Bio-Inspired Fabrication of Hierarchical Structure Superhydrophobic

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Bio-Inspired Fabrication of Hierarchical Structure Superhydrophobic Surfaces to Understand Droplet Bouncing Dynamics for Enhancing Water-Repellency Yizhou Shen, Jie Tao, Guanyu Wang, Chunling Zhu, Haifeng Chen, Mingming Jin, and Yuehan Xie J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01538 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 14, 2018

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The Journal of Physical Chemistry

Bio-Inspired Fabrication of Hierarchical Structure Superhydrophobic Surfaces to Understand Droplet Bouncing Dynamics for Enhancing Water-Repellency

Yizhou Shen,† Jie Tao,†,* Guanyu Wang,† Chunling Zhu,§,*, Haifeng Chen,‡ Mingming Jin† and Yuehan Xie,† †

College of Materials Science and Technology, Nanjing University of Aeronautics and

Astronautics, Nanjing 210016, P. R. China ‡

Department of Materials Chemistry, Qiuzhen School, Huzhou University, 759 East 2nd

Road, Huzhou 313000, P. R. China §

College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics,

Nanjing 210016, P. R. China

ACS Paragon Plus Environment

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ABSTRACT: Dynamic water-repellency refers to the capacity of droplets to rapidly detach from solid surfaces and is usually evaluated by the contact time; it has diverse applications such as anti-icing, water proofing, and self-cleaning, etc. Although various functional surfaces with non-wettability have been designed and fabricated to provide dynamic water-repellency with a certain extent of application potential, the underlying physics of bouncing dynamics of impact droplets is still needed to be studied for more rational explanation of some special phenomena, especially under low-temperature conditions. Based on experimental studies and theoretical calculations, we analyzed the critical condition between rebounding and splashing of impact droplets on the hierarchical structure superhydrophobic surfaces. Subsequently, the rebounding process was considered as the research object for revealing the action mechanism of triple-phase contact line on mediating the dynamic water-repellency. All these physics will help to analyze the anti-icing mechanism of anti-icing/icephobic materials with the aim to well repel the coming supercooled droplets.


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1. INTRODUCTION

Dynamic water-repellency of impact droplets on solid surfaces has attracted much attention for fundamental research over the past few years,1-4 because of great importance to diverse applications including anti-icing,5,6 self-cleaning,7 water proofing,8 and dropwise condensation.9 The main aim of these studies was to reduce the contact time of impact droplets and enhance the rapid droplet detachment from solid surfaces.10 Moreover, the underlying mechanism was to improve the hydrophobicity via a synergistic action of microstructure and surface free energy,11 and on this basis to design submillimeter macroridge structures to break the axisymmetric recoil of impact droplets on solid surfaces.12 Increasing the hydrophobicity (especially in the aspect of reducing the wetting hysteresis) is favorable for promoting the movement of triple-phase contact line of impact droplets on solid surfaces. This results in a lower work done against the viscous resistance during the solid-liquid contact process. Furthermore, various states of impact droplets on solid surfaces (such as spreading, recoiling, bouncing, and splashing) have also been widely investigated for clearly elucidating the mechanisms for these phenomena.2,13,14 Bussmann M., et al. modeled the splashing of a droplet impacting on a solid surface, and found that contact angle (CA) played an important role in the splashing capacity of impact droplet on a solid surface.15 James C. Bird reported that a macroridge could alter the hydrodynamics of impact droplets; therefore, the retract process generated more triggering locations, leading to a more rapid droplet separation.16 Essentially, this phenomenon is caused by the increase in triple-phase contact line length. Thus, contact line is a significant


