Patent Application
20100252177
This invention relates to fabricated adhesive microstructures, and to methods of their fabrication.
There has been significant interest in the fabrication of adhesive structures. Adhesive mechanisms in nature have been studied for a long time, but have not been fully understood or exploited. For example, geckos are recognised to be exceptional in their ability to climb up smooth vertical surfaces, and this has prompted several groups to attempt to fabricate adhesive structures which mimic the adhesive pads on the feet of geckos. Known proposed applications for exploitation of the remarkable adhesive properties of the gecko foot include areas where a dry, re-attachable adhesive bond would be of benefit, for example in high performance climbing robots (see M Sitti's paper on “High aspect ratio polymer micro/nanostructure manufacturing using nanoembossing, nanomoulding and directed self-assembly”, IEEE/ASME Advanced Mechatronic Conference, Kobe, Japan, July 2003). It has been suggested that the ability of geckos to climb and cling to surfaces is due to an intricate branching fibre structure comprising many micro/nanofibres which terminate in a pad or setal area which is in intimate contact with the surface (see for example M Sitti and R S Fearing's paper on “Synthetic gecko foot-hair micro/nanostructures for future wall-climbing robots”, JAST, 18, 1055, 2003). It is believed that this fibre structure confers compliance on a range of length scales sufficient to accommodate rough surfaces (see M Sitti and R S Fearing's above mentioned paper), and it is believed that the setal pad area achieves adhesion via intermolecular forces such as Van der Waals' forces (see for example London's seminal paper on “The general theory of molecular forces”, Transac. Faraday Soc. 1937, 33, 8-26 and K Autumn et al's paper on “Evidence for Van der Waals' adhesion in gecko setae”, PNAS, Sep. 17, 2002, Vol. 99, no. 19, 12252-12256).
Several groups have reported on the successful fabrication of synthetic adhesive microstructures. This includes, for example, electron-beam lithography of polyimide (see A K Geim et al's paper on “Microfabricated adhesive mimicking gecko foot-hair”, Nature Materials, Vol. 2, July 2003, 461), nanomoulding using silicon rubber (see N J Glassmaker et al's paper on “Design of biomimetic fibrillar interfaces: 1. Making Contact”, J. R. Soc. Lond. Interface 2004), polyimide (see A K Geim et al's above mentioned paper) and polyurethane (see D Campolo et al's paper on “Fabrication of gecko foot-hair like nanostructures and adhesion to random rough surfaces”, IEEE Nano. August 2003). Average bond strengths with glass substrates of 30 kPa have been reported by Geim et al (see their above mentioned paper) for 1 cm2 patches of microfabricated polyimide fibres of length of 2 μm, diameter 0.5 μm with separation between fibres of 1.6 μm. According to Geim et al's paper, these values compare with estimated values for the adhesive bond strength of gecko feet hair of approximately 100 kPa. It is to be also noted that Kesel et al have reported an adhesion strength of 224 kPa for the jumping spider (see Kesel et al's paper. “The J of Exp. Biol.”, 2003, 206, 2733).
It is an object of the present invention to provide adhesive microstructures having significantly improved adhesion strengths at least at one surface as compared to known fabricated adhesive microstructures.
It is a further object of the present invention to provide methods of fabricating such adhesive microstructures. A yet further object of the present invention is to provide adhesive microstructures which provide good immediate adhesion on a variety of surfaces. Another object of the invention is to provide a method of producing relatively large areas of the adhesive material. Another object of the invention is to provide a re-useable adhesive microstructure.
In broad terms, the present invention resides in the concept of using the properties of deformable materials in fabricated adhesive microstructures to provide significantly high adhesion strengths at one or more surfaces, and in the methods of fabricating adhesive microstructures incorporating deformable materials.
Accordingly, in one aspect, this invention provides a fabricated adhesive microstructure comprising a deformable material which, in use, deforms to provide an adhesion strength at a substantially smooth glass surface of at least 120 kPa in air at one atmosphere pressure and at least 10 kPa less (preferably at least 20 kPa less, or more preferably at least 50 kPa less) adhesion strength in vacuum than that at one atmosphere pressure.
The term “adhesion strength” is used in the present specification and claims to mean tensile pull-off adhesion strength. Furthermore, as will be described hereinafter, all values of “adhesion strength” in this specification (except where stated otherwise) are to be understood to correspond to tensile pull-off adhesion strengths which were measured by use of a purpose-built beam balance at The Advanced Technology Centre, Filton, BAE SYSTEMS.
As will be described hereinafter, we have carried out tests and experiments using smooth glass microscope slides. Such slides are commercially available and can be purchased from a number of suppliers including Menzel GmbH (see their website: www.menzel.de).
The adhesion force measurements of our fabricated microstructures made in vacuum will be described hereinafter. The term “in vacuum” (as used in the present specification and claims) is to be understood in this context.
