Adjusting the stability of plasma treated superhydrophobic surfaces by different modifications or microstructures

Abstract

Plasma induced hydrophilization of superhydrophobic surfaces is highly-efficient, reversible and less destructive, and has therefore been applied into fields like fabrication of wettability patterns; however, plasma treated surfaces tend to recover back to their original wettability during storage, and different time stabilities are required for diverse applications. This paper focuses on regulating the time stability of plasma treated superhydrophobic surfaces by different surface modification methods or microstructures, and the recovery time could be adjusted as either 10 hours or more than 100 days under normal ambient conditions. These differences in recovery could also be observed in wettability patterns prepared by dissimilar methods. The adjustment methods developed should facilitate applications of plasma induced hydrophilization, especially for those that require rapid recovery or long-time stability.

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1. Introduction

Wettability is a basic property of solid surfaces and is often characterized by water contact angle (WCA). Surfaces with extreme wettability generally refer to surfaces that are superhydrophobic (water contact angle > 150°) or superhydrophilic (water contact angle < 10°). In the past decade, these surfaces have been prepared, thoroughly studied, and applied in numerous fields, such as self-cleaning,^1–6 antibacterial,^7–9 anti-fog^3,10 and oil–water separation.^3,4,11,12 Recently, a new and interesting kind of extreme wettability surface containing both (super)hydrophobic and (super)hydrophilic regions, namely, the extreme wettability pattern, has drawn numerous researchers' interests.

Compared with superhydrophobic (SH) and superhydrophilic surfaces, extreme wettability patterns have some exclusive and promising applications in terms of fluid transport,^13–16 water collecting,¹⁷ and cell adhesion.^18,19 Therefore, there has been a lot of research aiming at simple, low-cost and high-efficient preparations of these patterns on different substrates. During preparation processes of these patterns, hydrophilization is an almost indispensable part, which can be realized by ultraviolet irradiation,¹⁴ laser processing,^20,21 lithography,¹⁹ high temperature annealing,^22,23 plasma modification,¹³ etc. Ghosh et al.¹⁴ realized pumpless fluid transport on wettability patterns via masked UV irradiation of SH surfaces. Kietzig et al.²⁰ employed femtosecond laser irradiation to prepare superhydrophobic metallic surfaces. Lai et al.¹⁹ fabricated extreme wettability contrasts on TiO₂ array nanotube surfaces by photocatalytic lithography. Chang et al.²² realized reversible wettability transition of CuO nanowire films via low and high temperature annealing. Plasma treatment is a high-efficient modification method,^13,24–26 and wettability of plasma treated surfaces could be reversed by immersion in low surface energy substances. Moreover, plasma treatment just modifies the outermost layers of materials,²⁷ and can minimize thermal degradation.^28–30 Thanks to these merits, plasma treatment has been widely used in hydrophilization and fabrication of extreme wettability patterns. Zhu et al.³¹ employed atmospheric pressure air plasma to treat SH surfaces and successfully achieved rapid wettability recovery. Her et al.³² used low pressure oxygen plasma to prepare patterned superomniphobic–superomniphilic surfaces. Garrod et al.¹⁷ fabricated microcondenser surfaces by plasma deposition of hydrophilic polymer arrays on SH surfaces. However, as a matter of fact, plasma-induced (super)hydrophilicity is not stable, and tends to recover to the initial (super)hydrophobic state. During the past two decades, a lot of studies have been focused on the wettability recovery of plasma treated polymer surfaces. According to these studies, the recovery is mainly attributed to structural rearrangement and migration of chains to minimize surface energy of plasma treated surfaces.^33–38

