Surface with an Anti-Adhesion Microstructure and Method for Producing Same

Abstract

The invention relates to a surface comprising a microstructure that reduces adhesion and to a method for producing said microstructure. Microstructures of this type that reduce adhesion are known and are used, for example, to configure self-cleaning surfaces that us the Lotus effect. According to the invention, the surface is produced electrochemically by means of reverse pulse plating, the known microstructure being first produced and a nanostructure that is overlaid on the microstructure is produced at the same time or in a subsequent step. To achieve this for example, the pulse length of the current pulse that is used during the reverse pulse plating lies in the millisecond range and has a pulse length ratio greater than 1:3. The microstructure that has been produced, consisting of peaks and troughs is then overlaid with peaks and troughs of a smaller size order belonging to the nanostructure.

Description

FIELD OF INVENTION

The invention relates to a surface with an anti-adhesion microstructure and a method for electrochemically producing such a surface.

BACKGROUND OF THE INVENTION

Anti-adhesion surfaces of the abovementioned type are used e.g. as so-called lotus-effect surfaces and are described, for example, in DE 100 15 855 A1. According to this publication, such surfaces are characterized by a microstructure which can be obtained by film deposition from solutions, but also by electrolytic deposition. This mimics an effect observed on the leaves of the lotus plant, according to which the resulting micropatterning, which for this purpose must have peaks and valleys with a radius of 5 to 100 μm, reduces the adhesion of water and dirt particles. This enables contamination of the corresponding surface to be counteracted. The formation of limescale, for example, can also be prevented.

SUMMARY OF INVENTION

The object of the invention is to specify a surface with an anti-adhesion microstructure and a production method for said surface, the adhesion-reducing effect being comparatively strongly marked.

This object is achieved according to the invention by a method in which the surface is produced by electrochemical pulse plating, a nanostructure overlying the microstructure being created by reverse pulse plating. According to the invention, the nanostructure is overlaid on the microstructure by producing, on the surface topology having surface profile bending radii in the micrometer range (microstructure), a surface topology whose bending radii are preferably in the range of a few nanometers to 100 nanometers (nanostructure). The formation of the nanostructure on the microstructure is achieved by reverse pulse plating using current pulses whose length is in the millisecond range. The microstructure can be produced simultaneously or separately depending on the process parameters such as pulse length and deposition density.

The surface's nanostructure in conjunction with the microstructure advantageously improves the effect of reducing the adhesion of substances to the surface, thereby advantageously improving the surface's lotus effect.

Although U.S. Pat. No. 5,853,897 discloses a method of electrodepositing films with a rough surface by means of pulse plating, the films produced according to this document are designed solely for optical applications, as they have excellent light absorbing properties in a wide optical wavelength range. For this purpose it merely suffices to create a dendritic microstructure without having to overlay same with a nanostructure.

Advantageously the pulse length for the process step of producing the nanostructure is less than 500 ms. This means that, during this step, favorable deposition parameters can be set on the surface to be produced, so that the resulting nanostructure differs sufficiently in its dimensions from the microstructure created.

The current pulses for reverse pulse plating are generated by reversing the polarity of the deposition current so that a significant time differential for the charge transfers at the surface can be advantageously achieved. In respect of their length, the individual current pulses are advantageously in the range between 10 and 250 milliseconds. It has been shown that advantageously, for the parameters specified, the surface's nanostructure is particularly pronounced.

It is particularly advantageous if during reverse pulse plating the cathodic pulses are at least three times as long as the anodic pulses. For the purposes of the invention, cathodic pulses are taken to mean those pulses resulting in deposition on the surface, whereas the anodic pulses produce dissolution of the surface. For the specified ratio between cathodic and anodic pulses it has been found that the needle-like basic elements of the nanostructure are advantageously produced with a high density on the microstructure, to the benefit the lotus effect to be achieved.

