Superhydrophobic nanotextured polymer and metal surfaces

Abstract

A method of manufacturing a multiscale (hierarchical) superhydrophobic surface is provided. The method includes texturing a polymer surface at three size scales, in a fractal-like or pseudo-fractal-like manner, the lowest scale being nanoscale and the highest microscale. The hydrophobic polymer surface may be converted to hydrophobic metal surface by subsequent deposition of a metal layer onto the polymer surface.

Claims

1. A method of manufacturing a hydrophobic surface having an apparent contact angle of at least 150, the method comprising: i) providing a base substrate and a polymeric material; ii) forming a layer of said polymeric material on said substrate, said layer having a bottom surface attached to said substrate base, and an upper surface, wherein the polymeric material in said layer is structured at two different size scales, the first scale ranging from about 0.1 to 2 m, and the second scale between 0.5-50 m; and iii) further structuring said polymeric material at a third size scale by forming indentations on said upper surface, wherein an average distance between adjacent indentations is from 20 to 200 m, wherein the hydrophobic surface having the three size scales is arranged in pseudoperiodic areas of higher and lower mass density, measured along a line going through the polymeric material and oscillating in a fractal-like manner; thereby obtaining a hydrophobic surface having an apparent contact angle of at least 150.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other characteristics and advantages of the invention will be more readily apparent through the following examples, and with reference to the appended drawings, wherein:

(2) FIG. 1. is a photograph of droplets deposited on the polymer fractal-like relief (FIG. 1a) and on a gold-coated relief (FIG. 1b);

(3) FIG. 2. is a scheme illustrating the manufacture of highly developed surfaces;

(4) FIG. 3. shows SEM images of the surface relief comprising PDVF bead-aggregates (Relief A), FIG. 3a shows the aggregates comprising PDVF nanobeads at 4800 magnification, FIG. 3b shows mainly a single aggregate of PDVF nano-beads at 18000 magnification;

(5) FIG. 4. shows SEM images of the random fractal-like interface composed of PVDF beads aggregates deposited on PE substrates (relief B), FIG. 4a shows lower magnification, and FIG. 4b shows higher magnification detailing a channel surrounded with PVDF beads aggregates; and

(6) FIG. 5. is a scheme showing a drop of water on a flat surface, and illustrating how the local contact angle is defined, FIG. 5a showing a hydrophilic surface, FIG. 5b a hydrophobic surface, and FIG. 5c showing a drop of water deposited on the textured surface and illustrating how the apparent contact angle is defined.

DETAILED DESCRIPTION OF THE INVENTION

(7) It has now been found that certain purely structural features confer to an otherwise hydrophilic material a strongly hydrophobic character. For example, when structuring a polymeric material at three different size scales in a fractal-like manner, a surface was obtained that exhibited an apparent contact angle (ACA) of more than 150; the lowest of said three levels was represented by 0.1 m polymer beads, the second level being formed by 10 m aggregates of said beads, and the highest level was created by channels carved in 100 m distances into the deposit of said polymer deposited on a smooth underlying matrix.

(8) In one embodiment of the invention, a superhydrophobic surface was created by depositing polyvinylidene fluoride (PVDF) particles of industrial grade onto a polyethylene or polypropylene substrate. The manner of depositing was such that particles of PVDF, about 0.1 m in diameter, were layered onto said substrate while forming aggregates of about 10 m in size, followed by carving approximately equidistant channels into the aggregate layer, the adjacent channels being about 100 m from each other, so forming a three-level structure in a fractal-like manner. What was surprising was that PVDF is inherently a hydrophilic material, showing an ACA of about 75, when measured on flat, unstructured, samples. However, when the same PVDF was structured according to the invention, and comprised nanosized beads, its ACA increased up to 160 (see FIG. 1a).

(9) The apparent contact angle (ACA) is a measure of hydrophobicity, and its meaning is illustrated in FIG. 5c. When a water drop is placed on a plane made of the measured material, the tangent to the water surface is taken at the contact point with the plane, and the angle formed by tangent and the plane (the angle that encompasses the drop) is measured. It can be seen, that the higher the ACA, the higher hydrophobicity of the plane surface is, the theoretically highest value being 180.

(10) When using the scheme of FIG. 2a of producing the said superhydrophobic surface from solution, the Relief A was obtained comprising PVDF nanobeads of 0.1 m (100 nanometers) in diameter which were assembled in aggregates of about 10 m, as shown in FIG. 3. Applying the powder technique, illustrated in FIG. 2b, led to the further structured Relief B with still higher structural level in its pseudo-fractal system, i.e. with approximately linear and roughly equidistant channels. The distance between channels was about 100 m. Scanning-electron-micrographs (SEM) of Relief B is shown in FIG. 4. The measured ACA values for Relief A was about 95, and for Relief B about 160. The structuring at said third level may be effected by known imprinting technique, or by modifying such technique.

(11) The obtained superhydrophobic surface was subsequently coated with a thin layer of gold, providing a metal surface. When measuring hydrophobicity of the produced metal surface, a surprising result was obtainedthe ACA value was 150. Whereas a smooth golden plane shows ACA of about 40, the same metal material textured to copy the relief of the underlying plastic layer exhibited superhydrophobic behavior, demonstrating that texturing a surface in the fractal-like manner according to the invention may confer water-repellent properties to any materials that can be deposited onto an underlying textured surface, provided that the unevenness of the lowest size scale be not smoothed out.

(12) While a smooth surface of a plastic material yielded ACA of about 75, a structured, pseudo-fractal surface of Relief A, comprising two structuring levels, exhibited by about 20 higher ACA, and when adding a third structuring level, a further increase by 50-60 more, depending on the material eventually deposited on the underlying plastic textured layer, was observedfalling not too far from the theoretical maximum of 180.

(13) The invention provides a simple and inexpensive method of producing superhydrophobic surfaces, comprising structuring polymer material simultaneously on more size levels, and layering the multi-textured material on a desired substrate. In a preferred embodiment of the invention, a superhydrophobic surface is formed, comprising hot pressing of PVDF powder on polyethylene substrate. The obtained partially ordered pseudo-fractal surface shows super-hydrophobicity with the apparent contact angle as high as 160.

(14) The invention enables to manufacture superhydrophobic surfaces from industrial grade polymer materials. In one embodiment, the surface comprises partly disordered pseudo-fractal arrays of PVDF globules. A superhydrophobic metallic surface can be produced, using the polymer surface as a template.

(15) In a preferred embodiment of the invention, a polymeric multi-structured superhydrophobic surface is coated with a metal (for example gold, silver, aluminium, titanium, molybdenum), the measured apparent contact angle for gold, for example, being 150, but other hydrophilic metallic layers may be employed. The method of the invention, thus, enables, in one aspect, to confer a high degree of hydrophobicity to a metal surface. Thus, the invention provides a method for converting inherently wettable materials to superhydrophobic ones.

(16) The invention will be further described and illustrated in the following examples.

EXAMPLES

Materials and Methods

(17) Polyvinylidene fluoride nano-beads were purchased from Aldrich, molecular weight M.sub.w=534 000, T.sub.g=38.0 C., density =1.74 g/cm.sup.3. The average diameter of particles was established as 130 nm. Polycarbonate (PC) Lexan 141 was purchased from GE Plastics. Chloroform (pure for analysis) was obtained from Karlo Erba Reagenti.

(18) Apparent contact angle (ACA) was measure using the goniometry technique and magnifying optical system. Droplets of bidistilled water were dripped carefully on the coated templates. The volume of the droplets was 2-5 l.

(19) Scanning electron microscopy was performed for reliefs A and B coated with 360 gold films by a sputtering procedure in argon atmosphere. A thickness of coating was determined by time of sputtering.

Example 1

(20) Highly hydrophobic polymer surfaces were obtained (FIG. 2a-b). Tilted base substrates were coated with chlorinated solvents-based solutions comprising polycarbonate and PVDF beads. Polycarbonate is soluble in the chlorinated solvents, whereas PVDF is not, thus a suspension was formed and dripped on the substrates as depicted in FIG. 2a, e.g. quartz glass substrate, and dried with a room temperature air current. The structure of the dry film studied by means of scanning electron microscopy (SEM) shows interconnected colloidal arrays (ICA), such as presented in FIG. 3a-b. These arrays are micro-scaled aggregates consisting of PVDF particles embedded in PC that filled the porosity between PVDF beads. The most frequent size of these aggregates was determined as 3-10 m. The aggregates incorporate 10.sup.4-10.sup.6 PVDF nanoparticles. The relief displayed in FIG. 3 is mentioned further as Relief A.

(21) At the first stage solutions containing 2-5 wt % of PC dissolved in chloroform were prepared. Then particles of PVDF (2 wt %) were added under stirring (PVDF is insoluble in chlorinated solvents). Two types of substrates, quartz glass and polypropylene (PP), were coated, in a manner depicted in FIG. 2a. The slope of the substrate was =19-22.

Example 2

(22) A layer of PVDF beads in powder form has been spread at the surface of the low density polyethylene (PE) substrate (see FIG. 2b). Then the sandwich has been exposed to hot pressing with a randomly riffled stamp. The PE has been softened under the pressing and traps single PVDF particles (which are still solid under the pressing temperature) and globular aggregates were formed comprising PVDF beads. The aggregates, composed of nano-scaled beads frozen in the PE matrix, formed highly developed interface photographed in FIG. 4a-b. This random pseudo-fractal surface is mentioned below as Relief B. Pressed stamp indentations form channels important for increasing the hydrophobicity. Hot pressing was carried out under t=85 C. The characteristic distance between indentations of the riffled stamp was 100 m, the depth of the indentations was 20 m.

Example 3

(23) At the next stage, Reliefs A and B were coated with 360 gold films by a sputtering procedure in argon atmosphere. Then double distilled water droplets were dripped carefully on the coated templates. The volume of the droplets was 2-5 l. Apparent contact angles are summarized in Table 1.

(24) TABLE-US-00001 TABLE 1 ACA values of the textured surfaces Gold coated Gold coated Relief A relief A Relief B relief B Measured 95 5 95 5 160 5 150 5 ACA

(25) While this invention has been described in terms of some specific examples, many modifications and variations are possible. It is therefore understood that within the scope of the appended claims, the invention may be realized otherwise than as specifically described.