Rapid recovery of plasma-induced superhydrophobicity is sometimes desired, as it is necessary for the use of some specific smart devices.^31,39 The self-healing surfaces^40–43 may be suitable for this purpose. Li et al.⁴¹ successfully fabricated self-healing superhydrophobic coating by preserving healing substances composed of reacted fluoroalkylsilane in the rigidly flexible coatings, and the plasma-treated superhydrophilic coating can easily recover to be superhydrophobic in a humid storage condition. However, for other applications like microfluidic chip and directional liquid transport, loss of hydrophilicity would result in invalidation of the entire system.³⁹ Therefore, it should be of high importance to adjust recovery time of plasma-induced (super)hydrophilicity according to the specific applications. To date, numerous studies have been dedicated to changing stability of plasma treated polymer surfaces via different methods, such as adjusting characteristics or treatment parameters of plasma,^44–46 changing surface topography,^39,47,48 pretreating these surfaces^36,44 and storing under different ambients.^25,49–53 Nguyen⁴⁴ obtained the optimal plasma treatment time for modifying several kinds of methyl-silicone materials, and successfully improved time stability of treated surfaces by vacuum pretreatment. Tsougeni⁴⁷ found that PMMA surfaces etched by oxygen plasma for longer time had better time stability thanks to the highly roughened surface morphology. Van Deynse²⁵ prolonged recovery time of plasma-treated polyethylene surfaces by decreasing storage temperature or relative humidity of the surrounding air. These works are very meaningful to promote applications of extreme wettability patterns; nevertheless, most of them are confined to common polymer surfaces, and the recovery mechanism of metal surfaces still needs further and thorough research. More importantly, few works study stability of plasma treated SH surfaces, which might have different recovery behaviors or mechanisms from the hydrophobic polymers due to the micro/nanostructures and/or modification methods that are indispensable for their preparations. As a result, further applications of numerous interesting plasma-induced extreme wettability surfaces^{13,15,17,31,54,55} containing SH regions or metal substrates might be regretfully hindered.

Our previous work⁵⁶ found that plasma treated SH surfaces could remain hydrophilic after more than 16 days of low temperature storage. However, the surfaces recovered to be hydrophobic within just 5 days under normal ambients, and their recovery time could not be adjusted under constant storage conditions. On the other hand, to the best of our knowledge, long-time stable plasma-induced (super)hydrophilization of SH surfaces storing under normal conditions has rarely been reported. In fact, most practical applications of plasma-induced (super)hydrophilicity require rapid or slow wettability recovery under normal ambients; therefore, it should be significant to study stability regulation of plasma treated SH surfaces under normal storage conditions.

SH surfaces can be prepared by different modifications and microstructures, and variations of these parameters are therefore likely to influence the time stability of plasma treated SH surfaces. This paper focuses on regulating time stability of plasma treated SH surfaces on metal substrates by different modifications or microstructures. The wettability variations of treated surfaces were characterized by WCA measurements and X-ray photoelectron spectroscopy (XPS). Recovery time of plasma treated surfaces could be adjusted to 10 hours or more than 100 days under normal ambients by different modification methods or microstructures. Furthermore, time stability of plasma treated wettability patterns was also investigated, and the patterns showed similar variation trends as the treated surfaces, which should be potentially valuable for specific applications that require stable wettability or rapid wettability recovery.

2. Experimental

2.1 Materials

1060 aluminium alloys sheets (2 mm thick) were purchased from Suzhou Metal Material Manufacturer (China). Fluoroalkylsilane [FAS, C₈F₁₃H₄Si(OCH₂CH₃)₃] was purchased from Deguassa Co. (Germany). All chemicals were analytically pure and used as just received.

2.2 Fabrication of SH surfaces

Shown in Fig. 1 is the treatment process of aluminium samples. Aluminium sheets were firstly polished and then electrochemically etched with 1 mol L⁻¹ NaCl aqueous solution as electrolyte. After drying, some plates were directly modified by 1 wt% fluoroalkylsilane or stearic acid ethanol solution to lower surface energy, while the others were processed via boiling water for 20 min before modifications to produce dual structures, as shown in Fig. 1 and Table 1. The three groups of modified surfaces all showed superhydrophobicity with WCAs in excess of 150°. Superhydrophobicity of surfaces modified via fluoroalkylsilane and stearic acid was mainly contributed by –CF₂ and –CH₂ chains, respectively.^57,58

Fig. 1 Treatment process of aluminium sheets.