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medium parameter to determine the dynamic behavior of impact droplets on solid surfaces; this is controlled by many factors such as surface wetting, droplet surface tension, and ambient environment.17-21 Here, we studied on the bouncing dynamics of impact droplets on the hierarchical

structure

superhydrophobic

surfaces

under

low-temperature

conditions; it is shown how heat transfer from a droplet to a cold surface retarded the movement (and contact time) of droplet by affecting the triple-phase contact line. This helps to analyze the application potential of superhydrophobic surfaces, especially in the antifreezing field. It has been demonstrated that under low temperature conditions, the water molecules at triple-phase contact line position first lost the partial movement capacity and produced interface action forces (electrostatic interaction (dominant) and van der Waals forces) on the solid surface. This pinned the triple-phase contact line to hinder the droplet movement and increased the contact time of impact droplets. Particularly, the retracting process of impact droplets on superhydrophobic surfaces produced a remarkable distinction as the temperature decreased. The physics of these experimental phenomena is described in details below by developing appropriate physical models and performing theoretical calculations.

2. EXPERIMENTAL SECTION

2.1. Materials


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Ti6Al4V sheets were purchased from Baoji Titanium Industry Co., Ltd (Shaanxi, China). Heptadecafluorodecyl trimethoxysilane (FAS-17, analytical grade) was provided by Tokyo Chemical Industry Co., Ltd (Japan). Other analytical-grade experimental chemicals such as CH3COCH3, CH3CH2OH, NaOH, and HCl were provided by Sinopharm Chemical Reagent Co., Ltd (in China). Deionized (DI) water was prepared in our laboratory. 2.2. Experimental Ti6Al4V sheets were cut into squares with the size of 10 mm × 10 mm × 5 mm, and then was thoroughly cleaned ultrasonically with acetone, ethanol, and distilled water. Subsequently, the microscale pit structures were constructed by means of sand blasting with 150-grit alumina at 0.5 MPa for 10 s. After cleaning again, the sample plates were placed into a 220 °C autoclave (containing 30 mL of 1 M NaOH solution) for 8 h to conduct the hydrothermal reaction and grow a layer of nanowires. The as-prepared samples were neutralized by placing in a 0.1 M HCl solution for 0.5 h. Finally, an annealing treatment was carried out to change the crystal form of nanowire arrays, and the main processing parameters were designed as 500 °C (2 °C/min rise in temperature) and 3 h. To obtain the low surface energy for hydrophobicity, all the asprepared micro-nanostructure surfaces were immersed in a 1 wt% FAS-17 ethanol solution. After reacting for 24 h, these samples were dried in a 120 °C oven for 2 h to obtain the final superhydrophobic samples. 2.3. Characterizations and wettability test


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A field-emission scanning electron microscopy (FE-SEM; Hitachi S4800, Japan) was used to observe the morphologies of the as-prepared micro-nanostructure surfaces. The chemical compositions of micro-nanostructures before and after FAS-17 modification were measured using an X-ray photoelectron spectrometer (Kratos AXIS UltraDLD, Japan). The superhydrophobicity of surfaces was indicated by the apparent contact angle (APCA), contact angle hysteresis (CAH), and sliding angle (SA), which were measured using a CA analyzer (Kruss DSA100, Germany). During this measurement, a 4 µL reference water droplet was selected to characterize the superhydrophobicity, and the average of three measurements was determined. 2.4. Evaluation of dynamic water-repellency Dynamic water-repellency is usually determined from the contact time of impact droplet on solid surfaces. A high-speed camera (Photron Mini 100) filming at 5,000 frames per second was used to record the contact and movement of impact droplet on the as-prepared micro-nanostructure surfaces. The droplets were released from a different height over sample surfaces, and the temperature of superhydrophobic sample surfaces was also tuned using a temperature controller, as shown in Figure 1. Furthermore, dimensionless Ohnesorge (Oh) and Bond (Bo) numbers were calculated to describe the characteristics of impact droplets, and the two numbers were kept constant in this study. Here, Oh and Bo can be expressed as follows

Oh= µ

ρσ 2R0

Bo= (2 R0 ) 2 ρ g σ

(1)

(2)


