“Lotus-effect” tape: imparting superhydrophobicity to solid materials with an electrospun Janus composite mat

Abstract

Making superhydrophobic structures on the surface of materials is always an intricate and various subject, which always requires special facilities and techniques. Herein, a “lotus-effect” tape (LET) was designed to impart solid materials with superhydrophobicity using a technique as simple as taping. The so-called LET was prepared with a Janus structure using dual-nozzle electrospinning, and consisted of a lotus-effect upper layer and a thermo-cohesive bottom layer. The LET can be pasted tightly onto the surface of various substrates via an ironing treatment with a household flatiron. The lotus-effect property was then endowed to the substrates. It is worth noting that the LET can also be detached easily from the substrate involved, but without any damage to the original surface of the substrate. This work provides a novel strategy to impart lotus-effect properties onto various materials without the limitations of special facilities and techniques.

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Introduction

Superhydrophobic surfaces can be found almost everywhere in nature,¹ in particular, the leaves of the lotus plant display an amazing self-cleaning effect, wherein water droplets can easily roll off and pick up dirt particles.^2,3 The secret of the so-called lotus-effect lies in the low surface energy wax covering on the hierarchical micro/nanostructure of the leaves, which allows air to be trapped, acting as a cushion for the water droplets.^4,5 Inspired by this natural wisdom, various materials with lotus-effect surfaces, i.e. with a water contact angle (CA) greater than 150° and a sliding angle (SA) lower than 10°,^6,7 have attracted considerable attention from scientific and industrial communities. Many review papers have summarized the various methods to fabricate hierarchical micro/nanostructures on different materials and the possible approaches to reduce the surface energy.^8–13

However, the current fabrication methods for lotus-effect surfaces sometimes depend on the surface structures of the substrates, where the required hierarchical micro/nanostructure should be constructed on a smooth surface,^14–17 and at other times the chemical composition has an effect as low energy chemical agents should be deposited on high surface energy materials.^18–21 These processes always require special facilities and techniques, and even skilled workers. Getting rid of the limitations of facilities and techniques is a new route worth exploring in order to fabricate lotus-effect surfaces independent of the chemical and physical properties of the pristine substrates. On the other hand, whereas many studies have considered the fabrication and repair of the lotus-effect surface, not as many have investigated their removal.^22,23 Indeed, when the lotus-effect surface of a material is destroyed during practical use, it is hardly repaired under site conditions unless it is returned to a factory. If the destroyed lotus-effect surface on the substrate can be replaced by a new surface easily at site, it would provide much convenience in practical use and would allow the broad application of superhydrophobic materials or devices.

As we know, various kinds of tapes are used in daily life, with a plastic or aluminium film as the upper layer and a bottom layer coated with a glue, which can be easily pasted onto the surface of different materials such as cardboard, wood, glass, metals, and so on. Inspired by this strategy, we plan to construct a “lotus-effect tape” (LET), which could be used to impart a lotus-effect surface on various materials by simple taping, and then be completely detached through a simple treatment, releasing the pristine surfaces of the substrate, when the LET is destroyed in use. For this purpose, a Janus structure of the LET is needed, which consists of a superhydrophobic upper layer and a cohesive bottom layer.

Recently, various superhydrophobic materials with hierarchical micro/nanostructures prepared using an electrospinning technique have been attractive²⁴ and applied in many fields, including membrane distillation,^25–27 biomaterials,²⁸ oil/water separation,^29–31 air filtration,^32,33 self-cleaning,^5,34,35 and so forth. For instance, X. Wang et al. prepared a stable superhydrophobic organic/inorganic composite nanofibrous membrane for the direct contact membrane distillation of saline water by electrospinning hydrophobic silica nanoparticles (hSiO₂ NPs) and poly(vinylidene fluoride) (PVDF) mixed colloids. Benefiting from the utilization of hSiO₂ NPs, the electrospun nanofibrous membranes were endowed with high porosity and superhydrophobic properties, which resulted in excellent waterproofing and breathability.²⁵ Meanwhile J. Lin et al. presented a surface of fibres that was prepared via electrospinning a polystyrene solution containing silica nanoparticles exhibiting a peculiar structure with the combination of nano-protrusions and numerous grooves due to the rapid phase separation in electrospinning. The amount of silica NPs incorporated into the fibres was proved to be the key factor affecting the fibre surface morphology and hydrophobicity.⁵

