Characterization of Methyl-Functionalized Silica Nanosprings for

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Surfaces, Interfaces, and Applications

Characterization of methyl functionalized silica nanosprings for superhydrophobic and defrosting coatings Giancarlo corti, Nickolas C. Schmiesing, Griffin T Barrington, Morgan. G Humphreys, and Andrew D. Sommers ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18873 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Applied Materials & Interfaces

Characterization of Methyl Functionalized Silica Nanosprings for Superhydrophobic and Defrosting Coatings Giancarlo Corti,∗ Nickolas C. Schmiesing, Griffin T. Barrington, Morgan G. Humphreys, and Andrew D. Sommers∗ Miami University, Department of Mechanical and Manufacturing Engineering, Oxford, OH, USA E-mail: [email protected]; [email protected] Phone: +1(513)529 0747; +1(513)529 0718 Abstract Thin non-PFA superhydrophobic coatings are desirable for heat exchangers due to their lower thermal resistance, and reduced environmental concerns. Coatings requirements must also include robustness and longevity and facilitate high defrosting rates in refrigeration applications to warrant their adoption and use. Methyl functionalized silica nanosprings (SN) possess water droplet static contact angles above 160° with contact angle hysteresis values as low as 6.9° for a submicron thick coating. The methyl functional groups render the silica surface hydrophobic, while the geometrical and topographical characteristics of the nanosprings make it superhydrophobic. Results show that SN are capable of removing 95% of the frost from the surface at a lower temperature than the base aluminum substrate. The submicron SN coating also decreases the time to defrost by ≈1.5 times and can withstand more than 20 frosting-defrosting cycles in a high humidity environment akin to real working conditions for heat exchangers.

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Keywords silica nanosprings, deicing, superhydrophobic, hexamethyldisiloxane

1

Introduction

Self-defrosting surface coatings for cables, marine structures, aircraft wings, and heat exchangers are a desirable technology that has received considerable attention in recent years. In particular, heat exchangers present additional unique challenges since the coating used should minimally affect the air-side pressure drop and heat transfer and pose a small additional thermal resistance. There are only a few studies, however, on the effect that coatings which alter the surface wettability of the fin and tube have on the overall thermal efficiency of heat exchangers, and even less that report the influence of surface wettability modification on defrosting performance. As a result, refrigerator evaporators still require periodic defrosting, which typically involves an external source of energy and/or long down times which both represent inefficiencies from the system standpoint. Therefore, a robust surface coating that minimizes the time to defrost and/or the accompanying energy requirement is still desired. Several different anti-icing solutions such as incorporating a lubricant into the surface, or coating the substrate with long-chain alkalis and/or perfluoroalkoxy (PFA) have been explored to prevent ice formation and improve de-icing. 1–6 The degree of ice-accretion resistance of these coatings depends on the coating application and test procedures (e.g., nano and microstructure coatings shed water droplets easily, but they suffer from poor mechanical stability under high humidity and pressure). Infused lubricants in the microstructural coating have shown improved mechanical stability and de-icing properties; however, these lubricants can be lost over time, leading to a deterioration in the desired properties of the coating. Thermally, these solutions can also degrade the air-side heat transfer performance since they involve either depositing a solid or liquid insulator on the surface, thereby affecting 2


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the efficiency of the heat exchanger. Environmentally, long-chain PFAs are also a concern, especially near water and households where heat pumps and refrigerator evaporators are commonly located. 7 Kim and Lee studied the influence that the surface coating wettability has on heat pump fins under typical outside winter conditions. 8 Kim and Lee found that hydrophilic fins had lower water retention and air-side pressure drop, and higher thermal efficiency than hydrophobic fins under wet operating conditions. 8,9 However, under frost conditions, hydrophobic surfaces provided the higher thermal efficiency and lowest air-side pressure drop due to frost retardation. 9 This frost retardation effect was reduced when the refrigerant temperature was lowered to -12 ℃. 9 Differences in the defrost time however between hydrophilic and hydrophobic surfaces were minimal. Kim and Lee attributed this finding to the thicker frost layer of the hydrophilic sample and the lower thermal conductivity of the hydrophobic one. 8 Other studies have similarly offered a mixed message with regards to the effect of surface modification on frost growth characteristics and defrosting behavior. Kim and Lee found that the frost layer was thinner and the average frost density was higher on a hydrophilic surface. 8 They also found that the ratio of residual water was smaller on the hydrophilic surface making it preferred to other treated surfaces. 8 In contrast, Rahimi et al. found that flat hydrophobic surfaces exhibited slower ice growth and overall denser ice layers making this surface treatment more preferred for aluminum heat exchangers. 10 Likewise, Wang et al. found that frost growth was delayed by as much as 60 minutes on a hydrophobic-coated surface and that strong interfacial adhesion of the coating was observed suggesting good longevity. 11 Although there are a significant number of publications and mathematical models addressing frost formation and properties of anti-icing coatings, there are only a limited number of studies on the influence of surface wettability on defrost efficiency and dynamics. Boreyko et al., Van Dyke et al. and Schmiesing and Sommers are a few studies addressing the 3



