Method for manufacturing a superhydrophobic surface, surface so obtained and use of it

Abstract

The invention is directed to a method for manufacturing a hydrophobic or superhydrophobic surface comprising the steps of: (a) providing a substrate with a surface roughness R.sub.a between 0.1 and 1.0 m and (b) exposing the substrate to a filamentary atmospheric pressure dielectric barrier discharge plasma which is fed by a reaction gas and siloxane-forming material in order to form a superhydrophobic siloxane layer over at least a portion of the surface of the substrate. Step (b) is operated with an electrical excitation frequency of 15,000 Hz to 35,000 Hz and a power density between 0.5 to 10 W.Math.cm.sup.2. The siloxane layer produced in step (b) shows thereby a micro-structure and a nano-structure with droplet sticking properties (high water sliding angle).

Claims

1. A method for manufacturing a superhydrophobic layer, said method comprising: (a) providing a substrate with a surface; (b) exposing the surface to a filamentary atmospheric pressure dielectric barrier discharge plasma which is fed by a gas flow comprising reaction gas and siloxane-forming material in order to form a superhydrophobic siloxane layer over at least a portion of the surface, wherein the surface of the substrate in step (a) has a roughness R.sub.a between 0.1 and 1.0 m, and wherein the filamentary atmospheric pressure dielectric barrier discharge plasma is operated with an electrical excitation frequency of 15,000 Hz to 35,000 Hz and a power density between 0.5 to 10 W.Math.cm.sup.2.

2. The method according to claim 1, wherein the concentration of the siloxane-forming material in the gas flow comprises between 50 and 10,000 ppm.

3. The method according to claim 2, wherein the superhydrophobic siloxane layer shows a micro-structure and a nano-structure.

4. The method according to claim 3, wherein the substrate is aluminum foil and the roughness R.sub.a between 0.1 and 1.0 m is achieved by cold-rolling.

5. The method according to claim 3, wherein the substrate comprises at least one of copper, steel, cardboard, wood, plastic, polypropylene fabric and glass.

6. The method according to claim 5, wherein the flow of gas comprises less than 500 ppm of oxygen.

7. The method according to claim 6, wherein the flow of gas comprises nitrogen.

8. The method according to claim 7, wherein the electrical excitation frequency is between 20,000 Hz to 30,000 Hz.

9. The method according to claim 8, wherein the siloxane-forming material is at least one of a organosilicon gas, vapour, liquid and aerosol.

10. The method according to claim 9, wherein the siloxane-forming material is selected from a group consisting of hexamethyldisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane, hexaethyldisiloxane, tetraethylorthosilicate, aminopropyltrimethoxysilane, tetramethyldisiloxane, pentamethylcyclopentasiloxane, octamethylcyclooctasiloxane, polydimethylsiloxane and its derivatives.

11. The method according to claim 10, wherein the siloxane layer comprises agglomerated particles having a size between 50 and 750 nm.

12. The method according to claim 11, wherein the filamentary atmospheric pressure dielectric barrier discharge plasma is operated with a power density between 1 to 5 W.Math.cm.sup.2.

Description

DRAWINGS

(1) FIG. 1 illustrates the contact angle of a water droplet on a hydrophobic surface and on hydrophobic surfaces according to the Wenzel and Cassie-Baxter models.

(2) FIG. 2 illustrates a hierarchical structure or double roughness of a superhydrophobic surface.

(3) FIG. 3 is a schematic view of a filamentary atmospheric pressure dielectric barrier discharge reactor.

(4) FIG. 4 shows water droplets on a first surface at different orientations, the surface being a siloxane film produced on a cold-rolled aluminium foil at electrical excitation frequency of 10,000 Hz according to a first sample (sample 2 of table 1) which is not part of the invention.

(5) FIG. 5 shows water droplets on a second surface at different orientations, the surface being a siloxane film produced on a cold-rolled aluminium foil at electrical excitation frequency of 22,500 Hz according to a second sample (sample 5 in table 1) in accordance with various embodiments of the invention.

(6) FIGS. 6 to 8 are a set of a scanning electron micrographs of the siloxane film produced on a cold-rolled aluminium foil at electrical excitation frequency of 10,000 Hz according to the first sample.

(7) FIGS. 9 to 11 are a set of a scanning electron micrographs of the siloxane film produced on a cold-rolled aluminium foil at electrical excitation frequency of 22,500 Hz according to the second sample.

(8) FIGS. 12 to 14 are a set of a scanning electron micrographs of the siloxane film produced on a polished aluminium foil at electrical excitation frequency of 22,500 Hz, this being a third sample (sample 4 in table 1) not being part of the invention.

(9) FIG. 15 is a Fourier transform infrared spectroscopy (FTIR) of the films according to the first and second samples, i.e. with a 10,000 Hz and 22,500 Hz respectively electrical excitation frequency on the cold-rolled aluminium foil.