Herein, we prepared a Janus composite mat consisting of a lotus-effect upper layer and a thermo-cohesive bottom layer. A low surface energy polymer, poly(vinylidene fluoride) (PVDF), together with low surface energy nanoparticles consisting of fumed SiO₂ modified by hexamethyl disilazane (denoted as mSiO₂), were electrospun into mSiO₂@PVDF ultrafine fibres to fabricate the lotus-effect upper layer. However, the low surface energy of the electrospun mats also represents an obstacle for making tight cohesion with the substrate using ordinary glue. In order to overcome this issue, we employed a dual-nozzle electrospinning process,^36,37 in which poly(vinyl acetate) (PVAc), a thermo-cohesive and thermoplastic polymer with low softening and melting points (38 and 65 °C, respectively),^38,39 was also electrospun into the fibres and mixed with the PVDF fibres to comprise the bottom layer of the composite mat. The resultant mat with a mSiO₂@PVDF upper layer and a PVDF/PVAc bottom layer was pasted onto various substrates such as glass, paper, wood, plastics, and aluminium (Al) foil by ironing with a household flatiron. The micro-morphology and lotus-effect property of the Janus composite mats after ironing were investigated and the peel strength of the Janus composite mats pasted onto the substrates, the detaching process and the recyclability were also carefully evaluated.

Experimental

Materials

Poly(vinylidene fluoride) (PVDF, M_w = 420 000) in powder form was purchased from Solvay Chemicals Co. (Belgium). Poly(vinyl acetate) (PVAc, M_w = 30 000–50 000), dimethyl formamide (DMF) and anhydrous alcohol were purchased from Sinopharm Chemical Reagent (China) Co., Ltd. Hexamethyl disilazane modified fumed silica (mSiO₂) (trade name TS530) was obtained from Cabot (China), Ltd. as a gift.

Fabrication of Janus composite mats

PVDF solution (10 g of PVDF powder dissolved in 90 g of DMF, 10 wt%) was selected as a precursor solution, and mSiO₂ was added at mass ratios (mSiO₂/PVDF) of 0, 0.5, 1.0, 1.5, and 2.0. The mixtures were stirred vigorously for 24 h at 60 °C for complete dispersion. PVAc solution (20 wt%) was prepared by dissolving 20 g of PVAc in 80 g of DMF. Here, PVDF, mSiO₂/PVDF and PVAc solutions are the feed solutions of the subsequent electrospinning process. The setup of the two-nozzle electrospinning process can be found in our previous report.³⁷ The electrospun mats with Janus structures were prepared using the following procedure. Firstly, PVAc and PVDF fibres were electrospun respectively from two different needles, which formed a bottom layer by interpenetrating PVAc and PVDF fibres to give a network, and then mSiO₂/PVDF fibres with different mSiO₂ amounts were electrospun subsequently from the bottom layer surface, and served as the upper layer of the whole mat. The distance between the needle tip and the top edge of the drum was 15 cm; the applied voltage was 12.5 kV; the flow rate of all liquids was 0.4 mL h⁻¹ and both the bottom and upper layer were electrospun for 12 h.

Analyses

The surface morphology of the Janus composite mats was observed using a field-emission scanning electron microscope (FESEM, Hitachi S-4800, Japan). The samples were sputter-coated with a 10 nm gold layer and examined at an acceleration voltage of 10 kV. The three-dimensional morphology of the fibres was obtained using an atomic force microscope (AFM, E-Sweep, Seiko Instruments Inc., Japan) in taping mode. Contact angles (CAs) were measured using an Attension Theta system (KSV Instruments Ltd., Finland), with a volume of 5 μl of deionized water. The average value of five measurements performed at different positions on the same sample was adopted as the contact angle. The inclination angles of the samples were gradually increased until the water droplets rolled down, then the sliding angles (SAs) were obtained. The bond strength of the Janus composite mats on the substrates was tested using a 90° peel strength testing machine (MK-BL-X90, MaiKe Instruments Ltd., China). The chemical stability of the Janus composite mats was also tested. The mats were completely soaked in aqueous solutions with different pH values (prepared with HCl or NaOH) and saturated saline (NaCl) aqueous solution for 12 h. And the contact angles were measured after washing and drying.

