Durable and Scalable Candle Soot Icephobic Coating with Nucleation

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Durable and Scalable Candle Soot Icephobic Coating with Nucleation and Fracture Mechanism Muhammad Imran Jamil, Xiaoli Zhan, Fengqiu Chen, Dang-Guo Cheng, and Qinghua Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09819 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 3, 2019

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ACS Applied Materials & Interfaces

Durable and Scalable Candle Soot Icephobic Coating with Nucleation and Fracture Mechanism Muhammad Imran Jamila, Xiaoli Zhana,b, Fengqiu Chena,b, Dangguo Chenga,b and Qinghua Zhanga,b* aZhejiang

Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China. bInstitute

of Zhejiang University-Quzhou, Quzhou 324000, China.

ABSTRACT. Ice formation and accretion affects the residential and commercial activities. Icephobic coatings decrease the ice adhesion strength τice < 100 kPa. However, rare icephobic coatings remove the ice under the action of gravity or natural winds. The icephobicity of such coatings depend on materials

Cassie-Baxter State

with low-interfacial toughness. We develop durable candle soot icephobic coating with RTV-1 as a low modulus and binding material. The heterogeneous nucleation on 20-40 nm candle soot particles and their fracture mechanism is discussed. The developed coating always show durable Cassie-Baxter superhydrophobic state with low ice adhesion (18 kPa) and maintain the τice about 25 kPa after severe mechanical abrasion, 30 liquid nitrogen/water cycles, 100 frosting/defrosting cycles, 100 icing/deicing cycles, acid/base exposure, under UV and exposure to natural freezing rain in Hangzhou. In addition, the proposed technique is time-efficient, inexpensive and suitable for large-scale applications. KEYWORDS: icephobic coating, durable Cassie-Baxter state, ice nucleation, elasticity, cracks. 1 ACS Paragon Plus Environment

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1. INTRODUCTION Ice formation on surfaces is a severe problem in several industries including transportation, wind turbines, power plants and telecommunications. Frost formation and ice accretion also affect the efficiency of heat exchangers and refrigerators.1 The impact of super-cooled water on surfaces is generally known as freezing rain or atmospheric icing. The solid-phase material of gaseous or liquid form of water is defined as frost (formed from desublimation of vapors), glaze (clear and hard ice which is formed from the freezing rain of large droplets with diameters in the range of 70 mm to even a few millimeters), rime (White and feather-like ice which is formed from freezing of super-cooled water droplets with diameters ranging from 5-70 mm creating from fog/clouds) and wet/dry snow (mixture of water and ice. Snow is ‘wet’ when the air temperature is very near to freezing point but snow is ‘dry’ at 1-2 °C.2 Icephobicity is defined as the repulsion of super-cooled water droplets, reduction in ice adhesion, inhibition of frost formation, delay in heterogeneous ice nucleation, delay in freezing point and prevention from Cassie to Wenzel transition1,3. Different icephobic methods such as infrared heating and electro-thermal systems are applied to keep the surfaces free from ice. But these methods are expensive and energy consuming.4 The glycols and salts are used to depress the freezing point. However, this causes environmental pollution. Deicing methods remove the accreted ice with the help of mechanical forces. However, accessibility is a drawback to this technique.3 A more attractive and general strategy to combat the icing issue is to engineer the surfaces. It reduces the amount of water and adhesion of accreted ice on a surface. The icephobic surface show very low ice adhesion τice < 20 kPa such that the accreted ice could fall off naturally due to gravity, mild wind or vibration.5 2 ACS Paragon Plus Environment

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Superhydrophobic surfaces repelled the water droplets before their freeze.6,7 In contrast, Varanasi and Kulinich reported that frost nucleation occurs on all areas of the superhydrophobic textures, resulting the loss of superhydrophobicity,8 gradual damage of textures9 and increase in ice adhesion strength10. Hejazi and Nosonovsky discussed the forces acting upon a water droplet and ice piece on a rough solid substrate.11,12 Inspired by Nepenthes pitcher plants, a slippery liquid-infused porous surface (SLIPS) has been developed to inhibit ice nucleation.13-15 However, the main weakness of SLIPS is its poor stability and high cost of lubricants. Varanasi and coworkers showed the loss of lubricant through capillary attraction onto frozen drops and entrainment in the water droplets as they are shed from the surface.16 Meanwhile, inspired by ice skating, Jiang, Wang, and coworkers fabricated SLIPS with a self-lubricating layer of water layer for anti-icing applications.17,18 In addition, organogels19 were also used for anti-icing application by swelling the cross-linked network of PDMS with liquid paraffin.20,21 Elastomers are the latest class of icephobic material. Golovin et al. used the elastomers to reduce the ice adhesion strength from the surface. They reduced the shear modulus of elastomers by reducing the cross-link density of the structure and introduced the interfacial slippage by inserting miscible polymeric chains.22,23 In another approach, Beemer et al. used the fracture mechanics and made durable PDMS gels with ultra-low ice adhesion strength.24 After that He et al. introduced the multiscale crack initiator mechanism and produced micro-holes in the PDMS structure to make cracks at the interface and designed the super-low ice adhesion surfaces (5.7 kPa).25 Irajizad et al. described the stress-localization concept to develop durable icephobic surfaces with ice adhesion in order of 1 kPa.26 Yu et al. developed highly stable anti-icing amphiphilic organogels from copolymer P(PDMS-r-PEG-r-GMA) and then infiltrated with amphiphilic lubricants.27 Recently, Golovin et al. used the low interfacial toughness material for 3 ACS Paragon Plus Environment


