Hydrophobicity, Freezing Delay, and Morphology of Laser-Treated

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Hydrophobicity, freezing delay and morphology of laser treated aluminum surfaces Victor J. Rico, Carmen Lopez-Santos, Martin Villagrá, Juan P. Espinos, Germán de la Fuente, Luis A Angurel, Ana Borras, and Agustín R. Gonzalez-Elipe Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00457 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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Hydrophobicity, freezing delay and morphology of laser treated aluminum surfaces Víctor Rico Gavira,*1 Carmen López-Santos,*1Martín Villagrá, 1Juan P. Espinós,1 German F. de la Fuente,2Luis A. Angurel,2Ana Borrás,1Agustín R. González-Elipe1

1. Nanotechnology on Surfaces laboratory. Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla). Avda. Américo Vespucio 49. 41092 Sevilla. Spain. 2. Instituto de Ciencia de Materiales de Aragón (CSIC-Univ. Zaragoza). c/María de Luna 3. 50018 Zaragoza. Spain.

KEYWORDS: freezing delay, hydrophobicity, aluminum, laser treatment, Cassie-Baxter, plasma coatings

ACS Paragon Plus Environment

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ABSTRACT Until recently superhydrophobicity was considered as a hint to predict surface icephobicity, an association of concepts which is by no means universal and that has been proven to depend on different experimental factors and material properties, including the actual morphology and chemical state of surfaces.

This work presents a

systematic study of the wetting and freezing properties of aluminum Al6061, a common material widely used in aviation, after being subjected to nanosecond pulsed IR laser treatments to modify its surface roughness and morphology.

All treated samples,

independently of their surface finishing state, presented initially an unstable hydrophilic wetting behavior that naturally evolved with time to reach hydrophobicity or even superhydrophobicity. To stabilize the surface state and to bestow the samples a permanent and stable hydrophobic character, laser treated surfaces were covered with a thin layer of CFx prepared by plasma enhanced chemical vapor deposition. A systematic comparison between freezing delay (FD) and wetting properties of water droplets onto these plasma/polymer-modified laser-treated surfaces reveals that, under


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conditions where a heterogeneous nucleation mechanism prevails, surface morphology rather than the actual value of surface roughness parameter is the key feature for long FD times. In particular, it is found that surface morphologies rendering a Cassie-Baxter wetting regime favor longer FDs than those characterized by a Wenzel-like wetting state. It is found that laser treatment, with or without additional coverage with thin CFx coatings, affects wetting and ice formation behaviors and might be an efficient procedure to mitigate icing problems on metal surfaces.

INTRODUCTION Due to the importance for safety issues (e.g., in aeronautics) and the economic impact of surface icing, the past few years have witnessed an increasingly growing research effort to understand the multiple facets of icing and the surface properties that promote surface icephobicity. Bioinspired surfaces,1-4 lithographic patterns,5,6 surfaces with natural anti-freezing proteins7-9 and other ideal surfaces have been developed in laboratory settings, e.g., using expensive and time-consuming lithography methods, complicated wet-chemical routes and


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similar approaches. These investigations have permitted the development of rather effective model icephobic surfaces and advance new concepts accounting for the factors controlling icephobicity.

1,4,5

These accomplishments, however, are difficult to translate to large-area

industrial applications (i.e., real surfaces) for which clear correlations between surface properties, surface manufacturing methodology and icing behavior are still required.10,11 Moreover, there is a clear need of specific paradigms to describe the icing behavior of these real surfaces that surpass the general but by no means universal relation between superhydrophobicity and icephobicity which, in reality, depends on icing conditions, the actual surface topography and other surface properties. 4, 12-16A specific aspect of surface icephobicity is their anti-icing capacity defined as the capacity of a surface to delay ice formation.

