Icephobic/anti-icing potential of superhydrophobic Ti6Al4V surfaces with hierarchical textures

Abstract

The main purpose of this paper was to investigate the icephobic potential of hierarchical superhydrophobic surfaces, which were prepared by modifying micro-nanostructures (constructed by the combination of sand blasting and hydrothermal treatment) on the surfaces of Ti6Al4V alloy with fluoroalkylsilane (FAS-17). We previously reported that this hierarchical superhydrophobic surface displayed excellent non-wettability with apparent contact angle of 161° and sliding angle of 3°. Thus, the present study focused on the systematic characterizations and analyses of the icephobic potential of the superhydrophobic surfaces around three parameters, including icing-delay time, ice adhesion strength, and contact time of an impacting droplet on cold superhydrophobic surfaces. The results indicated that the icing-delay time of a droplet on the superhydrophobic surface was many times longer than that of a droplet on the smooth Ti6Al4V substrate, and the ice adhesion strength on superhydrophobic surface was greatly reduced, which was attributed to the Cassie wetting state of a droplet on the surface. Additionally, the dynamic droplet impact and rebound assay demonstrated that water droplets always bounced off of the superhydrophobic surfaces before freezing under subzero conditions.

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Introduction

Ice formation and accumulation on the surfaces of aircraft have been a major challenge, endangering the safety of flights especially in colder regions.¹ In some extreme cases, aircraft icing may cause disastrous events such as crashes and contact loss, which results in severe economic impacts and large loss of life. Over the last several decades, an effort has been made to attain a more detailed understanding of the icing process and develop a more efficient method to prevent aircraft icing or mitigate its consequences.^2–5 In the stratosphere, a large number of supercooled water droplets gather in the cloud. When an aircraft flies through these clouds, the supercooled water droplets will freeze into ice on the surfaces of the plane, especially on the leading edge of the wings, tail, and propulsion system components.⁶

Currently, many popular strategies of anti-icing and de-icing, including mechanical, electrothermal, and liquid hybrid methods, have been developed and used widely to avoid and reduce ice formation and accumulation.^7–9 However, these techniques are considered to be power consuming and costly, even work to prevent or remove ice.^10,11 They do not only consume a great amount of energy for heat generation, but also lower the fuel efficiency because of the extra weight of anti-icing device.¹² Furthermore, during the de-icing process, heat stress induced by alternate cooling and heating also has an adverse effect on the operating life of an aircraft.¹³

A more exciting approach is to design icephobic coatings that can reduce ice build-up on the solid surfaces, increasing the energy efficiency as well as the safety of materials.¹⁴ Considering that the first step of the icing process is that the solid surfaces are wetted by supercooled water droplets, candidate icephobic coatings should possess very strong non-wettability to retard ice formation.¹⁵ Recently, inspired by the lotus leaf, peanut leaf, cicada wing, and butterfly wing, superhydrophobic surfaces with water contact angles greater than 150° and sliding angles less than 10° have aroused researchers' intense interest for their potential in anti-icing applications.^16–18 Superhydrophobic surfaces can delay ice formation owing to the existence of air pockets trapped inside the surface microstructures. The air pockets can form a thermal barrier between the surface and supercooled water droplets to prevent heat transfer.¹⁹ Furthermore, water droplets have a very small contact area with superhydrophobic surfaces, further reduce thermal conversion efficiency. Hence, superhydrophobic surfaces may be ideal potential anti-icing materials for protecting aircraft in colder regions.