We do not understand fully the role which the deformable material to be used plays in the adhesion of the fabricated microstructures of the invention. We suggest, however, without intending to limit the scope of the invention in any way, that the reason for the improved adhesion strengths of our adhesive microstructures is the significant atmospheric “suction cup” force contribution which the deformable material provides in atmosphere, in addition to the Van der Waals' contribution. In support of this, as will be described hereinafter, we have surprisingly found that adhesion force measurements of our fabricated adhesive microstructures alternately in vacuum and in air indicate there to be a significant atmospheric contribution of up to about 100 kPa in air for a range of smooth glass contact surfaces. Advantageously, we have further found that the adhesion strength of fabricated microstructures of the invention for a range of smooth glass contact surfaces may be in the range of between about 125 kPa and 220 kPa in air at one atmosphere pressure and in the range of between about 25 kPa and 120 kPa in vacuum.
Preferably, the deformable material is an elastomer. Conveniently, synthetic elastomers are used. Conveniently, the elastomer is a silicone polymer. The polymer material may comprise polydimethylsiloxane (PDMS) which is known to contain units of the formula
where n is the number of monomer units in the polymer molecules.
Optionally, the PDMS is Sylgard 170, Sylgard 184 or Sylgard 186. It is to be noted that the silicone elastomers Sylgard 170, Sylgard 184 and Sylgard 186 are commercially available and can be purchased from a number of suppliers including Dow Corning Corporation (see the Dow Corning website).
Optionally, the elastomer is a polyurethane. Conveniently, the polyurethane may comprise monothane A30. It is to be noted that monothane A30 is commercially available from Chemical Innovations Limited of 217, Walton Summit Road, Walton Summit Centre, Preston, Lancashire, United Kingdom (see website www.polycil.co.uk).
In one embodiment, a first level of hierarchical compliance with the surface is provided in the structure by means of formation of a first number of protrusions on a first set of stalks, the protrusions and the stalks being formed of the deformable material and the protrusions being arranged to provide the adhesive strength at the surface. The stalk lengths may be in the range of between about 20 μm and 100 μm, and the protrusions may have generally mushroom-shaped head formations with head diameters in the range of between about 10 μm and 40 μm and thicknesses in the range of between about 1 μm and 3 μm. Advantageously, we have found that such structures can provide a generally uniform stress distribution at the interface between the stalks with mushroom-shaped head formations and the surface. Further, we have found that such structures have a level of compliance which permits improved contact and adhesion to a range of surfaces which may be rough on a variety of scales. Advantageously, such structures can be fabricated via different routes using moulding. Conveniently, these structures have been found to be sufficiently robust as to permit multiple reattachment with adequate adhesion to a number of surfaces. In addition, such structures have been found to work in the presence of fluids, for example water, and are amenable to cleaning procedures when inevitably dirt and contamination arise.
In another embodiment, one or more additional levels of hierarchical compliance with the surface are provided in the structure by combination of the above described set of stalks and protrusions with one or more additional sets of stalks and additional numbers of protrusions, the additional stalks and the additional protrusions being formed of the above described deformable material. Because such structures have at least one additional scale of compliance, it is possible to achieve significantly improved adhesion and contact of the structures to a range of surfaces. Advantageously, such structures can be fabricated using a moulding technique.
Optionally, a double-sided adhesive microstructure may be provided by providing the above described deformable material as a first layer on one surface of the structure and as a second layer on an opposing surface of the structure. Such a structure can be conveniently fabricated using a moulding process.
It is to be appreciated that the above described fabricated adhesive microstructures of the invention enjoy various benefits over currently available glues and adhesives. For example, our structures can be (a) reapplied effectively to various surfaces many times if desired, (b) applied to surfaces without relying on the use of messy glues, (c) used without requiring any special surface preparation, and (d) applied easily and rapidly. Additionally, our structures can stick to a wide range of surfaces. Furthermore, our structures are inert and biocompatible.
In another aspect, this invention provides a method of fabricating an adhesive microstructure comprising the steps of (i) providing a mould structure; (ii) introducing a curable liquid polymer into the mould structure; (iii) curing the polymer in the structure; and thereafter (iv) separating the polymer from the mould structure to form the microstructure.
In one example of the method, the mould structure may be provided by forming first and second arrays of cavities at opposing surfaces of a base material, and forming an array of channels which extend through the base material at predetermined regions between said first and second arrays of cavities. The cavities of the first array may have a significantly different size from the cavities of the second array. The cavities of the first array may have diameters of approximately 40 μm and the cavities of the second array may have diameters of approximately 20 μm. Optionally, in this example, the method may include a step of providing a support made of pyrex or SD2 glass, and bonding the support to the surface of the base material at which the 40 μm diameter cavities are formed.