effects of nano- and microstructured coatings on defrost dynamics. 12–14 Liu and Kulacki also demonstrated that the time to defrost is smaller on superhydrophobic surfaces, when the melting finishes without water retention, as compared to plain substrates. 15 Moreover, repeated frost-defrost cycles have been shown to damage nano- and microstructured superhydrophobic coatings. 11,16 Thus, the damaged coating will accrete a higher frost mass under the same conditions. High humidity environments, can also condense water inside of the microstructure promoting frost formation within the substrate coating. 17–20 This Wenzel type mechanism promotes ice adhesion, increasing the frost mass as compared to a flat substrate thereby permanently damaging the superhydrophobic coating. 21,22 In this work, differences in frosting and defrosting behavior of methyl-functionalized silica nanosprings (SN) were studied under multiple frosting-defrosting cycles and compared to a bare aluminum substrate. Emphasis was given to the geometry of the silica nanospring mat topology, rather than the hydrophobic coating. Silica nanosprings are made of an average of ten nanowires that cohesively coil together to form a nanospring with an average wire diameter less than 100 nm, an average coil pitch and diameter smaller than 300 nm, and an overall length of tens of micrometers. 23,24 The SN geometry (Figure 1) provides a unique combination of nano and micro dimensions within the SN itself and the SN mat. Wojick et al. identified that the twisting and bundling of the nanowires that form SN minimize the free energy, and thus bifurcation is energetically disfavorable. 23 Suggesting that longer growth times will produce thicker and longer silica nanosprings, effectively altering the SN mat topology. Silica nanosprings are a non-porous high surface area nanomaterial with a high ratio of air to solid. Thus, air can flow through them minimizing the thermal penalty of the hydrophobic coating and because they are a flexible material SN present a unique opportunity and a solution for defrosting improvement on heat exchanger fins. Though SN are not abrasion resistant, SN are strong to support fluid flow, and thus the application of this SN coating will be limited for heat exchangers. 25,26

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a)

200 nm c)

b)

Figure 1: Silica nanosprings, a) Transmission Electron Microscope (TEM) image of silica nanosprings, the black hexagonal shape is the SN catalyst; b) CAD representation of the silica nanospring coil structure and geometry, and c) SN cross section schematic.

2 2.1

Experimental Test Samples

Samples were prepared using 3.17 mm thick aluminum 6061-T6 strips that were cut into plates approximately 100 mm × 81 mm in size. The aluminum samples were machined with four 6 mm through-holes to support the sample on the test fixture. Nylon screws with Teflon spacers were used to hold the samples to the fixture to minimize water retention and thermal losses from the surface. T-type thermocouples were inserted in a 1.7 mm hole drilled into each short side of the sample to monitor the sample temperature during testing. The top and bottom surfaces were kept "as-is". The samples were cleaned in subsequent sonication baths of ethanol and deionized water (DI) prior to testing. Cleaned samples were divided into two groups, namely, baseline (B) and silica nanosprings (SN). The baseline sample was placed 5



directly on the test fixture, while silica nanosprings were grown on the SN samples. SN samples were first sputtered for 20 sec with gold, the catalyst to grow the silica nanosprings. Nanospring were then grown following the procedure described by McIlroy et al. and Corti et al. 24,27 All growth conditions were kept constant with the exception of the mat thickness which directly depends on the growth time. To address the effect that silica nanosprings have on the frost and defrost efficiency, four different growth times were used, 30, 15, 5, and 2 minutes. Silica nanosprings OH bonds were activated by plasma treatment using a PlasmaPreen with an oxygen flow rate of 20 sccm at 30 mbar for 60 sec. After activation, SN samples were vapor functionalized with Hexamethyldisiloxane (HMDSO), at 200 ℃ for 30 min. The naming convention for the SN test surfaces indicates the duration of the growth period for the SN mat. Table 1 provides details of the different surfaces along with each of the surface names that were used in this study. Table 1: Identification of the SN surfaces and coating properties Sample Growth Period SN Mass Name (min.) (mg) B 0 0 SN30 30 15.7 SN15 15 8.9 SN5 5 5.4 SN2 2 1.1

Description Baseline Baseline+SN Baseline+SN Baseline+SN Baseline+SN

The silica nanospring surfaces were characterized using a Perkin Elmer Spectrum One Attenuated Total Reflection (ATR) infrared spectroscopy to ensure a uniform silane coating. ATR measurements were collected at 1 cm−1 with 32 scans. Hydrophobicity was studied via contact angle measurements using a Ramè-Hart contact angle goniometer with a resolution of less than 1°. Measurements of static, advancing, and receding contact angles were taken, with at least 25 measurements being recorded for each at varying locations on the sample. Contact angle hysteresis was calculated as the difference of the advancing and receding contact angles. The contact angle measurements were taken before and after the frost-defrost 6


