09/15/09 Notes:

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Misc Wet Adhesion Articles


Wet Adhesion can be divided into two categories: Adhesives intended for use in Wet Environments and Liquid Adhesives. This section lists some useful papers and articles relating to miscellaneous natural and synthetic wet adhesion

  1. Federle et al. "An Integrative Study of Insect Adhesion: Mechanics and Wet Adhesion of Pretarsal Pads in Ants. Integrative and Comparative Biology 2002 42(6):1100-1106. This paper discusses the adhesive mechanism on the legs of tree-climbing ants. These ants employ both a liquid adhesive secretion and either a suction cup-like mechanism or adhesive setae. The liquid adhesive generates adhesive forces using both capillary force and forces due to viscosity.
  2. Lee, Lee and Messersmith. "A Reversible Wet/Dry Adhesive Inspired by Mussels and Geckos". Nature 19 July 2007. 448: 338-341. Dry, ecko-inspired adhesives augmented by mussel-inspired liquid adhesives are used to provide strong adhesion in wet environments.
  3. Autumn, K. 2006. Properties, principles, and parameters of the gecko adhesive system. In Smith, A. and J. Callow, Biological Adhesives. Springer Verlag.
    • This is a lengthy chapter from a larger textbook on biological adhesives. Introduction includes physical description of gecko setae and desirable traits. The adhesive properties of the setae are described in detail, including associated body and leg dynamics, molecular mechanisms, and the JKR model. Mechanical properties and scaling of setae are also addressed.
    • A section on material properties summarizes contemporary work with gecko inspired adhesives in wet environments. Autumn rejects the conclusion that water could promote capillary adhesion, because of evidence that gecko setae are hydrophobic. (This could be refuted by Israelachvili, 2008; see below). Autumn proposes two opposing outcomes of water presence on setal adhesion on rough surfaces: reduction of adhesion due to prevention of spatular penetration into gaps or improvement of adhesion due to the water filling in gaps and increasing the contact fraction.
    • Autumn presents a summary of contemporary research on synthetic gecko structures, as well as a comparision of conventional and gecko adhesives.
  4. J. Israelachvili. Intermolecular and Surface Forces, Second Ed.; Academic Press Limited: London, 1995.
    • Book provides extensive background in surface forces. Includes lengthy section on adhesive forces between two surfaces with mathematical formulae that account for surface geometry and distance, composition, and interlayer composition.
    • Relevant material includes Van der Waals Forces between Surfaces (Chapter 11), Electrostatic Forces between Surfaces in Liquids (Chapter 12) and Adhesion (Chapter 14). Chapter numbers based on 1985 version.
  5. J. Israelachvili. Pre-Publication: Water Makes Beta Keratin in Gecko Setae Change from Hydrophobic to Hydrophilic. Private Communication 2008.
    • Describes molecular of gecko setae in the presence of water. Structural changes in proteins found in gecko setae cause the normally hydrophobic material to behave like a hydrophobic substance.
    • The implications of the paper are that adhesion in wet environments could be caused by capillary forces.
  6. Lee, H. et al. (2007) A reversible wet/dry adhesive inspired by mussels and geckos. Nature 448, 338341
    • Paper introduces Geckel material: gecko-inspired features combined with a thin mussel-mimetic polymer film. Force graphs show improved adhesion over biological gecko adhesion in both air and water. However, the presence of the mussel-mimetic film introduces a need for significant forces during detachment. The results were consistent over more than 1000 adhesion cycles.
  7. Mahdavi, A. et al. (2008) A biodegradable and biocompatible geckoinspired tissue adhesive. Proc. Natl. Acad. Sci. U. S. A. 105, 23072312.
    • Paper introduces gecko-inspired features on bandages with a tacky sugar-based coating. The bandages are used as biodegradable sutures during surgery. The sutures are meant to be permanent, so resistance to detachment was not included in the report.
  8. M.T. Northen, K.L. Turner, A batch fabricated biomimetic dry adhesive, Nanotechnology 16 (2005) 11591166.
    • Paper discusses the manufacture and testing of a synthetic, dry, gecko-inspired adhesive.
  9. Sun, W., Neuzil, P., Kustandi, T. S., Oh, S. & Samper, V. D. The nature of the gecko lizard adhesive force. Biophys. J. 89, L14L16 (2005).
    • Study of natural gecko setae properties and abilities.
  10. Miyake, T., Ishihara, H., Yoshimura, M.. Basic Studies on Wet Adhesion System for Wall Climbing Robot. IEEE Proceedings November 2, 2007 San Diego, Conference on Intelligent Robots and Systems, 1920-1925.
    • Study of Adhesion and Sliding Friction as a function of vacuum pressure, pulling speed, and liquid interlayer viscosity. The study was directed towards the design of a window-cleaning, wall-climbing robot ("WallWalker") which uses suction cups to adhere to very smooth surfaces.
  11. B. Bahr, et al., Design and suction cup analysis of a wall climbing robot, Computers Elect. Eng. Vol. 22, No. 3, pp. 193-209, 1996
  12. Mclaren, A.D. Adhesion of High Polymers to Cellulose. Journal of Polymer Adhesion, 1948. Vol. 3, No. 5, pp. 652-662.
    • Contains a general review of wet adhesion before launching into a specific case for adhesion to cellulose
    • discusses factors that affect strength of an adhesive such as long chain polymer structure, cohesion, surface tension, wetting, absorption, molecular orientation, inter-molecular forces, nature and spacing of chain substituents, tensile strength, shear strength, elasticity, electrical properties/polarity

Miscellaneous Wet Adhesion Links

Encyclopedia article on adhesion, comparing covalent, van der Waals, other bonding: http://www.substech.com/dokuwiki/doku.php?id=fundamentals_of_adhesive_bonding

Micro piezo dispensing heads: http://www.microfab.com/equipment/devices.html (cyanoacrylate is among the acceptable fluids)

http://www.dispensinglink.com/manual_diaphragm.htm

http://dx.doi.org/10.1016/j.snb.2007.10.064 review of MEMS micropumps

http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=00659772 - gripping with low viscosity liquids

http://www.xavitech.com/?gclid=CJmQ6rO6j5cCFQ8Qagod7yF7jg

variously Google "mems micropump adhesive dispense"

Fishman Corporation for precise dispensing of cyanoacrylate adhesives (CAs).

new trends in cyanoacrylate adhesives (bonding to silicone rubber, rubber-filled for complaince...)

http://www.edn.com/pressRelease/2140242289.html

for stickybot repair? http://www.shop3m.com/urethane-adhesives.html

http://www.shop3m.com/3m-scotch-weld-polyurethane-reactive-adhesives.html


Frog-Inspired Wet Adhesion Project


A few links: MarkCutkosky 11-17-03


STATE OF THE ART

Green, D. (1981). "Adhesion and the Toe-Pads of Treefrogs." Copeia(4): 790-796

Emerson, S. and D. Diehl (1980). "Toe Pad Morphology and Mechanisms of Sticking in Frogs." Biological Journal of the Linnean Society 13(3): 199-216.

Wet adhesion includes capillarity (surface tension and Laplace pressure) and stefan adhesion (ie - viscous forces). Capillarity is caused by surface tension at the air/liquid interface, producing a concave meniscus. