Spraying Fabrication of Durable and Transparent Coatings for Anti

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Spraying fabrication of durable and transparent coatings for anti-icing application: dynamic water repellency, icing delay, and ice adhesion Yizhou Shen, Yu Wu, Jie Tao, Chunling Zhu, Haifeng Chen, Zhengwei Wu, and Yuehan Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19225 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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ACS Applied Materials & Interfaces

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Spraying fabrication of durable and transparent coatings for anti-icing application: dynamic water repellency, icing delay, and ice adhesion

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Yizhou Shen,†,* Yu Wu,† Jie Tao,†,* Chunling Zhu,§ Haifeng Chen,‡ Zhengwei Wu,†

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Yuehan Xie,†

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†

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Astronautics, Nanjing 210016, P. R. China

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‡

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2nd Road, Huzhou 313000, P. R. China

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§

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

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

College of Aerospace Engineering, Nanjing University of Aeronautics and

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* Professor Jie Tao, E-mail: [email protected]. A/Professor Yizhou Shen, E-mail: [email protected].

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KEYWORDS: coatings; spraying; water repellency; icephobicity; durability

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ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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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ABSTRACT: Anti-icing/icephobic coatings, typically applied in the form of surface

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functional materials, are considered to be an ideal selection to solve the icing issues

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faced by daily life and industrial production. However, the applications of anti-icing

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coatings are greatly limited by the two main challenges: bonding strength with

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substrates and stability of the high anti-icing performance. Here, we designed and

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fabricated a kind of high-performance superhydrophobic fluorinated silica (F-

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SiO2)@polydimethylsiloxane (PDMS) coatings, and further emphasized the

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improvement of the bonding strength with substrates and the maintenance of high anti-

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icing performance. The resultant coatings exhibited excellent water repellency with a

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contact angle up to 155.3° and a very short contact time (~10.2 ms) of impact droplets.

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At low temperatures, the coming droplets still rapidly rebounded off the coating surface,

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and the superhydrophobic coatings displayed a more than fiftyfold increase of freezing

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time comparing with bare aluminum. The ice adhesion strength on the coatings was

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only 26.3 kPa, which was far less than that (821.9 kPa) of bare aluminum. Furthermore,

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the nanoporous structures constructed by anodic oxidation could tremendously enhance

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the bonding strength of the coatings with substrate, which was evaluated through a

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standard method (ASTM D3359). The anti-icing properties still retained high stability

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under the conditions of 30 icing/deicing cycles, soaking and scouring of acid solution

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(pH=5.6). This work can effectively push the anti-icing coatings towards a real-world

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application.

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INTRODUCTION

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Icing is a natural phenomenon, which usually occurs in many infrastructures and

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industrial products including power lines, buildings, and aircrafts, etc, and finally gives

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rise to severe safety issues and enormous economic losses.1,2 Up till now, a variety of

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strategies have been applied to prevent ice formation / accretion and remove ice, such

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as melting-ice agents,3 mechanical vibration,4 vapor heating, and electro-thermally

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melting.5 However, these active approaches are costly, design-complex, and also

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considered to be not environment-friendly. The continuous vibration or thermal cycle

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certainly affects the service life of surface function materials, therefore these strategies

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should be avoided or minimized in the future whenever possible. As an alternative,

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passive anti-icing materials have received widespread attentions in the last decade due

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to its “zero-energy” consumption,6-9 and it is considered to conform to the three typical

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features: excellent superhydrophobicity to repel the coming supercooled droplets,

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outstanding icing delay performance (or lower icing temperature), and ultralow ice

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adhesion.10-12 As well known, freezing is an inevitable process under the condition of

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sufficient supercooling, and the ultralow ice adhesion strength can allow the ice to be

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readily removed with the assistance of external force or even wind-force and self-

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gravity.13,14

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Currently, there are two main strategies to design the anti-icing materials, i.e.,

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microscopic rough superhydrophobic surfaces and slippery liquid-infused porous

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surfaces (denoted as SLIPS).15-18 Based on the ultra-slippery characteristic and

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chemical homogeneity, the SLIPS exhibit the extremely low wetting hysteresis and ice

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adhesion strength.19,20 However, the SLIPS still face great challenges for practical

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application due to the key limitation, where the infused lubricant is usually expensive

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and prone to be damaged by mechanical abrasion or several icing/deicing cycles.21,22

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Considering a typical ice formation and accretion process, supercooled droplets will

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firstly wet the solid surface and rapidly freeze into ice.23 In this case, superhydrophobic

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surfaces (contact angle (CA) > 150° and sliding angle (SA) < 10°) are naturally

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considered to be the ideal candidates as the anti-icing materials.24 It has been widely

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reported that the superhydrophobicity is caused by the trapped air pockets inside the

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surface micro-nanostructures. Meanwhile, the air pockets can effectively act as a

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thermal barrier between solid surface and supercooled water droplet to delay the

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freezing process. Even if ice can be formed on the solid surface, the special solid-ice

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interfacial state will result in an ultralow ice adhesion strength.25 According to the

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previous report, Li et al. prepared the superhydrophobic Co3O4 surface by employing

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solvothermal treatment to in-situ construct micro-nanostructures, and the resultant

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surface exhibited excellent anti-icing property with icing-delay time of more than 1 h

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at about -5 °C.26 Also, the superhydrophobic aluminum surface prepared by Chen et al.

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through a combination method of anodizing in H2SO4 solution and etching in myristic

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acid can greatly reduce the ice adhesion strength to 65 kPa.27 However, the in-situ

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constructed micro-nanostructures on superhydrophobic surfaces are also a huge

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challenge to resist the mechanical abrasion or the scour of freezing rain.

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Based on the excellent icephobicity of superhydrophobic materials, many

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researchers have preferred to prepare the superhydrophobic coatings for the anti-icing

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purpose, because the coatings matrix can play a supporting role in the immobilization

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of surface microstructures, and enhances the ability to resist the mechanical abrasion or

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damage. Ruan et al. adopted polytetrafluoroethylene (PTFE) to fabricate hierarchical

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microstructures on polydimethylsiloxane (PDMS), and finally prepared the

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superhydrophobic PTFE/PDMS coatings, where the high icephobicity was maintained

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well under the conditions of mechanical abrasion.28 Furthermore, a kind of porous

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superhydrophobic polyvinylidene difluoride (PVDF) coatings were fabricated through

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a method of phase separation, and the as-prepared superhydrophobic coatings were

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confirmed to possess higher icephobicity with the great ability to resist the scour of

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supercooled water droplets for ~50 min.29 Despite a notable progress being made on

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the superhydrophobic anti-icing coatings, studies on the durability (such as coatings

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adhesive force with substrate, maintenance of icephobicity under multiple icing/deicing

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cycles, and soaking and scouring of acid solution) are still insufficient, yet these are of

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vital importance for applications.30,31

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In this work, we firstly fluoridized the SiO2 nanoparticles, which were further

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sprayed onto the PDMS coating matrix to obtain the superhydrophobic F-SiO2@PDMS

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coatings. Meanwhile, the coatings bonding strength with substrate was contrastively

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investigated with the two types (nanopores and nanopits) of nanostructures on the

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aluminum. The icephobicity was systematically discussed and evaluated around the

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three aspects of dynamic water repellency, icing-delay time, and ice adhesion. 5



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Additionally, the durability of superhydrophobic F-SiO2@PDMS coatings was verified

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under the conditions of 30 icing/deicing cycles, soaking and scouring of acid solution.

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EXPERIMENTAL SECTION

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Materials

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Silica nanoparticles (20 nm, 99.0%) were supplied by Nanjing XFNANO Materials

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Tech. Co., Ltd. Polydimethylsiloxane (PDMS) prepolymer (Sylgard 184A) and the

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corresponding curing agent (Sylgard 184B) were obtained from Dow Corning Co., Ltd

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(USA).32 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (PFDTES, 96%) was purchased

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from Shanghai Macklin Biochemical Co., Ltd. Anhydrous ethanol (99.7%), ammonium

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hydroxide (25.0-28.0 wt%), and tetrahydrofuran (THF, 99.0%) were provided by

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Sinopharm Chemical Reagent Co., Ltd. Aluminum sheets (20 mm × 20 mm × 1 mm)

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were bought from Shenzhen Baogang Metal Co., Ltd. All chemical reagents were used

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directly without further purification in present work. Deionized water used throughout

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experiments was prepared in Milli-Q system in our laboratory.

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Pretreatment of aluminum sheets

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First, the aluminum sheets were cleaned ultrasonically with acetone and alcohol for

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15 min in sequence, followed by drying under a stream of nitrogen. Second, two

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common techniques were utilized to treat clean aluminum sheets and produce the

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corresponding nanostructures. One treatment: the clean aluminum sheets were 6


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chemically etched in the solution of 1.5 M HCl for 10 min to construct the nanopits

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structures. Another treatment: the anodic oxidation was performed in the solution of

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120 g L-1 H3PO4 with the processing parameters of 10 V applied voltage and 20 min,

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and the nanopores structures were grown on the surface of aluminum sheets.

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Subsequently, the nanostructured aluminum sheets were washed with alcohol and dried

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under nitrogen flow.

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Fabrication of PDMS coatings matrix

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PDMS (3 g) and curing agent (0.3 g) were dissolved in 25 mL THF under vigorous

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stirring for 60 min. Following this, the mixed solution was coated onto the treated

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aluminum sheets by means of airbrush at 0.4 MPa and with a distance of 10-15 cm.