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A

common test to correlate wettability and anti-icing capacity is the measurement of the freezing delay (FD) time of sessile water droplets left onto cold surfaces. 3, 4, 18-25 In the present paper we study both the basic wetting behavior and anti-icing capacity expressed in terms of FD time of a common material in aviation, the Al6061 alloy which, made of up to a 98% of aluminum, is deemed representative of the behavior of this element. 26, 27 In the first part of this study we show the possibility of modifying the surface topography of large aluminum


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substrates by laser irradiation. Because of the straightforward scalability and the wide range of possibilities offered by laser methods, they have been extensively used to modify the surface roughness, topography, chemical composition, as well as wetting properties, of aluminum and other metals.28-29/34 Laser treatment conditions using a nanosecond (ns) IR laser have thus been adjusted to properly tailor the roughness and topography (e.g., to produce a directional pattern) of aluminum surfaces in order to modify their wetting properties. In agreement with recent results in the literature, with

time,

from

35-37

we have observed that wetting of laser-treated samples evolved

hydrophilic/superhydrophilic,

in

the

as

prepared

state,

to

highly

hydrophobic/superhydrophobic after aging. In the second part of this work, in order to stabilize the wetting behavior and to increase the water contact angle (WCA) of these laser treated surfaces, approaching or reaching in some cases superhydrophobicity, samples were covered by a very thin (i.e., 50/100 nm nominal thickness) layer of a CFx polymer prepared by plasma enhanced chemical vapor deposition (PECVD). 38, 39 This thin layer leaves practically unmodified the surface topography of the laser treated surfaces, but contributes to enhance hydrophobicity as expected for a low surface energy coating.18, 40- 43 On these surfaces, we study the wetting properties (contact and sliding angles, as well as hysteresis) and FDs4, 17-25and try to correlate


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these parameters with roughness and surface topography. FD experiments carried out under conditions favoring heterogeneous vs. homogeneous nucleation processes

4, 21, 25, 44show

that

longest FD time appear on some superhydrophobic surfaces but not on others with apparently a similar wetting behavior. The critical analysis of the observed wetting and freezing behaviors on the studied laser treated surfaces has provided key clues to correlate FD times and the specific wetting states (i.e., Cassie-Baxter, Wenzel, etc.) in each case. The rational analysis of the obtained results sustains that, in a similar manner to ideal surfaces, tailoring the wetting and freezing behavior of real samples of aluminum alloy is possible by laser treatments compatible with large area applications in the industry.

RESULTS AND DISCUSSION

Surface topography of samples A-E Laser treatments of aluminum surfaces are commonly used to enhance their roughness and to produce specific structures at micro and nano- scales. These processes have been widely studied and modeled in the literature where various laser ablation regimes, denoted as thermal


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and explosive, have been identified.

31, 32, 45-48

The use of laser techniques to modify the wetting

properties of aluminum has been also reported in the literature. 29, 31-37 In the course of this investigation, different laser treatment parameters including output power, pulse width, beam scan rate and distance between scanning lines were systematically modified in order to obtain different surface topographies and roughness. For the sake of clarity, we will concentrate this study in five selected samples (named A to E) obtained with the laser conditions summarized in Table 1 using a power of 16 watts and a spot size of approximately 90 microns. This table also includes values of roughness parameters (Sq, defined as the medium

quadratic height and calculated according to 𝑆𝑞 =

1

, and Sp, defined as the average of

𝑛 𝑛∑𝑖 = 1𝑦𝑦𝑖2

the maximum height of the peaks in the area of study) as deduced from confocal microscopy maps of treated surfaces. Roughness values were averaged over observation areas of 350x265 µm2 and measurements on various zones of the sample surface (4 measurements on average). Two additional Al6061 aluminum samples, supplied by INTA (Spain), were considered as reference and are included in the table for comparison. These were a mirror polished (MP) and a reference rough (RR) sample prepared by sand blasting. A very flat quartz sample named QF was also used as reference to study FD times and wetting properties (see experimental and


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next sections). From the analysis of the laser parameters reported in Table 1, the highest fluence and energy incubation values in the scanning line have been used in sample D where, in consequence, ablation induced through a laser explosion mechanism should be expected.

45-

48

Table 1.- Summary of samples and preparation conditions Sample

Laser conditions: υ (KHz),

Sq (µm)2

Sp (µm)2

WCA (º) at

scan rate (mm/s) and

15ºC after

distance between scanning

one month

lines (μm)

aging

A

80, 2000, 4

0.7 + 0.1

3.5 +1.1

70

B

80, 2000, 20

1.7 + 0.3

9.5 + 2.1

75

C

20, 2000, 20

5.7 + 0.5

22.6 +

140

3.4 D

20, 100, 20

14.6 + 1.2

47.8 +

155

4.7 E

20, 2000, 4

22.0 + 1.4

42.0 +

*

3.0


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MP

--------

0.03 +0.02

0.32 +

100

0.1 RR

--------

0.49 + 0.21

0.88 +

82

0.3 2.-Values determined from confocal maps of the samples *Sample E was still hydrophilic one month after its preparation