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experiments. Finally, a Zeiss Supra Field Emission Scanning Electron Microscope (FE-SEM) was employed to measure the SN geometry and verify their integrity after the frost-defrost cycles. All SEM images were taken at 2KV to minimize charging of the silica coating. The liquid-to-air fraction fla of the SN coated samples was measured using at least three SEM images of each sample with a 5KX magnification. The SEM micrographs were converted to binary images, and the fla were estimated as the percentage of the black pixels (holes or air in the mat) with respect to the whole image. All image processing and measurements was performed with the NIH software ImageJ.

2.2

Environmental Test Chamber

An environmental test chamber (Figure 2) was used to carry out the testing for this study. The Plexiglas test chamber has a dual chamber design and overall dimensions of 400 mm × 257 mm × 285 mm. A hinged top allows access to the thermoelectric cooler (TEC) (B) and the test surface (A) under consideration, while also providing a seal to enclose the test surroundings. A vertical divider (E) inside the chamber is used to separate the surface surroundings from the TEC, allowing for more precise humidity control as well as higher relative humidity percentages to be achieved near the sample. The dual chamber design minimizes the amount of heat transfer from the TEC to the front chamber that was used for testing, thus maintaining a constant surrounding air temperature. The internal separator (E) was secured to the walls of the test chamber to eliminate any interaction with the Sartorius GP5202 balance (C). The TEC was used to mount the test sample and control its surface temperature. Insulation tape was affixed to the mounting stage around the test sample, minimizing heat loss and boundary condensation. A layer of thermal paste was applied between the test sample and the TEC to minimize the thermal contact resistance between the TEC stage and the surface. Four nylon screws with Teflon spacers were used to secure the samples to the stage. The bottom edge of the sample remained free allowing the melted frost layer to drain from the sample surface and be collected in a drip tray (D) for 7



accurate measurement of the mass removed from the sample surface. The mass of the frost growth and subsequently the water retained on the surface during defrosting was recorded every ten seconds during the experiments. The relative humidity and the surrounding air temperature were measured using an OMEGA OM-71 temperature and humidity data logger (F) with an accuracy of ±2% RH and ±0.5 ℃. The relative humidity within the chamber was regulated using an ultrasonic cool mist humidifier (H) and an Auber TH102 controller (not shown). Additional temperature measurements were collected via two T-type thermocouples mounted on the TEC stage, and two more thermocouples inserted in ≈6 mm deep holes on each side of the test surfaces. Data from the sample surroundings and the average surface temperature was gather (Figure 3) during a representative test. (Note: The peaks in this plot correspond to the three defrosting events that are performed during each test.) This plot shows that the surrounding temperature was held constant within ±1.0 ℃, while the relative humidity remained within ±3% of the set-point value. Additional information about the test setup can be found in the work of Sommers et al. 28

Figure 2: Environmental test chamber used for testing. A) test sample, B) thermoelectric cooler (TEC), C) balance, D) drip tray, E) chamber divider, F) temperature and RH data logger, G) environmental temperature and RH sensor, and H) inlet humidification port.

2.3

Testing and Data Collection

Cyclical frosting and defrosting tests were carried out during the present study to evaluate the defrosting performance of each sample surface, as well as their response to repeated frosting 8


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Temperature (oC)

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50 40 30 20 10 0 -10

80

Relative humidity Air temperature Sample temperature

70 60 50 40 30

0

2

4 6 8 10 Time (s x 103)

12

Relative Humidity (%)

e

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20

Figure 3: Air temperature (black), sample temperature (blue) and humidity measurements (red) from a representative test. and defrosting events. These tests consisted of exposing each sample to a frost-growth period lasting one hour, followed by a ten minute defrost period. This pattern was repeated for three cycles, resulting in a total test duration of three hours and thirty minutes. For each test, the sample surface was initially covered using a thin plastic sheet to prevent premature condensation and frost growth on the sample before the desired set-point surface temperature was achieved using the TEC. Each sample was evaluated for three temperatures; -8℃, -10℃, and -12℃, resulting in a total of nine defrost cycles per sample. The relative humidity was controlled during testing and kept constant at 60% during the whole test interval, while the surrounding environment temperature was monitored. Mass, temperatures, and humidity were recorded every ten seconds for the duration of each experiment. Further information about this testing procedure can be found at Schmiesing and Sommers. 12

3 3.1

Results and discussion Surface Charactherization

Previous studies on silica nanosprings by Wang et al. identified the amorphous nature of SN and its FT-IR spectrum (Figure 4), which agrees well with the NIST IR spectrum for amorphous silica. 29 Thus, nanosprings in their uncoated state have an innate hydrophilicity as shown (Figure 4) by the broadband stretching at 3400 cm−1 and its shoulder at ≈3650 cm−1 . 9