The base material is conveniently formed of silicon. As will be described hereinafter, we have found that the above described structures having a first level of compliance with the surface can be fabricated according to this example of the method.
In another example of the method, the mould structure may be provided by forming an array of channels through a base material which is supported on an etch-stop backing material. Conveniently, the base material is formed of silicon and the etch-stop backing material is formed of silicon oxide. As will be described hereinafter, we have found that the above described structures having a first level of compliance with the surface can be fabricated according to this example of the method.
In yet another example of the method, the mould structure may be provided by the following steps: (a) forming a first array of cavities at a surface of a first base material; (b) forming an array of channels through a second base material which is supported on an etch-stop backing material; (c) attaching the first base material to the second base material at a surface such as to provide an alignment between the cavities in the first base material and the channels in the second base material at said surface; and (d) forming a second array of cavities at an exterior exposed surface of the attached base material, and forming an array of channels therefrom which extend through the base material at predetermined regions between said second array of cavities and said surface at which the cavities in the first base material and the channels in the second base material are aligned. Conveniently, the first base material is attached to the second base material using a bonding process. Alternatively, the first base material is attached to the second base material by clipping the first and second base materials together. Optionally, the first and base materials are formed of silicon, and the etch-stop backing material is formed of silicon oxide. The cavities of the first array may have a significantly different size from the cavities of the second array. The cavities of the first array may have diameters of approximately 40 μm and the cavities of the second array may have diameters in the range of between about 7 μm and 20 μm. As will be described hereinafter, we have found that the above described structures having one or more additional levels of hierarchical compliance with the surface can be fabricated according to this example of the method.
In each of the above examples of the method, each said array of cavities and each said array of channels are formed by applying lithography and etching techniques through the use of masks.
Optionally, the curing step of the method may comprise applying heat to the polymer in the structure at elevated temperature for a predetermined duration. The elevated temperature may be approximately 65° C. and the predetermined duration may be approximately 4 hours. Preferably, in the method the liquid polymer cures to an elastomer. The liquid polymer may comprise monothane A30. Alternatively, the liquid polymer may comprise polydimethylsiloxane (PDMS) which is known to contain units of the formula
Optionally, in the method the liquid polymer is introduced into the mould structure by (a) distributing the polymer across the channels of the structure; (b) placing the structure inside a chamber in vacuum and controllably extracting air from the channels; (c) restoring the chamber to atmospheric pressure; and thereafter (d) infiltrating the polymer into the channels. The liquid polymer introduced in this way may comprise monothane A30. Alternatively, the liquid polymer which is introduced may comprise PDMS (Sylgard 170, Sylgard 184 or Sylgard 186).
The present invention extends to a method of fabricating a double-sided adhesive microstructure comprising the steps of (i) forming a first adhesive microstructure according to the above described method; (ii) partially forming a second adhesive microstructure according to steps (i) and (ii) of the above described method; (iii) pressing the formed first microstructure onto the partially formed second microstructure whilst the polymer, PDMS for example, in the mould structure is in liquid condition; (iv) curing the pressed structure of (iii); and thereafter (v) separating the cured structure of (iv) from the mould structure so as to form the double-sided microstructure. Preferably, in this method the curing step comprises applying heat to the pressed structure at elevated temperature for a predetermined duration. Optionally, heat may be applied to the pressed structure inside an oven at approximately 150° C. for approximately 10 minutes.
The present invention further extends to a method of fabricating a double-sided adhesive microstructure comprising the steps of (i) defining a structure with a cavity region by juxtaposing first and second mould structures; (ii) introducing liquid polymer into the cavity region and subjecting the defined structure of (i) to vacuum conditions thereby to cause filling of the cavity region by said polymer; (iii) curing the filled structure of (H); and (iv) removing the first and second mould structures to leave a formation of the double-sided microstructure. Preferably, the first and second mould structures are in juxtaposed spatial alignment by providing a nylon spacer between said first and second mould structures.
Optionally, the first and second mould structures are removed in aforesaid step (iv) by mechanical release.
Alternatively, the first and second mould structures are removed in aforesaid step (iv) using a chemical etching process.
Conveniently, in this method, the aforesaid curing step (iii) may comprise applying heat to the filled structure at elevated temperature for a predetermined duration. The heat may be applied to the filled structure inside an oven at approximately 150° C. for approximately 10 minutes.
Optionally, the first and second mould structures are formed of silicon.
Alternatively, the first and second mould structures are formed of polyimide.
The polymer used may comprise PDMS (Sylgard 184 for example).
The present invention further extends to a method of removably attaching a fabricated adhesive microstructure to a surface comprising the steps of (i) applying the above described structure to the surface at a first location; and (ii) removing the structure for re-application to the surface at the same location or at a different location.
