The Criterion of Supercooled Bionic Lotus Surfaces Repelling Impacting Droplets for Anti‐Icing Engineering

Icephobic material is of great importance in power transportation, communication, aerospace, and so on. Bionic lotus superhydrophobic surfaces show good application prospects in anti‐icing by repelling impacting droplets. However, the boundary (criterion) between droplet bounce and deposition on superhydrophobic surface under cold freezing rain is unclear. Here, from the view of statistics, the boundary and internal heat transfer mechanism of the droplet bounce, pinning, and three kinds of deposition including icing at later, early retraction, and spread. are clarified Droplet viscosity increase reduces droplet contact with surface and inhibited splash at high We. Surfaces with temperature above −20.6 °C showed 100% droplet bounce after impact on We = 64.7, and the probability of water adhesion and ice nucleation decreases to ≈0%, resulting in completely anti‐icing. Droplet bounce transition mainly ascribe to the decrease retraction driven force caused by interface freezing and transition temperature increases approximately linearly from −25.9 to −21.1 °C within 21.6 < We < 75.5. Ice nuclei brought by condensation in humid environment (60–100 RH%) further cause droplet eccentric retraction and transition temperature increases to −6 to −10 °C. The fundamental understanding of droplet behaviors on supercooled superhydrophobic surface is beneficial for icephobic surfaces applications.


The Criterion of Supercooled Bionic Lotus Surfaces Repelling Impacting Droplets for Anti-Icing Engineering
impact on these surfaces can save a large part of initial energy in the pancake liquid film at maximum spread, [10][11][12] then go through a fast retraction [13][14] and bounce away from surfaces. This ability of repelling impacting droplets greatly shortens the contact time with surfaces [15][16][17][18] and allows droplets bounce away before freezing. [19][20] Since Mishchenko et al. first proposed to realize ice-free nanostructured surfaces by repelling impacting water droplets, [21] droplets impact and freezing on cold surfaces have been widely investigated. Compared with room temperature, the influence of supercooled surface on droplet impact mainly includes the droplet physical properties and droplet deposition behavior below bounce transition temperature. [22][23][24] Maitra et al. found that viscosity dissipation at low temperature reduces the droplet maximum spread, and the meniscus penetrates into surface texture results in a decrease of retraction velocity. [25] Zhang et al. suggested that it is the ice nucleation that causes droplet adhesion rather than viscosity, the contact of droplet with surface texture will result in a higher nucleation rate. [26] Kanta et al. developed total internal reflection technology and observed the solidification of droplets propagates continuously from the impact center to the edge of the liquid film on cold surfaces. [27] Zhu et al. summarized the four frozen droplet morphologies of different elliptical caps and rings on cold-inclined hydrophilic surfaces. [28] Although some progress has been achieved, the boundary between droplet bounce and deposition on superhydrophobic surface is not clear. Since nucleation in supercooled water droplets is a stochastic process and affected by temperature and local surface morphology, [29][30][31] single droplet behavior cannot get a stable result statistically.
Moreover, most of the studies about droplet impact on cold surfaces are carried out under dry environment, [32][33] which are significantly different from freezing rain environment (0 to −5 °C, >50 RH%). Actually, the frosting and droplet impact freezing in freezing rain should not be studied separately. For example, condensation and desublimation from vapor changes the surface wettability and affects droplet impact dynamics, [34] while the freezing of supercooled droplets leads to the formation of ice bridge or frost halo and further promotes solidification propagating within the frost layer. [35][36] The lack of clear criterion for bionic superhydrophobic surface repelling impact Icephobic material is of great importance in power transportation, communication, aerospace, and so on. Bionic lotus superhydrophobic surfaces show good application prospects in anti-icing by repelling impacting droplets. However, the boundary (criterion) between droplet bounce and deposition on superhydrophobic surface under cold freezing rain is unclear. Here, from the view of statistics, the boundary and internal heat transfer mechanism of the droplet bounce, pinning, and three kinds of deposition including icing at later, early retraction, and spread. are clarified Droplet viscosity increase reduces droplet contact with surface and inhibited splash at high We. Surfaces with temperature above −20.6 °C showed 100% droplet bounce after impact on We = 64.7, and the probability of water adhesion and ice nucleation decreases to ≈0%, resulting in completely anti-icing. Droplet bounce transition mainly ascribe to the decrease retraction driven force caused by interface freezing and transition temperature increases approximately linearly from −25.9 to −21.1 °C within 21.6 < We < 75.5. Ice nuclei brought by condensation in humid environment (60-100 RH%) further cause droplet eccentric retraction and transition temperature increases to −6 to −10 °C. The fundamental understanding of droplet behaviors on supercooled superhydrophobic surface is beneficial for icephobic surfaces applications.