deicing purpose. The interfacial toughness is associated with two effects; one is the bonding energy between ice and coating. The other come from localized losses within the coating, associated with the high-stress region at the crack tip. 28 The intermolecular adhesion force is produced upon the contact of two solid surfaces. The intermolecular adhesion force may be of van der Waals interaction, hydrogen or covalent bonding. The adhesion strength is defined as the force per unit contact area required to break all the bonds across the interface. The adhesion strength can be reduced with the help of interfacial cracks and flaws induced by surface roughness, trapped air and contaminants. Under the action of external force, stress concentration occur in the vicinity of these flaws and eventually leads to the breakage of joints through crack propagation.29 The surface contaminants on metals and dissolved salts in ice decrease the ice adhesion due to small contact area between metal and ice.30 According to nucleation theory, the heterogeneous nucleation barrier exhibits a significant increase only when the structure size of the substrate is comparable to, or smaller than, the critical size of ice nucleation.31 It was found experimentally that Cassie-Baxter superhydrophobic surfaces with nanoscale roughness (R < Rc) show longer freezing delays as compare to surfaces with roughness (R > Rc). The Cassie-Baxter state32,33 and crack formation at the interface cause to delay the freezing point, decrease the ice shear strength, decrease the contact area between water/ice and the substrate. The ice nucleation rate also decreases, since it is directly proportional to the contact area.34 The adhesion mechanics postulated by Griffith and further explained by Kendall35 and Chaudhury, the critical shear stress (τice) required to separate a rigid object (ice) from a thin film of certain material is given as

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𝜏𝑖𝑐𝑒 ∝

𝑊𝑎𝑑ℎ 𝜇 𝑡

(1)

Here, Wadh, 𝜇 and t represent the adhesion work between ice and the film, modulus of the material and film thickness respectively. Thus, in order to get ultra-low ice adhesion strength, low shear modulus, low work of adhesion, and high coating thickness is desirable. The work of adhesion between ice and superhydrophobic surface with suitable roughness is lower than that between ice and hydrophobic surface. Therefore, an elastic surface that supports both the CassieBaxter superhydrophobic state and crack formation at the interface is required. Esmeryan et al. obtained the carbon soot from the combustion of rapeseed oil in a conical chimney and it was chemically functionalized with ethanol solution and aqueous fluorocarbon. The hydrophilic functionalized carbon soot showed high ice adhesion strength and the pristine carbon soot was removed even after one frosting/defrosting cycle.36,37 Here, we propose the fracture mechanics and develop durable candle soot icephobic surfaces with RTV-1 as an elastomer as well as binder. The 20-40 nm candle soot particles were embedded into RTV-1 to introduce the cracks and Cassie-Baxter superhydrophobicity. RTV-1 silicone is an elastic, non-toxic, chemically stable, hydrophobic and readily curable in nature. In addition, RTV-1 has low modulus, high temperature resistance and non-deformable property. The prepared elastic superhydrophic surface of candle soot suppress the ice nucleation and lower the ice adhesion due to the following reasons i) minimum contact area between water and candle soot nanoparticles ii) high nucleation energy barrier iii) presence of voids/cracks between the substrate and water/ice iv) low work of adhesion due to high contact angle v) weak bonding interaction between candle soot particles with water/ice and structural mismatch of the ice embryo due to the convex interface. 5 ACS Paragon Plus Environment


a)

Spin coating

Glass substrate

RTV-1

b)

Candle soot

c)

Non-embedded soot particles

RC

R < Rc unstable unstable R

Embedded soot particles

F= mg

d)

Weak interaction cracks

Ice

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Figure 1. a) Schematic represents the fabrication of candle soot coating (CS+RTV-1). b) candle soot nanoparticles with and without binder (RTV-1) c) Heterogeneous nucleation on a curved candle soot nanoparticle, the ice embryo on candle soot nanoparticle with small radius of curvature is unstable and needs extra cooling to become stable (Gibbs–Thomson effect). d) Spontaneous fall off ice under the action of gravity. 2. MATERIALS AND METHODS 2.1. Materials. RTV-1 silicone adhesive was obtained from Shen Zhen Xing Yong Wei Silicone Co., LTD. Paraffin wax candles were purchased from local market. The microscopic glass slides and silicon wafers were used as a substrate for coating. Ethanol and acetone solvents were used to clean the glass slides and silicon wafers. The deionized water was used to measure the 6 ACS Paragon Plus Environment

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wettability and icephobicity of the coating. Sulfuric acid, nitric acid, hydrochloric acid and sodium hydroxide were purchased from Chemical Reagent Co. The salts for artificial sea water were purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Fabrication of durable candle soot coating The durable candle soot coating was fabricated as follows; first, the microscopic glass slides were washed with ethanol and acetone respectively. Then, RTV-1was spin coated (WS-400B6NPP-LITE/AS, Laurell Technologies) for 60 s with the speeds of 6000, 5000, 3000, and 2000 rpm to obtain the layer of RTV-1with thicknesses of 40, 90, 150, and 200 µm, respectively. After spin coating, the RTV-1 coated glass slide is held above the flame of a paraffin candle (2.9 cm above the wick of a paraffin candle, supporting information Figure. S1) and moved across back and forth to obtain uniform soot deposition. The soot deposition was continued for duration of 2.0 min until the entire RTV-1 coated glass slide was covered with soot particles and turned black as shown in supporting information Figure S2. To obtain 20-40 nm of carbon particles and high water repellency, the soot was deposited near the top portion of candle flame. The extra non-embedded soot nanoparticles (as shown in Figure 1b and supporting information S3 b) were washed with water or removed with the help of air pressure before performing the mechanical durability tests. 2.3. Surface topography and wettability The surface topography structure and roughness of the samples was characterized by field emission scanning microscopy (SEM, SU-8010, Japan) at an accelerating voltage of 20 kV and atomic force microscopy (AFM, TT2-AFM, USA) operated by multimode in tapping mode, respectively. The contact angle (CA) was measured by SDC-100 optical contact angle goniometer (SINDIN Co., Ltd., China). The contact angle hysteresis (CAH) was measured by 7 ACS Paragon Plus Environment