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After being cured in 80 °C oven for 2 h, the PDMS coatings matrix were finally

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synthesized, and would be used in the following experiment procedures.

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Preparation of superhydrophobic F-SiO2@PDMS coatings

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Regarding the modification of SiO2 nanoparticles, PFDTES (0.6 mL) was firstly

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mixed with ethanol (80 mL), and the solution system was kept with the constant stirring

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for 60 min. Afterwards, 4 mL ammonium hydroxide was diluted to 20 mL with DI

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water, and then 3 g SiO2 nanoparticles were added into the solution to form

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homogeneous SiO2 suspension for the continuously ultrasonic-dispersion time of 10

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min. The SiO2 suspension would be slowly transferred into the above-prepared

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PFDTES solution followed by continuous magnetic stirring for 24 h at 40 °C. The

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resultant precipitate was further centrifuged at 10000 rpm for 5 min, and the products 7



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finally were washed with ethanol for twice. It was necessary to take the vacuum dried

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treatment for obtaining the hydrophobic SiO2 nanoparticles, which were labelled as F-

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SiO2.

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Subsequently, 0.2 g F-SiO2 nanoparticles were ultrasonically dispersed into 25 mL

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THF solution for 30 min. Following this, the mixed system was sprayed on the PDMS

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coating matrix, which was pre-cured at 80 °C for 10 min (the other steps were same as

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ones for PDMS coatings matrix). The superhydrophobic F-SiO2@PDMS coatings were

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obtained after the further curing at 80 °C for another 2 h.

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Characterizations and water repellent tests

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The surface morphologies of PDMS coatings matrix and superhydrophobic F-

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SiO2@PDMS coatings with a thin gold layer were captured by Field-Emission

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Scanning Electron Microscope (FE-SEM, Hitachi S4800, Japan) at an acceleration

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voltage of 5.0 kV. Energy Dispersive Spectrometer (EDS) attached to FE-SEM

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instrument was utilized to analyze element compositions of coatings. The surface

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groups and chemical compositions of F-SiO2 nanoparticles were directly detected by

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Fourier Transform Infrared spectrometer (FTIR, Nexus 670, USA) and X-ray

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photoelectron spectrometer (XPS, K-Alpha, USA), respectively. The surface 3D

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topography and roughness were obtained from Atomic Force Microscope (AFM, NT-

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MDT Prima, Russia) in tapping mode with a scan area of 6 μm × 6 μm. Both water

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Contact Angle (CA) and Contact Angle Hysteresis (CAH) were measured with a

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contact angle measured system (JC2000D2, Shanghai Zhongchen Digital Technology 8


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Apparatus Co., Ltd, China) using a 4 μL volume of DI water droplet. Water contact

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angle (CA) could be directly obtained by the contact angle analyzer. Advancing CA

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was recorded by the computer, when the contact area with surface changed owing to

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expansion of droplet. If the contact area decreased via shrinking the droplet, Receding

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CA could be recorded. CAH was the difference of Advancing CA and Receding CA.

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Therefore, CAH can be calculated from the directly tested values. Furthermore, five

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different locations of the sample coatings were measured for obtaining an average value.

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Evaluations of anti-icing performance

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Regarding the dynamic water repellency, the reference droplet (4 μL) fell from the

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height of 5 cm under the action of gravity and impacted onto the sample surface at 1 m

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s-1. In the process of free falling and impacting, the movement and deformation of water

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droplet were captured by a high-speed camera (i-SPEED 713, UK) at 5000 frames per

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second.

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According to our previous report,33 the icing-delay test was performed in a custom-

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built device controlled at -15 °C, which consisted of cooling system, sample chamber,

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and monitoring system (including camera and computer). Droplets (4 μL) were dropped

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on the coatings surface after the test specimen was placed on cooling stage for 5 min.

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This made sure that the surface temperature was same as the temperature of cooling

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stage. Subsequently, the freezing time and process of the reference droplet were record

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by monitoring system.

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The ice adhesion strength test was carried out with home-made system, as shown

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in Figure S1, and the corresponding operation process was described as follows. The

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coated substrate was placed on and faced to a cuvette (1 cm × 1 cm × 4.5 cm) with full

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of DI water. The setup was then kept in a refrigerator at -20 °C for 12 h to form the ice

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columns on the coatings surface. Subsequently, the sample with ice column was fixed

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on the cold stage, and a digital force transducer (HANDPI Instruments Co., Ltd, China)

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pushed the ice column to separate it from the coatings surface. The peak force (Fm) was

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acquired directly from force transducer, and the ice adhesion strength (τice) could be

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calculated by:

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𝜏𝑖𝑐𝑒 =

𝐹𝑚 𝐴

(1)

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Where A is the contact area between coatings surface and ice column, which was kept

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in a constant value of 1 cm2. At least three parallel samples were measured to obtain an

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average value.