Introduction
In the freezing rain environment, raindrops fall on cold surfaces and immediately freeze into glaze ice. Ice accumulation brings severe challenges to various engineering fields such as electric power transmission, aerospace, energy communication, and so on. [1][2] Bionic lotus surfaces with higher water contact angle (WCA > 150°) and lower water sliding angle (WSA < 10°) [3][4][5] show a good prospect in anti-icing engineering. [6][7][8][9] Droplets droplet in engineering environment with different humidity largely restricts the development of new icephobic coatings for practical application.
Based on our previous research of bionic superhydrophobic surface construction, [5] icing delay, [37] natural deicing and droplet impact dynamics, [38][39] this present work systematically studied the effect of viscosity on droplet impact dynamics and the droplets statistical behaviors on cold superhydrophobic surfaces. By adjusting the substrate temperature from -35 to -5 °C, environment humidity from 0 to 100 RH%, we found droplet bounce, pinning, and three kinds of deposition including icing at later/early retraction and spread icing and discussed internal mechanical and heat transfer mechanism of droplet behaviors conversion. On this basis, we proposed a criterion from the view of ice nucleation rate for surface repelling impact droplet to guide the application of icephobic coatings in freezing rain environment.

Effect of Droplet Viscosity on Impact Dynamics
Based on our previous research, PTFE/PPS porous bionic superhydrophobic surface (PBS) with micro/nanostructure was constructed by a conventional curing process. [5] As shown in Figure 1a,b, a large number of micropores with diameters of 20-50 µm are covered with 200 nm nanofibers, leading to a superior hydrophobicity with a water contact angle of 160° and a water sliding angle of 8°. Droplet impact on supercooled surface was carried out in a self-built enclosed chamber with adjustable temperature and humidity. Figure 1c shows the illustration of the experimental setup. Droplets are generated from a needle driven by peristaltic pump. By adjusting the recycled water flow rate, droplet temperature is controlled to be −1∼ −1 °C and the water in silicone hose just does not freeze.