Since the quite high surface roughness of some of the studied samples precludes the use of atomic force microscopy for their analysis, SEM and confocal microscopies were used for a thorough surface topography analysis of samples A-E. The results, presented respectively in Figure 1 and in the supporting information, Figure S1, reveal clear differences in roughness and surface morphology (see the laser scan direction in the supporting information, Figure S1) that prove the great versatility of the laser technique to tailor roughness and topography of aluminum surfaces. Table 1 and Figure 1 evidence the high value of the roughness parameters Sq and Sp of samples D and E (Table 1), the linear row patterns in the direction perpendicular to the laser scan line of samples C and E or the nodular surface topography of sample B. Unlike the relatively closer values of roughness parameters of samples D and E, their topography was


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completely different, as revealed by their linear profiles and high magnification micrographs reported in Figure 1. According to its linear profile in this figure, the topography of sample E consisted of tenths micron size roughness oscillations with superimposed micron height features which do not penetrate deep into the material. Meanwhile, sample D topography depicted a highly tortuous surface where deep and narrow hollow features penetrate tenths of microns inside the material (see in Figure 1 the line profiles of these two surfaces). This surface topography defines a dual scale surface roughness at micro and nano- scales, this latter clearly appreciated in the high magnification SEM micrograph of this sample.


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Figure 1.- Topography and roughness analysis of samples. Normal view SEM images of the different aluminum samples presented at high and low (inset) magnification scales and linear profiles (highlighted in red) obtained from confocal optical micrographs. Surface composition, wetting behavior and aging of samples A-E Static WCAs of samples aged for one month are gathered in Table 1. Just after laser treatment, all samples were hydrophilic or superhydrophilic (WCA< 5º), but their WCA slowly increased up to a constant value after they were handled in air for one month, in most cases. After this period, some samples were slightly hydrophilic (samples A-aged and B-aged with WCA approaching 90º), highly hydrophobic (samples C-aged and D-aged) or remained superhydrophilic (sample E, the which only became hydrophobic after a much longer time). Since the change in wetting behavior due to aging might be linked with a change in surface composition and/or surface contamination, we analyzed the surface state of the aged samples by XPS. Figure 2 shows a typical XPS analysis of laser treated samples exemplified with the results of sample D. In the spectra reported in this figure, and in similar spectra recorded for samples A-E, it is apparent a surface composition typical of Al2O3 (i.e., Al2p BE close to 74 eV), indicating that the laser treatment in air produces the surface oxidation of aluminum. Negligible differences in the


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surface concentration of carbonaceous contaminants (always below 12-13%) could be also deduced from the analysis for the fresh treated samples A-E (see results for sample D in supporting information, Table S1).The spectra in Figure 2, the fitting analysis of the C1s peak in Figure S2 and the surface composition in Table S1 for sample D-aged shows that the aluminum surface was highly contaminated with adventitious carbon, likely resulting from the surface incorporation of airborne hydrocarbons. It is noteworthy, that a laser treated sample D carefully stored in a desiccator for one month did not show any significant increase in surface contamination (see Figures 2, S2 and Table S1) and remained highly hydrophilic. We therefore attribute the observed change from hydrophilic to hydrophobic or even superhydrophobic states to the surface contamination by airborne hydrocarbons of the laser treated samples. A similar behavior has been recently reported by other authors.35-37Assuming a similar surface contamination, recent experiments have reported that laser treated aluminum can be converted into permanently superhydrophilic when laser treated pieces are boiled in water, or into superhydrophobic if heated in air at 100 ºC. 35, 36


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Figure 2.- O1s, C1s and Al2p photoelectron spectra of sample D just after laser treatment, stored for one month in a desiccator and aged air for one month without particular precautions. Spectra have been y-axis shifted for more clarity.