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and calculated as 2.63×10-3 and 0.54, respectively, where ρ and µ are the density and viscosity of water droplets, respectively; R0, g, and σ are the radius, gravitational acceleration, and surface tension.22

3. RESULTS AND DISCUSSION

3.1. Microscopic topographies and non-wettability The superhydrophobic surface was prepared on a Ti6Al4V substrate using a combined method of sand-blasting and thermal treatment. Figures 2a and b show the FESEM images of micro-nanostructures of the superhydrophobic surface. The constructed micro-nanostructures possess a multiscale gradient from ~50 µm to hundreds of nanometer, and evenly distribute on the substrate surface. Subsequently, a hydrogen-bond-driven technique was used to self-assemble the low-energy groups of FAS-17 onto the micro-nanostructure surface,23,24 achieving the superhydrophobicity. As shown in Figure 2c, the sample surface has high-intensity peaks of F1s and FKLL and low-intensity peaks of Ti2p and O1s after modifying with FAS-17. Moreover, the high-resolution spectrum (see inset of Figure 2c) shows that the peaks corresponding to –CF2 and –CF3 appear at 291 eV and 294 eV, respectively. This clearly indicates that the hydrophobic groups have been self-assembled on the surfaces of micro-nanoscale hierarchical structures. The resultant surface achieves a robust non-wettability with the APCA reaching 161° as well as the CAH being


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only 1.5° (as shown in Figure 2d), and a 4 µL droplet nearly suspends on the sample surface. 3.2. Movement characteristics of impact droplets at different Weber numbers Based on this superhydrophobic surface, the dynamic behavior of impact droplets was studied in details. Droplets impacting on superhydrophobic surfaces at different Weber numbers (We) exhibit some distinct phenomenasuch as Rebounding and Splashing.25-27 Here, We = ρV02 D0 / σ , and it only reflects the change in impact velocity because the water droplets have the same volume, where V0 is the impact velocity of water droplets with values of 0.5 m/s, 1.0 m/s, 1.5 m/s, 2.0 m/s, and 2.5 m/s. Figure 3 shows the movement of a 4 µL room-temperature droplet impacting on the superhydrophobic surface at different Weber numbers (We) ranging from 7 to 172. The impact droplets with lower Weber numbers (less than 110) were observed to perfectly rebound off the surface, and the entire movement are completed within ~11.5 ms, indicating robust dynamic water-repellency. Based on this perfectly rebounding process, much effort has been devoted to determine the dynamic behavior of impact droplets on a superhydrophobic surface to further reduce the contact time,28-30 because it was considered to be crucial for the applications of superhydrophobic surfaces, particularly in anti-icing field. The contact time directly reflects the extent of thermal and energy conversions between a water droplet and a solid surface.31,32 However, another phenomenon is also shown in Figure 3 that the impact droplet at We= 172 splits into many small droplets after spreading to the maximal


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deformation, and the schematic diagram for breakup can be seen in Figure 4. The splashed small droplets rapidly bound off, and a remaining large droplet continues to complete the retract and rebound process until 13.4 ms. By comparatively analyzing the five movement processes of impact droplets, the contact time is found to remain almost at the same level (~11.5 ms) with the increase in Weber number, except the breakup phenomenon of impact droplets at We = 172 (the contact time reaching 13.4 ms), as illustrated in Figure 5a. This can be attributed to the distinct retracting energy of impact droplets between rebounding and splashing processes.33,34 At a high Weber number, when the impact droplets complete the spreading process and reach the maximal deformation, the small droplets split from the edge of impact droplets and consume partial kinetic energy to overcome the work done against surface tension.35 Thus, the remaining large droplet has a lower retracting velocity, resulting in the impact droplet at We = 172 spending 13.4 ms to completely rebound off the surface. However, the spreading time almost maintains a constant value of ~2.0 ms, and the difference in the contact time among these cases is mainly caused by the recoiling process. The recoiling time has the same trend as the contact time with the variation in Weber number. Figure 5b shows the change in triple-phase contact line positions of impact droplets with time under various Weber numbers. Here, we use the ratio of D/D0 (D is the diameter of contact area of impact droplet with surface (is also the diameter of the contact line), and D0 is the initial diameter of droplet, D0 = 2 mm) to represent the change in contact line position. Clearly, all these impact droplets reach their maximum deformation diameter (Dmax) at 2.1 ms, indicating the time