using Dataphysics OCA 20 California USA with sessile drop (needle in) method. The average value was obtained by measuring at least five different positions on the same sample. 2.4. Measurement of shear modulus The rheometer (TA Instruments, ARES-G2) with 8 mm parallel plate geometry was used to determine the shear modulus G of RTV-1. The chamber temperature was adjusted at -20 °C to match the temperature of the ice adhesion strength measurements. First, we determined the linear viscoelastic regime of RTV-1 by performing strain sweeps and then the frequency sweeps ( 10 rad s-1 to 100 rad s-1 ) were performed within the linear viscoelastic regime to measure the storage modulus G' and the loss modulus G" in the plateau region. The storage modulus G' was found to be smaller than the loss modulus G". Thus the shear modulus was measured as, G  G2  G2 2.5. Anti-icing Experiments The anti-icing performance of the prepared samples was measured by determining the delay in freezing point of water, crystallization point of water and the adhesion strength of ice. The samples were put on a Peltier cooler and adjusted the temperature of the substrate at -20 ± 0.5 °C. The environmental temperature was 20.0 ± 2 °C with humidity (55 ± 5 %). The delay in freezing point of water on glass, RTV-1 coated glass and candle soot coated glass (CS+RTV-1) was recorded using a contact angle goniometer (SINDIN Co., Ltd., China). After maintaining the surface temperature at -20 ± 0.5 °C, we started to calculate the freezing delay time of water droplet (7 μL). The sample appearance was recorded after every second until a sharp tip was appeared at the top of water droplet. The delay in freezing point of water droplet on three surfaces was measured over four times to get the average value. The crystallization point of 8 ACS Paragon Plus Environment

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water on the prepared samples was examined using a differential scanning calorimeter (DSC, Q200 system, TA instruments). Before the DSC measurements, the sample pan was coated with RTV-1 and CS+ RTV-1. The blank and coated sealed crucible sample pans containing 3-6 mg of deionized water were placed in the instrument and the temperature was adjusted from room temperature to -70 °C with a cooling rate of 5 °C/min. We calculated the ice adhesion strength on candle soot coated sample before and after durability tests by using three independent measurements. The sample (CS+ RTV-1) was placed on a plastic cuvette (10 mm × 10 mm × 45 mm) having (1.5 mL) deionized water. The whole sample was turned down and put in the refrigerator, in which the inner temperature of refrigerator was 20 °C. The samples were kept in the refrigerator for 4 h to ensure the complete freezing of water. Before performing the test, the samples were placed into the cooling chamber for their stabilization at -20 °C for 6 minutes. Then the ice adhesion strength was measured by using dynamometers (NK-50) to remove the ice from the sample surface. 2.6. Anti-frosting Experiments The anti-frosting tests were performed by adjusting the sample temperature at -20 ± 0.5 °C on a Peltier cooling controller with ambient temperature (22 ± 0.5 °C). The relative humidity of the environment was kept at 75 ± 5% by using a spray humidifier. The samples CS+ RTV-1 coated glass and bare glass slide were placed on Peltier cooling controller. The frosting process on the sample surfaces was monitored carefully and captured the images after each minute until the whole sample surfaces were covered with the frost. In addition the outdoor anti-frosting experiment was also performed in January 2019 at Zhejiang University of China. 2.7. Mechanical durability tests



The mechanical durability of the prepared candle soot icephobic coating (CS+RTV-1) was examined by performing frosting/defrosting cycles, liquid nitrogen/water cycles, icing/deicing cycles, tape-peeling, sandpaper abrasion, super-cooled water-impacting, natural freezing rain, UV, acidic/basic medium and artificial sea water. Three acidic solutions (pH < 4) of strong acids HCl, H2SO4 and HNO3 were prepared to immerse the prepared candle soot coating. The basic solution (pH >12) of strong base NaOH was also prepared. The artificial sea water was prepared according to the method described in literature.38 The 100 frosting/defrosting cycles were performed on prepared microscopic glass slide of candle soot coating (CS+RTV-1) at -20 °C temperature with 75% humidity as mentioned in the anti-frosting experiment. Under extreme conditions in the range of liquid nitrogen temperature, the coating (CS+RTV-1) was dipped (30 times) in liquid nitrogen and water successively. The 100 icing/deicing cycles were performed on prepared microscopic glass slide of candle soot coating (CS+RTV-1) at -20 °C temperature as mentioned in the anti-icing experiment. Tape peeling was performed with a method described in the literature.39 The surface was pressed with Scotch-600 adhesive tape and further 1 kg weight was put on the sample to make a firm contact between the sample and the tape, then the tape was peeled off. The same test was repeated 100 times. The abrasion test was performed by applying 50 g weight on the candle soot coating (CS+RTV-1) then the coated sample was abraded by moving it in forward direction. The super-cooled water-impact test was carried by dropping 500 mL beaker of super-cooled water on the sample coating. The candle soot coated sample was also placed in the natural freezing rain for 20 hours. The coated sample was dipped in highly acidic, basic and artificial sea water respectively for one month. An UV chamber equipped with a 500 W ultraviolet high-pressure


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mercury lamp (Belsri/UV model, Beijing Institute of Electric Light Source) was used for irradiation, and the sample was placed under the lamp for 2 hours.