Optionally, the aforesaid removing step (ii) comprises a peeling action.
Advantageously, the aforesaid removing step (ii) may be effected or assisted by application of a chemical agent at the contact location between the surface and the microstructure. The chemical agent may comprise Skydrol liquid.
The present invention further extends to a fabricated adhesive microstructure comprising an elastomer which, in use, deforms to provide an adhesion strength at a substantially smooth glass surface of at least 120 kPa in air at one atmosphere pressure and at least 10 kPa less (preferably at least 20 kPa less, or more preferably at least 50 kPa less) adhesion strength in vacuum than that at one atmosphere pressure.
It is to be appreciated that the present invention has utility for many applications including (i.e. not limited to) the following: automated inspection robots, rapid reattachment of panels with no special surface preparation, for example in rapid field repair, attachment of access panels, “Spiderman gloves” etc.
The above and further features of the invention are set forth in the appended claims and will be explained in the following by reference to various exemplary embodiments and the specific Examples and Experiment which are illustrated in the accompanying drawings in which:
Table 1 is a table of etch parameters and processing conditions used in the method of
Table 2 is a table of properties of the moulding polymers used in the Examples of the invention;
Table 3 is a table of results of adhesive measurements for a number of structures produced according to the invention;
a) and (b) are further SEM images of another structure of the invention on a painted CFRP surface (note detachment of polymer from mushroom-head);
a) is a schematic plan view of a mould structure obtained using the method of step 2 in
Table 4 is a table of pull-off loads (adhesion strengths) as measured by different workers on different synthetic and real gecko materials;
a), (b) and (c) are images of another structure of the invention after (a) contamination with hairs, dust and dirt; (b) after cleaning using water droplets; and (c) after a water jet clean;
a) and (b) are images of a new double-sided adhesive microstructure produced by another method;
a) and (b) are images of another new hierarchical structure having multiple levels of compliance;
Referring first to
Two masks (not shown in
As will be readily understood by the man skilled in the art of lithography and etching techniques, etch parameters and procedures were used in this embodiment as given in Table 1 below.
The wafer was then anodically bonded to a Pyrex (or SD2 glass) substrate 15 (Step C), positioning the 40 μm cavities at the glass/silicon interface. The purpose of the substrate was to provide mechanical support for the wafer and give a flat surface for moulding the mushroom-shaped structures. The 20 μm mask was then used to pattern the top side of the wafer, which was then etched using the same procedures specified above to produce 20 μm diameter holes 20 through the entire thickness of the wafer, meeting the 40 μm cavities with a common axis (Step D). The mould was coated in fluorocarbon release agent, and a polymer PDMS solution 25 was then spun onto the mould (Step E). This was then cured for about 10 minutes at 150° C. The resulting casting comprising stalks and mushroom heads was then pulled out through the mould in a single peeling process. This produced microstructures 40 like that shown in the image 39 of
Conveniently, it is to be noted that the resulting mould made by using this method was suitable for making multiple casting operations.
It is to be further appreciated that the above described method 1 can be suitably modified of provide alternative new hierarchical structures having multiple levels of compliance.
In one possible modification example shown in
In another possible modification example, the steps A to D of the above described method 1 are performed to provide a structure with cavities on to which is bonded a silicon wafer with holes formed through its entire thickness. The two wafers are thus attached to each other at a surface in such a way that the formed hole/cavities in the wafers are made to coincide at the surface. Bonding is effected by forming a eutectic between the wafers, or by means of adhesive bonding. PDMS polymer is then introduced into the mould in exactly the same way and under then same conditions as described before in method 1 (see step E,
Because the structure 311 provides an additional level of elastic compliance, it is envisaged that this kind of structure can provide improved contact with a surface (for example, a matt painted CFRP surface) having a large scale of roughness.
In yet another possible modification example, the steps A′ to E′ of the above described method of
Referring next to one of the steps (step 2.) of
Wafers consisting of a 20 μm thick silicon layer on top of an oxide were obtained. These were patterned using negative versions of existing “coarse” and “fine” masks where, as in method 1 described above, blanking regions now defined the regions between hairs, rather than the hairs themselves. An example of a mask 45 defining the required features is shown in
Referring again to
As shown in
With the cavity and channel features so formed and aligned, a new silicon mould structure is thus achieved (see
It is to be appreciated that the above described method 3 can be suitably modified to provide alternative new hierarchical structures having additional levels of compliance if desired. It is also to be appreciated that the silicon layer dimension and/or the cavity diameter dimensions in this embodiment could be varied typically by several μms, if desired, so as to provide the same inventive effect.