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required to spread to the maximum diameter is the same irrespective of Weber number. In addition, by comparing with the impact droplets at the other Weber numbers (i.e., 7, 28, 62, and 110) due to the breakup of the impact droplet at We = 172, the contact line position shows a distinct slump at 2.1 ms. Also, the subsequent retracting process makes a change with the contact line position first gathering quickly and then slowly under the action of inertia force,36 making the retracting time longer (up to 11.3 ms). 3.3. Rebounding and Splashing Based on the above observation and analyses about the dynamic behavior of a 4 µL room-temperature droplet impacting on a superhydrophobic surface under various Weber numbers, we believe that it is important to explore the critical condition between Rebounding and Splashing to evaluate the dynamic behavior of impact droplets, and also it is important to develop a theoretical calculation model to deduce the critical condition. We assume that the droplet is a perfect sphere, which then spreads out to become a cylinder. According to the law of energy conservation, the energy relationship during the spreading of impact droplets can be expressed as follows: EK 1 + ES1 + EH 1 = EK 2 + ES 2 + EH 2 + W

(3)

where EK1, ES1, and EH1 are the initial kinetic energy, surface energy, and potential energy of impact droplet, respectively. When the impact droplet spreads to the maximal deformation, the kinetic energy, surface energy, and potential energy of impact droplet are labeled as EK2, ES2, and EH2, respectively. W is the work done against viscosity during


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the spreading process. By comparing with the initial kinetic energy ( EK1 = πρ D03V02 12 , 2 EK 2 = 0 ) and surface energy ( ES1 = π D02σ , ES 2 = π Dmax (1 − cos θ )σ 4 ), the potential energy

2 δ 8 ) is found to be so small (with the difference of two ( EH 1 = πρ gD04 12 , EH 2 = πρ gDmax

orders of magnitude) that it can be ignored. ρ and σ are the density and surface tension of impact droplet; θ is the APCA of a 4 µL droplet on the surface; D0 and V0 are the diameter and velocity before the impact of droplet onto the surface and Dmax is the maximal wetting length (i.e., the diameter of maximal deformation). Furthermore, based 2 δ , where δ is the thickness of droplet spreading to the on mass balance, π 6 D03 = π 4 Dmax

−2 D0 . Also, the work done (W) maximal deformation, and Dmax D0 = ξmax , and δ = 2 3ξ max

against viscosity can be expressed as follows:22

W =∫

t

∫

0 Ω

Ψd Ωdt = ΨΩt

(4)

and Ψ = µ (∂µ ∂y + ∂Ω ∂x) ∂V0 ∂y ≈ µ (V0 δ )2 .37,38 Therefore, the equation can be replaced by 4 W = πµV0ξmax D02 , where µ is the viscosity of water droplets, and t is the time required for the

impact droplet to spread to the maximal deformation. Finally, the energy conservation relationship of impact droplets on the superhydrophobic surface can be expressed as follows: 2 4 3(1 − cos θ )ξ max − 12 6ξ max + =1 2We Re

(5)

When the wetting length of impact droplets reaches Dmax, the impact droplet will produce a splash under the action of residual kinetic energy. Hence, the Equation (5) can be


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considered to be the critical condition between Rebounding and Splashing for an impact droplet on the superhydrophobic surface. We define that: f (We, R e) =