Additionally, the free silanol groups at ≈960 cm−1 and the Si-O-Si groups at ≈760 cm−1 correspond to amorphous silica. Hexamethyldisiloxane (HMDSO)-coated silica nanosprings, show the C-H bending mode at ≈1260, 860 and 800 cm−1 , a narrow stretching peak at 2900 cm−1 , and the disappearing of the broad OH and free silanol bands which indicates its chemical conversion into a hydrophobic surface. Thus the methyl functional groups are responsible for rendering the SN surface hydrophobic.The superhydrophobicity of the SN coated samples is achieved by the nanostructured topology of the silica nanosprings mats. 100 Transmittance (AU)

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90 80

OH

CH3

70

Si-CH3

60

Uncoated SN HMDSO coated SN 40 4,000 3,000 2,000 1,000 Wavenumber (cm-1) 50

Figure 4: FTIR spectra of the silica nanosprings (black) with its stretching OH band at 3400 cm−1 and the methylated SN (red) with its peak at 2900 cm−1 . Surface topology of the SN samples was controlled by the thickness of the silica nanospring coatings. High-resolution FE-SEM images (Figure 5) were used to measure the gaps within the silica nanospring mats and their correspondent coil diameters. These topographical differences between samples were further verified by contact angle (CA) measurements (Figure 5) which contain the optical images of 10 µl water droplets used to measure the static contact angle of each sample. Table 2 summarizes the thickness, geometry, and superhydrophobicity of the tested substrates as a function of the mass, dimensions ranges, and contact angle respectively. The identification number refers to the growth time in minutes, i.e., SN30 had a growth time of 30 minutes. Based on these results, thicker SN coatings, i.e., SN30 and SN15 have similar static contact angles and CA hysteresis, ≈154° and ≈30° respectively. The gap distance of SN30 and SN15 are in the range of 1811 to 674 nm (Note: The gap distance refers to the approximate distance between individual nanosprings on the 10


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surface). It is also important to note that although SN30 and SN15 samples have a contact angle hysteresis greater than 10°, water droplets still possess high mobility on these surfaces. This phenomenon is attributed to the length and flexibility of SN, which deflects under the weight of the water droplet. Thinner SN coatings have a reduced gap distance, in the range of 400 - 135 nm and they had their static contact angles increased by ≈10°. The main geometric difference between the SN5 and SN2 samples is the silica nanosprings coil diameter distribution, which decreased from 50±15 nm to 39±10 nm. These differences increased the fraction of the liquid-air interface (fla ) for samples coated with the thinner SN. More importantly, as the thickness of the SN mats decreases and fla increases their contact angle hysteresis decreased significantly down to 10.2°±3.7° and 6.9°±2.7° for the SN5 and SN2 samples, respectively. These findings are in accordance with the Cassie−Baxter relation, and previous works in superhydrophobicity. As with other similar superhydrophobic nanostructured surface, smaller water of droplets are able to nucleate in the walls, and valleys of silica nanospring coating , resulting in a mixed Cassie and Wenzel type droplets during condensation. 17 Due to the 3D nature of SN mats, it is expected that as these internal water droplets grow the hydro Overall these finding suggests that the superhydrophobicity of SN coated samples can be controlled by the growth time of silica nanosprings. Table 2: Contact angle measurements, topology, and physical properties of tested surfaces Sample

Gap Coil d. range (nm) range (nm) B SN30 1811-675 189-95 SN15 1540-674 121-65 SN5 401-179 65-35 SN2 335-135 49-29

fla (%) 45.2±0.6 50.4±0.3 58.1±0.1 59.3±0.2

Thickness (nm) 4072±240 1358±57 754±100 488±25

11

Θstatic (°) 99.0 ± 3.6 154.6±1.6 152.3±1.8 164.7±1.8 164.1±1.7


Θadv (°) 110.4±4.1 161.4±1.9 161.6±1.6 166.9±1.1 166.9±1.4

Θrec Θadv − Θrec (°) (°) 70.5±2.7 39.9±6.8 130.6±1.6 30.8±3.6 129.9±2.3 31.6±3.9 156.7±2.7 10.2±3.7 160.0±1.3 6.9±2.71


B

2 μm

SN30

500 nm

2 μm

500 nm

SN5

SN15

2 μm

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500 nm

2 μm

500 nm

SN2

2 μm

500 nm

Figure 5: High-resolution FE-SEM images and optical images of a 10 µl water droplets from contact angle measurement show the topographic and superhydrophobic differences a between the tested samples.