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RESULTS AND DISCUSSION

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Chemical composition of F-SiO2 nanoparticles

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Grafting low-energy groups onto the surface of SiO2 nanoparticles will benefit the

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bonding with PDMS coatings matrix, where the firm chemical covalent bonds can be

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formed to enhance the bonding strength. The FTIR spectra of SiO2 nanoparticles before

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and after modifying are shown in Figure 1a. Therein, a distinct peak at 1618.6 cm-1 of

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the both FTIR spectra indicates the existence of H2O from air.34 After modifying the 10


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SiO2 nanoparticles, these characteristic peaks at 3418.4 cm-1 (caused by -OH stretching

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vibrations35), 947.7 cm-1 (corresponding to -OH out of plane bending36), 1111.6 cm-1

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(belonging to Si-O-Si asymmetric stretching vibrations37), and 800.7 cm-1 (belonging

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to Si-O-Si bending mode38) have an enhanced intensity comparing with raw SiO2

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nanoparticles. The appeared -OH and Si-O-Si are mainly generated owing to the

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hydrolysis of PFDTES and self-assembly of PFDTES on the surface of SiO2

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nanoparticles, respectively. More importantly, the as-modified SiO2 nanoparticles (F-

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SiO2) have a new chemical bond and locate around 1198.7 cm-1, which corresponds to

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the C-F stretching vibration of -CF2- and -CF3 function groups.39 This C-F chemical

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bond is derived from PFDTES molecules, which further demonstrates the successful

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grafting of low-energy groups.

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Figure 1. (a) FTIR spectra and (b) XPS survey spectra of SiO2 nanoparticles before

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(blue line) and after (red line) modifying. (c-f) The XPS high-revolution spectra of F-

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SiO2 nanoparticles for Si 2p, O 1s, C 1s, and F 1s, respectively.

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Furthermore, XPS spectra were obtained to further confirm the surface chemical

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compositions of raw SiO2 and F-SiO2 nanoparticles. As shown in Figure 1b, the

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modification of SiO2 nanoparticles with PFDTES further increases the intensity of Si

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2p and O 1s peaks. Also, the Si 2p high-resolution spectrum can be resolved into four

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components assigned to Si-C (at 100.8 eV), Si-O (at 102.6 eV), Si-O-Si (at 103.4 eV),

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and Si-OH (at 104.5 eV), respectively (see Figure 1c).40 All these Si-C components

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originate from PFDTES, while the Si-O (at 284.8 eV), Si-O-Si (at 288.5 eV), and Si-

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OH (at 290 eV) are the newly formed chemical covalent bonds between PFDTES and

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SiO2 nanoparticles. Furthermore, the O 1s peaks are located at 532.1 eV, 533.7 eV and

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540.0 eV, and mainly assigned to the oxygen of -OH, SiO2 and CO2, respectively, as

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shown in Figure 1d. It is clear that there are four components in C 1s high-resolution

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spectrum (see Figure 1e), which are located at 284.0 eV (C-C/C-H), 285.6 eV (C-Si),

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292.0 eV (C-F), and 288.8 eV (C=O). The bonds of C-C/C-H, C-Si and C-F are ascribed

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to PFDTES molecules, and C=O attributed to CO2 from air.41 Notably, it can be seen

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that the F element only exists on the surface of the F-SiO2 nanoparticles (see Figure 1f).

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This prominent peak is associated with the -CF2- (at 689.3 eV) and -CF3 (at 684.9 eV)

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groups of PFDTES molecules, which have low surface energy and endow the F-SiO2

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nanoparticles with excellent hydrophobicity.42 These analyzed results also indicate the

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polycondensation reactions take place to form the new chemical covalent bonds

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between PFDTES molecules and SiO2, which is in a line with FTIR analysis.

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Non-wettability and morphologies of the superhydrophobic F-SiO2@PDMS

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coatings 12


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The high-dispersibility F-SiO2 nanoparticles in THF solution will be sprayed onto

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the semi-cured PDMS coatings matrix to increase the surface microscopic roughness.

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As a response, the non-wettability is also greatly enhanced and shown in Figure 2.

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Comparing with the intrinsic hydrophilicity of bare aluminum substrate, the non-

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wettability of smooth PDMS coatings is enhanced and reaches the category of

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hydrophobicity with CA increasing from 79.4° to 113.9° (see Figure 2a and b), because

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the PDMS materials have lower surface free energy. With addition of F-SiO2

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nanoparticles, the excellent superhydrophobicity is obtained on the surface of the F-

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SiO2@PDMS, and the CA reaches 155.3° and the CAH reduces to 2° (see Figure 2c).