Results and discussion

Fig. 1a shows that no matter what the mass ratio of mSiO₂/PVDF in the upper layer fibres of the Janus composite mats is, the water contact angles (CAs) are all in excess of 150°, and increase as the mass ratio increases gradually. We also find that the droplets of some common liquids, including milk, orange juice and coffee can stand on the surface of the mats as almost a perfect ball (see the inset of Fig. 1a). Interestingly, the sliding angles (SAs) of the water droplets on the mats decrease monotonically as the mass ratio increases and when the mass ratio is up to 1.0, the SAs of the mats are less than 10°, which is an example of the lotus-effect of superhydrophobic behaviour. Water droplets, as well as milk, orange juice and coffee droplets, stuck firmly to the mat even when it was turned vertically upright (Fig. S1†), although the CA of the water droplets on the PVDF mat reached 155°. This indicates that the surface of the PVDF mat shows the “rose petal effect” of superhydrophilic behaviour, in which the surface has a strong adhesion to water,^18,40 and a mat with a lotus-like surface can be prepared by introducing mSiO₂ NPs in the fibres of the upper layer.

Fig. 1 (a) Water contact angles and sliding angles of electrospun Janus composite mats with different mass ratios of mSiO₂/PVDF in the upper layer fibres; the inset is a digital picture of different liquid droplets on the Janus composite mat (mass ratio: 1.5). (b) The stability of Janus composite mats (mass ratio: 1.5) and PVDF mats in aqueous solutions with different pH values.

In order to evaluate the chemical stability of the Janus composite mats, we immersed the mats completely in aqueous solutions with different pH values and saturated saline solution as shown in Fig. S2.† As Fig. 1b shows, after immersion in a strong acid (pH = 0) or alkali (pH = 14), the CAs of the Janus composite mats decreased by about 5° in comparison to those before immersion (ca. 170°), but were still much higher than 150°. In contrast, the CAs of the PVDF mats decreased by around 10°, and the CAs of the PVDF mats after strong acid and alkali immersion went beneath 140°. The same trends were also found from the results of the saline solution immersion (Fig. S3†). This indicates that introducing mSiO₂ into the PVDF fibres is helpful to enhance the chemical stability of the composite mats.

Generally, the microstructural morphology of a material plays a very important role in the material’s surface wettability, and the hierarchical micro/nanostructure is an indispensable feature of materials constructed with a lotus-effect surface.³ Here, we investigated the micromorphology of the obtained electrospun composite mats using field emission scanning electron microscopy (FE-SEM). As Fig. 2a and b show, the PVDF mat is a web of PVDF fibres of smooth appearance and with uniform diameters around 400 nm. After introducing mSiO₂ NPs into the PVDF fibres, the fibres became more and more rough as the amount of mSiO₂ NPs (mass ratio) increased (Fig. 2c and d, and S4a–f†). Fig. 2d shows that there are many nano-protrusions and numerous grooves pervading over the mSiO₂@PVDF fibres’ surface, which formed during the rapid phase separation while electrospinning the mSiO₂/PVDF mixture solution.⁵ When the mass ratio is 0.5, some spindle-like knots and roe-like aggregates embed into the line of the mSiO₂/PVDF fibres, and the diameter of some fibres was increased to 1000 nm (Fig. S4a and b†). These mSiO₂/PVDF fibres were further thickened to about 1800 nm in diameter when adding mSiO₂ in a mass ratio of 1.0, and the fibres exhibited a more uniform diameter and no spindle-like knots appeared (Fig. 2c and d). It could be reasoned that mixing mSiO₂ NPs into the PVDF solution has increased the non-volatile ingredients and reduced the fluidity of the mixture solution (due to the excessively high surface area of the mSiO₂ NPs), which results in the diameters of the fibres becoming thicker. When the mass ratio was increased to 1.5 and 2.0, the fibres got more and more defects and had uneven diameters, with some big agglomerates scattered in the composite mats (Fig. S4c–f†). It is possible that the fluidity of the mixture solution was reduced dramatically by introducing too many mSiO₂ NPs, which leads to the electrospinning jets becoming unstable and intermittent. However, the surface of the fibres are completely covered with nano-protrusions, which is the reason that the SAs of the Janus composite mats are lower than 10°.