3.2

Nanospring Mat Thickness Performance and Repeatability

The test chamber employed in these experiments was previously used and characterized by the authors and demonstrated high repeatability over tests performed several days apart. 12 Defrosting performance of the samples was measured as a function of the mass of water released after each defrost cycle for three surface temperatures, -8℃, -10℃, and -12℃, being -12℃ the first test condition and -8℃ the last one. The nondimensional defrost metric (Φ) shown in Table 3 is the maximum value for each sample calculated using the following defrost metric: Φ=

mr t f , td mi

(1)

where m and t are the mass and time respectively, the subindices f and d represent the frost and defrost cycle respectively, and the subindices r and i stand for removed and initial mass of water, respectively. This metric was introduced by Schmiesing and Sommers to eliminate small variances in the frost and defrost time. 12 Larger Φ values correspond to better defrosting performance.

12


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Table 3: Identification of the SN surfaces and coating properties Sample B SN30 SN15 SN5 SN2

-8℃ 3.76 3.58 3.38 3.3 4.18

Defrost metric (Φ) -10℃ 4.34 2.31 5.39 8.7 8.59

-12℃ 4.81 3.89 7.97 8.26 7.54

Mass data (Figure 6) for each sample subjected to three defrost cycles was collected following the procedure stated earlier. The frost mass accretion for all samples is higher at the lower temperatures, i.e., -12℃. The baseline sample performed relatively well at 12℃, but as the test progressed it was possible to see an “aging effect”. The frost rate of the baseline (B) and SN30 samples increased at higher temperatures, i.e., -8℃, and with each cycle, the retained water increased as well. This pattern is similar to the maximum defrost efficiency of these samples, as shown in Table 3. All other samples, however, showed a more consistent behavior with previous works, with the frost rate decreasing as the tested temperature increased. 12 SN15 showed increasing frost rates and higher amount of retained water during the last temperature test, -8℃. SN5 did not show increased frost rates, but the amount of retained water during the -8℃ test is higher than B. In terms of efficiency, all samples have higher efficiency at lower temperatures, i.e., -12℃, since the frost accretion is higher at lower temperatures, but the remanent amount of water is approximately similar for all tested temperature. Surface B had approximately half of the efficiency of the SN5 and SN2 samples at lower temperatures. This is due to the higher frost mass that the baseline surface (B) collected in each test and the amount of retained water after defrosting. Overall, the SN2 and SN5 samples showed the highest defrost efficiency and the most consistent frost rate for all tested conditions. The time to defrost is another important parameter due to its relation to the defrost efficiency. In this study, all samples were defrosted without an external heat source. Sample 13


6 a) -8 oC SN30 5 SN15 SN5 4

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SN2 B

3 2 1 0 6

0

2

4

6 8 Time (s x 103)

10

12

4

6 8 Time (s x 103)

10

12

4

6 8 Time (s x 103)

10

12

b) -10 oC

Mass (g)

5 4 3 2 1 0 6

0

2

c) -12 oC

5 Mass(g)

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Mass (g)


4 3 2 1 0

0

2

Figure 6: Frost-defrost cycles of the four SN coated samples tested at three different temperatures a) -8℃, b) -10℃, and c) -12℃. defrosting times and temperatures (Figure 7) were extracted from the experimental data, which was collected every ten seconds. tm is the melting time since defrosting began until the time the sample had lost 95% of its frosted mass, td is the time to reach defrosting from 0℃, and Td is the recorded sample temperature at the time when defrosting began. Regardless of the coating, the melting time (Figure 7a) and time to defrost (Figure 7b) increased proportionally to the testing temperature. Frost that grew at colder temperatures 14


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,i.e., -12℃ took longer to defrost than frost grown at a warmer temperature, i.e., -8℃. This is in agreement with previous observations of defrosting time depending on different crystal morphologies formed during frost growth at different temperatures. 30,31 td (Figure 7a) and tm (Figure 7b) are higher on SN30 and SN15 than the SN5 and SN2 samples. This shows a direct correlation between the surface coating thickness and the defrost time. In addition, Td (Figure 7c) increased as the testing temperature was reduced and decreased with the thickness of the SN coatings. This is likely due to the higher thicknesses slowing down the conduction of heat through the mat since silica is an insulator as well as the increased travel

cycle 1 cycle 2 cycle 3

Time (s)

length for the melted water. 200 a) Melting time (tm) SN30 SN2 SN15 B 150 SN5 100 50

Time (s)

0

-8 -10 120 b) Time to defrost (td) 100 80 60 40 20 0 -8 -10 4 c) Temperature at defrost (Td) 3

Temperature (oC)

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-12

-12

2 1 0

-8

-10 -12 Test temperature (oC)