3. RESULTS AND DISCUSSION 3.1. Robust candle soot coating and its morphology The candle soot coated surface shows extreme repellency to water but the weak physical interaction between individual carbon nanoparticles and with the substrate display its poor stability as shown in supplementary Figure S3 a. To overcome this problem, we used the binder (RTV-1) to enhance its surface adhesion and robustness under extreme environmental conditions. The RTV-1 is used due to its synergetic effect of robust binder (hydrophobic) as well as low modulus material. The binder (RTV-1) was spin coated on the microscopic glass slide then the soot layer was embedded into RTV-1 coated surface as shown in Figure 1a and supplementary Figure S2. The soot deposition was completed within 2 minutes and RTV-1 maintained its rubber like thin film due to its heat resistant as well as non-deformable property. In Figure 2a the SEM image shows the fragile structure and porous morphology of the candle soot nanoparticles without binder (RTV-1). The embedded candle soot nanoparticles into the binder are shown in SEM image of Figure 2b and supplementary Figure S3b and S4. The candle soot nanoparticles have spherical shape with an average size of 30 nm. In order to get small size of carbon nanoparticles and extreme water/ice repellency, the soot was deposited from the top portion of the flame (2.9 cm above the wick of candle, supplementary Figure S1). The surface topography of the candle soot coated surface (CS+RTV-1) is examined by atomic force microscopy (AFM). According to the AFM results, the average root mean square roughness (R rms) of the candle soot coated surface is around 106 nm as shown in Figure 2 c and 2d. The increase in average root 11 ACS Paragon Plus Environment


mean square roughness (R rms , 205 nm) of the soot coated surface was found with the increase in concentration of soot nanoparticles as shown in the supplementary Figure S5. The candle soot coated surface (CS+RTV-1) showed extreme repellency to water droplets with contact angle of 158° as compare to RTV-1 coated surface (100°). The wettability results of the coated and uncoated surfaces are shown in supporting information Figure S6. The increase in static contact angle and decrease in contact angle hysteresis (CAH) of water was observed on soot coated surface with the increase in concentration of soot particles as shown in Table 1. 3.2. Cassie-Baxter superhydrophobic state of candle soot coating When superhydrophobic surfaces with suitable roughness are dipped into water then a silvery mirror-like sheen is observed at their submersed surface. The silvery mirror-like sheen is due to light reflection from a sheathing layer of air retained in between the cavities of candle soot coating as shown in Figure 3. This is a sign of Cassie-Baxter superhydrophobic state of candle soot coating underwater. The low values of contact angle hysteresis also indicate the existence of Cassie-Baxter superhydrophobic state of candle soot coating before and after the mechanical durability test. The results about the advancing, receding and hysteresis contact angles are shown in the table 1.


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b)

a)

RMS = 106 nm c)

d)

Figure 2. SEM images of candle soot particles a) without binder (RTV-1). b) with binder. c) 2-D AFM image of candle soot coating (CS+RTV-1). d) 3-D AFM image of candle soot coating (CS+RTV-1).



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b)

a)

Cassie-Baxter state

Figure 3. Schematics of (a) Candle soot coating (CS+RTV-1) under water with silvery mirror due to reflection of light from trapped layer of air (b) a droplet of water in Cassie-Baxter state on candle soot coating (CS+RTV-1) in air. Table 1. Contact angle (CA), advancing CA, receding CA and contact angle hysteresis values on candle soot coating (CS+RTV-1) with different concentration of soot particles and after mechanical durability tests.

Candle Soot Coating (CS+RTV-1)

1- No soot deposited at 0 sec 2- Soot deposition after 30 sec 3- Soot deposition after 60 sec 4- Soot deposition after 120 sec 5- After Frosting-defrosting 6- Liquid Nitrogen-Water cycles 7- Icing deicing cycles

Contact angle

Advancing contact angle

𝛉 (°) 97 ± 3 133 ± 2 142 ± 1 157.3 ± 1 152.4 ± 1 151.2 ± 1 151.4 ± 1

𝛉𝑨 (°) 107 ± 2.4 138 ± 1.6 145 ± 2.1 158.75 ± 0.2 154.6 ± 2.4 153.25 ± 2.8 153.42 ± 0.8

Receding contact angle 𝛉𝑹 (°)

76 ± 1.5 114 ± 1.2 134 ± 1.4 157.95±3 153.1 ± 0.3 151.4 ± 2 153.2 ± 0.7

Contact angle hysteresis

∆𝛉 = 𝛉𝑨 - 𝛉𝑹 (°) 31 ± 0.9 24 ± 0.4 11 ± 0.7 0.8 ± 0.2 1.5 ± 2.1 1.85 ± 0.8 0.22 ± 0.1

3.3. Removal of water droplet before freezing The superhydrophobic surfaces repelled the water droplets before the freezing process.40,41 The water adhesion force is low with icephobic coating at room temperature.42 We observed the water roll off phenomenon on candle soot coating even at -10 °C substrate temperature. We 14 ACS Paragon Plus Environment

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applied the large water droplet (10µl) then pressed it hard but the water droplet showed no adhesion with candle soot as shown in the Figure 4. This shows that the candle soot nanoparticles have passive anti-icing property due to negligible water adhesion force even below -10 °C. The trapped air in between the nanoparticles and low energy of candle soot are responsible for extremely low water adhesion force. The icephobicity of candle soot coating (CS+RTV-1) is also examined by impacting the super-cooled water (-10 °C). The candle soot coating, not only repelled the super-cooled water (-10 °C) but also maintained its Cassie-Baxter superhydrophobic state in super-cooled water (-10 °C) as shown in the supporting information Figure S9 b. The results can be shown in the supporting information video S1.