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This mainly attributes to the microscopic rough structures caused by the F-SiO2

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nanoparticles, as shown in Figure 2d and e. The as-formed microstructures evenly

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distribute on the surface of PDMS coatings matrix, and the spacing distance between

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the adjoining micropits is approximately 50 μm. Meanwhile, it can be found that the

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PDMS coatings matrix has a strong supporting action on the microstructures, and there

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are many nanoscale pores inside the microstructures (see inset in Figure 2e). Also, the

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3D topography in Figure 3a exhibits lots of microscopic protuberances and cavities. It

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can be calculated from 2D image that the root-mean-square roughness (RMS) is about

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229.5 nm (see Figure 3c), showing the great size effect of microstructures on

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superhydrophobicity. Besides, the EDS analyzed results (see Figure S2) further confirm

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that the surface of F-SiO2@PDMS coatings has been successfully treated with low

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surface free energy by grafting fluorine-containing groups original from PFDTES.43

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Finally, the synergistic action of microscopic rough structures and low surface free 13



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energy can induce to entrap numerous air pockets underneath water droplets, and make

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the coatings water-repellent.

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Figure 2. The photographs of 8 μL water droplets on (a) aluminum sheet, (b) PDMS

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coatings, and (c) superhydrophobic F-SiO2@PDMS coatings. Insets are 4 μL droplets

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for measuring CA and CAH. (d) SEM images of PDMS coatings and (e)

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superhydrophobic F-SiO2@PDMS coatings, and inset is the high-resolution image.

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Figure 3. (a) AFM 3D surface topography, (b) corresponding 2D image, and (c) section

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roughness profile extracted from 2D image of the superhydrophobic F-SiO2@PDMS

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coatings.

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The water droplet almost suspends on the surface of superhydrophobic F-

6

SiO2@PDMS coatings. This phenomenon has been well elucidated by the Cassie-

7

Baxter wetting model, indicating that the microscopic rough structures further reduce

8

the apparent surface free energy to repel water. In this case, the apparent contact

9

interface is actually composed of solid/liquid and liquid/gas. Since the CA of a water

10 11

droplet on air is considered as 180°, the apparent CA (θ*) can be expressed as:

cos  *  f cos   f  1

(2)

12

Where f and θ are the area fraction and Young CA of the solid, respectively. The

13

microscopic rough structures produced by F-SiO2 nanoparticles have a high aspect ratio, 15



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which results in a large amount of air pockets being trapped. Reducing the solid contact

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area fraction f (i.e., capturing more air pockets) is beneficial to the improvement of the

3

water repellency. When the F-SiO2 nanoparticles with suitable quantity is added into

4

the PDMS coatings matrix, the resultant microstructures can almost perfectly hold up

5

the water droplet, causing the ideal superhydrophobicity with the CA up to 155.3° and

6

the CAH of only 2°.

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Enhancement of adhesion strength with substrates

8

The adhesion of superhydrophobic F-SiO2@PDMS coatings to substrate materials

9

was measured according to the standard method of ASTM D3359 and divided into

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different grades from best adhesion to worst adhesion (denoted as 5B, 4B, 3B, 2B, 1B,

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and 0B).44 As a reference, the fact that nearly 50% of the coating is removed by test

12

tape is considered to be classified as 3B. In order to enhance the adhesion strength of

13

the coatings, we performed anodic oxidation and chemical etching on the aluminum

14

substrate to produce the microscopic rough structures. It can be observed that there is a

15

layer of nanoporous (treated by anodic oxidation) and nano-ravine (treated by chemical

16

etching) structures evenly distributing on the aluminum substrate, respectively, as

17

shown in right column of Figure 4. Even though the produced nano-ravine structures

18

increase the roughness to a certain extent, the adhesion of the as-prepared

19

superhydrophobic coatings has no obvious enhancement with a major removal from the

20

chemically-etched substrate. According to the experimental results, the adhesion

21

strength only belongs to 2B grade. While there is no removal of the superhydrophobic 16


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F-SiO2@PDMS coatings on the anodized aluminum after tape peeling off, indicating

2

that the superhydrophobic coatings have a better adhesion which is rated as 5B. This is

3

also evident from SEM images that the anodic oxidation greatly increases the

4

microscopic roughness of the aluminum substrate, and a large number of nanopores

5

appear on the surface. These nanoporous structures play an anchoring role between the

6

superhydrophobic F-SiO2@PDMS coatings and substrate. Consequently, anodic

7

oxidation on the aluminum substrate is needed to enhance the adhesion strength of the

8

as-prepared superhydrophobic F-SiO2@PDMS coatings.

