Abstract
An object includes at least two faces which are suitable for dry adhesion and differ in their adhesion parameters. By suitable structuring, it is possible, where appropriate in combination with a suitable contact pressure, to selectively control the detachment of surfaces contacted on these faces.
Claims
1. An object which has, at least on two surfaces, in each case a face capable of dry adhesion, wherein the two faces differ in at least one adhesion parameter.
2. The object as claimed in claim 1, wherein at least one of the faces has a structure comprising perpendicular projections.
3. The object as claimed in claim 1, wherein both faces have a structure comprising perpendicular projections.
4. The object as claimed in claim 1, wherein the adhesive force of the two faces is different.
5. The object as claimed in claim 1, wherein the two faces differ in structure and/or modulus of elasticity.
6. The object as claimed in claim 1, wherein at least one face has a structure whose adhesive force is reducible by Euler buckling or shear loading.
7. The object as claimed in claim 6, wherein the adhesive force resulting after the Euler buckling or shear loading is less than the adhesive force of the other face.
8. A method for selective adhesion for an object as claimed in claim 1, comprising: contacting the first face with a first surface and contacting the second face with a second surface; moving at least one of the surfaces away from the object until detachment of one of the two surfaces.
9. The method as claimed in claim 8, wherein at least one of the faces has a structure and, before moving away the surfaces, the Euler buckling is brought about by a sufficient contact pressure in one of the structures.
Description
[0070] The exemplary embodiments are schematically illustrated in the figures. Identical reference signs in the individual figures here designate identical or functionally identical elements or elements which correspond to one another in terms of their functions. Specifically:
[0071] FIG. 1 shows a schematic illustration of an object having opposite structures which differ in the height (length) of the projections (example 1), the diameter (example 2) or the modulus of elasticity (example 3). Illustrated at the bottom is the side view of an object having two structures according to example 2 with D.sub.1=0.8 mm and D.sub.2=0.4 mm and also L.sub.1=L.sub.2=1.6 mm and E.sub.1=E.sub.2=2 MPa;
[0072] FIG. 2 shows an illustration of the measured force under loading and load relief (distance) of the structure shown in FIG. 1;
[0073] FIG. 3 shows a schematic illustration for producing the double-sided reversible adhesive structures;
[0074] FIG. 4 shows an illustration of the measuring arrangement for determining the adhesion force as a function of the penetration depth;
[0075] FIG. 5 shows the controlled detachment of the object according to the invention;
[0076] FIG. 6 shows the adhesive force as a function of a structure according to example 1 of FIG. 1; and
[0077] FIG. 7 shows the adhesive force as a function of a structure according to example 2 of FIG. 1.
[0078] FIG. 1 shows, in the upper region, various examples of objects having two opposite structures which have column-like projections which in turn have slightly widened end faces (mushrooms). In example 1, the structures differ in the height of their projections (L.sub.1 not equal to L.sub.2). In example 2, the diameter of the projections differs (D.sub.1 not equal to D.sub.2). In example 3, the modulus of elasticity of the structures is different (E.sub.1 not equal to E.sub.2). These differences result in the fact that, in addition to different adhesive force, in particular the Euler buckling in the structures occurs under different forces.
[0079] FIG. 2 shows the behavior of the object depicted in FIG. 1 (at the bottom) under different loading. For this purpose, the two structures of the object are contacted with a surface. If an external pressure is now exerted on the object perpendicular to the contact faces (contact pressure), the two structures are compressed (distance is negative). If the pressure is now reduced again, that is to say the contacted surfaces or one of the contacted surfaces move/moves away from the object, an adhesion force can be measured (tension in FIG. 2) until detachment of the object occurs. Which of the structures detaches depends on their adhesion force. This behavior is illustrated in FIG. 2 by the solid line.
[0080] If, during pressing on, the pressure now exceeds the limit for Euler buckling, there occurs elastic buckling and thus a reduction in the contact face of the buckling structure with the surface contacted on said structure. A decrease in the measured force during detachment of the surfaces occurs. The force to be applied is now considerably less, and the surface can be detached with considerably less force. Here, the structure for which the Euler buckling has been triggered is released.
[0081] FIG. 3 shows one possibility for producing double-sided adhesive structures. An uncrosslinked, liquid polymer (prepolymer) is poured into a multipart casting mold. The casting mold includes inserts which serve as a template (negative mold) for the adhesive structures. After crosslinking, the double-sided adhesive structure is removed from the mold.