Fig. 2 SEM images of the upper layers of the electrospun PVDF mat (a, b) and the Janus composite mat with a 1.0 mass ratio of mSiO₂/PVDF in the upper layer fibres (c, d). AFM 3D images of a fibre in the PVDF mat (e) and (f) the upper layer of the Janus composite mat (mass ratio: 1.5).

Furthermore, we also used an atomic force microscope (AFM) to observe the hierarchical micro/nano-structure of the Janus composite mats’ surfaces. As the AFM 3D images of a fibre in the PVDF mat (Fig. 2e) and the upper layer of the Janus composite mat (Fig. 2f) show, there is an apparent contrast with the uniform thickness and smooth surface of the pure PVDF fibre, as the as-prepared mSiO₂@PVDF fibre shows marked fluctuations on the surface, which are due to the accumulation of mSiO₂ NPs in the undulating surface of the fibres.

As mentioned above, the Janus composite mat here consists of an upper layer prepared by electrospinning a mSiO₂/PVDF mixture solution and a bottom layer obtained by the dual-nozzle electrospinning of PVDF and PVAc solutions. The SEM image of the bottom layer (Fig. S5†) shows that the micromorphology of the fibres in the bottom layer is thinner and smooth, very different from that of the upper layer, which clearly confirms the Janus structure of the composite mats, with not only a chemical composition difference between the upper layer and the bottom layer. Considering that PVAc is thermo-cohesive and is an effective constituent of organic glues, we designed a simple approach to attach the Janus composite mat onto the substrate as illustrated in Fig. 3a. The Janus composite mat was laid onto a substrate, such as a glass slide, wood or paper, then ironed using a household flatiron under about 3.6 kPa of pressure, at a pre-set temperature and for a pre-set duration. After ironing treatment at 120 °C for 30 s, the Janus composite mat was tightly pasted onto a glass slide and could not be peeled off even with two 200 g weights hanging on the end of the mat (Fig. 3b). Moreover, the water droplets easily rolled off from the surface of the Janus composite mat pasted glass slide, which was laid on the floor with an inclination angle of about 5° (Fig. S6†). This demonstrates that the lotus-like surface can be effectively imparted to substrates via an ironing treatment with the Janus composite mat. In other words, the Janus composite mat works as a “lotus-effect” tape (LET) to transmit a lotus-like surface to the substrates. For brevity, the Janus composite mat is hereafter expressed as LET instead.

Fig. 3 (a) Illustration of the process of imparting substrates with a lotus-like surface from the Janus composite mat (mass ratio: 1.5) by ironing treatment; (b) demonstration of the high peel strength of the Janus composite mat (mass ratio: 1.5), performed by hanging two 200 g weights; the effect of (c) temperature and (d) duration of the ironing treatment on the peel strength of the Janus composite mat (mass ratio: 1.5) pasted onto a glass slide; and (e) comparison of the peel strength of the Janus composite mat (mass ratio: 1.5) and commercial tape pasted onto different substrates.