Bio-inspired superhydrophobic surfaces have been fabricated and investigated widely for non-wettability. The principle for the fabrication of superhydrophobic surfaces is based on the combination of surface texture and chemical composition.²⁰ On a flat surface, the water contact angle can reach only 120° by lowering the surface energy (the lowest surface energy value yet recorded, 6.7 mJ m⁻², was attained for a surface with regularly aligned closest-hexagonal-packed CF₃ groups).²¹ However, the contact angle may be raised to 150° or beyond by designing surface textures on the basis of low surface energy. W. Li's group constructed five types of microstructures on aluminum alloy using an anodic oxidation and chemical etching method to prepare superhydrophobic surfaces with a large apparent contact angle (159.1°) and a small contact angle hysteresis (4.0°).²² Moreover, the icing-delay time of water droplets on the superhydrophobic surface was postponed from 406 s to 676 s compared to substrate alone at temperatures of −2.2 °C to −6.1 °C. The latest report from L. Jiang's group said that a composite micro-nanostructured superhydrophobic surface prepared using poly vinylidene difluoride polymer (PVDF) in combination with ZnO materials displayed excellent icing-delay properties. The water droplets on this surface did not completely freeze at −10 °C until 7360 s.²³ These researchers have also promoted the improvement of anti-icing properties of superhydrophobic surfaces to a certain extent. However, the icephobic potential (including icing-delay time, ice adhesion strength, and the contact time of an impacting droplet) of superhydrophobic surfaces at lower temperatures is as yet unknown. Meanwhile, some researchers also have a little doubt about the anti-icing properties of superhydrophobic surfaces due to their instability and limitation to the droplet size.^24–26

In this paper, we fabricated hierarchical superhydrophobic surfaces via modifying the micro-nanostructures (constructed by the combination of sand blasting and hydrothermal treatment) on the surface of Ti6Al4V alloy with fluoroalkylsilane (FAS-17). The present work focused on the systematic characterizations and analyses on icephobic potential of the superhydrophobic surfaces at lower temperatures of −10 °C, −20 °C, and −30 °C, including static analysis of ice adhesion strength and icing-delay time, and the dynamic contact process of an impacting droplet.

Experimental

Materials

All experimental chemicals purchased from Sinopharm chemical reagent Co., Ltd (China) were analytical grade. Ti6Al4V alloy was obtained from Baoji titanium industry Co., Ltd (China) and fabricated into samples with a dimension of 10 mm × 10 mm × 1 mm via a wire-electrode cutting machine. Commercial grade heptadecafluorodecyl trimethoxysilane (FAS-17, purchased from Tokyo chemical industry Co., Ltd (Japan)) was used to modify the microstructures on the surfaces of samples.

Sample preparation

First, the substrates were ultrasonically cleaned in sequence with acetone, alcohol, and deionized water for 10 min each and dried in the air. The substrate was then sand-blasted (with 150 grit alumina at 0.5 MPa for 10 s) to construct the micrometer-scale pits with an adjacent distance of ∼30 μm. Subsequently, the samples with micrometer-scale pit structures were placed in a autoclave with 30 mL 1 M NaOH aqueous solution at 220 °C for 8 h. After hydrothermal treatment, the samples were sufficiently rinsed with deionized water and immersed in a dilute 0.1 M HCl aqueous solution for 0.5 h. Annealing at 500 °C (heating rate is 2 °C s⁻¹) for 3 h was necessary to produce TiO₂ nanowire arrays on the features of micrometer-scale pits.^27,28 This method is much easier than thermal oxidation (reported in some literatures^29–31) to prepare TiO₂ nanowire arrays without high temperature and mixed gas of Ar gas and oxygen. Finally, these samples were modified in 1 wt% FAS-17 ethanol solution for 24 h and dried for 2 h at 120 °C to obtain superhydrophobic surfaces. In order to make a comparative study, we designed four kinds of surfaces, (SS) Ti6Al4V substrate surface without modification; (MS) micrometer-scale pit surface with FAS-17 modification, (NS) nanowire structured surface with FAS-17 modification, and (MNS) micropit-nanowire structured surface with FAS-17 modification.

Characterization and anti-icing property test

Sample morphologies were characterized by field emission scanning electron microscopy (FE-SEM; Hitachi S4800, Japan). An atomic force microscope (AFM; Dimension Edge from Bruker, Germany) was used for visualization of the micro-features, and surface roughness of the samples was measured via a surftest (Mitutoyo SJ-210, Japan). The apparent contact angle (APCA), advancing contact angle (ACA), receding contact angle (RCA), and contact angle hysteresis (CAH) of a 4 μL water droplets on these surfaces were measured by a contact angle analyzer (Kruss DSA100, Germany). APCA could be directly obtained by the contact angle analyzer. ACA was recorded by the computer, when the contact area with surface changed owing to expansion of droplet. If the contact area decreased via shrinking the droplet, RCA could be recorded. CAH was the difference of ACA and RCA. Each contact angle measurement was repeated three times and the average value was calculated.