(10) FIG. 16 is a confocal microscopy 3D topographic image of the siloxane film of FIGS. 9 to 11, formed according to the second sample, i.e. with a 22,500 Hz electrical excitation frequency on the cold-rolled aluminium foil.

(11) FIG. 17 is a confocal microscopy 3D topographic image of the siloxane film of FIGS. 12 to 14, formed according to the third sample, i.e. with a 22,500 Hz electrical excitation frequency on the polished aluminium foil.

(12) FIG. 18 is an atomic force microscopy (AFM) 3D topographic image of the films of FIGS. 6 to 8, formed according to the first sample.

(13) FIG. 19 is an atomic force microscopy (AFM) 3D topographic image of the films of FIGS. 9 to 11, formed according to the second sample.

(14) FIG. 20 shows for both first and second samples the measured gas applied external voltage waveforms.

(15) FIG. 21 shows for both first and second samples the current density of the plasma as a function of the frequency of the plasma electrical excitation.

DETAILED DESCRIPTION

(16) FIGS. 1 and 2 which schematically illustrate the contact of a water droplet on a hydrophobic surface according to the Wenzel and Cassie-Baxter models as well as on a double rough structure have already been discussed in the section Background art.

(17) FIG. 3 illustrates a dielectric barrier discharge reactor 2 comprising two flat or cylindrical parallel high voltage electrodes 4 (for example 21.530 cm.sup.2) covered with alumina and a moving table 8 as a grounded electrode. The dielectric barrier discharge (DBD) plasma operates at atmospheric pressure (AP) and is generated by supplying a reaction gas like nitrogen (N.sub.2) into a discharge gap formed between a high voltage electrode 4 and a grounded electrode. Optionally, the moving table 8 can include a heating unit that allows the substrate 10 to be heated up to a temperature of 80 C. The plasma can be generated using for example a Corona generator 7010R from Softal Electronic GmbH generating a 10,000 to 100,000 Hz sinusoidal signal of 100 W.

(18) A siloxane-forming material, like for example a hexamethyldisiloxane (HMDSO), is injected into the reactor using a classic bubbler system made of a cylinder and a frit (not illustrated). The respective flow rates of nitrogen through the process gas line and the HMDSO bubbler are kept constant to 19.5 L.Math.min-1 and 0.5 L.Math.min-1 (1000 ppm) respectively for all experiments. The siloxane-forming material, HMDSO, was obtained from Sigma-Aldrich and used without further purification (98%). Nitrogen (99.999%) gas can be obtained from Air Liquide. The deposition time is 60 seconds for all the experiments.

(19) Deposition Experiments:

(20) The substrates in the following examples were foils of an aluminium alloy. The precise composition of the aluminium alloy foil is not critical for the invention.

(21) In these examples the aluminium alloy foil with a surface roughness Ra of 0.18 m was obtained by cold rolling, but many other methods well known to the skilled person could be used to obtain the same kind of surface roughness. Polished aluminium foil with a surface roughness Ra of 0.06 m was also used as a substrate to illustrate the effect of a smoother surface substrate.

(22) The films were deposited on the foil substrates by submitting the substrates to dielectric barrier discharge plasma operating at atmospheric pressure under filamentary regime, as described above in relation with FIG. 3. The excitation frequency was set at various levels and the temperature within the plasma discharge apparatus was also set at two different levels. The substrate roughness and water contact angle of the siloxane layers produced for different frequencies and power densities are shown in Table 1.

(23) TABLE-US-00001 TABLE 1 Sample Frequency Power density T R.sub.a [m] R.sub.z [m] WCA ID [Hz] [W .Math. cm.sup.2] [ C.] [a] [a] [] 1 10,000 1 20 0.06 [b] 0.40 [b] 98 2 10,000 1 20 0.18 0.89 112 3 10,000 1 80 0.18 0.89 103 4 22,500 1 20 0.06 [b] 0.40 [b] 103 5 22,500 1 21 0.18 0.89 151 6 22,500 0.9 80 0.18 0.89 155 7 30,000 1.5 21 0.18 0.89 150 8 40,000 1 21 0.18 0.89 102 [a] Measured by confocal microscopy [b] Polished aluminium foil Ra designates the arithmetic average of the absolute deviation from the mean line of all points of the profile. Rz designates the arithmetic average of the five largest maximum peak-to-valley heights.

(24) Samples 5 to 7 are in accordance with the invention whereas samples 1 to 4 and 8 are examples not being part of the invention.

(25) Samples 2 and 3 show an increase of the WCA compared to sample 1. Samples 5, 6 and 7 show a substantial increase of the WCA compared to sample 4. It results that the use of a rough substrate (Ra of 0.18 m) according to the invention increases the hydrophobic properties of the resulting surface.