Figure 7: Defrosting comparisons of a) melting time (tm ) from maximum defrost rate until 95% of the water was removed, b) time to defrost (td ) is the time from 0℃ until the maximum defrost rate was reached, and c) Td is the temperature at the maximum defrost rate. The advantage of the thinner SN mats was more pronounced at colder test temperatures, 15



i.e., -12℃. SN2 not only had one of the highest defrost efficiencies, but also the lowest amount (Figure 8) of retained water after three frosting-defrosting cycles. Although the SN5 sample had a similar amount of retained water in all tests as SN2 suggesting a similar defrost capacity, SN2 showed the lowest overall amount of water left on the sample surface after each one of the three cycles. Sample SN2 holds on average the same amount of water after each defrost cycle regardless of the testing temperature; cycle 1: 0.62±0.1 mg, cycle 2: 0.95±0.19 mg, and cycle 3: 1.24±0.12 mg for T=-12℃. Optical images before and after each defrost cycles (Figure 8) show the amount of frost on the surface before defrosting and the amount of water left after the defrost cycle. These images agreed with the mass data collected during the experiments, and it is possible to observe the small number and size of water droplets retained on sample SN2. It is also worth mentioning that most of the water left on sample SN2 remained around the nylon bolts and the edges of the sample which serve as natural “pinning” sites for droplets. The rate of increased water retention (Figure 8) on the surface after each test is an indicator of surface degradation and water retention within the SN coating. However, it is important to identify if the water retention was due to surface damage or partial evaporation of the sample due to the short defrosting time without an external heat source. Durability is also an important factor when considering the potential lifetime of antifrosting surfaces and deicing coatings. To assess the robustness to frost and defrost of the silica nanospring coating, SN samples were tested at least once at each temperature following the procedure stated in the experimental section; however, with the exception of the SN30 and B surfaces, all samples were tested two or three additional times at randomly selected temperatures. Sample SN15 was tested four times at a temperature of -12℃ following the same three frosting-defrosting cycles procedure. To entirely evaporate the remanent water after each test, sample SN15 was left overnight inside the environmental chamber at room temperature. The mass versus time plots (Figure 9) for these four tests followed a similar path. Also, the average defrosting efficiency metric and retained water mass (Figure 9b) 16


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a) -8 oC SN30 SN15 2.5 SN5 2.0

Mass (g)

3.0

SN2.5 B

SN30 (cycle 1)

1.5 1.0 0.5 cycle 1

cycle 2 cycle 3

b) -10 oC

SN15 (cycle 2)

3.0 Mass (g)

2.5 2.0 1.5 1.0 0.5 cycle 1 cycle 2 cycle 3 c) -12 oC SN2 (cycle 3) 3.0 2.5 Mass (g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0 1.5 1.0 0.5 cycle 1

cycle 2 cycle 3

Figure 8: Retained water mass per cycle e and optical images of the samples before and after the third frost cycle at different test temperatures, a) -8℃, b) -10℃, and c) -12℃. after each cycle remained within 10% of the four-test mean. These measurements help to corroborate the robustness of the silica nanosprings coating for multiple frosting-defrosting cycles. Moreover, this is proof that the short ten minutes of natural defrosting did not entirely remove the water inside the SN mat. Unfortunately, as a results, this water lead to accelerated frost formation in the subsequent cycles, increasing the frost accretion (Figure 8) and previously studied by Schmiesing and Sommers. 12 In addition, SEM images of the tested samples were taken after completing the frostingdefrosting tests to analyze the integrity of the SN coatings. These images were converted to binary images (Figure 10) to enhance the effect of the water expansion during the frost cycle. Table 4 summarizes the estimated fla and the average gap length distribution of the SN coated surfaces before and after the frosting-defrosting tests. The binary images of the SN30 17



Mass (g)

6

a)

test 1 test 4

test 6 test 8

4 2 0

0

2

4

6 8 Time (s x 103)

10

12

Mass (g)

4 b) 3 2 1 0

1

2 Cycles

3

10 8 6 4 2 0

Defrost Metric Φ (ND)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 9: SN15 sample a) multiple tests at -12℃ of three frost-defrost cycles, b) the defrosting metric (Φ) and retained water mass after each cycle. Together, these results suggest a durable and robust SN coating. and SN15 samples (Figures 10 a and b) showed a reorganization of the silica nanosprings and an increased air fraction (black areas) of these samples (i.e., the fla ratio for SN15 increased to 58.0%). Samples SN5 and SN2 (Figures 10 c and d), on the other hand, showed a slight reduction fla due to the SN motion during the frosting-defrosting cycles (i.e., the fla ratio for SN2 decreased to 57.8%). High-resolution SEM images (Figures 11) revealed the extent of the SN mat deformation. However, there were not fractured SN observed. The SN mat rearrangement is believed to happen due to the expansion of frozen water inside the SN mat, which compresses the SN between each other. The coil geometry of the SN provided an anchor for the nanosprings themselves, preventing them from returning to their original place while also creating additional support points. The flexibility, shown in Video S1 in the Supporting Information, of silica nanosprings, allowed the SN mats to rearrange without breaking (Figure 10) or losing their defrost efficiency (Figure 9b). Considering the topographic changes of the SN coatings, contact angles (Figure 12) were also measured for all samples after the frosting-defrosting cycles. The contact angle hysteresis

