Robust superhydrophobic surfaces from small diffusion flame treatment of hydrophobic polymers

Abstract

When pure perfluorinated acrylic or natural wax-perfluorinated acrylic polymer blend coating surfaces are treated with small diffusion flames for a short duration, deposition of superhydrophobic nanostructured carbonaceous films is observed. The carbonaceous films display a good degree of surface binding which cannot be removed by impinging and rolling water droplets. The resultant surfaces have very low droplet roll-off angles and remarkable resistance to saturation against fast impacting water streams as well as to disintegration in ultrasonic bath treatment.

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Superhydrophobic surfaces fabricated by simple, inexpensive, rapid and ambient conditions have created a substantial amount of academic and industrial interest due to ease of fabrication and various potential industrial scale applications.^1–8 In some cases, these materials also display additional functionalities such as electrical conductivity, antimicrobial property, and mechanical resistance, tunable wetting, high temperature and solvent resistance as well as low ice adhesion.^9–20 Therefore, exploration of additional novel and simple methods of producing superhydrophobic surfaces is still highly attractive. To this end, nanostructured carbon materials would offer many possibilities as they can be synthesized by many diverse processes and in large quantities.^21,22

Amorphous low surface energy carbon nanoparticles can be deposited on surfaces stagnated against diffusion (non-premixed) flames.^18,23–27 Various surfaces have been used to synthesize such carbon nanostructure films such as metals, ceramics, silicon wafers etc.^28–30 Impinging diffusion flames on glass surfaces for instance generate an axial temperature distribution on the surface.³¹ Detailed flame–wall interaction measurements indicated that glass (silica) walls stagnated against small diffusion flames at an average distance of 3 cm experience surface temperatures around 200 °C.³¹ This temperature is not prohibitively high due to the fact that perfluorinated acrylics thermally degrade above 300 °C (ref. 32) and may allow treatment of polymer surfaces for short durations (a few seconds) so that nanostructured carbon films can be deposited on polymeric surfaces.

Earlier studies have shown that amorphous carbonaceous films (α-C) synthesized by impinging diffusion flames are highly hydrophobic but lack durability.³³ In other words, water droplets impinging on such structures can pick up and permanently remove these carbonaceous films as they roll over. However, surfaces coated with properly chosen polymeric materials can eliminate this drawback such that impinging nanostructured carbon nanoparticles can be embedded in the polymeric surface if the impinging flame temperature maintains a local polymer melt during deposition. In fact, this concept has been recently shown to render hydrophobic carbon nanotube–polymer composite surfaces superhydrophobic.¹⁸ Although the nanostructured carbon particles deposited on the carbon nanotube–polymer composite surface did not permanently adhere to the surface, the local melting of the polymeric matrix during flame impingement resulted in the restructuring of the surface texture and subsequent generation of superhydrophobicity after the carbon nanoparticles were washed away.¹⁸

In this study, we demonstrate that small diffusion flames such as the ones generated by cigarette or utility lighters can be used to form robust superhydrophobic nanotextured surface structures on hydrophobic polymer surfaces coated on glass. The superhydrophobic carbonaceous films display remarkable resistance to saturation against impacting water streams. The films were found to be saturated by ultrasonic processing when immersed in water. However, upon drying the superhydrophobicity was preserved.

Two types of polymer solutions were used for spray coating glass slides. First, a solution of perfluoroacrylic polymer in acetone and second, an emulsion of carnauba wax and the same perfluoroacrylic polymer. Polymer coatings were deposited on microscope glass slides using a standard airbrush atomizer with 3 bar air atomizing pressure and from a distance of approximately 10 cm. A commercial aqueous perfluoroacrylic polymer suspension named Capstone ST-100 was obtained from DuPont, USA.^1,18,19 It contains 20% polymer by weight in water. The perfluoroacrylic polymer, originally dispersed in water, was precipitated by mixing equal volumes of as received Capstone ST-100 and trifluoroacetic acid (TFA) at room temperature. After mixing a few minutes in a vortex mixer, the solid precipitate was collected by decanting the supernatant. The precipitate had an elastic rubbery form (chewing gum-like). It was washed several times with distilled water, dried and dissolved in acetone at 10% by weight.

The carnauba wax/perfluoracrylic emulsion was prepared by first dissolving carnauba wax flakes in hot toluene at 2.6% by weight. The wax solution was then mixed with the as-received aqueous Capstone ST-100 dispersion using a vortex mixer forming a partial emulsion. The polymer to wax weight ratio in the emulsions was adjusted to be 1. The homogenization of the emulsion was achieved by ultrasonic processing using a probe sonicator for 30 seconds (Sonics & Materials, Inc, USA. VC505, 20 kHz). Several standard microscope glass slides were coated with the polymer solution and the wax–polymer emulsions. The coatings were left to dry under ambient conditions for about 5 hours.