The reference water droplets (4 μL) were deposited on the sample surfaces and the temperature was controlled in a range from −10 °C to −30 °C. The freezing process was observed by a CCD camera. The icing-delay time of water droplets on the sample surfaces could be directly obtained from the CCD camera. Ice adhesion strength measurement was performed by a self-made ice adhesion strength measurement device including a cooling plate with a temperature in the range from 0 °C to −40 °C, as shown in Fig. 1. To carry out this measurement, we filled a cuvette with distilled water and placed a sample on the rabbet of the cuvette. This setup was kept in a refrigerator for more than 24 h to form an ice column sticking to sample surface. A force transducer was used to record in real time the force of separating the ice column from sample surfaces. Ice adhesion strength was calculated using the equation: τ = F/A, where τ was ice adhesion strength; F was the critical force; A was the contact area of ice column with sample surface.

Fig. 1 Schematic diagram of a self-made ice adhesion strength measurement device.

A high speed CCD camera was used to record the contact process of an impacting water droplet (4 μL) to sample surfaces, and the contact time was obtained from the high speed camera. In this measurement, the water droplets were released from a fixed height of 50 mm over the surface.

Results and discussion

Morphology and wettability

In this section, we present the demonstration and analysis of correlations between surface morphologies and wettability. Fig. 2a and b illustrates the SEM and AFM images of MS, NS, and MNS. Micrometer-scale pits (size ∼30 μm) on Ti6Al4V alloys fabricated by sand-blasting evenly distribute on the surface, and nanowires synthesized via hydrothermal treatment are also planted on substrate. The composite MNS can be obtained successfully by the combination of the two methods. In the Fig. 2d, the surface roughness of MNS is 1.352 μm, which falls in between the surface roughness of MS (2.287 μm) and NS (0.271 μm). Moreover, the surface features of MNS are considered to favor water-repelling due to the high amount of air trapped under the droplet by the micro-nanostructures (i.e., Cassie wetting state).

Fig. 2 Images of hierarchical morphologies and characterization results of the wettability of SS, MS, NS, and MNS. (a) SEM and (b) AFM images of MS and NS; (c) APCA, ACA, RCA, and CAH of water droplets on the four surfaces. (d) Surface roughness of MS, NS, and MNS.

The wetting properties of these surfaces were examined via a contact angle analyzer. Fig. 2c depicts the results of contact angle measurement on the SS, MS, NS, and MNS. We can find that the APCA of the MS is up to 135°, and the CAH has a large value of 40°. This result indicates that the MS has good hydrophobic performance, but the water droplet has no ability to roll off from the surface. According to the Wenzel model, this large contact angle is expected since the water droplet is completely impregnating the MS, resulting in the high CAH.

However, the APCA of the droplet on the composite MNS reaches 161°, and the CAH reduces to 2° owing to the existence of air pocket under the droplet induced by the effect of the combination of micro- and nanostructures.³² This phenomenon has been elucidated in detail by the Cassie–Baxter Model, indicating composite micro-nanostructures on a low surface energy material significantly improves superhydrophobicity.³³ In this case, the apparent contact interface of the water droplet is actually composed of solid/liquid and liquid/gas. Since the contact angle of a droplet on air is considered as 180°, the APCA (θ*) can be expressed as:

cos θ* = fcos θ + f − 1 (1)

where f and θ are the area fraction and Young contact angle of the solid, respectively. Thus, we can draw the conclusion that reducing the solid contact area fraction (i.e., capturing more air pockets) is important for improving the superhydrophobic properties. The nanowire produced by the hydrothermal method has a high aspect ratio, which results in a large amount of air being trapped. With the addition of micrometer-scale structures, a certain size of water droplet can be almost suspended on the MNS. According to results reported by Y. Song's group,³⁴ the APCA of a droplet on the superhydrophobic surfaces decreases with the decrease of the surface temperature. But the APCA is still greater than 150° even at the dew-point, indicating that superhydrophobic surfaces with hierarchical structure exhibit excellent stability of their APCA and CAH.^35,36 Therefore, the MNS may be considered as a potential surface for anti-icing.