With the exception of sample E which, despite being exposed to air for one-month, was still superhydrophilic, WCAs in Table 1 for samples A/D-aged follow the normal behavior predicted by the Wenzel49 and/or Cassie Baxter50 models in the sense that, for a similar chemical state, hydrophobicity of surfaces increases with roughness.41,

42

Another common feature in all

samples was that the WCA of droplets measured at -5º was slightly smaller (varying between 5º and 15º, depending on samples) than at 15ºC. A decrease in WCA is a common tendency found


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in wetting experiments with supercooled water21,23,51 and may reflect the decrease in surface tension of water with temperature52 and/or the effect of other phenomena such as changes in the volume of water, the accommodation to roughness features or a different equilibrium between liquid and vapour. FD experiments were also carried out on samples A/D-aged (sample E-aged was still superhydrophilic). The general tendency was that FD times were longer for the roughest sample (i.e., sample D-aged, 40 min), than for the rest (i.e., A-aged, 2 min, B-aged, 3 min and C-aged, 20 min) and slightly increased for these samples aged for longer periods. This preliminary assessment of FD times, even if not completely reliable since the surface state of laser treated aluminum samples changed with time, support that the higher the surface roughness of laser treated aluminum (i.e. aged samples C/D-aged vs. A/B-aged) the longer the FD times. This observation and the evolution in the same direction of the WCA of these samples might support the controversial assumption that superhydrophobicity and anti-icing capacity run parallel.4, 12-16, 44In

the following sections, working with aluminum samples coated with a very thin CFx film that

depict a stable wetting behavior over time, we will show that this is not an universal rule.


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CFx thin coatings, wetting and FD behavior A way of changing the surface composition of the laser treated Al6061 substrates and to make them wetting and freezing stable over time without significantly affecting their surface roughness consists of covering them with a thin CFx (i.e. Teflon like) layer deposited by PECVD.38,

39A

similar approach was used by Heydari et al.18,19working with this and other thin film materials prepared by plasma. Unlike other methods of applying thicker Teflon layers by spraying, sol-gel or similar chemical techniques,

40, 41, 51, 53

plasma deposition methods provide a precise control

of thickness, even in the nanometer range, a conformal coverage and a rather homogeneous lateral distribution of the coating material.42,

43

In other words, applying thin CFx coatings by

PECVD would preserve the surface roughness and morphology of the laser treated samples but change their intrinsic surface tension as expected for a teflon-like terminated surface. Following this idea, we studied the study the wetting and freezing behaviors of samples A-E coated with 50 or 100 nm thick layers of CFx (hereafter named as A/E-CFx (thickness in nms)). For comparison, FD tests were also carried out with reference samples MP and QF which, prepared as described in the experimental section, were also covered with a CFx thin film (i.e., samples MP-CFx and QF-CFx).


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The fitted C1s XPS spectrum in Figure 3 corresponds to sample D-CFx(100). This spectrum, taken as example to illustrate the surface chemistry of all samples, clearly reveals that the surface is completely covered by a CFx layer characterized by a F/C ratio of 1.3 and where CF3, CF2, CF and C-CF functionalities are identified by fitting analysis of the C1s peak

38, 43

(see the

inset in Figure 3). The absence of Al2p peaks in the general spectrum of this sample confirms that the CFx film is quite conformal and covers completely all the roughness features of the aluminum laser treated surfaces.


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Figure 3.- Fitted C1s fitted spectrum taken for sample D-CFx(100). The different fitting bands used to reproduce the whole spectrum can be attributed to C-C, CF, CF2 and CF3 (BEs at 284.3eV, 286.9eV, 289.1eV, 291.3eV and 293.4eV respectively) as indicated (refs. 38, 43); (au) in the y-axis means arbitrary units. The wetting properties of these CFx covered samples did not vary with time and their static WCAs at 15ºC, hysteresis and sliding angles have been summarized in Figure 4. It is noteworthy that, in a similar manner than on samples C/E, the WCA on samples D-CFx and ECFx only decreased slightly when firstly cooling from 15ºC to -5ºC. During successive freezingthaw cycles WCA only decreased by 20º after the ice-water transformation (see results for samples E-CFx and D-CFx in the supporting information, Figure S3). This behavior agrees with some results in literature reporting little or no change in WCA during freezing-thaw cycles,23,54 but disagrees with other showing that the WCA of water droplets during the melting/heating process significantly decreased with respect to the values attained after firstly dripping and cooling.18,19 According to Heydari et al.,18, 19when cooling in a humid environment, water vapor condenses and eventually freezes onto the surface. This makes that, upon heating and melting, a thin liquid water film is covering the surface. Under these conditions, the large decrease in WCA of sessile droplets during freezing-thaw cycling would reflect the interaction of the water


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droplet not with the pristine surface but with a water layer. Our experiment and those of refs 54are

23,

conducted under a practical absence of humidity and changes in WCA are smaller or

negligible because the rest of sample surface remains dry.