18


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a) SN30 before

SN30 after

10µm

10µm

b) SN15 before

SN15 after

5µm

5µm

c) SN5 before

SN5 after

2µm

2µm

d) SN2 before

SN2 after

1µm

1µm

Figure 10: Binary SEM images of the samples before and after the frost-defrost tests, SN shows as white and holes or gaps as black. Table 4: Liquid-air fraction before and after frosting-defrosting tests Sample SN30 SN15 SN5 SN2

fla before (%) 45.2±0.6 50.4±0.31 58.1±0.11 59.3±0.16

fla after (%) 55.6±0.17 58.0±0.41 56.9±0.15 57.8±0.63

(Figure 12a) for the SN30 and SN15 samples decreased following testing to 15° and 10°, while it increased for the SN5 and SN2 samples to 11° and 17°, respectively. Even though the surfaces with low contact hysteresis retain less water than those with high contact angle hysteresis, due to the changes in the hysteresis SN2 and SN5 are no longer superhydrophobic by definition. The static contact angle (Figure 12b) had similar behavior. SN30 and SN15 19



a) SN2 before

SN2 after

100nm

100nm

b) SN15 before

SN15 after

SN15

200nm

200nm

Figure 11: High resolution SEM images before and after the frost-defrost tests og the samples a) SN2 and b) SN15. both increased to ≈164°, while SN5 remained statistically unchanged at ≈165°, and SN2 decreased to ≈164°. Thus, these changes in the contact angle hysteresis and static contact angles complement the observed topographic changes (Figure 10) of the SN coated mats. For example, the holes in the SN15 mat are more organized and evenly spaced than the ones in the untested mat. Based on these results and when examined together with the earlier findings, the changes to the silica nanosprings topology do not appear to have a significant effect on the defrost efficiency of the SN coated samples (particularly for the thinner SN mats); instead, the SN surfaces upheld their hydrophobicity after several frosting-defrosting cycles. Due to the nature of the frost growth (i.e. condensation frosting) as well as the high relative humidity (RH) of the experiments and the SN geometry and mat topography, frosting on the SN samples originated from inside the silica nanospring mat. This is in agreement with the existing literature on frost formation on microstructured substrates. 17,22 Thus, understanding the initial formation of silica nanosprings on the surface and their catalyst evolution at the early stages of SN formation could help explain the proposed defrosting mechanism. Silica nanosprings, as employed on the SN samples, are grown via a vapor-liquid-solid (VLS) process. As the SN reaction begins, the SN catalyst diffuses and nucleates on the sample surface creating islands, which later catalyze the precursor to form 1-D nanowires. 23 During this 20


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50 a) 40

Untested Tested

30 20 10 0

SN 30 180 b) Static Contact Angles (∘)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Hysteresis Contact Angles [Adv-Rec] (∘)

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SN 15

SN 5

SN 2

SN 15

SN 5

SN 2

170 160 150 SN 30

Figure 12: Before and after testing comparison of the a) contact angle hysteresis and b) static contact angles. island formation step, large areas of the substrate are depleted of the catalyst exposing the underlying base material, in this case, aluminum. In addition, these silica nanowires merge as their catalyst nanoparticles coalesce into a single catalyst with a bundle of nanowires that cohesively coil together forming the silica nanosprings. Since the energy required to bifurcate a catalyst nanoparticle is larger than the energy required to merge them, there is no smaller or individual SN as they grew longer. 23 This suggests that as the SN mat grows thicker, silica nanospring coalesce into larger and ticker SN (Figure 5) as shown by the SN measurements of Table 2. The growth mechanism of silica nanosprings and the SN mat dimensions at different thickness shown in Table 2, suggest a unique structure with a pore size and a silica nanospring that gradually increase in dimension with the thickness of the SN mat. These geometric characteristics of the nanosprings and the SN-aluminum interface are associated with the increased defrost efficiency and lower time to defrost of the SN2 and SN5 samples. The proposed deicing mechanism thus begins with the water melting on the exposed aluminum, the hydrophilic surface. The exposed aluminum, which was created during the 21



SN island formation stage, is expected to have higher hydrophilicity than the SN. As the melted water grows in volume is repelled by the silica nanosprings. The decreased gap or void space size near the plate (larger gap or void space near the top) creates a small net surface tension force that tends to drive the melted water away from the plate surface towards the top of the mat. The melted water is thus transported to a larger fraction of the frost as seen in Video S2 in the Supporting Information. As the frost further melts, the hydrophobicity of the SN mat keeps repealing the melted water, and eventually, when gravitational force is larger than the surface tension, large sheet-like pieces of the frost layer may actually separate from the aluminum and fall off the surface. This proposed mechanism is further supported by the (Figure 7) time and temperature at defrost data. It should be noted that Td actually decreased in some cases here for the thin SN-coated samples, SN2 and SN5, suggesting a lower energy requirement to defrost (as compared to the baseline) and thus an improved transport mechanism for the melted frost water away from the surface.