Icing-delay time of the water droplet

To examine the anti-icing properties of these surfaces, we investigate the icing-delay time of the water droplet on these surfaces under different temperatures (−10 °C, −20 °C, and −30 °C). The water droplets are deposited on these surfaces as a reference droplet that is used to observe how long the water droplet is maintained before completely freezing. Fig. 3a shows the icing process of water droplets on SS, MS, NS, and MNS at −10 °C. Water droplets on SS and MS are quickly frozen (11.3 s and 12.1 s). However, water droplets on the NS and MNS take a long time to completely freeze, especially the droplets on the MNS (750.4 s). Moreover, pre-cooling the water droplet (before ice growing) spends most of the time. For the sake of data integrity, we also test the icing-delay time of water droplets at −20 °C and −30 °C, and the results are shown in Fig. 3b and c. With reducing temperature, the icing-delay time of water droplets decreased across all surfaces. However, the icing-delay performance of MNS is obviously superior to the other surfaces (i.e., SS, MS, and NS).

Fig. 3 The results of icing-delay time measurement, (a) optical images show the icing process of 4 μL water droplets on SS, MS, NS, and MNS at −10 °C; (b) the relationship of the pre-cooling time (before ice growing) and temperature; (c) the ice growth time of the water droplet versus temperature.

From the view of thermodynamics, the reduction of droplet's energy can be expressed as:^37,38

δQ = ρ_wC_p(t₀ − t_s) (2)

where δQ is the reduced energy of droplet; ρ_w is the water density; C_p is the specific heat capacity of water; t₀ is the starting temperature of the droplet; t_s is the final temperature. If Δh is the reduction of droplet's energy in unit time, the relationship between Δh and time (t) can be expressed as:

δQ = Δh × t (3)

If δQ can be considered as a constant value, the small Δh will make large t. For the micro-nanostructured superhydrophobic surfaces, Cassie wetting model can be formed. A large amount of air are trapped under the droplet, resulting in a very small apparent contact interface between the water and surface, and composing of solid/liquid and liquid/gas. Moreover, the small area fraction of solid/liquid interface also results in low efficiency of heat conduction, and the air inside the micro-nanostructure provides efficient thermal insulation. Thus, the less heat loss (Δh) in unit time causes the longer icing time. This is why the MNS shows excellent icing-delay performance at a low temperatures (subzero).

Ice adhesion strength

Ice adhesion strength, another important parameter, is also used to evaluate anti-icing properties. The ice adhesion strengths on these surfaces (i.e., SS, MS, NS, and MNS) under low temperature were measured via a self-made ice adhesion strength measurement device. As shown in Fig. 4, the ice adhesion strength (approximately 80 kPa) on the MNS is much lower than that (760 kPa) on SS at −10 °C. Furthermore, the ice adhesion strengths on these surfaces increase slowly as the temperature decreased from −10 °C to −30 °C. The change rule of ice adhesion strength was in agreement with the reported result by B. J. Basu.³⁹

Fig. 4 The relationship between ice adhesion strength and temperature.

No matter what temperature was used, the ice adhesion strength on the MNS was the lowest among these sample surfaces. The low adhesion strength can be explained by the large air pockets that MNS traps. When the water droplet deposits on the MS, an impregnating contact model (i.e., Wenzel model) can be formed. At the lower temperatures, stronger ice adhesion strength could be due to the mechanical interlocking of ice in micrometer-scale structures.⁴⁰ However, for a composite heterogeneous wetting regime, a spherical water droplet is formed on the MNS (i.e., Cassie model), leading to the water droplet being suspended, as demonstrated by the wetting experiment. Therefore, there is a layer of air between the ice column and MNS, which causes the lowest ice adhesion strength.

According to the report from Ludmila B. Boinovich's group,⁴¹ a polymolecular water layer at the ice/air and ice/hydrophobic surface interfaces acts as a lubricating layer and thus makes it possible to decrease considerably the ice adhesion strength. Additionally, the breakage of the contact between ice and superhydrophobic surface practically occurs only along the real contact interface between ice and solid. Thus, the measured ice adhesion strength on MNS is much smaller than that on SS, MS, or NS owing to the small actual contact area.