Figure 4. Schematic displays no water adhesion force with the candle soot coating (CS+RTV-1) even at -10 °C. 3.4. Delay in the crystallization of water The delay in crystallization of water droplet was measured through differential scanning calorimeter (DSC) analysis by continuously lowering the temperature and observing the whole freezing process of individual water droplet at -20 °C by using a CAM 200 optical contact-angle goniometer. The results of the DSC analysis reveals that the superhydrophobic candle soot coated (CS+RTV-1) crucible possess small heat flow (released during crystallization of water) as compare to hydrophobic (RTV-1) and hydrophilic (blank) crucible. The reason behind the small 15 ACS Paragon Plus Environment


heat flow is the insulated trapped air and very small contact area between the water droplet and the candle soot coated substrate.43 The delay in the freezing point of water is attributed to the interfacial geometry and roughness of the candle soot coated substrate. It was found experimentally that surfaces with nanoscale roughness with R < Rc showed longer freezing delays than surfaces with R > Rc. The interfacial geometry of the candle soot coated (CS+ RTV-1) substrate affects the ice nucleation on the basis of Gibbs-Thomson effect. It explains the high vapor pressure of water droplet on a curved interface. According to the Gibbs-Thomson effect, the interface becomes thermodynamically unstable due to an increased chemical potential or vapor pressure on a curved interface, especially if the surface has a high degree of curvature. Therefore, the nuclei at a curved interface need an extra cooling to persist. Thus the curved interfacial structure size of the candle soot coating plays its role to delay the freezing point of water. The Kelvin’s law and Clapeyron equation also explain the delay in freezing point of water on candle soot Cassie-Baxter superhydrophobic coating. The following equation of Kelvin’s law shows that the vapor pressure and radius of water droplet are inversely related to each other. The small radius of water droplet (r) on candle soot superhydrophobic surface produces large vapor pressure (Pr). 2γVm

( )

𝑃𝑟 = P exp

RTr

(3)

Where Pr and P indicate the vapor pressure and saturated vapor pressure at temperature T, while r, γ, Vm, and R indicate the radius of water droplet, surface energy, molar volume and gas constant respectively. The following Clapeyron equation shows that the vapor pressure P and temperature T have inverse relation. In short, the small radius of a condensed water droplet and 16 ACS Paragon Plus Environment

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large its vapor pressure causes the delay in freezing point of water droplet on candle soot coated superhydrophobic coating. The small condensed water droplet can be shown in supporting information Figure S9 a.

𝑇2

𝑙𝑛𝑇1 =

∆𝑠 𝑉𝑚 ∆𝑠 𝐻𝑚

(𝑃2 ― 𝑃1)

(4)

Here the ΔsVm and ΔsHm show the volume change and enthalpy during the freezing process. The uncoated blank hydrophilic crucible possess low vapor pressure due to large contact area with water droplet, so the water droplet on it crystalizes at a high temperature (-9.9 °C) as compare to hydrophobic (RTV-1, -10.3 °C) and superhydrophobic (CS+ RTV-1, -12.3 C°) as shown in Figure 5. Thanks to Cassie-Baxter superhydrophobic candle soot coating with small r and high vapor pressure of water droplet, this leads to delay (2.9 °C) in the crystallization point of water.



35

Cooling 5 ° C /min

30

-12.8 ° C

25

33.6 W/g

20

Space degree

15 10 5

CS +RTV-1 Coated

0 100

-10.3 ° C 101.15 W/g

80

Heat flow (W/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

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60

40

20

RTV-1 Coated 0 180

160 140

- 9.9 ° C 171 W/g

120 100 80 60 40 20

Uncoated

0 2

4

6

8

10

12

Time (min)

Figure 5. DSC curves of super-cooled water droplet on uncoated crucible (Hydrophilic), crucibles coated with RTV-1 (Hydrophobic) and candle soot CS+RTV-1(Superhydrophobic).


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The delay in freezing point of individual water droplet was also observed on glass, RTV-1 coated and candle soot coated (CS+ RTV-1) at -20 ± 0.5 °C. The disappearance of transparent center of water droplet and appearance of sharp peak of ice indicates the complete freezing time. The investigated delay in freezing point of water droplet (7ul) on three surfaces was in this order CS+RTV-1 > RTV-1 > Glass substrates. The candle soot coated (CS+RTV-1) surface showed 4 times delay in freezing point of water as compare to uncoated glass surface as shown in the Figure 6.

0s

48 s

Glass

55 s

48 s

0s RTV-1

0s

48 s

55 s

110 s

200 s

CS+RTV-1

Figure 6. Photographs of the freezing process of individual water droplet on uncoated glass surface (Hydrophilic), RTV-1 coated (Hydrophobic) and candle soot coated (CS+RTV-1) surface (superhydrophobic). 3.5. Delay in frost formation According to classical nucleation theory, a new thermodynamic phase (e.g frost formation/ice nucleation) occurs on foreign particles or solid substrate by overcoming the certain nucleation barrier. The foreign particles lower the surface free energy and reduce the nucleation barrier.



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Therefore, the frost formation/ice nucleation favorably occurs on foreign particles through a phenomenon known as heterogeneous nucleation. The effect of foreign particles on reducing the nucleation barrier can be calculated with the help of interfacial correlation factor,44, 𝑓(𝑚,𝑅′) which is defined as 𝑓(𝑚,𝑅′) = ∆𝐺 ∗ 𝐻𝑒𝑡𝑒𝑟/∆𝐺 ∗ ℎ𝑜𝑚𝑜

(6)

Where ∆𝐺 ∗ 𝐻𝑒𝑡𝑒𝑟 indicates the heterogeneous nucleation barrier and ∆𝐺 ∗ ℎ𝑜𝑚𝑜 represents the homogeneous nucleation barrier. The interfacial correlation factor 𝑓(𝑚,𝑅′) shows that how much reduction in nucleation barrier occurs due to the presence of foreign particle as compare to homogeneous nucleation barrier. The interfacial correlation factor 𝑓(𝑚,𝑅′) strongly depends on m (the interfacial free energy) and R′ (the interfacial structure size of the substrate) as shown in Figure 7. According to the equation (2 in supporting information) the parameter m in 𝑓(𝑚,𝑅′) depends on wettability and the binding affinity between the water/crystal. When, 𝑚 = ―1 (CA 180°), 𝑓(𝑚,𝑅′)→ 1, then there is no affinity between the water/crystal and the substrate. In this situation the substrate does not have any effect on the nucleation process and promote the homogeneous nucleation process. On the other hand, when 𝑚 = 1 (CA °0), 𝑓(𝑚,𝑅′)→ 0, then there is strong attraction between water/crystal and the solid substrate to promote the heterogeneous nucleation process34. The convex small-scale roughness provides high energy barrier against its wetting than flat, concave or rod-like shape.45 In our case we have CassieBaxter superhydrophic surface of candle soot coating (CS+RTV-1) with 𝑚 = -0.927 and 𝑓