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Figure 4- Wetting contact angle, wetting hysteresis and sliding angles as a function of the roughness parameter Sq of samples A/E-CFx and reference RR-CFx. WCA values of samples


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A/E-aged and references RR and RR-CFx are included for comparison in the top panel (note that one month aged sample E had not yet reached superhydrophobicity).

Data in Figure 4 show that, with respect to samples A/E-aged, WCA increases when samples A/E are covered by CFx. This tendency reveals that for Sq higher than 4µm (i.e., samples CCFx, D-CFx and E-CFx), surfaces behave as highly hydrophobic (sample E-CFx) or superhydrophobic (i.e. WCA>160º, samples C-CFx(100) and D-CFx(100)). The hysteresis and sliding angles reported in this figure for these three samples are typical of a superhydrophobic state. Under the assumption that hydrophobicity runs in parallel with icephobicity,

4, 12-16, 44

the

set of results in Figure 4 would suggest that FD times should be similar for samples C/D/E-CFx. However, we will show that FD times of samples C/D/E-CFX are completely different, thus demonstrating that this common assertion does not hold for these samples. FD times for samples C/D/E and C/D/E-CFx, as well as for samples RR, MP and Q-CFx included for comparison, are gathered in Figure 5. Droplet volume in these tests was 2µl, except for sample D-CFx which required 8 µl (its superhydrophobic character prevented the use of smaller droplets, see experimental section). Since large droplet volumes are known to produce a


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decrease in FD time,

16(this

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effect was also verified in our case for droplets deposited onto

partially hydrophobic MP and RR samples, see supporting information, Figure S4), the value of this parameter for sample D-CFx using smaller droplets is expected to be longer than that reported in Figure 5. The differences of FD times in the bar diagram in Figure 5 provides useful information relating roughness, surface morphology and surface tension with anti-icing capacity. For example, although both samples MP and Q-CFx are rather flat (see the AFM image and RMS value of QCFx sample in the supporting information Figure S5), FD time is much longer for the latter. In the latter, this behavior can be accounted for by the external teflon like film (moderate hydrophobicity and good anti-icing performance are typical features of low roughness surfaces of polymer or teflon-like materials

10, 16, 40, 41, 51)

and the much lower heat conductivity of the

quartz substrate.55-58 Thus, the higher thermal conductivity of the aluminum substrates and the availability of nucleation sites at the laser treated surfaces are surely contributing to shorten the FD time in samples C-CFx and E-CFX which, despite their superhydrophobic or highly hydrophobic character (see wetting parameters in Figure 4), presented much shorter FD times than sample QCFx. Only samples D-aged and, particularly D-CFx, both of them


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superhydrophobic (a fresh laser treated sample D in the superhydrophilic state had FD times in the order of seconds), presented FD times that, respectively, were comparable and significantly longer than the value found for sample Q-CFx. Therefore, the completely different anti-icing behavior of samples C/D/E-CFx confirms that superhydrophobicity is not synonymous of icephobicity, at least regarding the FD times onto CFx and polymer-modified aluminum samples.4, 12-18, 44 Reasons behind the very long FD times in sample D-aged and, particularly, sample D-CFx must be looked for in their particular surface morphology. In fact, samples D and D-CFx(50) have roughness parameters values Sq and Sp in the order of tenths of microns which are within a similar order of magnitude than the values found for sample E (c.f., Table 1). However, according to the topography analysis in Figure 1 and the SEM images of sample D-CFx(50) in Figure 6, this sample presents a completely different surface topography than sample E. According to Figure 1, sample E presents a sinusoidal-like roughness profile and roughness features at the nanoscale that do not penetrate deep into the bulk. This surface microstructure will be likely to yield a Wenzel wetting regime at low temperatures where liquid is always in contact with the solid substrate.