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The PBS is placed on a Peltier cooling stage with an adjustable temperature of 25 to −35 °C. The sprayer and refrigeration unit of chamber are used to control environment temperature and humidity. Thermocouple sensors are employed to simultaneously measure droplet and PBS temperature. Droplet impacting on surface is recorded by a high-speed camera system. Figure 1d-f shows the selected snapshots of droplets bounce (c-d) and deposition (e) after impact on PBS with different temperatures at We = 53.9. Here, the Weber number We = ρV 2 D 0 /γ, where ρ, V, D 0 , and γ are the density, impact velocity, initial diameter, and surface tension of the droplet, respectively. After a similar spread during 0-6 ms, droplet impact at room temperature shows a higher maximum spread β m = 2.4, compared with the bounce (2.29) and deposition (2.08) on supercooled PBS. Here, β m = D m /D 0 and D m is maximum spread diameter. Then droplet on 25 °C and −18 °C PBS retracts rapidly with the dynamic contact angle at the retraction front showing hydrophobic, and finally bounce off at 24 ms. In contrast, droplet on −26.6 °C PBS retracts slowly and shows a hydrophilic front after 9 ms, which indicates that wettability at interface has changed. Droplet finally stops retraction after 16 ms and deposits on PBS.
Droplet impact on PBS is governed by the balance of inertia, viscosity, and capillary force. The inertial force determined by impact condition dominates the droplet spread, while capillary and viscous force are determined by the droplet physical properties. The capillary force makes interface maintain smooth and dominates droplet retraction. The viscous force continuously consumes droplet kinetic energy slows down its motion. Compared with room temperature, viscosity increase and interface freezing caused by supercooled PBS lead to a translation from droplet bounce to deposition. Considering the difficulty of accurate controlling droplet temperature, the glycerolwater solution with different mass concentrations at room temperature is used to investigate the influence of droplet viscosity increase (see Figure S1, Supporting Information). The properties of glycerol-water droplets at 25 °C are shown in Table 1. Here, the Ohnesorge number as the ratio of viscous force to capillary force is expressed as , µ is droplet viscosity. Figure 2 shows the selected snapshots of droplets behavior after impact PBS at We = 10.8-86.2 and Oh = 0.00167-0.08286. For droplet impact at low We and Oh (Figure 2a), with the continuous inward movement at the contact line, a Worthington jet is formed at impact point and drives droplets to bounce from PBS. As We increases, retraction velocity increases and a satellite droplet is separated at top jet during bounce ( Figure 2b). When We increased the splash parameter K = We·Re 0.5 > 3000, droplet splashes at spread and instability appears at the bottom jet ( Figure 2c). Here, the Reynolds number Re = ρD 0 V 0 /µ. In particular, for We ≥ 75.5 and Oh ≤ 0.00249, droplets splash violently at spread and are unable to form an upward jet. The bounce height decreases significantly ( Figure 2d). And when droplet viscosity is high enough (We ≥ 64.7 and Oh = 0.08286), droplet deposition on surface after the first bounce due to viscosity dissipation (Figure 2e). Figure 3a shows the phase diagram of droplet behaviors in Figure 2 as a function of We and Oh. Droplet splash and the instability of jet are significantly inhibited as Oh increases. For supercooled PBS, viscosity increase prevents droplet from breaking apart at high We (We > 75) and preserves more initial kinetic energy, which is a benefit for droplet rebound. As shown in Figure 3b, the fitting results of β m and We obey the power law β m ∼ aWe b . At maximum spread, the initial kinetic energy of the droplet is converted to the surface energy at the gas-liquid interface γ l-g and the solid-liquid interface γ l-s at the maximum spread, the viscous dissipation can be estimated as. [40] 0 0 Here, τ s is characteristic spread time, /4 2 π Ω = LD m is the viscous dissipation volume and L = 2D 0 /(2Re) 0.5 is the boundary layer thickness. The larger the droplet viscosity, the more viscous dissipation, and the smaller β m . Under the condition that the capillary force that dominates retraction is almost constant, the reduction of the total retraction distance even makes droplet contact time τ break the limit of 2.6τ 0 . Here, / 0 0 3 τ ρ γ = R is the capillary time. As shown in Figure 3c, droplets with higher viscosity show a smaller contact time with PBS. Figure 3d shows the restitution coefficient ξ = V 0 /V = (h 0 /h) 0.5 as a function of We and Oh. Here, the V, h 0 , and h are the velocity of the first bounce from the PBS, initial height, and maximum height after the first bounce, respectively. As We and Oh increase, viscous dissipation of droplet increases during droplet spread and retraction, ξ decreases. In particular, two sudden ξ decreases are observed when droplet behavior converts to splash ( Figure 2c) and violent splash without Worthington jet (Figure 2d). Droplet kinetic energy is consumed to create more interfaces during splash. When We is high enough, stochastic splashes of satellite droplets result in eccentric retraction. The Worthington jet cannot be generated, and droplet bounce at a lower ξ. In summary, droplet viscosity decrease caused by low supercooled PBS is not conducive to continuous bounce (lower ξ) or move on PBS, but significantly reduces droplet contact with a surface (lower β and τ) during first bounce and inhibited splash at high We, which is a benefit for repelling impacting droplets.