(𝑚,𝑅′) = 0.95 which promote the homogeneous nucleation process and provide the maximum nucleation barrier to remove the water droplet before its freezing. In addition, the interfacial geometry i.e small convex bumpy structure size of candle soot nanoparticles helps to delay the frost formation (Gibbs-Thomson effect). The diameter of candle soot nanoparticles was in the 20 ACS Paragon Plus Environment

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range of 20-40 nm and we took their average radius (12 nm). The calculations for interfacial correlation factor 𝑓(𝑚,𝑅′) and interfacial structure size 𝑅′ of candle soot nanoparticles can be shown in the supporting information.

RC

T = -20 °C Rc = 21.76 nm

R

Figure 7. Interfacial correlation factor 𝐟(𝐦,𝐑′) vs interfacial structure size of 𝐑′ candle soot coating (CS+RTV-1). We also performed the outdoor experiment of frost formation at -10 °C on candle soot coated surface. There was no frost formation for almost seven hours and the crystals of frost fell down from the candle soot coated surface as shown in the supporting information video S2. The coating showed no attraction with the crystals of frost even after 20 hour contact. All the frost was removed from the candle soot coating (CS+RTV-1) with a gentle vibration while the frost show strong attraction with uncoated glass surface. The results can be shown in supporting information video S3. After performing the outdoor experiment, the candle soot coated (CS+ 21 ACS Paragon Plus Environment


RTV -1) surface remain same as shown in Figure 8a. The same coating was dipped in a beaker of water as shown in Figure 8b. The shiny surface indicates that the Cassie-Baxter state of candle soot coating remain stable during outdoor frosting. The Figure 8c shows the delay in frost formation at -20 °C with 75% humidity on candle soot coating (CS+RTV-1) as compare to uncoated surface. The uncoated surface was covered fully with the frost after 1 hour but the soot coated surface was not covered fully with the frost even after 4 h. After 4 hours small amount of ice is also formed on soot coated surface but the weight of the frost was half as compare to the uncoated surface. This delay in frost formation is due to the delay in condensation of small water droplets on superhydrophobic soot coated surface. The geometry and low energy of candle soot coating (CS+RTV-1) creates less stable interface and the water droplet need more time to freeze. On the other hand the water condenses quickly on uncoated hydrophilic surface.


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a)

c)

b)

0 min

60 min

120 min

180 min

240 min

Glass

CS+RTV-1 Figure 8. a) Photograph of the candle soot coating (CS+RTV-1) after outdoor frosting (-10 °C) b) Cassie-Baxter state of the candle soot coating after outdoor frosting. c) Photographs of the frosting process on glass and candle soot coating at cooling temperature T = -20 °C with 75% relative humidity. 3.6. Ice adhesion performance. When the surfaces are covered with ice, the suitable strategies are applied to make them ice free. The ice adhesion strength is an important parameter to measure the icephobic performance. Based on fracture mechanics, the ultra-low ice adhesion strength can be achieved by applying the materials with properties like, low shear modulus, low work of adhesion and high coating thickness. We developed an elastic Cassie-Baxter superhydrophobic coating by embedding the candle soot nanoparticles into the low modulus soft binding material (RTV-1). The candle soot 23 ACS Paragon Plus Environment


nanoparticles provide the low surface energy and high contact angle of water droplet that is required for low work of adhesion. The candle soot nanoparticles show very weak interaction with ice and the trapped air in between them acts as cracks, voids and stress concentrators as illustrated in Figure 1d. In order to get further low ice adhesion strength (according to the equation 1 of adhesion mecnanics), we increased the coating thickness by changing the speed of spin coating. The ice adhesion strength of candle soot (CS+RTV-1) is 25 ± 1.5 kPa at 45 µm coating thickness. When the coating thickness is increased to 225 µm, the lowest possible ice adhesion strength obtained was 18 ± 1.5 kPa. The results of decrease in ice adhesion by increasing the coating thickness are shown in Figure 9. Ice can also be removed from the candle soot coating only under the action of its own weight. For this we used two small petri dishes, one was coated with candle soot (CS+RTV-1) and other was uncoated. Both were filled with same amount of deionized water and put into refrigerator at -20 ° C until the whole water was frozen into ice. Then both the petri dishes were clumped with a small tilt. We found that the ice was removed from the coated petri dish and the ice in uncoated petri dish remained same as shown in the supporting information video S4. The same video also shows that a very small and light weight ice piece can be removed from the candle soot coating (CS+RTV-1, 45 µm) under the action of gravity.


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30

RTV-1 Modulus, G = 3.91 kPa

Ice Adhesion Strength t ice (kPa)

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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10 40

60

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100

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Coating thickness, t (µm)

Figure 9. Ice adhesion strength of candle soot coating (CS+RTV-1), decreases by increasing the coating thickness. We also observed the removal of ice under the action of small wind as shown in the supporting information video S5. For this the temperature of the coated glass slide was maintained at -20 °C and water droplet was placed on the coated surface to make its ice. Then the ice was removed with the help of small wind. These experiments proved that the candle soot coated surfaces are icephobic in nature and remove the ice naturally due to gravity, mild wind or vibration. 3.7. Mechanical durability The poor mechanical stability is a major issue that hinders the use of icephobic surfaces on largescale. The abrasive forces, scratches, acid rain and frosting destroy the surface topography and its icephobic behavior. In an effort to measure the mechanical durability of the prepared candle soot icephobic coating, the frosting/defrosting cycles, liquid nitrogen/water cycles, icing/deicing cycles, tape-peeling, sandpaper abrasion, super-cooled water impact, natural freezing rain, UV, acidic/basic medium and artificial sea water test were performed.