The strength of the LET pasted substrate was measured by means of peel strength tests in the direction of 90°. Fig. 3c and d show the influence of the ironing temperature and duration on the peel strength of the LET from a glass slide. It can be found that the peel strengths shows a strong dependence on the ironing temperature, where a high ironing temperature results in high peel strengths (Fig. 3c). Meanwhile the duration of the ironing treatment has a relatively small effect on the peel strength, and the maximum can be obtained after about 30 s (Fig. 3d). It means that the LET can be rapidly (just within 60 s duration) pasted onto substrates by ironing treatment, which is a good signal for practical usage. The optimized conditions for the ironing treatment of the LET on a glass substrate is at 120 °C for 30 s, which leads to a resulting peel strength of up to 232 N m⁻¹. This strength is equivalent to saying that a 2.5 cm wide strip of LET pasted onto glass could withstand a weight of 580 g in the vertical direction, which was demonstrated by using two 200 g weights, as shown in Fig. 3b. As a comparison, PVAc glue was previously coated onto the surface of glass and a single-layer mSiO₂@PVDF electrospun mat, and after drying was ironed at 120 °C for 30 s. But the resulting peel strength was only 57.5 N m⁻¹, much lower than that of the LET. Moreover, the peel strength of the LET was compared with that of commercial tape attached on glass, wood, plastic, and aluminium (Al) foil, and the results show that the interactions between LET and the substrate after ironing treatment are much stronger than those measured for the commercial tape pasted at room temperature (Fig. 3e).

The ironed LET was peeled off from the glass slide and its upper and bottom layer were characterized using FE-SEM. Fig. 4a shows that the micro-morphology of the upper layer is almost unchanged compared with that before ironing (Fig. 2c). That is because the melting point (ca. 175 °C) of PVDF is much higher than the ironing temperature (120 °C), so the mSiO₂@PVDF fibre in the upper layer can withstand and keep its shape during the ironing process. But after the ironing treatment, some fibres in the bottom layer were transformed to a melt lump, and others still kept their fibre form and were stuck together by the melt lump (Fig. 4b), which is very different to the pristine bottom layer (Fig. S5†). The reason for this can be illustrated by Fig. 4c. As described previously, the bottom layer consists of PVDF fibres and PVAc fibres. The PVAc fibre can be melted at the ironing temperature due to its melting point (ca. 65 °C) being much lower than the ironing temperature. The melted PVAc fibres turn into a semi-continuous phase, gluing the PVDF fibres and substrate together tightly, which contributes to the high peel strength of the LET with the substrates.

Fig. 4 SEM images of the (a) upper and (b) bottom layer of the Janus composite mat (mass ratio: 1.0) after ironing treatment at 120 °C for 30 s. (c) The schematic of the Janus composite mats being pasted onto the substrates by ironing treatment. (d) The lotus-effect on the glass slide pasted with the Janus composite mat (mass ratio: 1.5).

It is particularly gratifying that the lotus-effect properties of the LETs were retained after being pasted onto the glass slides by ironing treatment. As Fig. 4d shows, the water droplet can roll off rapidly from the surface of the LET-covered glass slide that was laid with an inclination angle of about 4.3°, which is similar to that of the LET before the ironing treatment (Fig. 1a). Dust particles, such as carbon black, can be easily carried off from the LET-covered glass slide by the rolling water droplets (Fig. S7†). Those should be attributed to that the hierarchical microstructure of the mSiO₂@PVDF fibres in the upper layer of the LET were not damaged during the ironing process. Therefore, the LET coupling with the ironing treatment is a simple way to impart lotus-effect properties to substrate surfaces. Furthermore, the LET can also be applied onto soft substrates such as paper. A water droplet on the LET-covered area of printed paper can roll over freely (video S1 in ESI†), which means the lotus-effect property has been imparted onto the surface of paper.

As far as we know, the superhydrophobic structures constructed on the substrates by conventional methods are almost disposable, and they are very difficult to be refreshed without damage to the substrates. But then, this problem is not a handicap yet for the LET. As is shown in Fig. 5a and b, the LET covered glass slide was soaked in ethanol at room temperature for 30 s, and the LET can be detached easily and completely from the glass slide, without any damage. This is owed to the alcohol that is able to swell the PVAc and reduce the surface adhesion energy of the PVAc and the glass slide (Fig. 4c), which results in the exfoliation of the LET.