Table 1 Preparation process of SH surfaces

Groups	Fluoroalkylsilane modification	Stearic acid modification	Boiled water treatment
Group 1	✓
Group 2		✓
Group 3	✓		✓

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2.3 Hydrophilization

Each SH sheets was treated to become superhydrophilic by a nitrogen atmospheric pressure plasma jet (APPJ) described in our previous work⁵⁹ for 3 min, as shown in Fig. 1. Vertical distance between SH sheets and outlet of APPJ was set as approximately 8 mm. Frequency of the AC power supply was adjusted at 60 kHz, and the discharge power was approximately 3.0 W. Pressure of nitrogen (purity 99.999%) was firstly reduced by a pressure reducing valve (PRV), and flow rate of nitrogen was then controlled to be 12.0 standard litre per minute by the mass flow controller (MFC). After inducing nitrogen and applying high voltage, a plasma jet whose diameter and length were respectively 4 mm and 15 mm could be generated, and hyrophilization of SH surfaces was thereby realized.

2.4 Characterization

WCA measurements were carried out using an optical contact angle metre (DSA100, Krüss, Germany) at ambient temperature with a relative humidity of 30%. A 5 μL injector was used to dispense the water droplets and the investigation was repeated for at least 4 times. To investigate time stability of the plasma-induced hydrophilicity, the WCAs were measured from 0 h to 100 days.

Surface morphologies of aluminium sheets were observed by a scanning electron microscope (SEM, JSM-6360LV, Japan). Surface composition of the sheets was analysed by an X-ray photoelectron spectroscopy (Thermo ESCALAB 250Xi, USA), using a monochromatic aluminium Kα X-rays source (hν = 1486.6 eV) with the power being 150 W. The pressure of the analysing chamber was maintained to be 7.1 × 10⁻⁵ Pa after loading the sheets. Spectra were acquired with the take-off angle being 45°, and all binding energies were in reference to carbon (C 1s) at 284.8 eV. Curve fitting of the C 1s peak was accomplished by using XPS peak 4.1 software.

3. Results and discussion

Fig. 2 shows the variation in WCAs of different groups with storage time. As shown in the images, all surfaces became superhydrophilic after 3 min plasma treatment. Then, the treated surfaces were stored in open air with the temperature and relative humidity being room temperature and 30%, respectively. After storing for 4 days, sheets in group 1 and group 2 begun to recover, changing from superhydrophilic to hydrophilic; while that in group 3 was still superhydrophilic with a contact angle of 0°. When the storage time was 11 days, the sample in group 1 became hydrophobic with the WCA being approximately 95°; whereas that in group 2 remained to be hydrophilic and the WCA was less than 60°, and the one from group 3 was still superhydrophilic. After 15 days of storing, the original superhydrophobicity of the sheet in group 1 had almost fully recovered, as the WCA was nearly 150°; while WCA of that in group 2 did not significantly change, showing hydrophilicity, and WCA of the one in group 3 was still 0°. The wettability for each groups almost remained unchanged after further storage for as long as 100 days, as group 2 was still hydrophilic, and group 3 was still superhydrophilic.

Fig. 2 Variation in WCAs of sheets in different groups with storage time.

Fig. 3 shows surface morphologies of the etched superhydrophilic sheets before and after boiled water treatment, and the three groups of SH sheets before and after plasma treatment. It can be observed from the low magnification images that all surfaces have similar micro scale structures with some differences in details. From the high magnification surface morphologies of the three groups of SH sheets [Fig. 3(c2–e2)], it can be appreciated that the structures of sheets in group 1 and group 2 are still similar, while the samples in group 3 exclusively have some nano scale pin-like structures, namely, the dual structures. On the other hand, it can be easily found that there are no obvious structural differences between the merely etched surfaces, SH surfaces and plasma treated surfaces. This indicates that surface modification and plasma treatment did not evidently change surface structures, and that the changes of wettability should be attributed to variation of surface composition.^52,60 According to the previous research,²² the changes in chemical composition of the upmost layer can result in wettability variation or recovery. Therefore, time stability differences among the three groups of plasma treated sheets were presumably due to the differences in the recovery of surface composition. To further investigate this, XPS was conducted to characterize the surface chemistry of sheets from different groups.