2 4 3(1 − cos θ )ξ max − 12 6ξ max + −1 2We Re

(6)

where based on many test results,39 ξ max = 0.9 ⋅ (2We)1/ 4 . Re is the Reynolds number ( Re = ρV0 D0 µ ). When f (We,Re) ≥ 0, the impact droplets can only undergo the rebounding phenomenon. If not, the impact droplet will produce the splash phenomenon instead, as shown in Figure 6. To validate the deduced critical condition, some experimental results were incorporated with the impact velocity ranging from 0.5 m/s to 3 m/s and substituted into the Equation (6), demonstrating perfect consistency with the rule (red symbols in Figure 6). However, it should be noted that the volume of impact droplet with a diameter of 2 mm is constant, and the superhydrophobicity has an APCA of 161° and CAH of 1.5° in this study. Therefore, it can be predicted that the splashing phenomenon will always take place with the increase in impact velocity. 3.4. Effect of temperature on dynamic water-repellency Subsequently, by considering the substrate temperature as another important factor affecting the dynamic behavior of impact droplets,40 the rebounding process (at We = 28, i.e., an impact velocity of 1.0 m/s) was used to evaluate the effect of surface temperature on impact dynamics based on the above-analyzed results. The temperature of superhydrophobic substrate surface was reduced, and the movement process of room-temperature impact droplets was observed. Figure 7 shows the rebound processes of impact droplet at We = 28 on the


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superhydrophobic surface with the temperature ranging from room temperature to -30 °C. With the reduction in surface temperature, the rebounding capacity of impact droplets exhibits a gradual decrease, increasing the contact time from 11.4 ms at room temperature to 16.8 ms at -20 °C, and even not rebounding off the surface at -30 °C. Impact droplets require more time to complete the spreading process to maximal deformation (from 2.0 ms at room temperature to 2.8 ms at 30 °C), and the retracting time also displays the same trend. All these apparent phenomena indicate that a cold surface makes a difference to the movement of triple-phase contact line during the impact process. Figure 8a shows the triple-phase contact line positions of impact droplets on the superhydrophobic substrate surface at various temperatures from 25 °C to -30 °C, and as a function of time. Clearly, the time required for contact line to reach its maximal value gradually increase (marked using dotted line) under the condition of heat transfer between room-temperature droplet and cold surface. During the retracting process, the contact line takes more time to retract until rebounding off with decreasing temperature. Thus, the contact line plays a significant role in the movement of impact droplets on the superhydrophobic surface, and its discontinuity directly determines the movement capacity. 41-43 3.5. Mechanism of triple-phase contact line affecting droplet bouncing The triple-phase contact line of a droplet on the hierarchical structure superhydrophobic surface has a high extent of discontinuity, resulting in a lower CAH of 1.5° and dynamic water-repellency (contact time of 11.4 ms at room temperature). However, with a decrease in temperature, the movement capacity of


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the discontinuous contact line decreases along with a partial heat transfer process between the droplet and the superhydrophobic surface. This leads to the pinning of a large number of water molecules, and the contact line is the most severe locations,44 as shown in Figure 8c. This can be mainly attributed to many unstable factors at the contact line position caused by complex phase interface relationships. Under the condition of heat loss, it is very difficult for the water molecules to keep moving, hindering the macroscopic movement of impact droplets. It can also be deduced that if the droplet can be frozen, the contact line is the initial position of ice nucleation.45 For the preference of ice nucleation at the contact line, Gurganus C.W. et al. designed an experiment using a textured fiber (with the same chemical composition as the substrate) to pass through a supercooled droplet, and then recorded the freezing process using a high-speed camera.46 In this experiment, high-speed images show that the nanoscale texture on the surface of fiber causes the ice nucleation events to occur preferentially at the contact line. Also, the preference of ice nucleation events occurring at contact line highly depends on the length scales of textures. Furthermore, using classical nucleation theory, Fu Q. T. et al. calculated the ice nucleation rate RL* (Ti ) at the contact line of supercooled droplet with the texture superhydrophobic surface.47 Here RL* (Ti ) = R (Ti ) 2π r , where R (Ti ) is the whole nucleation rate at a temperature of Ti , and r is the radius of the contact line. The calculated results strongly support the viewpoint that the dominant ice nucleation sites are present along the contact line rather than at the solid/liquid contact interface. Qualitatively analyzing, when the temperature is less than the freezing point (Tm), the liquid will undergo ice nucleation, and this process depends on the structure


