfor applications is in progress. aircraft applications in aircraft progress. available online at http://www.mdpi.com/2079-4991/6/12/232/s1, Supplementary Supplementary Materials: Materials:The Thefollowing followingare are available online at http://www.mdpi.com/2079-4991/6/12/232/s1, Video Video S1: S1: The The water water droplets droplets bouncing bouncing off offphenomenon. phenomenon.

Acknowledgments: This work was supported by CleanSky II-EU initiative GAINS (Grant Agreement No: 671398). The work forms a part of the project to develop a suitable icephobic and hydrophobic coating on aircraft wing surfaces. Author Contributions: Junpeng Liu, Fang Xu, Kwing-So Choi, and Xianghui Hou were involved in the design of the experiment, performed the experiments and drafted the manuscript. Zaid A. Janjua and Barbara Turnbull contributed to the design, set-up, measurement and calculation of the icephobicity performance test. Martin Roe contributed to the characterization of materials. Each contributor was essential to the production of this work.

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Acknowledgments: This work was supported by CleanSky II-EU initiative GAINS (Grant Agreement No: 671398). The work forms a part of the project to develop a suitable icephobic and hydrophobic coating on aircraft wing surfaces. Author Contributions: Junpeng Liu, Fang Xu, Kwing-So Choi, and Xianghui Hou were involved in the design of the experiment, performed the experiments and drafted the manuscript. Zaid A. Janjua and Barbara Turnbull contributed to the design, set-up, measurement and calculation of the icephobicity performance test. Martin Roe contributed to the characterization of materials. Each contributor was essential to the production of this work. Conflicts of Interest: The authors declare no conflict of interest.

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