(26) In samples 5 and 6, the WCA is lowest, i.e. 151, for the films formed at room temperature and slightly increased, i.e. to 155, for the film deposited at 80 C.

(27) Additionally, like rose petals, the superhydrophobic surfaces of samples 5 and 6 obtained by the method according the invention have high adhesive force to water. During tilting experiments, it was observed that the water droplets clung to the surface, as shown in FIG. 5. FIG. 4 illustrates a 3 L water droplet on a surface according one of the samples 2 and 3 which correspond to a hydrophobic surface (i.e. not superhydrophobic). For both FIGS. 4 and 5, the left image illustrates the droplet resting on the horizontally oriented surface, the central image illustrates the droplet sticking on the vertically oriented surface and the right image illustrates the droplet suspended at the reversed horizontally oriented surface. The spherical water droplet does not roll off nor slide on the PDMS coating, even when the surface was tilted at 90 or turned upside down. However, this effect failed for water droplets larger than 5 L because the droplet's weight overcomes the adhesion forces of the PDMS films. After one year storage in air, new WCA and WSA measurements matched the observations made just after the film preparation, indicating the high chemical stability of the deposited films coatings.

(28) Evaluation of the Surfaces:

(29) Scanning electron micrographs (SEM) of the aluminium foil substrates of sample 2 are shown in FIGS. 6 to 8 at different magnifications (indicated at the bottom left of each image). FIGS. 9 to 11 are corresponding SEMs of sample 5 and FIGS. 12 to 14 are corresponding SEMs of sample 4.

(30) At low magnification, i.e. 1,000 in FIGS. 6 and 9, the films deposited at 10,000 Hz and 22,500 Hz on the cold-rolled aluminum foils are indistinguishable from each other. It reveals long parallel rolling lines characteristic of cold rolled foil. The observed rolling lines are due to the topography transfer of the work rolls during the rolling process of the aluminum foil.

(31) At a higher magnification, i.e. 10,000 in FIGS. 7 and 10, pinholes surrounded by numerous particles are visible. They show a size distribution varying from 100 to 500 nm. The use in the reactor (FIG. 3, reference 2) of a reduced gap between the electrodes (about 1 mm) at atmospheric pressure and the high precursor concentration imply that the nitrogen plasma discharge operates in the filamentary mode. Under the filaments or current channels, electron and ion bombardment, pinholes and heterogeneities are formed in the film. As observed on the micrographs, in the early steps of the deposition process, the filaments, in various embodiments, anchor to surface defects and in particular to the rolling lines. Moreover, etching of the surface under the impact of the filaments induced a high concentration of condensable vapours, which quickly generated particles. The local higher power density at the filament position also induced a faster gas depletion and nucleation of particles.

(32) At even higher magnification, i.e. 100,000, FIGS. 11 and 14 reveal that the films deposited using a 22,500 Hz frequency are composed of agglomerate particles. These particles, with sizes varying from 100 to 500 nm, are constructed from smaller agglomerated primary particles of about 50 nm. As is visible in FIG. 8, the film deposited at 10,000 Hz has a smoother appearance, with weaker evidence of particle formation. FIG. 12 shows in contrast that the film deposited on polished aluminum foil is relatively smooth and particle-free. However, the higher magnification SEM pictures in FIGS. 13 and 14 reveal that the films deposited on polished aluminum foil using a 22,500 Hz frequency are composed of 100 to 500 nm agglomerate particles made of smaller primary particles of about 50 nm. The SEM picture in FIG. 8 obtained at high magnification for the film formed with a 10,000 Hz frequency is smoother.

(33) Evaluation of the Composition

(34) FIG. 15 illustrates the Fourier Transformed InfraRed spectroscopy (FTIR) of the films according to the samples 2 and 5, i.e. with a 10,000 Hz and 22,500 Hz respectively electrical excitation frequency on the cold-rolled aluminum foil. The FTIR curves confirm the formation of PDMS-like thin films. All the spectra are almost identical with a broad and intense band between 1000 to 1200 cm-1 and a narrow peak at 1260 cm.sup.1. These peaks correspond respectively to different SiOSi chain vibrations and Si(CH.sub.3).sub.2 bending modes. The intense bands observed between 700 and 900 cm.sup.1 are attributed to vibrations from SiOSi (736 cm.sup.1), Si(CH.sub.3).sub.1 (778 cm.sup.1), (CH.sub.3) and (SiC) from Si(CH.sub.3).sub.3 (764 cm.sup.1; 830-850 cm.sup.1), Si(CH.sub.3).sub.2 (800 cm.sup.1; 904 cm.sup.1), (SiO) (810-849 cm.sup.1) and (HSiO) (848 cm.sup.1; 890 cm.sup.1). Around 1400 cm.sup.1 and 2900 cm.sup.1, peaks with shoulders corresponding to asymmetrical CH.sub.3 deformation vibrations and CH.sub.3 stretching modes can be observed. The peaks observed at 1555 and 1655 cm.sup.1 demonstrate the existence of CN (sp.sup.2) bonds in all the films deposited.