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fluctuation (N1) and energy fluctuation (N2) under the supercooling condition (see Figure 8d). The classic nucleation theory indicates that ice nucleation occurs under three conditions: supercooling, structure fluctuation, and energy fluctuation. Here, structure fluctuation means that the crystal nucleus in supercooled droplet is unstable and becomes bigger or smaller, and the energy is also unstable at the contact line. Because of the complex and unstable factors at the contact line of solid/liquid/air phases, the structure and energy fluctuations take place easily compared with the solid/liquid interface, causing ice nucleation preferentially occurring at the contact line, i.e., heterogeneous nucleation. As a consequence, this produces the pinning action of contact line to hinder the movement of impact droplets on the superhydrophobic surface. Additionally, ice nucleation at the contact line also produces electrostatic interaction (dominant) and van der Waals forces with the surface, further affecting the dynamic behavior of impact droplets. This is mainly because the formed ice consists of polar water molecules that strongly interact with the solid surface due to the difference between solid surface substance and the ice preferentially formed at the contact line.48,49 Also, the electrostatic interaction induces a higher energy than chemical bonding energy, i.e., van der Waals forces. Thus, the droplet impacting on the superhydrophobic surface at -20 °C spends about 16.8 ms to complete the entire contact process and leaves a small droplet on the surface, and the surface at -30 °C cannot even make the impact droplet to rebound off (see Figure 8b). This means that the electrostatic interaction (dominant) and van der


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Waals forces have completely prevented the movement of contact line of impact droplets on the surface. To further verify the internal physical mechanism of contact line pinning mediating the dynamic behavior of impact droplets on the superhydrophobic surface, some images depicting the rebounding off of impact droplets are shown in Figure 9. Clearly, the impact droplets rebound off the surface at room temperature without any stagnancy in an agile manner, resulting from the extremely low action of contact line pinning. After rebounding, the droplet displays the shape of a slender upper half and a bulky bottom half. However, the rebounding process is slowed down with the reduction of surface temperature, showing the loath detachment of impact droplet (marked using a red circle in Figure 9). Particularly, the impact droplet on the superhydrophobic surface at -20 °C exhibits a strong loath detachment, finally leaving a small droplet adhering to the surface. During this period, the droplet undergos mass redistribution, leading to its shape changing to the bulky upper half and slender bottom half. Furthermore, the impact droplet on the surface at -30 °C cannot rebound off due to higher interface action forces with the cold surface, demonstrating that the contact line is strongly pinned. All these phenomena further reveal the underlying physical mechanism: Under the action of heat transfer between liquid and cold superhydrophobic surface, the water molecules at contact line position partly loss the movement capacity and produce interface action forces (electrostatic interaction (dominant) and van der Waals forces) with surface to prevent the movement of contact line, finally changing the dynamic behavior of impact droplets at a macroscopic level.


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As a consequence, the movement velocity (rate of D/D0) of contact line exhibits a corresponding variation under the action of heat transfer between droplet and cold superhydrophobic surface. Because the time required to spread to the maximal deformation is very short, the change in movement velocity in spreading process is not distinct, yet the retracting process does not conform to the situation (see Figure 10). Clearly, the movement velocity of contact line of impact droplets on the surface at -30 °C is the minimum compared with those on the surface at other temperatures during the retracting process. With a decrease in the temperature difference between droplet and surface, the retracting velocity of the contact line gradually increases, leading to the impact droplets spending less time to complete the entire movement and rebound process. Thus, the variation of movement velocity in contact line laterally demonstrates the underlying physical mechanism that the dynamic behavior of impact droplets is mediated by contact line pinning.