Fig. 5 (a) Soaking of the LET-pasted glass slide in ethanol for 30 s. (b) Detaching the LET from the glass slide after soaking in ethanol. (c) The CA and SA of the LET-covered glass slides prepared with detached LET. (d) Peel strength of the LET-covered glass slides prepared with the detached LET. (e) SEM image of the bottom layer of the LET after 5 reusing cycles.

Considering that PVAc still exists within the bottom layer of the LET, the recyclability of the detached LET was tested by repeating the process of ironing and alcohol exfoliation. Fig. 5c shows that the CA of the LET-covered glass slide is decreased, and the SA is increased slightly as the number of recycling cycles increases. After recycling 6 times, the CA decreases to around 157°, and the SA increases to 6.5°, which indicates that the lotus-effect of the LET is still retained after recycling multiple times. It also can be confirmed by the SEM image of the upper layer of the LET after recycling 5 times (Fig. S8†), which shows that the morphology of the nano-protuberances that comprise the hierarchical network structure is retained, keeping almost the same appearance as in the as-prepared sample shown in Fig. S4d.† However, the peel strength of the LET-covered glass slides prepared with the detached LET obviously decreased, and after recycling 5 times it reached a value less than half that of the fresh LET, and close to the corresponding value of the coated PVAc (Fig. 5d). The SEM image of the bottom layer of the LET after 5 reusing cycles, presented in Fig. 5e, shows that PVAc appears mainly as a film attached to the bottom layer, which means that most PVAc in the bottom layer was deposited from the network of fibres, due to the repeated melting processes. Therefore, the LET can be reused 5 times at least to build up a lotus-effect surface on substrates after preparation.

The recycling experiment on the smooth and rigid surface of a glass slide may not be able to comprehensively prove thesuccess of the detaching process. A LET-covered paper was prepared by ironing treatment (Fig. 6a), and subsequently, some alcohol was sprayed onto the back of the LET-covered paper (Fig. 6b). The LET was completely detached from the paper and the ink-printed logo in the centre was still clear (Fig. 6c), even though some patterns at the top and bottom appeared blurred due to being soaked by the water droplets. This confirms that the detaching procedure using alcohol causes only minimal damage to various substrate surfaces, even soft and vulnerable substrates.

Fig. 6 (a) A LET-covered paper with printed logos of SINAP (Shanghai Institute of Applied Physics, Chinese Academy of Sciences). (b) The LET-pasted paper after having been sprayed with alcohol on the back. (c) The restored paper and LET after detaching.

Conclusions

In conclusion, a Janus composite mat with a lotus-effect upper layer and a thermo-cohesive bottom layer was designed and it is really able to impart its lotus-effect property onto substrates through a technique as simple as taping assisted by ironing. The hierarchical microstructure of the mSiO₂ embedded PVDF fibres contributes to the lotus-effect property of the upper layer, and the PVDF and PVAc fibre blended network prepared by dual-nozzle electrospinning transmits the ability of thermo-cohesion to the bottom layer. In this manuscript, we showed that the Janus composite mat can be pasted onto various substrate surfaces tightly and impart its lotus-effect property onto substrates easily through an ironing process. Interestingly, the LET can be easily detached from the substrate by soaking or spaying with alcohol, but brings almost no damage to the surface of the substrate, no matter whether it is a smooth and rigid or rough and soft substrate. Furthermore, the detached LET can be reused at least 5 times to build up a lotus-effect surface on a substrate by simply repeating the ironing process. All in all, the approach reported here definitely provides a novel strategy to impart lotus-effect properties to various substrates without the limitation of special facilities and techniques.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (Grants No. 51473183, 11305248 and 11475246), and the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA02030200). We sincerely appreciate Dr Yue Huang from Cabot (China), Ltd. for the kind supply of mSiO₂ NPs as a gift.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Pictures of various liquid droplets sticking on the electrospun PVDF mat surface; pictures of the immersion of the as-prepared mats in saline solution and aqueous solutions with different pH; contact angles of the electrospun mat before and after chemical treatment; SEM images of the mats; pictures of water droplets rolling off a LET-covered glass slide and so on. See DOI: 10.1039/c5ra24632f

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