Interfacial Freezing Inhibiting Droplet Bounce in Dry Environment
The effect of interface freezing on droplet behaviors on supercooled PBS was further investigated in a dry environment (< 20 RH%). Figure 4a-e shows the selected snapshots of 0 °C droplet bounce, pinning, or deposition of icing at until the Worthington jet is formed, [39] the droplet thickness h increases, retraction slows down gradually and finally droplet bounces from PBS at 23 ms. As PBS temperature decrease, droplet starts pinning on surface. Retraction is significantly slowed down after Worthington jet formation. Although main droplet bounce from PBS, a residual droplet is left on the surface and results in a longer contact time of 31 ms. As a further decrease of PBS temperature, the retraction velocity is significantly slower in the decelerating retraction (icing at later retraction) and uniform retraction (icing at the early retraction) stages respectively compared with bounce behavior. Droplet finally deposition on PBS. When the PBS temperature is low enough, droplet barely retracts after a short period of dynamic wetting angle change (icing at spread), showing a similar behavior to droplet on bare al. Since nucleation in supercooled water droplets is a stochastic process, repeated experiments were conducted every 0.2 °C to calculate probability distribution of 0 °C droplet behaviors after impact on supercooled PBS at We = 64.7. As shown in Figure 4h and Figure S2 (Supporting Information), when the PBS temperature is higher than −20.6 °C, the probability of droplet bounce is 100%, probability of water adhesion and ice nucleation decreases to ≈0%, resulting in a completely anti-icing in engineering application. As temperature decreases, droplet shows pinning at the PBS temperatures of −20.8, −22.4, and −24.4 °C and gradually transform to icing at later retraction, early retraction, and spread. The probability of droplet bounce, pinning, and icing at later retraction within −25 to −23 °C is 10%, 10%, and 80%, respectively. While for temperature within −35 to −33 °C, droplet shows a 40% probability of icing at early retraction and 60% of icing at spread. According to the principle of minimum variance (Supporting Information Discussion S1), the bounce transition temperature As shown in Figure 5, the contact sequence between different parts of the main droplet and supercooled PBS causes the different heat transfer time and temperature gradient inside the droplet, and further forms the viscosity gradient from bottom to top. Since droplet spread and retraction depend on the advancing and receding of contact line, the droplet increases viscosity at interface brings additional viscous dissipation. At early spread, droplet spread velocity can be written as V ∼ (D 0 V 0 /t) 0.5 , which means this part liquid (dash area) have the longest heat transfer time with PBS. [41,42] As surface temperature decrease, the interface freezes outward from the impact point. When the SHS is cold enough, freezing happens at spread and the shear force on ice crystals diffuses this seed crystal to the whole interface during spread, result in the PBS to be covered by an ice layer during the subsequent retraction. Droplet on ice layer shows a bare Al like retraction. For a lower supercooling degree, central interface freezes outward during retraction. When the ice layer meets the contact line that keeps retraction inward, the original PBS-droplet contact converts to an ice-droplet superhydrophilic contact. The adhesion work of substrate to droplet turns from W adh = A·(γ sg + γγ sl ) = Aγ(1+θ PBS ) to Aγ(1+θ ice ), here A is the contact area when ice layer meets the contact line. Droplet is strongly adhered by ice layer and difficult to bounce from surface. The surface temperature at which the droplet retraction velocity slows to 0 before it is about to bounce is the bounce transition temperature.
The effect of We on the bounce transition temperature and droplet behavior evolution was further investigated.  Figure 6d shows the dimensionless contact length L/D when retraction stops under different We. Results show that as We and PBS temperature increase, L/D slowly decreases until the icing at spread behavior converts to retraction, and then rapidly decreases to 0, droplet bounce from PBS.
We further attempt to correlate the droplet behavior evolution with interfacial heat transfer. Since droplet spread is mainly driven by the deformation stress at the contact line and droplet retraction is the process of the rim at the front continuously col- c-e) deposition of icing at later retraction, early retraction, and spread. g) β as functions of time in different droplet behaviors of (a-f). h) Probability distribution of droplet behaviors after impact on supercooled PBS at We = 64.7 (10 impacts for each temperature interval).

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lecting the liquid in the central lamella, [9,39,42] the liquid in contact with the substrate at the center liquid film remains static during most of the contact time. Shiri [15] demonstrated that the convection heat transfer rate is only 0.002 that of heat conduction. For droplet impact on supercooled PBS in this work, thermal diffusion length 50 ατ ≈ µm is much smaller than substrate thickness and the droplet size of D = 4 mm is also much smaller than the sample size of 120 mm × 50 mm, the heat conduction can be approximated as 1D and semi-infinite transient process. Here, α = 1.087 × 10 −7 m 2 s −1 is the thermal diffusivity of PTFE. Since the interface keeps an ice-water mixture state during droplet contact with substrate, interface temperature is maintained 0 °C before freezing completely. For substrate, the interface temperature suddenly changes from the initial temperature at 0 ms to 0 °C and remains constant. Heat flux q of heat conduction is written as:   5 (b). c) The average droplet retraction velocity at uniform retraction stage of 9 -11 ms under different droplet behaviors. The fitting results of purple and green dotted line shows that interface freezing 59.7% and 89.9% for droplet icing at later and early retraction, respectively. d) Droplet contact length when retraction stops as functions of We and substrate temperature. (10 impacts for each temperature interval of 2 °C and take the average).

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Here, the x at interface is 0, T w , T 0 , λ, and t are the ice-water mixture temperature, initial substrate temperature, interface thermal conductivity, and heat transfer time, respectively. In order to obtain the heat transfer during impact, the droplet trajectory equation is simplified as 2 Figure S4, Supporting Information). Here, the r m is the maximum spread radius. The heat transfer Q can be expressed as: Indicating that the heat transfer is independent of droplet properties at the freezing point. Based on Cassie equation cosθ = f s cosθ 0 + f s − 1, the solid-liquid contact fraction f s is calculated as f s ≈ 0.092 with the apparent WCA = 160° and the intrinsic WCA = 110° for PBS in this work. Considering the thermal resistance brought by the air cushion, λ is calculated as λ = f s λ PTFE + (1-f s )·λ Air = 0.0444 W m −1 K −1 . The β = 0.7554We 0.2878 of droplet at Oh = 0.004 in Figure 2a was used to approximate the maximum spread of 0 °C droplet on supercooled PBS, the heat transfer can be expressed as a function of We and ΔT: Figure 7 describes the relationship between We versus T 0 versus Q and the phase diagram of droplet behavior according to the Equation 3 Here, Q is the heat transfer by assuming that droplet has finished bounce on PBS. As heat transfer decreases, droplet gradually changes from icing at spread to retraction, and then pinning and bounce. The bounce transition temperature (solid red line) increases approximately linearly from −25.9 to −21.1 °C within 21.6 < We < 75.5. When the heat transfer reaches an average of about 10.97 mJ, the droplet exhibits deposition on supercooled PBS.

Condensation Nucleation Increases Bounce Transition Temperature in Humid Environment
In actual freezing rain, surfaces are often exposed to higher humidity. The effect of environment humidity on 0 °C droplet behavior is investigated by adjusting the sprayer and refrigeration unit of chamber. The environment humidity is adjusted to moderate humidity (40-60 RH%) and high humidity (80-100 RH%) and the sample is placed on cooling stage for 4 min to ensure PBS is cooled to the set temperature. Figure 8a,b shows the probability distribution of droplet behaviors after impact on PBS at We = 64.7 in moderate humidity environment and high humidity environment, respectively. Compared with the bounce transition temperature of −22.1 °C in low humidity environment Figure 4h, droplet keeps icing at spread in high humidity environment until substrate temperature increases to −11.8 °C, and then rapidly changes to icing at later retraction and pinning. When temperature rises above −6 °C, droplet 100% exhibits bounce behavior. In contrast, droplet shows irregular bounce or deposition between −19 and 9 °C. Even at −18.6 °C, droplet exhibits bounce but pinning at −9 °C. Droplet 100% exhibits bounce behavior after temperature increases above −8.8 °C.
Droplet also shows eccentric retraction in humid environment. As shown in Figure 8c, 0 °C droplet shows a symmetric maximum spread at 6 ms after impact on −20 °C supercooled PBS at We = 64.7 in a moderate humidity environment. But in the subsequent retraction, the left droplet retracts rapidly while the right is pinned, eventually result in a movement to the right. Figure 8d shows the average eccentric distance Δx as functions of substrate temperature and environment humidity. The eccentric retraction mainly occurs in the droplet behaviors of icing at retraction. In a moderate humidity environment, droplet starts eccentric retraction as PBS temperature increases above −22 °C. After maintaining the maximum within −20 to −16 °C, Δx start decreases when the surface temperature reaches −15 °C until droplet 100% bounce or pinning at −12 °C, Δx = 0. For a high humidity environment, Δx suddenly increases within −14 to −12 °C and then decreases with surface temperature.
The eccentric retraction and decrease of bounce transition temperature are related to ice crystals formed by water vapor in humid environment. When there are ice crystals in the impact area, droplet does not need to form ice crystals by heat exchange with the surface. This ice crystals act as crystal nuclei leading to a fast interface freezing, and the probability of icing at spread significant increase. In this case, the number of nuclei and nucleation time on the surface (nucleation rate) become the key factors influencing