4

Conclusions

Superhydrophobic and deicing coatings are a desirable material for a variety of heat transfer and aerospace applications. However, thermal efficiency and environmental concerns are often a major impediment to their implementation and use in heat exchangers in air conditioning and refrigeration applications. Experimental results of methylated silica nanosprings as a superhydrophobic and defrosting coating were studied with encouraging results. Water droplet static contact angles increased, and the contact angle hysteresis decreased as the thickness of the SN coating decreased. The SN4 sample displayed the highest contact angle at 164.1°±1.7° and the lowest contact angle hysteresis at 6.9°±2.7°. During defrosting, thinner SN mats exhibited the lowest defrost time and temperature at the time to defrost, suggesting improved mass transport of the melted water outside of the silica nanospring mat as compared to the frosted aluminum substrate. Moreover, thinner SN coatings retained the

22


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lowest amount of melted water after each test while maintaining an overall maximum defrosting metric (Φ) above 7.5. Frosting cycles displayed a similar amount of frozen water at higher temperatures for all samples, but as the temperature decreased, the mass of the frozen water increased faster on thicker SN coatings. The SN coating also proved to be robust to multiple frosting-defrosting cycles. Even though the amount of retained water increased after each subsequent cycle (a phenomenon that is commonly observed), the defrosting efficiency of all samples returned to the original value after the sample was dried completely. This was further corroborated by static contact angle measurements of the tested samples, which revealed similar (and largely constant) values ≈164° for thinner samples and an increase of ≈12° on thicker samples. Moreover, coating the silica nanosprings with methyl functional groups achieved improved defrosting rates as compared to the base aluminum substrate. Since the methylated surface is not a low-energy surface as compared to fluoroalkyl- or alkyl-based chains, the geometry, and topology of the silica nanosprings are chiefly responsible for the observed hydrophobicity of the tested samples. Thus, by engineering submicron gaps and pillars on a surface, this study has shown that it is possible to design a robust hydrophobic and deicing surface without the use of long-chain PFA and alkyl-based coatings.

Acknowledgement The authors thank the Center for Advanced Microscopy and Imaging at Miami University for their assistance.

Supporting Information Available The following files are available free of charge. A listing of the contents of each file supplied as Supporting Information should be included. For instructions on what should be included in the Supporting Information as well as how to prepare this material for publications, refer to the journal’s Instructions for Authors. 23



The following files are available free of charge. • VideoS1bendingGCMU.mov: Shows a single silica nanosprings bending due to electrostatic charging during a SEM imaging. • VideoS2defrostingGCMU.mov Shows a defrosting cycle of a -12C test of samples SN2 at 60

References (1) Wang, Y.; Xue, J.; Wang, Q.; Chen, Q.; Ding, J. Verification of Icephobic/Anti-Icing Properties of a Superhydrophobic Surface. ACS Appl. Mater. Interfaces 2013, 5, 3370– 3381. (2) Wilson, P. W.; Lu, W.; Xu, H.; Kim, P.; Kreder, M. J.; Alvarenga, J.; Aizenberg, J. Inhibition of Ice Nucleation by Slippery Liquid-Infused Porous Surfaces (SLIPS). Phys. Chem. Chem. Phys. 2013, 15, 581–585. (3) Chen, J.; Dou, R.; Cui, D.; Zhang, Q.; Zhang, Y.; Xu, F.; Zhou, X.; Wang, J.; Song, Y.; Jiang, L. Robust Prototypical Anti-Icing Coatings with a Self-Lubricating Liquid Water Layer between Ice and Substrate. ACS Appl. Mater. Interfaces 2013, 5, 4026–4030. (4) Antonini, C.; Innocenti, M.; Horn, T.; Marengo, M.; Amirfazli, A. Understanding the Effect of Superhydrophobic Coatings on Energy Reduction in Anti-Icing Systems. Cold Reg. Sci. Technol. 2011, 67, 58–67. (5) Wang, W.; Salazar, J.; Vahabi, H.; Joshi-Imre, A.; Voit, W. E.; Kota, A. K. Metamorphic Superomniphobic Surfaces. Adv. Mater. 2017, 29, 1700295. (6) Perera, H. J.; Mortazavian, H.; Blum, F. D. Surface Properties of Silane-Treated Diatomaceous Earth Coatings: Effect of Alkyl Chain Length. Langmuir 2017, 33, 2799– 2809. 24


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Graphical TOC Entry Frosted

Defrosted

CH

CH

3

CH 3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 28

3

CH3 CH 3

CH

3

CH 3 CH

CH 3 CH 3

3

200 nm

28


Characterization of Methyl-Functionalized Silica Nanosprings for

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