Fig. 3 SEM images of surfaces. (a) Etched surfaces. (b) Etched surfaces after boiled water treatment. (c1–c2) Non-boiled SH surfaces modified by fluoroalkylsilane. (d1–d2) Non-boiled SH surfaces modified by stearic acid. (e1–e2) Boiled SH surfaces modified by fluoroalkylsilane. (f) Non-boiled SH surfaces modified by fluoroalkylsilane and treated by plasma. (g) Non-boiled SH surfaces modified by stearic acid and treated by plasma. (h) Boiled SH surfaces modified by fluoroalkylsilane and treated by plasma.

Fig. 4 shows XPS spectra of untreated SH sheets, sheets after plasma treatment and treated sheets stored for 100 days, respectively. The spectra of sheets from group 1 and group 3 both consist of C 1s, O 1s, F 1s, Si 2p and Al 2p, while the ones from group 2 are mainly composed of C 1s, O 1s and Al 2p. In addition, N 1s peaks around 400.7 eV could be observed from plasma treated sheets, which were probably contributed by N₂O or NO absorbed during plasma treatment.¹³

Fig. 4 XPS spectra of different sheets. (a) Non-boiled sheets modified by fluoroalkylsilane. (b) Non-boiled sheets modified by stearic acid. (c) Boiled sheets modified by fluoroalkylsilane.

The detailed contents of each element are firstly calculated to investigate the relations between element contents and surface wettability. As shown in Table 2, the fluorine content (F content) and oxygen content (O content) of the superhydrophobic surface in group 1 are respectively 10.4% and 26.4%, after plasma treatment, the F content plunges to 0%, while the O content increases to 37.5%, leading to the superhydrophilicity. Then, these contents almost recover back to the original state during long-time storage, which contributes to the wettability recovery.

Table 2 The relative contents (C, O and F) of samples in each group

Elements	Group 1			Group 2			Group 3
	Original	Treated	Stored	Original	Treated	Stored	Original	Treated	Stored
C	37.2	32.4	31.1	75.8	55.5	68.2	32	38.4	32.4
O	26.4	37.5	32.5	15.8	24.2	20.6	22.3	37.4	37.8
F	10.4	0	10.1	0	0	0	35.1	0	0.5

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The boiled sheets in group 3 show similar variation trends in element contents after plasma treatment, but the contents do not change obviously with long-time storage (the right three columns in Table 2), resulting in the relatively better time stability. On the other hand, for the stearic-acid modified surfaces in group 2, the carbon content (C content) and O content are 75.8% and 15.8%, respectively. These contents of the plasma-treated superhydrophilic surface change obviously, the C content decreases to 55.5%, whereas the O content increases to 24.2%. During long-time storage, the contents partially recover, but the C (O) content of the partially recovered hydrophilic surface is still lower (higher) than that of the superhydrophobic surface. The relations between wettability and contents are similar with the previous research,^61–65 which demonstrate that higher C contents and lower O contents are likely to result in better hydrophobicity. Since one peak in the full spectra might be induced by various functional groups with different characteristics, it might not be accurate to regard the relative intensities of different elements shown in these spectra as the only indicator for wettability. Therefore, high resolution XPS data of C 1s was collected to further investigate the differences in wettability changes among three groups.