Another advantageous possibility is that, for reverse pulse plating, the cathodic pulses are implemented with a higher current density than the anodic pulses. This also increases the deposition rate of the cathodic pulses compared to the erosion rate of the anodic pulses so that nanopatterning layer growth is advantageously produced. Self-evidently, the measures of modifying the pulse duration and varying the current density can be combined together, an optimum having to be found for the material to be deposited by adjusting the specified parameters.

According to one embodiment of the method it is provided that the pulse length is at least one second for an upstream microstructure producing step. With pulse lengths in the seconds range, the surface's required microstructure can be advantageously produced time-efficiently by electrochemical means if it is not produced, or not with sufficient markedness, in the nanostructure producing step.

According to another embodiment of the method, the surface is additionally produced with a macrostructure superimposed on the microstructure. The macrostructure can be produced electrochemically or by other means, e.g. mechanically. The term macrostructure is to be understood here as a surface topology whose elementary structural components' geometrical dimensions are at least an order of magnitude greater than those of the microstructure. In the case of a wavy macrostructure, this would mean e.g. for the radius of the waves that said radius is greater to a corresponding degree than the radii of the peaks and valleys of the microstructure. The macrostructure advantageously allows the anti-adhesion properties of the surface to be increased still further. In addition, the surface's macrostructure can advantageously assume additional functions such as improving the flow characteristics of the surface.

The surface according to the invention achieves its stated object by a nanostructure created by pulse plating being overlaid on the microstructure. This inventive surface composition enables the already mentioned advantages to be achieved, in particular improving the anti-adhesion properties of the surface.

According to a particular embodiment of the surface, same is superhydrophobic. This means that the adhesion of water or other hydrophilic substances is particularly greatly reduced. The superhydrophobic properties in particular cause poor wettability of the surface for water, so that water present on the surface forms individual droplets which, because of the surface's contact angle of more than 140°, readily roll off and possibly also entrain dirt particles present on the surface with them. Surfaces with superhydrophobic properties are therefore particularly suitable for making the surface a lotus-effect surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details of the invention will now be described with reference to the accompanying drawings in which the same or corresponding elements are provided with the same reference numerals and will only be explained more than once where they differ from drawing to drawing.

FIG. 1 schematically illustrates an embodiment of the surface according to the invention in schematic cross section,

FIG. 2 shows the surface profile of a lotus-effect surface as an example of the inventive surface in cross section and

FIG. 3 shows perspective views of the lotus-effect surface according to FIG. 2.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a body 11 having a surface with reduced adhesion properties. The surface 12 can be schematically described by an overlaying of a macrostructure 12 with a microstructure 13 and a nanostructure 14. The microstructure creates surface waviness. The microstructure is indicated by hemispherical peaks on the wavy macrostructure 12. The nanostructure 14 is represented in FIG. 1 by bumps on the hemispherical peaks (microstructure) and in the parts of the macrostructure 12 located between the peaks and forming the valleys of the microstructure 13.

The anti-adhesion properties of the surface formed by the superimposition of the macrostructure 12, the microstructure 13 and the nanostructure 14 are indicated by a water droplet 15 which form a pearl of water on the surface. Due to the low wettability of the surface on the one hand and the surface tension of the water droplet on the other, there is formed between the water droplet 15 and the surface a relatively large contact angle γ which is defined by an angle leg 16a running parallel to the surface and an angle leg 16b forming a tangent to the skin of the water droplet, said tangent running through the edge of the contact area of the water droplet 15 with the surface (or more precisely the angle leg 16a). FIG. 1 shows a contact angle γ of more than 140° so that the schematically represented surface is superhydrophobic surface.