[0082] FIG. 4 shows the measuring arrangement for determining the adhesion forces in dependence on the penetration depth. The adhesion is measured on both sides against glass substrates. A glass substrate (at the bottom) is mounted on a tilting table for orienting the adhesive surfaces with respect to the substrate surfaces. During the measurement, the upper substrate is brought into contact and pressed on to a defined penetration depth. Here, the pressing-on force (compressive force) is recorded. After the pressing on, the substrates are pulled apart and the adhesive force (tensile force) is determined.
[0083] FIG. 5 shows how this principle can be used with the object of FIG. 1. Use is made of a structure according to example 1 of FIG. 1, that is to say the projections on the two sides differ in terms of their height. Upon contacting of the two structures, the contact pressure, also referred to as penetration depth) can be used to control for which of the two structures the detachment takes place (with identical contacted surfaces). In the case of a contact pressure which does not lead to Euler buckling (FIG. 5, left-hand column), the structure which has a smaller adhesive force is detached during the movement apart. It can be seen in the bottom drawing that the upper structure of the object has released. This is also the side which has the shorter projections. If, by contrast, a contact pressure is selected which leads to the Euler buckling in one of the structures, the adhesion for this structure decreases considerably, which leads to the preferential detachment of this structure (FIG. 5, right-hand column).
[0084] FIG. 6 shows measurement values which have been obtained for an object according to example 1 of FIG. 1. The adhesive force has been measured in dependence on the penetration depth. In the case of small penetration depths, the double-sided structure is adhesive and, in the case of larger penetration depths, is low-adhesive. The detachment from the substrate changes from side 1 (filled boxes) to side 2 (unfilled boxes) with increasing penetration depth. Boxes correspond to experimental data. The dashed line corresponds to the fitted Sigmoid function for determining the asymptotic force values for the adhesive and low-adhesive range.
[0085] FIG. 7 shows measurement values which have been obtained for an object according to example 2 of FIG. 1. The adhesive force has been measured in dependence on the penetration depth. In the case of small penetration depths, the double-sided structure is adhesive and, in the case of larger penetration depths, is low-adhesive. The detachment from the substrate changes from side 1 (filled boxes) to side 2 (unfilled boxes) with increasing penetration depth. Boxes correspond to experimental data. The dashed line corresponds to the fitted Sigmoid function for determining the asymptotic force values for the adhesive and low-adhesive range.
[0086] The switching efficiency of all investigated structure types is summarized in table 1. The adhesive tensions, .sub.p,i, of both regimes (adhesive and low-adhesive) has been calculated from the asymptotic adhesive forces, F.sub.p,i, (cf. FIGS. 6 and 7) and the contact face, A: .sub.p,i=F.sub.p,i/A.
[0087] The efficiency, S, results from S=1o.sub.p,K/.sub.p,0, where .sub.p,0 is the adhesive tension without buckling (at small penetration depths) and .sub.p,K is the adhesive tension after the buckling of the structures (at high penetration depths). S can vary between 0 and 1, where 0 describes no switching behavior and 1 describes the maximum switching efficiency. The results in table 1 show that all double-sided adhesive structures have an efficiency of greater than 0.5, with some exemplary embodiments, with S0.8, having a very high switching efficiency. The thickness of the layer between the two switching structures has only minor influence on the switching efficiency in the examples.
[0088] Preference is given to systems having a switching efficiency of above 0.5, in particular above 0.7.
TABLE-US-00001 TABLE 1 Adhesive tension Adhesive Switching without tension after efficiency, buckling, buckling, S = 1 .sub.p,K/ .sub.p,0 .sub.p,K .sub.p,0 Example 1 28.0 kPa 7.5 kPa 0.73 (d = 1 mm) Example 1 34.8 kPa 6.0 kPa 0.83 (d = 2 mm) Example 1 22.7 kPa 10.6 kPa 0.53 (d = 3 mm) Example 1 28.1 kPa 13.6 kPa 0.52 (d = 5 mm) Example 2 31.8 kPa 13.5 kPa 0.58 (d = 1 mm) Example 2 33.3 kPa 7.6 kPa 0.77 (d = 5 mm)