Considering the real application, a high durability of this property is important as well.^42,43 The durability was investigated by applying 20 icing/ice breaking cycles to the MNS. The operating sequence was similar to that of ice adhesion test. Fig. 5 shows the images of the MNS before and after 20 icing/ice breaking cycles. It can be found that the surface of sample after the durability test has almost no change with the initial surface, and the nanowire arrays on the features of micrometer-scale pits are not taken off because of the hydrothermal synthesis of nanowire for a short time. Despite losing part of its hydrophobicity with APCA decreased from 161° to 155°, and CAH increased from 2° to 5°, the MNS still exhibit excellent hydrophobicity. Therefore, this hierarchical structure is robust enough to support the water droplet to suspend on the surface, and form an air cushion to reduce ice adhesion strength and increase icing-delay time.

Fig. 5 Images of the MNSs for durability test, (a) before 20 icing/ice breaking cycles; (b) after 20 icing/ice breaking cycles.

Contact process of an impacting droplet

To further characterize the anti-icing potential of MNS, dynamic droplet impact experiments on these surfaces were performed with an impact speed of 1 m s⁻¹ under different temperatures. For the measurements under subzero conditions, samples on the cooled Peltier plate were purged with nitrogen to prevent condensation. As shown in Fig. 6, in the dry environment, the impacting water droplet on MNS spread to a nearly uniform coating, retracted, and then completely lifted off of the surface within 12 ± 1 ms. Moreover, the impacting droplet on NS could also complete the entire process within 14 ± 1 ms. In contrast, there is an obvious difference in the recoiling mechanism of the impacting droplet on SS and MS that the droplets could not lift off owing to the weak water-repellent performance. Thus, for the MNS, we carried out the droplet impact measurement under subzero conditions.

Fig. 6 Water droplet impact measurement on SS, MS, NS, and MNS performed using water droplets of 4 μL at velocity 1 m s⁻¹ under 24 °C.

Fig. 7 shows the results of the measurement of the impacting water droplets on MNS under subzero conditions. At −10 °C, the droplet could not bounce off the surface until 21 ms. Furthermore, the contact time is extended to 24 ms as the temperature further decreases to −20 °C. The water droplet could not completely bounce off the surface, leaving a tiny droplet behind at −30 °C. According to the above analysis of icing-delay properties, the contact time of the impacting droplet on MNS is far less than the icing-delay time of the static droplets on MNS. The impacting droplets can bounce off before freezing under subzero conditions.

Fig. 7 Water impact measurement on MNS at 24 °C, 0 °C, −10 °C, −20 °C, and −30 °C.

As for the dynamic droplet impact and rebound process on MNS, the impacting water droplet can completely bounce off within 12 ± 1 ms in dry conditions owing to its robust water-repellent properties. In a subzero environment, the contact time of the impacting water droplet on MNS may be prolonged as the temperature reduces because of the adhesion caused by the cold surface. But the water droplet can always bounce off before freezing unlike the other surfaces (i.e., SS, MS, and NS). Therefore, MNS may be considered as a promising anti-icing surface.

Conclusions

In summary, we fabricated the hierarchical superhydrophobic surfaces (i.e., MNS) via modifying the micro-nanostructures (constructed by the combination of sand blasting and hydrothermal treatment) on the surface of Ti6Al4V alloy with FAS-17. Micro-nanostructures acted by trapping a large amount of air under the droplet, which resulted in the low efficiency of heat conduction between water droplet and cold solid surface. This process seemed to directly cause the long icing-delay time (750.4 s) and low ice adhesion strength (80 kPa) at −10 °C. In the dynamic droplet impact and rebound assay on MNS, although the contact time was extended with the reduced temperature, the water droplet could always bounce off before freezing regardless of temperature, owing to the robust water-repellency. These investigations demonstrate that MNS materials are a promising anti-icing surface, which have significant potential for application.

Acknowledgements

The authors gratefully acknowledge the financial support of the National Science Foundation of China (no. 51202112, no. 51475231), the Jiangsu Innovation Program for Graduate Education (KYLX_0261) and the Fundamental Research Funds for the Central Universities (NZ2013307).

Notes and references

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