The prepared microscopic glass slide of candle soot coating (CS+RTV-1) was put on Peltier cooling controller and the temperature was maintained at -20 °C with 75% humidity. The soot coated glass slide was remained in the chamber until it was fully covered with the frost. After that the coating was defrosted at room temperature and it was dipped into the beaker of water to observe the Cassie-Baxter superhydrophobic state. The dipped coating showed a shiny mirror in the water due to the reflection of light from trapped air in between the soot particle and water layer. The shiny surface of soot coated glass slide can be shown in the Figure 3a. Then the water droplets were impacted on the coating and the water droplets rolled off from the surface like a bead. The same test was performed 100 times and the coating showed same shiny surface in water due to its durable Cassie-Baxter state. At the end a small decrease in the contact angle (156° supporting information Figure S7) and increase in ice adhesion strength (22 ± 3 kPa) was observed as compare to its initial state as shown in the Figure 11. We examined the mechanical durability of the candle soot coating under extreme conditions i.e liquid nitrogen (~ -150 °C) and water cycles. The prepared microscopic glass slide of candle soot coating was put into the liquid nitrogen for one minute then quickly immersed into the water and removed back after getting a thick layer of ice. The thick layer of ice showed low adhesion strength and removed with gentle vibration. The thick layer of ice become melt very quickly and the water was also evaporated very quickly from the candle soot coating (CS+RTV-1) due to unstable interface (Gibbs Thomson effect) as shown in the supporting information video S6. After that the coating was dipped into the beaker of water to observe the Cassie-Baxter superhydrophobic state. The dipped coating showed a shiny mirror in the water due to the reflection of light from trapped air in between the soot particle and water layer. Then the water droplets were impacted on the coating in air and the water droplets rolled off from the surface 26 ACS Paragon Plus Environment

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like a bead. The same test was performed 30 times and the coating showed same shiny surface in water due to its durable Cassie-Baxter state. The candle soot coating maintained its durable Cassie-Baxter state in water even after 2 hours dipping in liquid nitrogen. At the end the decrease in contact angle (152° supporting information Figure S2) and increase in ice adhesion strength (28 ± 3 kPa) was observed as compare to its initial state as shown in the Figure 11. By cooling the substrate in liquid nitrogen, we observed the durability of candle soot coating (30 cycles) under the different kinds of ice like frost, glaze, rime, wet and dry snow. The definition of frost, glaze, rime, wet and dry snow has been discussed in the introduction. The tape peel test was performed by pressing the tape on to the soot coated sample with 1 kg load as shown in the supporting information Figure S10. Moreover, the Scotch-600 adhesive tape was applied to provide more adhesive force as compared to ordinary scotch tape. The tapepeeling test was performed at least 100 times and after that the coating was dipped into the beaker of water to observe the Cassie-Baxter superhydrophobic state. The dipped coating showed a shiny mirror in the water due to the reflection of light from trapped air in between the soot particle and water layer. Then the water droplets were impacted on the coating in air and the water droplets rolled off from the surface like a bead. At the end the decrease in contact angle (155° supporting information Figure S7) and increase in ice adhesion strength (23 ± 3 kPa) was observed as compare to its initial state as shown in the Figure 11. The durability of passive icephobic candle soot coating (CS+RTV-1) was observed during 100 icing/deicing cycles at -20 °C. The geometry of candle soot nanoparticles remain same and no damage was observed due to their very weak interaction with ice as shown in Figure 10. Then the coating was dipped into the beaker of water to observe the Cassie-Baxter superhydrophobic state. The dipped coating showed a shiny mirror in the water due to the reflection of light from 27 ACS Paragon Plus Environment


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trapped air in between the soot particle and water layer. Then the water droplets were impacted on the coating in air and the water droplets rolled off from the surface like a bead. At the end the decrease in contact angle (155° supporting information Figure S7) and increase in ice adhesion strength (25 ± 3 kPa) was observed as compare to its initial state as shown in the Figure 11.

Ice

100 icing /deicing cycle

Cassie-Baxter State

Ice

Cassie-Baxter State

Figure 10. Ice on candle soot coated surface (CS+RTV-1). The candle soot nanoparticles maintained their geometry and Cassie-Baxter superhydrophobic state after 100 icing/deicing cycle. The abrasion test was performed by using 400 grid SiC sandpaper with 50 g load on the prepared coating (CS+RTV-1) as shown in supporting information Figure S5. The surface was dragged in one direction for 30 cm under 50 g weight. The same abrasion test was repeated for 10 times and after that the coating was dipped into the beaker of water to observe the Cassie-Baxter superhydrophobic state. The dipped coating showed a shiny mirror in water due to the reflection of light from trapped air in between the soot particle and water layer as shown in supporting information video S7. Then the water droplets were impacted on the coating in air and the water droplets rolled off from the surface like a bead. At the end the decrease in contact angle (151° supporting information Figure S7) and increase in ice adhesion strength (30 ± 3kPa) was observed as compare to its initial state as shown in the Figure 11.