9 10

Figure 4. Cross-cut tape adhesion test for superhydrophobic F-SiO2@PDMS coatings

11

on the aluminum substrate with different treating types. T1: untreated aluminum

12

substrate; T2: anodic oxidation; T3: chemically etching. Left column is the actual

13

experimental images, and middle column is the schematic drawing of the test results

14

(red parts stand for the coatings not removed from the substrate). Right column is the

15

corresponding microscopic morphology.

17



1

Page 18 of 36

Dynamic water repellency

2

Dynamic water repellency is considered to be first element to anti-icing materials,

3

because it reflects the capacity of repelling the coming supercooled droplets. Dynamic

4

water repellency is mainly evaluated by the contact time and contact process of impact

5

droplets on the sample surface. Here, two important dimensionless parameters are the

6

Weber number (We) and the Reynolds number (Re), which are defined as follows:

7

𝑊𝑒 =

8

𝑅𝑒 =

𝜌𝐷0𝑉2

(3)

σ

𝜌𝐷0𝑉

(4)

𝜇

9

where ρ is the liquid density (1 g cm-3), σ is the liquid-gas interfacial tension (72 mN

10

m-1), V is the impact speed, μ is the liquid viscosity (1.0×10-3 Pa s), and D0 denotes the

11

initial diameter of droplet.45 The droplet with a radius of 1 mm impacts onto the surface

12

at a speed of 1 m s-1, therefore it is calculated that We is approximately 27.8, and Re is

13

around 2234.9 in this work. As shown in Figure 5a and Video S1, it takes a certain

14

amount of time (3.6 ms and 2.8 ms, respectively) for the impact droplet to spread to the

15

maximum deformation on the bare aluminum and PDMS coatings. Comparing with that

16

on bare aluminum, the spreading droplet on PDMS coatings has a higher retracting

17

degree owing to the relatively low surface energy of PDMS coatings.46 However, the

18

impact droplet fails to rebound off the surface of PDMS coatings. With the gradual

19

decrease of temperature, the impact droplet is more difficult to rebound off the PDMS

20

coatings, as shown in Figure 6. In contrast, for the superhydrophobic F-SiO2@PDMS

21

coatings, the impact droplet spreads rapidly (only 2.4 ms) and completely rebounds off

22

with a retracting time of 7.8 ms. Therefore, the entire contact time of the impact droplet 18


Page 19 of 36 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


1

is 10.2 ms from the contact to the detachment. Even at lower temperatures of -5 °C and

2

-10 °C (Figure 6), the impact droplet can also rebound off the surface of

3

superhydrophobic F-SiO2@PDMS coatings, which strongly demonstrated the superior

4

dynamic water repellency at low temperature.

5 6

Figure 5. (a) Time-lapse photographs of 4 μL water droplets impacting on bare

7

aluminum (S1), PDMS coatings (S2) and superhydrophobic F-SiO2@PDMS coatings

8

(S3) at room temperature. (b) The dimensionless parameter of spreading factor (D*=

9

D/D0, where D0 is the original diameter of droplet, and D is the diameter of droplet in

10 11

contact area with the surface) versus time after the droplet touched the surface. Furthermore, the spreading factor (D*) variation of the water droplets on the 19



Page 20 of 36

1

sample surfaces was also calculated and shown in Figure 5b.47 For comparison, the

2

impact droplet on superhydrophobic F-SiO2@PDMS coatings goes through three stages

3

(spread, retract, and rebound), while it cannot rebound off two other surfaces (i.e., bare

4

substrate and PDMS coatings). Also, the droplet mobility parameter (ξ) is determined

5

by the ratio of D*min and D*max.

6

𝐷 ∗ 𝑚𝑖𝑛

𝜉 = 𝐷∗

(5)

𝑚𝑎𝑥

7

where D*max denotes the maximum droplet spreading diameter, and D*min is minimum

8

diameter of contact area after retracting.10 It can be obtained that the value of ξ on the

9

bare aluminum is approximately equal to one, yet the PDMS coatings present a lower

10

value of ξ=0.48, demonstrating the PDMS coatings causing higher droplet mobility.

11

However, the superhydrophobic F-SiO2@PDMS coatings can rapidly rebound off the

12

surface, and the value of ξ can be considered to be zero. Therefore, the

13

superhydrophobic F-SiO2@PDMS coatings exhibit greater dynamic water repellency,

14

which also declares the huge anti-icing potential.

15 16

Figure 6. Time-lapse photographs of 4 μL water droplets impacting on (S2) PDMS 20


Page 21 of 36 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


1

coatings and (S3) superhydrophobic F-SiO2@PDMS coatings at -5 °C and -10 °C.