(35) FTIR shows that the large difference observed in the measured WCA is not related to any chemical disparity between the films produced, whereas SEM reveals some large differences in the morphology of the PDMS deposited.

(36) Micron and Nanometer Length Scales of the Superhydrophobic Surfaces:

(37) The micrometer and nanometer scale roughnesses observed by SEM can be also identified by confocal microscopy and atomic force microscopy (AFM).

(38) FIG. 16 is a confocal microscopy 3D topographic image of the siloxane film of sample 5 (FIGS. 9 to 11), i.e. with a 22,500 Hz electrical excitation frequency on the cold-rolled aluminum foil. It shows long parallel rolling lines that cover the whole surface of the samples. The observed rolling lines are due to the topography transfer of the work rolls during the rolling process of the aluminum foil.

(39) FIG. 17 is a confocal microscopy 3D topographic image of the siloxane film of sample 4 (FIGS. 12 to 14), i.e. with a 22,500 Hz electrical excitation frequency on the polished aluminum foil. It shows a smoother surface, only covered by several smaller peaks.

(40) The arithmetic average of the absolute deviation from the mean line of all points of the profile (R.sub.a) and the arithmetic average of the five largest maximum peak-to-valley heights (R.sub.z) are given in Table 1. The size of the rolling lines (10-20 m wide and 1-1.5 m height) observed on the roughness profile of the films prepared on the cold-rolled aluminum foil (FIG. 16) is consistent with the scale of the microscopic cells at the surface of the lotus leaf (3-11 m diameter and 7-13 m height) and the micropapillae of the rose petal (16 m diameter and 7 m height). While referring to these values, it can be noticed that similarly to the micropapillae observed on the rose petal, the rolling lines on the aluminum foil are wider than higher.

(41) FIGS. 18 and 19 are AFM 3D topographic images of the films according to sample 2 which is not part of the invention and sample 5 which is according to the invention, respectively. The AFM 3D topographic images of the films deposited at 22,500 Hz, irrespective of the aluminum substrate roughness, show a larger surface roughness than the film deposited a 10,000 Hz. The scale of the PDMS particles at the surface of the films (200-800 nm) is consistent with the size of the nanoscopic wax crystals of the lotus leaf (100 nm) and the nanofolds on the rose petal (730 nm). The roundness of the particles and the chink at their surfaces are quite comparable to the structure of the petal rose surface, whereas the nanoscopic wax crystals of the lotus leaf exhibit a needle-like morphology.

(42) Confocal microscopy and AFM emphasis the importance of the dual-scale roughness as the superhydrophobic property was only achieved when nanometer scale features in addition of the micrometer features was constructed.

(43) Plasma Frequency and Formation of Rough PDMS Surfaces:

(44) A double-roughness surface is important to achieve the desired superhydrophobic effect. It is the mechanism of polymer agglomeration that occurs during the DBD process that is the key to yielding such specific rough surfaces. The relatively high concentration of HMDSO used (1000 ppm) is known to favor the formation of particles in the gas phase. The high concentration of precursor induces the formation of a large amount of condensable species by the reaction with the reactive species produced by each filament of the AP-DBD. The high concentration of condensable species leads to high local concentration of nucleated particles, which grow by condensation and coagulation. The number of formed particles in the gas phase and at the surface of the film is therefore related to the precursor concentration, but also to the density and energy of the filaments of the AP-DBD. The mean dissipated power density, which is known to influence the formation of particles, has been kept constant to 1.1 W.Math.cm.sup.2 for both frequencies studied. However the plasma gas current density as illustrated in FIG. 20 varied with the electrical excitation frequency. The maximum value of current density at 22,500 Hz (ca. 1.2 mA.Math.cm.sup.2) was twice higher than the one measured at 10,000 Hz (ca. 0.6 mA.Math.cm.sup.2). The current density variation, which is used to estimate the variation of plasma density according to discharge parameters, indicated the formation of larger amount of active species at 22,500 Hz. The higher concentration of active species and the high concentration of HMDSO lead to the formation of a higher density of particles at the highest plasma frequency, which increased the surface roughness.

(45) The invention has been described above specifically in relation with an aluminum substrate. It is to be mentioned that the invention can also be applied on other type of substrates like copper, steel, cardboard, wood, polypropylene fabric and glass. Deposition tests have been successfully carried out on all these types of substrates and did all provide superhydrophobic properties with varying WSA going from 20 with substrates made of cardboard or wood to 90 (i.e. totally sticking) for a substrate made of electrodeposited copper foil.