An important variable controlling hair properties of our structures, including compliance, is recognised to be the modulus, hardness, tensile strength and tear strength. Proprietary brands of PDMS made and supplied by Dow Corning known as “Sylgard” are available in a range of different grades. In addition to the Sylgard 184 which was used in the Examples, Sylgard 186 and Sylgard 170 were also selected for evaluation using different moulds, including existing simple non-hierarchical moulds. As an additional option, Monothane with a Shore A hardness of 30 was obtained for evaluation. Monothane A30 is commercially available from Chemical Innovations Limited of Preston Lancashire UK (see website www.polycil.co.uk). Monothane is described as a single component, ester based, heat cure, castable polyurethane resin. Properties for each of these materials as used in the Examples are shown in Table 2 below. Further information on these materials was obtained by our own measurements or from the manufacturers' literature.
(3)G L Flowers and S T Switzer, “Background material properties of selected silicone potting compounds and raw materials for their substitutes”, 1978 May 01, Report No. MHSMP-78-18, http://www.osti.gov/energycitations/servlets/purl/7032853-hwLQRd/7032853.PDF.
(4)CIL Monothane Product Data, (4) Technical report “Empirical data on load extension for Monothane, PR-1564 and Neuthane 801”, TES 100770, Aug. 05, 2006.
(5)R. Pelrine, R. Kornbluh, J. Joseph, R. Heydt, Q. Pei, S. Chiba, High field deformation of elastomeric dielectrics for actuators, Mater. Sci. Eng. C 11 (2000) 89-100.
(6)http://mass.micro.uiuc.edu/publications/papers/136.pdf.
(7)http://www.lehigh.edu/~mkc4/our%20papers/She rolling.langmuir2000.pdf
Four different surfaces were used for assessment of attachment forces in our Examples. These were a smooth clean flat glass slide, a glossy painted aluminium surface typical of the quality used on the Hawk aircraft, a matt primer painted carbon fibre surface, and a matt primer painted aluminium surface. The matt painted aluminium surface had a small scale roughness with features of size typically ˜a whereas the matt painted carbonfibre reinforced plastic (CFRP) surface had both a small scale roughness and also a larger scale roughness with peaks and valleys with an amplitude of approximately 20 μm over distances of about 0.4 mm.
Attachment forces to the surface were measured in tension using a simple purpose-built balance at the Advanced Technology Centre, BAE SYSTEMS, Filton UK.
The specimen 87 was glued to the base of the balance and small mounting stubs were glued to the free surface of the glass slide.
Note that all examples of our adhesive material were bonded to a 12.5 or 25 mm diameter aluminium stub with Dow Corning Acetoxy Sealant 781—see 95 in
It should be noted that all adhesion measurements irrespective of surface required a pre-load compressive force to be applied to the specimen in order to obtain attachment. This was achieved using between ˜20 g-˜130 g dead weight applied for periods of a few seconds to a few minutes when undertaking multiple re-attachment tests. Exceptionally when making initial measures of the first attachment strength, a few specimens were left with the dead weight in-situ on the surface overnight. These long pre-load times were dictated by the need to cure the backing sealant over several hours with an applied load in order to ensure a uniform bond line. In general, larger values of adhesive pull-off strength were obtained when longer timers and larger values of the dead weight were used for pre-loading.
Thus, the pull-off adhesion force measurements were made on the specimens using the purpose-built balance according to the following procedure: (a) by mounting the adhesive microstructure specimen under consideration on a glass surface, loading at successively increasing loads, and measuring the adhesion force alternately in an evacuated vacuum chamber (typically 1 mbar or less) and in air, thereby effectively enabling an elimination of the atmospheric contribution by noting that load at which the specimen detached when in a vacuum.
An assessment was also made of separation distance between glass surface and hair surface using standard optical interferometry techniques when measurements were made with a glass substrate. In addition, when making measurement of an average tensile pull-off strength with a glass surface as the contact, estimates of bonding area were made by viewing the actual contact area from the non-contacting rear surface of the glass surface. An assessment of contact area and tensile pull-off strength was found not to be possible when opaque surfaces such as the painted Hawk or CFRP surface was used. All adhesion measurements for specimens based on Type 1, 2 or 3 specimens (see also the Examples) are summarised in Table 3 below.
1Notes: Stress was calculated based on the actual contact area of hairs with the surface, and not the total average contact area.
We envisage improvements in our fabrication techniques to increase the actual contact area of our specimens with the contact surface in proportion to the total contact area.
A pre-requisite for obtaining adhesion is that intimate contact is achieved between the top of the stalks of the specimen in question and the contacting surface. For Van der Waals' forces to operate, intimate contact between the stalks and the surface is achieved when the separation distances are typically less than 10 nm.
A key requirement to achieving intimate contact is the ability of the specimen structure in question to conform to the contact surface. A glass slide was used by the inventors as a suitable reference contact surface. This was found to provide a convenient surface which was flat, smooth and could easily be cleaned.