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Avogadro constant, and Boltzmann constant. The p and p 2 is the pressure of water vapor in environment and the saturated vapor pressure corresponding to the condensate water, respectively. And the p 2 is calculated as: [44] According to Arrhenius equation, the nucleation rate J ∼ exp(−ΔG 12 * /kT) ∼ exp(−lnp/p 2 ) 2 . For moderate (60 RH%) and high (100 RH%) humidity at −5 °C, the pressure of water vapor is calculated as 240 pa and 400 pa, respectively. As shown in Figure 8e, for PBS temperature higher than −12.2 °C (60 RH%) and −5.6 °C (100 RH%), ln(p/p 2 ) < 0 and ΔG > 0. Water vapor in environment cannot spontaneously condense to form little droplets, and the droplet behavior is similar to 100% bounce in dry environment (<20 RH%). While for PBS temperature lower than −21 °C (60 RH%) and −15 °C (100 RH%), ln(p/p 2 ) > 0.74, droplet show icing at spread behavior both in moderate and high humidity environment, indicating that ln(p/p 2 ) = 0.74 is a criterion that there are sufficient ice nuclei to freeze the whole interface. When 0 < ln(p/p 2 ) < 0.74, there are some ice nuclei at the interface and droplet behavior is a result of competition between clean interface (dry environment) and interface covered by ice nuclei (high humidity environment), which depends on the number of ice nuclei at impact region ( Figures S5 and  S6, Supporting Information). In particular, when ice nuclei are present on one side of the droplet but not on the other side, droplet exhibits eccentric retraction with the former rapidly retracts and the latter is pinned. The criterion for droplet behaviors in Figure 8e is well consistent with the droplet behavior statistics in ( Figure S7, Supporting Information).

Conclusion
In this work, the porous bionic lotus superhydrophobic surface (PBS) combines with temperature and humidity adjustable chamber and high-speed camera system is used to systematically investigate the effect of viscosity (0.0017 < Oh < 0.0829), impact condition (21.6 < We < 75.5) on droplet impact dynamic, the substrate temperature (−35 to −5 °C) and environment humidity (0-100 RH%) on 0 °C droplet behaviors. Although the droplet viscosity increase consumes the initial kinetic energy and reduces the restitution coefficient, the decrease of the maximum spread factor and inhibiting splash at We > 75.5 are conducive to reducing the contact between droplet and substrate. As substrate temperature decrease, 0 °C droplet bounce, pinning and three kinds of deposition including icing at later, early retraction, and spread are observed in turn on supercooled PBS. Droplet deposition mainly ascribe to the interface freezing. Droplets icing at early and late retraction show an average of 89.9% and 59.7% interface freezing respectively at the uniform retraction stage. The maximum heat transfer allowed for a 4 mm diameter droplet bounce from PBS is 10.97 mJ. In humid environment, the environment humidity and substrate temperature determine interface ice nuclei rate and droplet behavior by influencing the pressure of water vapor in environment p and the saturated vapor pressure p 2 respectively. When 0 < ln(p/p 2 ) < 0.74, droplet may show eccentric retraction because of the uneven distribution of ice nuclei. This research about droplet behaviors on supercooled PBS in freezing rain environment will provide a new criterion for icephobic material surfaces in anti-icing engineering application.

Experimental Section
Materials and Coating Preparation: The water-soluble polytetrafluoroethylene (PTFE) emulsion was prepared by adding irradiated PTFE powder (diameter < 10 µm) by suspension method, nonionic surfactant (C 8 H 17 -Ph-O(C 2 H 4 O) n H, n ≈ 10) and ammonium carbonate ((NH 4 ) 2 CO 3 ) to the mixed solvent (distilled water/ethanol/ isobutyl alcohol in a volume fraction of 2:5:1) and ultrasonic dispersing for 30 min. As a binding agent, PPS resin was mixed into the PTFE emulsion and dispersed for 10 min to form the coating precursors. Superhydrophobic coating was prepared by spraying the coating precursors on bare aluminum polished by 800# sandpapers with 0.2 MPa nitrogen gas, then curing at 150 °C for 1 h and 380 °C for 1.5 h, respectively, and natural cooling at room temperature.
Contact Angle Measurement: The WCA and WSA of Surf1-Surf4 were tested by a contact angle apparatus (DSA-100, KRÜSS GmbH, Germany) using 5 µL and 15 µL distilled water droplet, respectively. The advancing/ receding angle was tested by injection/extraction liquid into/from a droplet and measured from digital camera photos. Each surface was tested five times for repeatability.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.