A butane utility lighter (First Light Ltd, UK) was used to deposit the amorphous carbon nanoparticles on the polymeric surfaces. The surfaces were stagnated against the small diffusion flame at an approximate distance of 3 cm. The flame impinged on the substrate for approximately 5 seconds to deposit a coating of amorphous carbon nanoparticles. The deposited coating was measured to be about 300 nm thick. The substrate was then moved over the flame gradually keeping the same impingement distance to deposit the nanoparticles on unexposed regions of the surface (Fig. 1a). These α-carbon films are well characterized in literature including XPS. In general, hydrophobic α-C films have been reported to contain a 50–71% sp² hybridized structure. Detailed XPS analyses conducted on such hydrophobic carbon films indicate that the sp³/sp² ratio for the two regions is generally around ≈0.80.³³ An sp² structure pertains to planar hexagonal graphitic rings while sp³ is the tetrahedral structure that is present in diamond. In order to form three-dimensional concentric spheres, the conversion of sp² C-bonds into sp³ bonds is required. The relatively large presence of sp³ bonds explains the preponderance of the spherical spongy nanobead structures. The absence of a catalyst in these experiments and in ours as well as nearly equal distribution of sp² and sp³ hybridized structures strongly indicates that these structures are amorphous.³³

Fig. 1 (a) Schematic representation of the small diffusion flame deposition of an amorphous nanostructured carbonaceous film on polymer coated glass slides. (b) Surface morphology of the as-deposited amorphous carbonaceous film after about 5 seconds of flame treatment on the perfluoroacrylic coating. (c) Transmission electron microscope (TEM) image showing randomly formed aggregates of deposited carbon nanoparticles.

A flat thermocouple was attached to the uncoated side of the glass slide to measure the temperature on the glass substrate during flame impingement. An average temperature of 180 °C was recorded at the end of 5 seconds of coating deposition. Fig. 1b shows a scanning electron microscope (SEM) image of the nanostructured amorphous carbonaceous films deposited on the polymeric surfaces just after flame deposition. Fig. 1c shows the transmission electron microscope (TEM) image of carbon nanoparticles in an aggregated form. Detailed TEM image analysis indicated that the typical particle sizes ranged from 20 to 50 nm.

Flame treated samples were left to cool down to room temperature and afterwards the films were washed under a running tap water at an average velocity of 1 m s⁻¹. Loosely attached particles were carried away by the impacting water stream. However, the surface of the samples was never wet by the impacting water stream. After this, the sample surfaces were air blasted at 3 bars to ensure no loosely attached carbon particles remained on the surfaces. These surfaces were used for the contact angle measurements. Twenty contact angle measurements with water droplets of about 10 μL in volume were performed on randomly selected locations on both the spray deposited polymeric and superhydrophobic surfaces. Fig. 2a shows, as an example, the morphology and the roughness of the wax–polymer blend films right after thermal curing at 120 °C and before flame deposition measured by an atomic force microscope (AFM) in non-contact mode. The inset in Fig. 2a shows the roughness profile of the films based on a standard grain analysis.³⁴ The average surface roughness of the thermally cured as-sprayed films was approximately 0.64 μm.

Fig. 2 (a) AFM morphology and roughness data for a spray cast and thermally cured wax–polymer film. The image is a square 10 μm × 10 μm in size. (b) All contact angle data. The uncertainty in contact angle measurements on polymeric films was approximately ±4° and in flame treated superhydrophobic surfaces was ±3°. Filled circular symbols correspond to polymer and unfilled triangles correspond to wax–polymer films for both as deposited and flame treated cases.

Fig. 2b shows the results of the contact angle measurements for the polymeric coatings before and after flame deposition of nanostructured carbon films. Both the perfluoroacrylic and the wax–polymer emulsion films are hydrophobic with static contact angles around 110°. As seen in Fig. 2b, the flame treated surfaces are all superhydrophobic after washing and air blasting the loosely attached carbonaceous layers. Droplet roll-off angles, measured as the tilt angle of the substrate at which the droplets roll off the surface, are also reported in Fig. 2b. A slight tilting of the substrate was sufficient for droplet mobility. All droplets readily rolled off the superhydrophobic surfaces when the tilt angle was around 3°.