By careful arrangement of illumination and observation angle, it was possible to observe visually the formation of interference fringes formed in the cavity between the lower surface of the glass slide which was in contact with the structured PDMS surface, and the top surfaces of the PDMS stalks.
Hierarchical mushroom structure, Sylgard 184, hair length 100 μm, hair diameter 20 μm, head diameter ˜40 μm, head thickness 3 μm, 12.5 mm backing stub (above described Method 1).
Above described method 1 was used (refer to
A patterned mask defining circular features (40 μm diameter) was used (see
A patterned mask defining circular features (20 μm diameter) was then used to pattern the exposed top surface of the silicon wafer, and again by applying standard lithography and deep reactive-ion etching (DRIE) techniques well known to the man skilled in the art (see also DRIE references: R B Bosch Gmbh 1994 U.S. Pat. No. 4,855,017 and German patent no. 4241045C1; Lithography reference: Sze VLSI Technology, 2nd Ed., McGraw Hill Book Co. 1988), 20 μm diameter cavities were etched into the silicon to form channels extending through the entire thickness of the silicon and which meet the formed 40 μm diameter cavities about a common axis. Alignment of the formed 20 μm and 40 μm diameter cavities about a common axis to within an accuracy of ˜1 μm was achieved using an Electronic Visions EV620 Bottom-Side Aligner. With the cavity features thus formed and aligned, a new silicon mould structure with channel formations was obtained.
7.5 g of liquid PDMS (Sylgard 184 supplied by Dow Corning) was poured centrally onto the mould of
According to this Example, new adhesive structures (of the type shown in
Example 1 was repeated but instead of using SD2 glass substrate, a pyrex glass substrate was used.
A new structure of the type shown in
An example of a specimen of this type (see the image in
Inspection of
These results show that the best contact, as seen in
Given that the hierarchical head resembled very small suction pads, it was of interest to ascertain the extent to which the component of adhesion to a surface was due to molecular forces, such as Van der Waals' forces, and that due to atmospheric forces. As described previously using the balance, by mounting a new hierarchical specimen on a glass surface, loading at successively increasing loads, and measuring the adhesion force alternately in an evacuated vacuum chamber and in air, it was possible to eliminate the “atmospheric contribution” by noting that load at which the specimen detached when in a vacuum. This showed first detachment in vacuum at 300 g. A load of 300 g implied a molecular contribution of ˜111 kPa. Since a specimen of the same Type 1 (different sample—see Table 3) had already failed in air at 160 kPa, this implied there was also an “atmospheric contribution” of at least 49 kPa, and that potentially with a “full atmospheric pressure contribution” of 100 kPa such a specimen should ultimately give at least an adhesion strength of ˜211 kPa.
Hierarchical mushroom structure, Sylgard 184, hair length 100 μm, hair diameter 20 μm, head diameter ˜40 μm, head thickness 1 μm, 12.5 mm backing stub (Method 1).
Above described method 1 was used in this Example (refer to
In an attempt to improve adhesion to the rough painted CFRP surface, specimens with 1 μm thick mushroom heads were fabricated. Example 1 was repeated, but the shallow etch depth in the silicon was limited to 1 μm or so (instead of 3 μm). This was done by a routine variation of the etch parameters (based on the method 1 parameters), as would be understood by the man skilled in the art.
The resulting fabricated new structures were found to be similar in most respects to the type 1 specimens produced in Examples 1 and 2, except for the more compliant head feature which it was hoped would conform better to the small scale roughness of the surface.
An example of this specimen was tested successively for the tensile pull-off strength on a smooth glass slide, the glossy pointed Hawk surface and the rough painted CFRP surface.
Inspection of
Hierarchical mushroom structure, Sylgard 184, hair length 20 μm, hair diameter 8 μm, head diameter ˜10 μm, head thickness ˜1 μm (Method 2).
Above described method 2 was used in this Example (refer to FIG. 4—step 2.).
A wafer comprising a 20 μm thick, 100 mm diameter silicon layer on top of a 1 μm thick silicon oxide layer (the layer covering the entire wafer) was obtained. Such a wafer was purchased from the manufacturer Virginia Semiconductor Inc. (see their website: www.virginiasemi.com). A mask (of the type shown in
It has been thus found in this Example that the presence of the silicon oxide at the silicon/silicon oxide function resulted in increased etching of the side walls, resulting in the successful production of re-entrant mushroom head structures with disk features on stalk ends (as shown in the SEM images of
Hierarchical mushroom structure with enhanced mushroom head shapes, Sylgard 184, head diameter>10 μm (Method 2).