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The super-cooled water (-10 °C) was also impacted on to the prepared coating in air and the super-cooled water droplet showed jumping phenomenon. After that the half liter beaker of super-cooled water was dropped on to the prepared coating as shown in the supporting information video S1 then the coating was dipped into the beaker of super-cooled water to observe the Cassie-Baxter superhydrophobic state. The dipped coating showed a shiny mirror in water due to the reflection of light from trapped air in between the soot particle and water layer. At the end the contact angle (158°) and ice adhesion strength (18 ± 3 kPa) remained same. The coating was put outside the door in natural freezing rain to observe its durable icephobic behavior. The freezing rain test was performed at -10 °C in Zhejiang University Hangzhou China. The frost particles showed the jumping phenomenon on the prepared elastic superhydrophobic coating of candle soot and they were collected near the bottom of the coated plate as shown in the supporting information video S2. The frost particles grow gradually and covered the whole plate after 20 hours. These frost particles showed very weak interaction with the candle soot coating and were removed with small vibration as shown in the supporting information video S3. After that the coating was dipped into the beaker of water to observe the Cassie-Baxter superhydrophobic state. The dipped coating showed a shiny mirror in water due to the reflection of light from trapped air in between the soot particle and water layer. The shiny mirror of soot coated glass slide can be shown in the Figure 8b. Then the water droplets were impacted on the coating in air and the water droplets rolled off from the surface like a bead. At the end the decrease in contact angle (156° supporting information Figure S7) and increase in ice adhesion strength (22 ± 3 kPa) was observed as compare to initial state as shown in the Figure 11. The UV-durability of the prepared candle soot coating was observed after the irradiation of UV light for 2 h. After that the coating was dipped into the beaker of water to observe the Cassie29 ACS Paragon Plus Environment


Baxter superhydrophobic state. The dipped coating showed a shiny mirror in water due to the reflection of light from trapped air in between the soot particle and water layer. Then the water droplets were impacted on the coating air and the water droplets rolled off from the surface like a bead. At the end the contact angle (158° supporting information Figure S7) and ice adhesion strength (18 ± 3 kPa) remained same as shown in the Figure 11. Similarly the coating was dipped in highly acidic, basic and artificial sea water medium for almost one month. After that the coating was dipped into the beaker of water to observe the Cassie-Baxter superhydrophobic state. The dipped coating showed a shiny mirror in water due to the reflection of light from trapped air in between the soot particle and water layer. Then the water droplets were impacted on the coating in air and the water droplets rolled off from the surface like a bead. At the end the contact angle (158° supporting information Figure S7) and ice adhesion strength (18 ± 3 kPa) remained same as shown in the Figure 11. 35

25

ice

(kPa)

30

Ice adhesion strength 

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

20 15 10 5 0

In

10 1 na Ar 10 Ac 10 2h 3 iti tu 0f Ab 0 T 00 ic 0 li tif id al UV ra /B q ro ici i ra ap ng ui lf a st i a s d se ep ls re ion cy ng N ez e c e a les 2 / w el cy cy in wa cle gr te cle at te st s ain s er r cy cle s

Figure 11. Schematic shows the durability characterizations of the candle soot coating (CS+RTV-1) after different mechanical test. 30 ACS Paragon Plus Environment

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4. CONCLUSION Based on the classical nucleation theory, wettability theory and principles of fracture mechanics, this study conveys new visions to design icephobic coatings. We developed a durable and scalable icephobic coating of candle soot which always exists in Cassie-Baxter state. The candle soot nanoparticles not only repel the super-cooled water droplet before freezing due to their low energy and interfacial structure size but also introduce the cracks, voids and stress concentrators at the interface which leads towards the low ice adhesion coating. The developed coating showed, very weak interaction with super-cooled water/ice, four times delay in freezing point as compare to uncoated surface, low ice adhesion strength (18 kPa) and maintain the τice about 25 kPa after severe mechanical abrasions such as 30 liquid nitrogen/water cycles, 100 frosting/defrosting cycles at -20 °C, 100 icing/deicing cycles, 100 tape peel test, 10 sandpaper abrasions with 50 g weight, acid/base exposure, under UV and exposure to natural freezing rain in Hangzhou. The proposed technique is time-efficient, inexpensive and suitable for large-scale applications. ASSOCIATED CONTENT Supporting information The supporting information is available free of charge on the website. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +86-571-8795-3382. Fax: +86-571-8795-1227. Notes The authors declare no competing financial interest



ACKNOWLEDGMENTS The authors would like to gratefully acknowledge the National Natural Science Foundation of China (NSFC) for Award No. 21878267, 21576236 and 21676248 for supporting this research. Supporting information Figure S1. Height and position of substrate in the candle flame. Figure S2. Increase in concentration of candle soot nanoparticles with the passage of time. Figure S3. a) Candle soot nanoparticles without RTV-1(binder) b) Candle soot nanoparticles with RTV-1(binder) Figure S4. Difference between the embedded and non-embedded candle soot nanoparticles. Figure S5 a) 2-D AFM image of only RTV-1. b) 3-D AFM image of only RTV-1. c) 2-D AFM image of candle soot coating (CS+RTV-1) with more concentration of carbon soot. d) 3-D AFM image of candle soot coating (CS+RTV-1) with more concentration of carbon soot. (i)

Calculations for interfacial correlation factor 𝐟(𝐦,𝐑′) and interfacial structure size of 𝐑′ of candle soot nanoparticles.

(ii)

Work of adhesion between ice and candle soot coating.

Figure S6 water contact angle on sample surfaces Figure S7 Water contact angle on candle soot coating (CS+RTV-1) after different mechanical test. Figure S8 Representative Frequency sweeps for the storage modulus G' and loss modulus G" of RTV-1. Figure S9 a) small size of condensed water droplet on candle soot coating (CS+RTV-1) b) Cassie-Baxter state of candle soot coating (CS+RTV-1) in super-cooled water (-10 °C). 32 ACS Paragon Plus Environment

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Figure S10 Photographs of a) Tape peel test b) Sand abrasion test

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Cassie-Baxter State


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Durable and Scalable Candle Soot Icephobic Coating with Nucleation

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