49

In addition, according to the SEM images of sample D-CFx


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(50) in Figure 6, this sample exhibits deep hollows and a dual micro- and nano-scale roughness distribution which is typical of a Cassie-Baxter wetting regime where liquid/solid and liquid/air interfaces would exist and which presumably remains at low temperature.50In this regard, the comparison of the SEM images in Figure 6 for D-CFx samples with different thickness of the teflon-like overlayer reveals that the surface morphology did not significantly change after applying a thin layer of CFx (i.e., 50 and 100 nm, for which the coverage of is conformal and nanoroughness features are still visible), but becomes strongly modified for thicker layers as in sample D-CFx(1000). In this case pore entrances have been closed, surface features smeared out and big CFx aggregates formed on the surface (c.f. Figure 6 right panels). This sample, at the limit between hydrophobicity and superhydrophobicity (i.e., it presented a WCA 160º), presented very short FD times in the order of 40 s. We attribute this difference between CFx covered samples to the lost the nanometric features clearly visible in sample D-CFx(1000) (c.f., Figure 6) and, therefore, to the lack of a dual micro- and nanometer topography that, in this way, appears to be a requirement for an enhanced anti-icing response. Also, these evidences gained from this analysis of rough aluminum samples permit to conclude that, even on low energy


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surfaces made of a teflon-like termination, long freezing times require the dual roughness and air pockets typical of a Cassie-Baxter state.

Figure 5.- Bar diagram representing the FD times measured for the indicated samples. Error bars represent fluctuations for different measurements. Schemes of the different surface topographies and an orange line to denote a thin CFx layer (i.e.50 nm) are included for illustration. Sample E aged for one month was supehydrophilic and its FD time was in the order of 40seconds.


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Several studies with different ideal surfaces depicting a Cassie-Baxter state

56, 58, 59have

effectively shown that the air pockets at the interface between droplets and substrates contribute to delay freezing by the combination of a reduced heat transfer through the interface 56

and the lack of sufficiently large sites for ice nucleation.

21, 44, 55, 58-60At

-5ºC critical ice nuclei

size has been estimated in approximately 10 nm18,19,61 which, according to the inset in Figure 6, falls within the size of surface features of the sponge-like microstructure of sample D-CFx(50).58 We can therefore conclude that surface topography is playing a significant role in increasing FD in samples D-aged and D-CFx, an effect that becomes enhanced by the incorporation of a thin layer (i.e., between 50 and 100 nm) of CFx. We must emphasize that when sample D was covered with a thicker layer of CFx of approximately 1000 nm, FD times drastically decreased to several tenths of seconds (i.e., almost immediate freezing).


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Figure 6.- SEM micrographs at progressively higher magnifications from top to bottom (see the scales) of samples D-CFx(50) (left) and D-CFx(1000) (right). The series of images taken for sample D-CFx(50) prove that the surface topography of sample D is practically unaffected by


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the deposition of CFx. The inset in the bottom panels show and enlarged view of the surface microstructure of these samples.

Finally, we would like to remark that although the factors contributing to the transition between superhydrophilic and superhydrophobic behaviors in laser treated metals and its association with carbon contamination (c.f. Figure 2) is still an open question for debate, it is remarkable that sample D just as prepared, and therefore superhydrophilic, presented very short FD times in the order of a few seconds. The large droplet/substrate area developed in this sample due to its superhydrophilicity and the fact that surface hollows are completely flooded with water in this superhydrophilic state confirm the requirement of air pockets and a superhydrophobic state in order to increase the FD time.

CONCLUSIONS In this work, anti-icing surfaces have been prepared by laser treatment of aluminum Al6061. Their anti-icing capacity was determined by measuring the freezing delay time of water dripped onto cooled surfaces under conditions of low humidity but yet favoring a heterogeneous (at the


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solid-liquid interface) rather than a homogeneous freezing mechanism. It has been found that setting the laser irradiation conditions to reach an “explosive” regime leads to the generation of a very rough and tortoise micro/nano surface morphology prone to achieve a superhydrophobic state after prolonged aging. The observed evolution with time from the superhydrophilic to the supehydrophobic state has been associated with the surface contamination by airborne hydrocarbons. It has been also realized that a Cassie-Baxter wetting regime for the aged samples is a key issue to achieve long freezing delay times. Inefficient heat transfer kinetics between the supercooled water droplets and the substrate due to the formation of air pockets at the interface and the absence of large enough surface features where ice nucleation may take place are proposed factors contributing to the long freezing delays found on this sample. Delay time was even longer if the surface energy of aluminum was lowered by the plasma enhanced chemical vapor deposition of a very thin layer of a Teflon-like (C-Fx) polymer. Unlike the modification in surface topography achieved by the deposition of thick layers of this polymer compound, thin CFx layers in the order of tenths of nanometers deposited by PECVD do not significantly affect the pristine surface morphology of the laser treated aluminum. The combined effect of the low surface energy of the teflon-like coating and the Cassie-Baxter wetting regime


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provided by the surface morphology of the laser treated samples are revealed as critical factors rendering the longest freezing delay time among all studied surfaces.