4. CONCLUSION

We envisaged that it was essential to analyze the critical condition between Rebounding and Splashing of impact droplets on the hierarchical structure superhydrophobic surfaces to develop the fundamental theory. Under low-temperature conditions, it was confirmed that the pinning of triple-phase contact line will mediate the bouncing dynamics of impact droplets. The underlying physical mechanism is revealed that at low temperatures, the heat transfer from droplet to cold surface causes a great loss


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of movement capacity of water molecules at the triple-phase contact line position, resulting in the pinning of contact line. Moreover, the interface action forces (electrostatic interaction (dominant) and van der Waals forces) are also caused at low temperatures to further strengthen the action of contact line pinning, finally changing the bouncing dynamics of impact droplets on the hierarchical structure superhydrophobic surfaces. All these physics are favorable for analyzing the anti-icing mechanism of icephobic materials.

○s Supporting Information Movie 1 showing the moving processes of a 4 µL room temperature droplet impacting on the superhydrophobic surface at different Weber numbers. Movie 2 showing the moving processes of impact droplets (We = 28) on superhydrophobic surface under different temperature conditions ranging from room temperature to -30 °C. AUTHOR INFORMATION Corresponding Author * Professor Jie Tao, Tel/Fax: +86-25-52112911. E-mail: [email protected]. Present Addresses †

College of Materials Science and Technology, Nanjing University of Aeronautics and

Astronautics, Nanjing 210016, P. R. China ‡

Department of Materials Chemistry, Qiuzhen School, Huzhou University, 759 East 2nd

Road, Huzhou 313000, P. R. China


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§

College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics,

Nanjing 210016, P. R. China Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors acknowledge the support of the National Natural Science Foundation of China (51671105 and 51705244), the National Postdoctoral Program for Innovative Talents (BX201600073), the Natural Science Foundation of Jiangsu Province (BK20170790), the Project Funded by China Postdoctoral Science Foundation (2017M610329), General Project of Zhejiang Provincial Department of Education (Y201737320), Jiangsu Planned Projects for Postdoctoral Research Funds (1701145B), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.


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(18) Shen, Y.; Tao, J.; Tao, H.; Chen, S.; Pan, L.; Wang, T. Anti-Icing Potential of Superhydrophobic Surfaces: Ice Nucleation and Growth. Langmuir 2015, 31, 1079910806.. (19) Zhou, J.; Yang, J.; Gu, Z.; Zhang, G.; Wei, Y.; Yao, X.; Song, Y.; Jiang, L. Controllable Fabrication of Noniridescent Microshaped Photonic Crystal Assemblies by Dynamic Three-Phase Contact Line Behaviors on Superhydrophobic Substrates. ACS Appl. Mater. Interfaces 2015, 7, 22644-22651. (20) Cao, M.; Guo, D.; Yu, C.; Li, K.; Liu, M.; Jiang, L. Water-Repellent Properties of Superhydrophobic and Lubricant-Infused "Slippery" Surface: A Brief Study on the Functions and Applications. ACS Appl. Mater. Interfaces 2016, 8, 3615-3623. (21) Hao, C.; Liu, Y.; Chen, X.; Li, J.; Zhang, M.; Zhao, Y.; Wang, Z. Bioinspired Interfacial

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Caption list Figure 1. Schematic diagram of homemade measuring device for filming the moving process of impact droplet on the cold superhydrophobic surface. Figure 2. Characterizations of the micro-nanostructure superhydrophobic surface. (a,b) surface microscopic morphologies; (c) Survey and high resolution (inset) XPS spectrum of self-assembling low-energy group surface; (d) The nonwettability of the micro-nanostructure superhydrophobic surface. Figure 3. The moving processes of a 4 µL room temperature droplet impacting on the superhydrophobic surface at different Weber numbers. (For more details, please see Supplementary Movie 1). Figure 4. The breakup schematic diagrams (three typical moments) of droplets impacting on the micro-nanoscale structure superhydrophobic surface at high Weber number. (a-c) Side view; (d-f) Top view. Figure 5. (a) Variations of contact time, spreading time, and retracting time versus Weber numbers; (b) The change of triple-phase contact line position of impact droplet as a function of time. Figure 6. Critical condition of impact droplets on superhydrophobic surface taking place rebound and splash phenomena. The abscissa and ordinate are customized as Reynolds number (Re) and Weber number (We), respectively. Note that in this work the Reynolds number (Re) and Weber number (We) only reflect the variation (corresponding to 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0, 2.5, and 3 m/s respectively) of impact velocity of droplets on the given superhydrophobic surface.