The procedure as specified in Example 4 was used. Structures of the type fabricated in Example 4 were then modified to provide deliberately enhanced mushroom head shapes by controllably depositing layers of the etch-resistant polymer material into the mould structure. This modification step was effected in accordance with a known etching procedure known as “footing”, as applied to mushroom-type structures (see on “footing”, the paper by Hwang, Gyeong, Giapis, and Konstantinos: “On the origin of the notching effect during etching in uniform high density plasmas (1997), Journal of Vacuum of Science and Technology B, 15(1) pp 70-87).
Successive tensile pull-off strengths were measured on the Example 4 type 3 specimen using the surfaces of a smooth glass slide, the rough painted CFRP specimen and the glossy painted Hawk. A maximum load of 750 g was measured on the glass surface, 53 g on the glossy painted Hawk surface and ˜1 g on the painted CFRP surface. The hair contact area fraction was estimated to be approximately 27% over the whole stub area, giving an equivalent tensile strength of 219 kPa for a glass surface maximum adhesion force of 750 g. After the first detachment of the specimen from the glass surface it was again noted that hairs had become detached in a similar fashion to that shown for specimen type 1 in
In this Example, a new double-sided adhesive microstructure was fabricated via moulding. The fabrication steps are shown schematically in steps 1. to 6. of
A silicon mould 180 was obtained as described in Example 1 above. 7.5 g of liquid PDMS 181 (Sylgard 184 supplied by Dow Corning) was then introduced by pouring it into the mould 180, exactly as described in Example 1, and then the mould was thermally cured at about 65° C. for a duration of about 4 hours whilst ensuring excess PDMS material was scraped off the mould with a thin rubber blade to provide a very thin backing (of ˜200-300 μm)—step 1. The resulting cured structure was then pulled out through the mould (step 2.). The pull out step involved the following: (i) carefully cutting around the edge of the mould with a sharp scalpel blade to provide an easy to peel edge, (ii) prising up one edge of the cured adhesive material with the scalpel blade and (iii) peeling the cured adhesive material up very carefully and slowly by hand using a 90° peel angle. We found that the peeling of a 4 inch diameter adhesive material usually took 2-3 minutes. This prepared adhesive material 183 was then put to one side.
Next, the silicon mould 180 was refilled with more liquid PDMS material 181′ (7.5 g, Sylgard 184 as before) exactly as described before (step 3.), and whilst the PDMS 181′ was still liquid, the already prepared adhesive material 183 (as described in this Example) was pressed down onto the mould 180 ensuring that the hairs were facing up (step 4.). The structure was then cured in an oven at about 150° C. for about 10 minutes (step 5.). The cured structure was then pulled out through the mould (step 6.) to provide the double-sided adhesive structure 185. This pull-out step was effected in the same way as the first pull out step (already described in this Example).
a) and (b) show images 190, 191 of the double-sided adhesive structure as produced by the method according to this Example. Note in these images the formation of separate adhesive layers on opposing surfaces of the structure.
Referring next to
Two separate silicon moulds 196, 197 are obtained. Each of the moulds could be obtained as described in Example 1 above. The moulds 196, 197 are then positioned close together in face-to-face relationship with a small controlled spacing 198 between them, defining a new mould structure 199 having a cavity region 200. A nylon spacer 201 is used to control the spacing between the moulds (step 1.). Liquid PDMS is then injected into the cavity region through a narrow bore needle (not shown) and the structure is then put under a vacuum to provide adequate filling of the structure pores (step 2.). Once the structure pores are adequately filled 203, the structure is cured in an oven at about 150° C. for 10 minutes (step 3.). Thereafter, the moulds are removed by careful mechanical release or by chemical etching (step 4.) to leave behind formation of the doubled-sided adhesive structure 205.
It is to be understood that other kinds of mould could equally be used in this method instead of the silicon moulds—for example, the use of polyimide moulds is envisaged to be amenable to this method.
It is believed that the foregoing Examples and embodiments provide ample instruction to the man skilled in the art to put the present invention into effect, but for the sake of completeness there is also provided below a discussion of the results and of some further tests and experiments on the polymers specified in Table 2.
We have found that our fabricated adhesive structures surprisingly exhibit significantly improved adhesion strengths of up to ˜220 kPa at least at a smooth glass surface, as compared to ˜30 kPa adhesion strengths for known fabricated adhesive structures. Various workers have measured pull-off loads on specimens under different conditions, and for the sake of completeness, all the values collated to date are set out in Table 4 below.
It is suggested that the significantly raised adhesion strengths of ˜220 kPa of our structures at least on a smooth surface such as smooth glass are due to an atmospheric “suction cup” and molecular (Van der Waals') component of force which typically contribute in roughly equal measure; thus, it is likely that whereas on smooth surfaces such as glass or glossy paint this full adhesion strength can be achieved, on other rougher surfaces where it is not possible to obtain any such atmospheric “suction cup” contribution, the strengths are significantly reduced to a maximum strength of ˜100 kPa. It is also recognised that roughness of surface results in less intimate contact which in turn causes a reduction in the adhesion strength. We thus propose to undertake further studies to accommodate several scales of surface roughness. It was found to be possible in these studies to deal with very small scale roughness of ˜1 μm successfully by means of reducing head thickness of our structures. To accommodate several scales of roughness, it is suggested we fabricate that new structures in accordance with the invention having additional scales of hierarchical compliance in relation to surfaces having larger scales of roughness.