In order to test the superhydrophobic durability of the samples against saturation, flame treated coatings were exposed to impinging water streams from a running tap water at different speeds. Typical impinging velocities from 1 to 4 m s⁻¹ were used. The samples were kept under impinging water streams for a duration of 1 min. Under all impinging velocity conditions no degradation in the degree of superhydrophobicity was observed; static water contact angles and droplet roll off angles were maintained after impingement. Fig. 3a shows an SEM image of a sample superhydrophobic surface (wax–polymer blend) after the water stream impingement experiment (3 m s⁻¹). As seen in the image the impinging water stream modified the surface morphology of the carbonaceous film which resembles a foam-like morphology as compared to Fig. 1b. Fig. 3b and c show the superhydrophobic water droplets before and after water stream impact. Droplet roll-off angles were also maintained around 3° as before. It must be mentioned that above 2 m s⁻¹ water stream impingement, some of the carbonaceous film from the pure perfluoroacrylic polymer coatings was removed. However, the films surfaces were still visibly black and maintained their superhydrophobicity as well as their low droplet roll of angles. Carbonaceous film on wax–polymer blend coatings remained visibly intact even above 2 m s⁻¹ impact speeds.

Fig. 3 (a) SEM image of a superhydrophobic carbonaceous surface deposited on a wax–polymer blend after the water stream impingement experiment (3 m s⁻¹). (b) A sessile water droplet on the surface before water stream impingement. (c) A sessile water droplet on the surface after water stream impingement.

Finally, the durability of the superhydrophobic surfaces against saturation by ultrasonic bath processing was tested. The samples were kept in an ultrasonic processing bath (59 kHz, Falc Instruments, Italy) for 20 seconds under 6 cm of water in a glass container during sonication. Upon immersion into the water bath all films maintained a mirror like sheen due to the saturation resistance of the films to water immersion.^35,36 Once the sonication started, however, the mirror like sheen due to air cushion was lost and the surfaces were all wet after 10 seconds of sonic processing. Small amount of carbon nanoparticles were also released into the solution during sonication but there were no visible differences in the continuity of the carbonaceous coating on the surfaces before and after sonication. Fig. 4a shows as an example the AFM surface morphology of the flame treated superhydrophobic wax–polymer surface after sonication in the water bath for 10 seconds. The nanostructured features are due to the presence of carbon nanoparticles on the surface.

Fig. 4 (a) Morphology of the carbonaceous superhydrophobic surface deposited on wax–polymer blend coatings after sonication for 20 seconds in an ultrasonic bath. (b) Comparison of roughness grain analysis results of the same superhydrophobic coating in (a) before and after sonication. As seen, the average roughness (flat yellow line) of the superhydrophobic carbonaceous films remains practically the same at approximately 174 nm.

Melting of the polymer and/or the wax during flame impingement might allow the nanoparticles to get embedded into the polymer film and establish a better binding carbonaceous film compared to a film deposited on a ceramic or metallic surface which can be picked up by the rolling droplets (self-cleaning). Fig. 4b compares the surface roughness measurements on this superhydrophobic surface before and after being placed in an ultrasonic bath using a standard grain analysis. The average surface roughness of both surfaces remained at 174 nm. Although surface roughness of the film before and after sonication remained stable, its superhydrophobicity was lost during sonication. However, once the sample was dried and heat treated in an oven at 120 °C for a half an hour, the surfaces recovered their superhydrophobic state as well as low droplet roll-off angles. Although no superhydrophobic resistance against ultrasonic immersion was observed and the surfaces were saturated, the carbonaceous films on the flame treated coatings were not removed or even cracked by the ultrasonic treatment. This is considered quite satisfactory since the carbonaceous film is an external nanostructured deposit from the diffusion flame. The films show a certain degree of mechanical durability against soft abrasion. Rubbing the surfaces with a material having a Shore A hardness of 70–75 for several times do not degrade the superhydrophobicity, although some of the carbonaceous film is removed. The films do not have similar abrasion resistance reported in our earlier works¹³ (see Fig. S1 in the ESI†).

In conclusion, amorphous carbon nanoparticles deposited on hydrophobic polymer surfaces by flame synthesis³³ form saturation resistant superhydrophobic films which can withstand impacting water streams as well as ultrasonic bath processing by immersion. Surfaces demonstrate very high sessile droplet contact angles with low droplet roll off angles after undergoing water stream impacts and sonication. Ultrasonic processing does not remove or crack the nanostructured carbonaceous films indicating strongly bound nanoparticles to the polymeric films possibly due to local melting of the polymer during flame impingement. Further work will focus on the fundamental investigations on the interaction between the amorphous carbon nanoparticles and the polymeric surfaces during synthesis.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra44169e

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