EXPERIMENTAL METHODS

Materials, laser surface treatments and coatings Al6061 surface polished substrate pieces (15x15 mm2) were supplied by Goodfellow and used without further conditioning except for a gentle degreasing cleaning using detergent water solutions. Since this alloy basically consists of Al (around 98%) and minor concentration of Si, Cu and other elements, we will refer to these samples as “aluminum”. Surface morphology and roughness of these aluminum substrates were modified by treatment at room temperature with a 20 W diode-pumped Nd:YAG (Powerline E, Rofin-Baasel Inc.) unpolarized laser emitting at 1064 nm with a 100 ns pulse width and a 20 kHz repetition rate. Although there is not a specific restriction, the laser treatment was applied onto an area of 4 cm2. Laser treated samples are denoted A, B,C, D and E in the text, adding the term “aged” for samples kept in air for one month after the laser treatment.


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Surface topography of laser treated samples was characterized by scanning electron microscopy (SEM) and confocal microscopy. SEM analysis in a normal configuration (i.e., top view images) was carried out in a HITACHI S4800 field emission microscope in the Institute of Materials Science of Seville. Confocal microscopy analysis was carried out with a ZEISS LSM 7 DUO microscope in the CITIUS of the University of Seville. Roughness parameters were calculated from the confocal surface images of samples. Atomic force microscopy (AFM) analysis of some samples were roughness did not exceed a certain value was carried out in tapping mode with a Nanotec microscope and a Dulcinea electronics. AFM images were taken with high frequency levels and analyzed with the WSxM software. The roughness is expressed in terms of the RMS coefficient.

CFx thin films (50, 100 and 1000 nm thickness as estimated from thin films simultaneously deposited onto a flat silicon wafer) were deposited by PECVD according to a procedure previously described.38,39Thicknesses were directly measured in the cross section SEM images of diced samples of silicon. For the thinnest films, thickness was determined comparing their deposition time with that of thicker thin films (i.e., 200 nm) where the SEM analysis was carried


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out. Deposition was carried out in a parallel plate reactor using mixtures of C4F8 (50%) and Ar (50%) at a working pressure of 0.15 mbar. RF power (at 13.56 MHz) was applied to the top electrode and its value adjusted as to induce a negative self-bias voltage of 100 V at the bottom electrode acting as sample holder. Laser treated samples A-E, as well as flat quartz substrates (sample QF) covered with CFx are denoted in the text as A/E-CFx or QF-CFx, adding in parenthesis the thickness of the CFx layer (i.e., 50, 100 and 1000 nm) only when it is relevant for the discussion. Surface composition and chemical state analysis of laser treated and CFx covered samples were carried out by XPS (X-ray photoelectron spectroscopy). Samples were introduced in the analysis chamber and spectra recorded with the AlKα line. Binding energy (BE) scale of the spectra was referred to the C1s peak taken at 284.5 eV for the adventitious carbon contaminating the surface of samples. To follow the aging behavior of samples, specimen subjected to the same laser treatment were either stored in a desiccator or kept in air for at least one month before XPS analysis. Wetting and freezing tests were always carried out with samples exposed to air or stored in the desiccator, but not yet utilized for XPS analysis. This protocol was carefully followed because some hints indicated that surface contamination leading to a state similar to that attained in the


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air exposed samples may occur for samples long kept in vacuum systems run with oil rotary pumps (e.g., like in the pre-chamber of the XPS spectrometer).

Wetting behavior and FD time determinations Static, dynamic and sliding wetting contact angles were determined by the Young method with a Data Physic Instruments using small droplets of, otherwise stated, 2 µl of deionised and then bidistilled water (i.e., high purity water). The reported values, with an estimated error bar of 10%, are an average of a minimum of seven measurements taken for each examined surface. FD times were measured in a home-made set up consisting of an environmental chamber provided with lateral glass windows to follow the evolution in shape of water droplets deposited onto the studied samples. A scheme of this chamber is presented in Figure 7. A Peltier cooling system was automatically adjusted with the reading of a thermocouple placed onto the analyzed surface in a location close (i.e. approximately 1 mm) to the droplet. Another thermocouple could be placed in contact with the water droplet during its cooling process to determine its actual temperature, although measurements were done in separated freezing experiments in order to do not affect the freezing process by the thermocouple inserted in the water droplet. A small