Shown in Fig. 5 are the fitting results for the high resolution C 1s peaks of the original SH surfaces, plasma treated surfaces, and the treated surfaces with 100 days storage. The C 1s XPS spectra were deconvoluted into several components. The peaks at 293.6 ± 0.15 eV, 291.4 ± 0.2 eV and 290.2 ± 0.1 eV were respectively attributed to –CF₃, –CF₂ and C̲F₂–CH₂ groups, while the component at 288.8 ± 0.1 eV was assigned to carbonyl carbon in the ester group. The peak at 288.4 ± 0.1 eV was contributed by carboxylate moiety COO⁻ involved in aluminium stearate⁶⁶ and plasma treatment. The contributions of CF₂–C̲H₂ and C–O groups were correspondingly found at 286.7 ± 0.1 eV and 285.9 ± 0.2 eV, and the C 1s electrons from hydrocarbon (C–H/C–C and C–Si) had a binding energy of 284.8 ± 0.05 eV. The detailed contents of each group are shown in Table 3. The untreated SH surfaces in group 1 contained 24.8% fluorine containing groups (F-containing groups), contributing to the superhydrophobicity. After treated by plasma jet for 3 min, the F-containing groups were mostly removed, while content of oxygen containing groups (O-containing groups) increased to 21.8%, and the surface became superhydrophilic. Therefore, the wettability transition from SH to superhydrophilic should be due to the removal of hydrophobic F-containing groups and the inducing of hydrophilic O-containing groups. With the storage of 100 days, content of F-containing groups recovered to 21.2%, and that of O-containing groups plunged to 11.4%, thus resulting in the wettability recovery. The non-boiled surfaces modified by stearic acid in group 2 showed similar variation trends in relative contents of functional groups during plasma treatment: the hydrophobic C–H decreased while hydrophilic O-containing groups increased significantly. After the subsequent long-time storage, relative contents of functional groups partially recovered, which was corresponding to the incomplete wettability recovery from superhydrophilicity to hydrophilicity (Fig. 2). The higher C–C/C–H values are corresponding to better hydrophobicity, which is consistent with the previous research.⁶¹ Additionally, the boiled surfaces in group 3 also experienced similar variations in relative contents of groups during plasma treatment; however, the contents remained almost unchanged rather than recovered with long time storage, which should contribute to the interesting long-time maintenance of superhydrophilicity. According to the aforementioned data, the differences in changes of related functional groups should be responsible for the different wettability recoveries, i.e., dissimilar time stabilities of plasma treated surfaces. It should be meaningful to study the mechanisms causing the changes in surface chemistry.

Fig. 5 Peak-fitted C 1s of different surfaces. (a1–a3) Non-boiled surfaces modified by fluoroalkylsilane. (b1–b3) Non-boiled surfaces modified by stearic acid. (c1–c3) Boiled surfaces modified by fluoroalkylsilane.

Table 3 The relative concentrations (%) of the fitted peaks for C 1s of samples in each group

Moieties	Group 1			Group 2			Group 3
	Original	Treated	Stored	Original	Treated	Stored	Original	Treated	Stored
C–C/C–H/C–Si	68.8	78.2	67.5	94.6	65.1	83.1	45.4	77.2	77.2
C–O	1.7	10.5	6	0	13.9	7.1	5.6	16.1	10.6
–COO	0	0	0	5.5	21	9.8	0	0	0
C=O	4.7	11.3	5.4	0	0	0	5.3	6.7	9.2
CF2–C̲H2	5.9	0	5.4	0	0	0	8.2	0	3
C̲F2–CH2	4.2	0	3	0	0	0	4.7	0	0
–CF2	12.6	0	9.8	0	0	0	24.6	0	0
–CF3	2.1	0	3	0	0	0	6.2	0	0

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The recovery of plasma induced (super)hydrophilicity results from the tendency of treated surfaces to minimize energy and thereby maintain equilibrium.³⁷ When plasma treatment does not cause obvious surface damages, reorientation of hydrophilic polar groups from surfaces into the bulks and migration of untreated chains from bulks to the surfaces are considered to be the main factors resulting in the (super)hydrophobic recovery.⁴⁴ Since sheets in group 2 have similar structures with those in group 1, the difference of surface chemistry variation between them should be attributed to dissimilar modification methods. During the modification process, fluoroalkylsilane firstly hydrolyzes and then reacts with the –OH group on the aluminium oxide surfaces,^67,68 and the low energy group is mainly –CF₂; while stearic acid just chemically absorbs on the surfaces,^66,69,70 and the effective low energy group is –CH₂. After treated by plasma, the chemical absorption of stearic acid molecule is easier to be desorpted compared with the chemical bonds of fluoroalkylsilane. As a result, wettability recovery of stearic acid modified surfaces is partially retarded. Interestingly, the plasma treated superhydrophilic sheets in the two groups showed similar tendencies of wettability recovery during the first 4 days of storage, but behaved differently as the storage time increased (Fig. 2). The similar wettability recovery at first might be attributed to their similar reorientation of hydrophilic polar groups, for these treated surfaces had similar microstructures; while the interesting wettability differences after longer storage might be explained by their differences in migration of untreated hydrophobic chains. The migration process of these chains should be mainly determined by the dispersion forces between nonpolar groups remaining on the topmost layer, the nonpolar air environment and nonpolar groups from the untreated chains in the bulk. The nonpolar group of untreated chains of sheets in group 1 is mainly –CF₂, whereas that of group 2 is –CH₂. Since fluorine has stronger electronegativity, larger atom radius⁷¹ and better static polarizability^72,73 than hydrogen, –CF₂ should have better polarizability than –CH₂. As a result, dispersion forces between nonpolar groups of sheets in group 1 are stronger than the forces in group 2. F-containing chains of sheets in group 1 are therefore easier to migrate driven by the stronger interaction, thus causing the different behaviors during storage. Hence, reorientation might dominate wettability recovery of the two groups at first, and sheets of the two groups both recovered from superhydrophilicity to hydrophilicity. After that, migration of hydrophobic groups played a major role and the two groups consequently had different time stability. This was consistent with Bacharouche's study,⁷⁴ which reported that the reorientation was faster than the migration. Interestingly, after long time storage, the WCAs of sheets in group 2 (50–60°) kept to be approximately one-third of these of the original SH surfaces (more than 150°), and this was in accordance with the previous study on polymers, which reported that contributions of reorientation and migration to the wettability recovery were about respectively one-third and two-thirds.⁷⁵ Therefore, both surface structures and modification methods could have large influences on the time stability of plasma-treated surfaces, and this is consistent with the experimental results of Kietzig,²⁰ who claimed that the time variation of surface wettability is determined by surface morphology and surface chemistry.

The models shown in Fig. 6 were tentatively proposed to explain the excellent time stability of samples in group 3. As depicted in the models, the special dual nanostructures of sheets from group 3 along with plasma-induced crosslinks hindered the reorientation of polar groups and the migration of hydrophobic moieties. As a result, most plasma induced hydrophilic groups were trapped on the top layer of the surfaces, and the untreated hydrophobic groups could hardly migrate to the top layer, which could account for the long-time superhydrophilicity of plasma treated sheets in group 3. This might be somewhat inconsistent with the statements of Tompkins,³³ who found that crosslinks could not affect short-range rearrangement of plasma-induced polar groups. Presumably, crosslinks might have different influences on surfaces with different microstructures.

Fig. 6 Recovery models of (a) non-boiled and (b) boiled surfaces modified by fluoroalkylsilane.

To further investigate the influences of micro/nanostructures on time stability of plasma-treated surfaces, two other groups of aluminium sheets were prepared as follows: one group is ordinary aluminium sheets modified by fluoroalkylsilane, and the other one is firstly boiled and then modified by fluoroalkylsilane. The first group does not have micro or nano structures [Fig. 7(a1–a2)], while the other group has nanostructures [Fig. 7(b2)], but does not have microstructures [Fig. 7(b1)]. After modified by fluoroalkylsilane, contact angles of the ordinary aluminium sheet and the boiled sheet were approximately 104° and 140°, as shown in Fig. 7(c₀ and c₂). The two groups of sheets were then both treated by plasma jet for 3 min, and their variations of contact angles with time are shown in Fig. 7(c). After plasma treatment, contact angles of the non-boiled smooth surfaces quickly recovered to be hydrophobic within 12 hours, while the boiled surfaces with nanostructures remained hydrophilic for more than 480 hours (20 days). The obvious wettability difference is attributed to the dissimilar nanostructures of the two groups of surfaces. The nanostructures along with the crosslinks induced by plasma may hinder the reorientation and migration processes, thus contributing to the relatively better time stability.