2

Icing delay performance

3

To evaluate the anti-icing behavior, the icing process of water droplets on different

4

surfaces was record. As shown in Figure 7a, water droplet is turbid as soon as it touches

5

the bare aluminum, indicating the occurrence of ice nucleation. The droplet completely

6

freezes with a tip appearing on the droplet top after a short time of 4.8 s. The reason for

7

icing in a few seconds is its high thermal conductivity of 237 W m-1 K-1 without any

8

barriers to water droplet. In contrast, the PDMS coatings exhibit a freezing delay time

9

of more than 80 s with a long precooling time (~68.5 s). This phenomenon can be

10

mainly contributed to relatively low thermal conductivity of PDMS coatings (0.16 W

11

m-1 K-1). Therefore, the heat transfer process between substrate and water droplet is

12

greatly slowed down. Notably, the superhydrophobic F-SiO2@PDMS coatings show

13

longer freezing delay time of 276.2 s, compared with PDMS coatings (81.9 s) and bare

14

aluminum (4.8 s). On the whole, there are two reasons for these results. First, the

15

superhydrophobic F-SiO2@PDMS coatings have an apparent low-surface-energy

16

which greatly reduces the contact area between water droplet and substrate surface.

17

More importantly, the superhydrophobic coatings reach the Cassie-Baxter wetting state

18

and could entrap a large amount of air pockets underneath water droplet with ultralow

19

thermal conductivity of 0.024 W m-1 K-1.48 Actually, the interface wetting regime

20

(solid-liquid and solid-ice interfaces) is complex and depended on many factors, such

21

as size, morphology, temperature, etc. The presented work demonstrated that the 21



Page 22 of 36

1

structure-designed superhydrophobic coatings can display a higher extend of icing

2

delay performance than some reported research results.49,50 As a consequence, the

3

superhydrophobic F-SiO2@PDMS coatings can greatly delay the icing process of water

4

droplet. Besides, it is worth noting that icing delay time primarily depends on

5

precooling process, as shown in Figure 7b. The above-mentioned results verify that the

6

superhydrophobic F-SiO2@PDMS coatings show great potential on delaying ice

7

formation.

8 9

Figure 7. (a) Sequential images of water droplets on (S1) bare aluminum, (S2) PDMS

10

coatings, and (S3) superhydrophobic F-SiO2@PDMS coatings. (b) Required time for

11

water droplets precooling, icing growing, and freezing on three sample surfaces.

12

Ice adhesion strength

13

Ice adhesion strength is another important parameter to characterize the icephobicity.

14

As shown in Figure 8a, the ice adhesion strength of PDMS coatings is much lower than

15

that of bare aluminum. This can be explained by that interfacial slippage provided by

16

the miscible chains has a great effect on ice adhesion strength.12 In particular, the

17

superhydrophobic F-SiO2@PDMS coatings with ice adhesion strength of 26.3 kPa have

18

a 96.8% reduction over bare aluminum. The ultralow ice adhesion strength is mainly 22


Page 23 of 36 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


1

caused by the existent air pockets, which have a significant reduction in contact area

2

between ice and solid surface. Additionally, the air pockets in the concaves are

3

squeezed by surrounding ice and generate strain force to the ice, which also reduces the

4

ice adhesion strength to a certain extent.51

5 6

Figure 8. (a) Ice adhesion strength of different sample surfaces. (b) The change of the

7

ice adhesion strength on the superhydrophobic F-SiO2@PDMS coatings with the

8

number of icing/deicing cycles. (c) Effect of HCl solution immersion time on the ice

9

adhesion strength of the superhydrophobic F-SiO2@PDMS coatings. (d) Variation in

10

ice adhesion strength with different volumes of HCl solution impacting.

11

It is necessary to investigate the stability of icephobicity. The repeated

12

icing/deicing experiments were conducted in ice adhesion test system. Data presented

13

in Figure 8b show that the ice adhesion has no obviously increasing trend over 30

14

icing/deicing cycles. This is due to that the immobilization of PDMS is effective to 23



Page 24 of 36

1

improve robustness of superhydrophobic F-SiO2@PDMS coatings. We also performed

2

artificial rain erosion and scour through home-made devices. As shown in Figure 8c,

3

the coatings sustain at a relative low value of ice adhesion strength after immersing in

4

HCl solution for 48 h, implying that stable air pockets on the surface enable the high

5

corrosion resistance in HCl solution. It can be also found that superhydrophobic F-

6

SiO2@PDMS coatings still remain low ice adhesion strength after impinging by water

7

jet (7 mm diameter) at ~1 m s-1 (see Figure 8d). The excellent wash-out resistance of

8

coatings is related to the pinning and supporting action of PDMS, which guarantees the

9

F-SiO2 nanoparticles firmly cover on the surface. All the above experimental results

10

strongly prove that superhydrophobic F-SiO2@PDMS coatings have favorable

11

chemical stability and mechanical durability for extending their service life.