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flow of dry nitrogen (10 sscm) cooled at 0º or lower temperature by immersing the conducting tubes in a cooled bath at -4ºC was flowing through the chamber to perform the freezing experiments in the absence of ambient water vapor. Under these conditions, main source of ambient humidity in the chamber was the water vapor in equilibrium with the supercooled water droplets or the ice particles (i.e. around 400 Pa62). A retractable syringe filled with water was put inside the chamber at a temperature close to 0º and retrieved from the chamber after dripping the water. In most experiments the droplet volume was 2 µl, except for some superhydrophobic surfaces (e.g., for sample D) where, despite the horizontal position of the samples, these small droplets drifted away the observation areas and larger droplets of 8 µl had to be used in order to keep them fixed during the observation.


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Figure 7.- a) Scheme of the experimental chamber used to measure FDTs. The figure also presents an image with the thermocouple touching the surface of the analyzed sample under experimental conditions of freezing (left hand side of image) and the retractable syringe tip used for water dripping.

Water droplet freezing analysis was done dripping water onto the aluminum surface kept at 15º C. Then, the surface was fast cooled to -5ºC by switching on the Peltier. This process took less than 20 s. To ensure that water freezing occurs through a heterogeneous (i.e. at the liquid-solid interface) and not a homogeneous (i.e. at the liquid-gas interface) ice nucleation mechanism, 21, 25, 26, 44

4,

the flow of dry nitrogen was maintained at a minimum level to minimize water vapor


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removal and any additional cooling of droplet/ice surfaces due to water evaporation (i.e., to avoid homogeneous freezing). In all cases, heterogeneous freezing was confirmed by the observation of the progression of an ice front from the solid-liquid interface towards the top of the droplet (see images of different experiments provided in supporting information, Figure S6).No frost formation attributed to water vapor condensation and freezing was observed on the surface of analyzed substrates at any stage of the experiments. Reported FD times are the result of four different experiments. Under the conditions of the FD tests it was possible to follow the well-recognized sequential stages of the freezing process: water cooling, rapid kinetic freezing, isothermal freezing and ice cooling.18,

19, 21, 22

For example, the experiment reported in the Supporting information, Figure

S6, permits to identify these stages for a water droplet onto an aluminum surface covered by CFx. FD times reported in this work encompasses the three first periods. Experimentally, this FD time is determined as the time elapsed from the moment of dripping the water onto the surface at 15 ºC to the end of the isothermal freezing, this latter easily recognized in the sequence of snapshots of Figure S6. Water cooling times from 15ºC to -5ºC were determined in separate experiments placing a thermocouple into the water droplets (note that experimental


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conditions are not equivalent with or without thermocouple). Time intervals of approximately 30 s were measured in all cases for this water cooling process. Although FD times usually do not take in to account this water cooling period, since this time was very short in our experiment, values provided for FD times also include this contribution. WCAs of supercooled droplets during a series of freezing and thaw cycles (see supplementary information Figure S3) only showed a small change in this parameter after 3-4 cycles.

ASSOCIATED CONTENT Supporting information. The supporting information is available free of charge on the ACS Publications website at DOI: Confocal microscopy micrographs of samples, XPS data of laser treated aluminium samples, wetting/freezing cycling behavior, and effect of size of water droplet on freezing, AFM characterization of Teflon-like films and identification of different stages during freezing of water droplets are reported as supported information.

AUTHOR INFORMATION Corresponding authors


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E-mail:[email protected] 0000-0002-5083-0390 E-mail: [email protected] 0000-0001-8782-7331

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Notes The authors declare no competing financial interest

ACKNOWLEDGMENT This work has been carried out with the support of the EU project PHOBIC2ICE (ref. 690819). The authors also thank the European Regional Development Funds program (EU-FEDER) and the MINECO-AEI (201560E055 and MAT2016-79866-R and network MAT2015-69035-REDC) for financial support. Valuable discussions carried with the other members of the PHOBIC2ICE consortium and particularly with Dr. Julio Mora,


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Paloma García Gallego and Alina Aguero from INTA (Spain), who also supplied the reference samples used in this work, are acknowledged.


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TOC: Rough aluminum surfaces with a Cassie-Baxter state efficiently delay water freezing

Water

22222

Cassie-Baxter

Ice Wenzel


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Hydrophobicity, Freezing Delay, and Morphology of Laser-Treated

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