Fig. 7 SEM images and time variations of WCA. (a1–a2) SEM images of non-boiled aluminium surface; (b1–b2) SEM images of boiled aluminium surface modified by fluoroalkylsilane, both without etching. (c) Time variations of WCA. The insert images are WCAs of (c₀) untreated ordinary aluminium surface, (c₁) plasma-treated ordinary surface stored for 12 h, (c₂) untreated boiled surface and (c₃) plasma-treated boiled surface stored for 480 h.

It was assumed that extreme wettability patterns prepared by plasma hydrophilization of SH surfaces corresponding to the three groups should have different time stabilities. To confirm this, these patterns were fabricated and their time stabilities were investigated. As shown in Fig. 8(a1 and a2), both patterns could realize directional transportation immediately after plasma treatment. After storing at room temperature for 10 days, plasma treated part modified by fluorine had recovered to be SH, thus resulting in invalidation of the pattern (Fig. 8(b1)). By contrast, the part modified by stearic acid was still capable of transporting water droplets to designated area (Fig. 8(b2)). Shown in Fig. 8(a3 and b3) are patterning surfaces consisting of two areas prepared by different processes. The boiled area showed better time stability than the non-boiled one. The longer time validation of these functional patterns corresponding to sheets in group 2 or group 3 should have great application potential in fields that require stable hydrophilicity. On the other hand, reducing recovery time of plasma treated surfaces is also important, as rapid wettability transition of plasma treated surfaces is sometimes desired. Our previous work⁵⁶ has realized rapid wettability recovery by heat treatment, and this paper attempts to accelerate recovery process under normal ambients. This has been realized in sheets from group 1. As shown in Fig. 8(c), plasma treated sheet from group 1 could recover to be hydrophobic within 3 hours, and was almost SH after 10 hours storage. Presumably, shorter treatment time results in less break of F-containing groups and less generation of O-containing groups, thus causing the variation in time stability of plasma treated SH surfaces. This should be focused on in further studies.

Fig. 8 Wettability patterns and rapid recovery of WCA. (a1–a2) Non-boiled patterns modified by fluoroalkylsilane and stearic acid, respectively. (a3) Fluoroalkylsilane modified patterns containing both boiled and non-boiled areas. (b1–b3) The patterns in (a1–a3) with 10 days storage. (c) Rapid recovery of WCA. The insert images are WCAs of (c₀) original SH surface and plasma treated surfaces after storing for (c₁) 0 h, (c₂) 1 h, (c₃) 3 h, (c₄) 6 h and (c₅) 10 h.

4. Conclusions

In this paper, time stability regulation of plasma treated SH surfaces has been successfully realized by different modification methods or microstructures. Surfaces modified by stearic acid and boiled surfaces treated by fluoroalkylsilane both showed much longer recovery time than non-boiled surfaces modified by fluoroalkylsilane. The SEM images showed the differences in microstructures between boiled and non-boiled surfaces, and the XPS results were consistent with the different wettability variations. Stearic acid modification might retard migration of hydrophobic groups, while the dual structures of boiled surfaces along with plasma-induced crosslinks could probably hinder both reorientation and migration processes. Recovery time of treated surfaces could be controlled as 10 hours or more than 100 days, and the time stability of plasma treated patterns could also be regulated for different requirements, which should be quite meaningful in practical applications that require different wettability recoveries.

Acknowledgements

The authors are grateful to National Natural Science Foundation of China (NSFC, Grant No. 51305060 and No. 51275072), National Basic Research Program of China (Grant No. 2015CB057304) and the Fundamental Research Funds for the Central Universities (Grant No. DUT15RC(4)21 and No. DUT15ZD241).

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