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Figure 7. Moving processes of impact droplets (We = 28) on superhydrophobic surface under different temperature conditions ranging from room temperature to 30 °C. (For more details, please see Supplementary Movie 2). Figure 8. (a) Contact line position and (b) contact time of impact droplet (We = 28) on cold surface. (c) Schematic diagram of contact line pinning on cold superhydrophobic surface. (d) The relationship between ice nucleation rate at the contact line and the temperature is determined by the two kinds of factors (i.e., structure fluctuation (N1) and energy fluctuation (N2)), when the temperature is less than freezing point Tm. Figure 9. The process of impact droplets rebounding off the superhydrophobic surface at various temperatures. Impact droplets on the surface at room temperature can agilely rebound off, whereas the rebounding process is slowed down with the reduction of surface temperature, showing the loath detachment of impact droplet (marked in red circle). Figure 10. The moving velocity (rate of D/D0) of contact line as a function of time.


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Figures:

Figure 1. Schematic diagram of homemade measuring device for filming the moving process of impact droplet on the cold superhydrophobic surface.


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Figure 2. Characterizations of the micro-nanostructure superhydrophobic surface. (a,b) surface microscopic morphologies; (c) Survey and high resolution (inset) XPS spectrum of self-assembling low-energy group surface; (d) The nonwettability of the micro-nanostructure superhydrophobic surface.


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Figure 3. The moving processes of a 4 µL room temperature droplet impacting on the superhydrophobic surface at different Weber numbers. (For more details, please see Supplementary Movie 1).


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Figure 4. The breakup schematic diagrams (three typical moments) of droplets impacting on the micro-nanoscale structure superhydrophobic surface at high Weber number. (a-c) Side view; (d-f) Top view.


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Figure 5. (a) Variations of contact time, spreading time, and retracting time versus Weber numbers; (b) The change of triple-phase contact line position of impact droplet as a function of time.


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Figure 6. Critical condition of impact droplets on superhydrophobic surface taking place rebound and splash phenomena. The abscissa and ordinate are customized as Reynolds number (Re) and Weber number (We), respectively. Note that in this work the Reynolds number (Re) and Weber number (We) only reflect the variation (corresponding to 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0, 2.5, and 3 m/s respectively) of impact velocity of droplets on the given superhydrophobic surface.


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Figure 7. Moving processes of impact droplets (We = 28) on superhydrophobic surface under different temperature conditions ranging from room temperature to -30 °C. (For more details, please see Supplementary Movie 2).


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Figure 8. (a) Contact line position and (b) contact time of impact droplet (We = 28) on cold surface. (c) Schematic diagram of contact line pinning on cold superhydrophobic surface. (d) The relationship between ice nucleation rate at the contact line and the temperature is determined by the two kinds of factors (i.e., structure fluctuation (N1) and energy fluctuation (N2)), when the temperature is less than freezing point Tm.


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Figure 9. The process of impact droplets rebounding off the superhydrophobic surface at various temperatures. Impact droplets on the surface at room temperature can agilely rebound off, whereas the rebounding process is slowed down with the reduction of surface temperature, showing the loath detachment of impact droplet (marked in red circle).


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Figure 10. The moving velocity (rate of D/D0) of contact line as a function of time.


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Bio-Inspired Fabrication of Hierarchical Structure Superhydrophobic

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