12

Anti-condensation property

13

Finally, the anti-condensation property was also evaluated, and the condensation

14

experiments were conducted on a horizontal cooling stage at 2 °C with the relative

15

humidity is 80%. As shown in Figure 9a-b, condensed droplets cover immediately the

16

aluminum and PDMS coatings and grow in size gradually. Comparing with that on

17

aluminum, the condensed droplets on PDMS coatings have a lower growth rate and

18

form the more regular spheres due to low surface energy. It is interesting to note that

19

condensed droplets do not fully cover superhydrophobic F-SiO2@PDMS coatings, and

20

they are sphere shape with different size after 30 min. The reason for this phenomenon

21

is that the adjacent metastable droplets coalesce and lots of fresh areas are exposed for 24


Page 25 of 36 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


1

reforming smaller droplets. Furthermore, it is obvious that the small condensed droplets

2

continue to combine into lager droplet without any external force and slide readily (see

3

Figure 9d), which is caused by the decreased surface energy during the coalescence

4

process. This coalescencing and sliding behaviors contribute to the anti-condensation

5

and self-cleaning of the superhydrophobic F-SiO2@PDMS coatings at low

6

temperatures.

7 8

Figure 9. Time-lapse photographs of condensed droplets on different surfaces,

9

including (a) bare aluminum, (b) PDMS coatings and (c) superhydrophobic F-

10

SiO2@PDMS coatings. (d) Dynamic optical micrographs of condensed droplets

11

showing the coalescence and slide behaviors on superhydrophobic F-SiO2@PDMS

12

coatings.

13

CONCLUSION

25



Page 26 of 36

1

In summary, the superhydrophobic F-SiO2@PDMS coatings were successfully

2

fabricated by a facile spraying method, and the bonding strength to substrate materials

3

is greatly enhanced through anodizing pretreating of aluminum. In superhydrophobic

4

F-SiO2@PDMS coatings, high-viscous PDMS is introduced to bond and support F-

5

SiO2 nanoparticles. Due to the excellent superhydrophobicity, the coatings showed an

6

ultralow contact time (10.2 ms) of impact droplets at ambient temperature and could

7

compel water droplets to rebound off even at -10 °C. The superhydrophobic F-

8

SiO2@PDMS coatings displayed a more than fiftyfold increase of freezing time

9

comparing with bare aluminum. The ice adhesion strength on the coatings was only

10

26.3 kPa, which was far less than that (821.9 kPa) of bare aluminum. Finally, the anti-

11

icing properties still retained high stability under the conditions of 30 icing/deicing

12

cycles, soaking and scouring of acid solution (pH=5.6). This work is considered to be

13

effectively to push the anti-icing coatings towards a real-world application.

14

○s Supporting Information

15

Schematic diagram of ice adhesion test system; Element detection by EDS for PDMS

16

coatings and superhydrophobic F-SiO2@PDMS coatings. (PDF)

17

Video of the moving process of impact droplets on aluminum, PDMS coatings and

18

superhydrophobic F-SiO2@PDMS coatings. (AVI)

19

26


Page 27 of 36 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


1

AUTHOR INFORMATION

2

Corresponding Author

3

* Professor Jie Tao, Tel/Fax: +86-25-5211 2911. E-mail: [email protected].

4

A/Professor Yizhou Shen, Tel: +86-25-5211 2911. E-mail: [email protected].

5

Present Addresses

6

†

7


8

‡

9

2nd Road, Huzhou 313000, P. R. China

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

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

10

§

11

Nanyang Avenue 50, Singapore 639798

12

Notes

13

The authors declare no competing financial interest.

14

ACKNOWLEDGMENTS

15

J.T., Y.S. and C.Z. acknowledge financial support from the National Natural Science

16

Foundation of China (No. 51671105, 51705244, 11832012). Y.S. and Y.W. thank the

17

support of the Natural Science Foundation of Jiangsu Province (No. BK20170790), and

18

H.C. acknowledges the General Project of Zhejiang provincial department of education

19

(Y201737320). Z.W. and Y.X. thank the Project Funded by the Priority Academic

School of Materials Science and Engineering, Nanyang Technological University,

27



Page 28 of 36

1

Program Development of Jiangsu Higher Education Institutions and the NUAA

2

Innovation Program for Graduate Education (kfjj20170608, kfjj20180609).

28


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1

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TOC Graphic:

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We designed and fabricated a kind of high-performance superhydrophobic F-

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SiO2@PDMS coatings, and focused on the evaluations of anti-icing performance of the

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as-prepared superhydrophobic coatings around the three aspects of dynamic water

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repellency, icing delay, and ice adhesion.

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Spraying Fabrication of Durable and Transparent Coatings for Anti

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