As part of an experiment, reverse pulse plating has been used to produce a lotus-effect surface by depositing copper on a surface smoothed by electroplating, the following process parameters having been selected:

Production of the nanostructure in a process step:

Pulse length (reverse pulses): 240 ms at 10 A/dm² cathodic,40 ms at 8 A/dm² anodicElectrolyte contains 50 g/l Cu, 20 g/l free cyanide, 5 g/l KOH

The electrochemically produced surface was then examined using an SPM (Scanning Probe Microscope—also known as AFM or Atomic Force Microscope). An SPM enables surface structures down to the nanometer range to be identified and displayed. A segment of the surface produced is shown in cross section in FIG. 2 as an SPM test result, the profile being exaggerated. Relative to a zero line 17, there is drawn in FIG. 2 a waveform 18 making clear the macrostructure superimposed on the surface structure. Because of the exaggeration, the microstructure 13 is identifiable as a succession of peaks 19 and valleys 20. In addition, there can be identified, in particular regions, the nanostructure 14 resulting from a close succession of peaks and valleys which cannot be resolved further at the scale shown in FIG. 2 and are therefore only identifiable as a thickening of the profile line of the surface profile.

Further details may be obtained from FIG. 3a which provides a perspective view of the SPM recording of the copper surface. A region 100×1100 μm square was selected as the extracted segment, the needle-like peaks 19 determining the microstructure 13 being clearly visible. The resulting image has the appearance of a “coniferous forest”, with interspaces between the “conifers” (peaks 19) forming the valleys 20. The surface as shown in FIG. 3a is also represented in exaggerated form in order to make clear the peaks 19 and valleys 20 of the microstructure 13.

As can be seen from the perspective view of the surface according to 3b which constitutes a segment enlargement of the representation according to FIG. 3a, a nanostructure 14 is additionally superimposed on the microstructure 13. In the less exaggerated representation according to FIG. 3b, the peaks 19 and valleys 20 look more like a waviness of the surface (which, however, because of the different scale must not be confused with the waviness according to FIG. 2). Additionally superimposed on this waviness are very small peaks 19n and valleys 20n characterizing the nanostructure of the surface. These are again reminiscent in terms of their structure of a coniferous forest already explained in connection with FIG. 3a, their geometrical dimensions turning out to be about two orders of magnitude smaller, i.e. totally imperceptible at the scale selected in FIG. 3a.

In order to make the size relationships clear, the macrostructure 12, the microstructure 13, and the nanostructure 14 are each marked with a bracket in FIGS. 2 and 3. The bracket in each case encloses only one segment of the relevant structure, which contains a peak and a valley, so that, among one another, the brackets within a Figure allow the orders of magnitude of the structures in relation to one another to be compared. In the example shown, the contact angle measured for a water droplet was 152°. The superhydrophobic properties of the copper coating shown, which produce a lotus effect, are achieved by an interplay of at least the microstructure 13 and the nanostructure 14, the overlaying of a macrostructure 12 improving the observed effects still further. By selecting suitable process parameters, such lotus-effect surfaces can be produced for different coating materials (silver coatings have been successfully tested, for example) and for liquids with different wetting behaviors.

Claims

1.-9. (canceled)

10. A method for electrochemically producing a surface with an anti-adhesion microstructure, comprising: providing a macrostructure;arranging a microstructure on the macrostructure by electrochemical pulse plating; andarranging a nanostructure on the microstructure by reverse pulse plating.

11. The method as claimed in claim 10, wherein the reverse pulse plating pulse length for producing the nanostructure is less than 500 ms.

12. The method as claimed in claim 11, wherein a cathodic pulse is at least three times as long as an anodic pulse for reverse pulse plating.

13. The method as claimed in claim 12, wherein the cathodic pulse is implemented with a higher current density than the anodic pulse for reverse pulse plating.

14. The method as claimed in claim 13, wherein the pulse length for an upstream process step for producing the microstructure is at least one second.

15. A surface with an anti-adhesion microstructure, comprising: providing a microstructure; andsuperimposing a nanostructure on the microstructure by pulse plating to produce the surface.

16. The surface as claimed in claim 15, wherein the surface is superhydrophobic.

17. The surface as claimed in claim 16, wherein a macrostructure is superimposed on the microstructure and the nanostructure.

18. The surface as claimed in claim 17, wherein a pulse plating pulse length for producing the nanostructure is less than 500 ms.