In this connection, we have found that the above described method 3 can be used to fabricate new adhesive structures in accordance with the invention having multiple levels of hierarchical compliance with a surface. Significantly, it is to be noted that our proposed scheme bears the tremendous potential for producing large scale specimens with four levels of hierarchical compliance (see
In addition to Sylgard 184 which was used in the above described Examples 1 to 5, some tests on moulding and contact with surfaces were carried out using other polymers with properties shown in Table 2. Moulds of the kind described in Examples 1 to 5 were used in the tests.
The results obtained to date suggest to the inventors that, by using different new moulds for Monothane A30 and Sylgard 170, such polymers could be used to advantage in the present invention.
It was noted that the specimens used here exhibited superhydrophobic properties. This property is manifest as a very hydrophobic surface which exhibits no wetting, and is shown in the image 210 of
In order to exploit the superhydrophobic properties as a cleaning mechanism, a type 2 specimen was deliberately contaminated by dust and hair particles and measurements of adhesion made both in its pristine state, after contamination and after cleaning. In this instance, cleaning was obtained by both allowing water droplets to drop onto and roll off the surface without wetting in a manner akin to rain falling, and also using a jet of water ejected from a small squeezy bottle in which case some wetting of the surface occurred. Cleaning occurred in both cases. A “before” and “after” image for the droplet and water jet cleaning method for the specimen is shown in
Measurements of adhesion force recorded for the specimen were 200 g in its re-contamination state, 85 g when contaminated, 85 g after cleaning using droplets and 220 g after the water jet cleaning. These values are shown on a graph 230 schematically in
Skydrol is a common liquid used on aircraft and it was of interest to examine adhesion when the bond was contaminated by this liquid. An example of a type 2 specimen was tested by dropping fluid onto an already adhered specimen on a clean glass surface, and also by trying to establish adhesion on a Skydrol contaminated surface. Adhesion force dropped from 200 g to ˜40 g after 2 hours exposure to Skydrol on the pre-adhered clean glass surface, and was ˜10 g in adhesion testing on a pre-contaminated surface.
Thus, having regard to the foregoing, it is recognised that the requirement for easy detachment of our adhesive material from the surface is an important consideration, for example for effective multiple attachment applications. Detachment could be achieved via a mechanical peeling action in some circumstances. However, it is equally recognised that certain circumstances may arise where a peeling action is not possible and it is still necessary to remove and then re-attach the material to a surface (at the same location or at a different location). It is proposed that under such circumstances it might be possible to weaken temporarily the bond at the contact area between the adhesive material and the surface, as seen in the de-clumping and reduced adhesion action of Skydrol. It is possible that Skydrol is not unique in conferring poor adhesion, and that there are other more environmentally-friendly liquids which could perform a similar function to Skydrol in respect of providing the de-clumping/reduced adhesion action. Given such liquids can be identified that rapidly penetrate between the hairs of the adhesive structure, rapidly evaporate after use and result in no deleterious effects, it is envisaged that such a scheme might be suitable for use as a multiple attachment methodology for large non-peelable adhesive materials.
Whilst we have described the use of particular polymer materials (as listed in Table 2 above) in the invention, the man skilled in the art will appreciate that other elastomers can be used in accordance with the present invention, with a reasonable amount of trial and experiment. Such other elastomers may be conventional elastomers or thermoplastic elastomers. They may be natural or synthetic. They may contain for example, styrene, butadiene, isoprene, chloropene, urethane, acrylonitrile, ethylene, propylene, ester, and/or amide units. If copolymers, they may be random or block copolymers.
A further envisaged application of the method of the present invention is in the production of adhesive microstructures based on a different combination of the above described methods. Cylindrical cavities are etched into a first silicon wafer extending through the thickness of the silicon based on steps A, B and D of method 1, omitting the step C (i.e. omit the bonding step to Pyrex/SD2 glass substrate). This wafer is then positioned and aligned on a second silicon wafer with cavities which is formed by method 2. PDMS is then introduced into the channels of the resultant mould structure in the same way as described in method 3 (see
It is to be understood that any feature described in relation to any one embodiment or Example may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments or Examples, or any combination of any other of the embodiments and Examples. Further, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07254250.9 | Oct 2007 | EP | regional |
| 0721044.6 | Oct 2007 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2008/003619 | 10/27/2008 | WO | 00 | 8/11/2009 |
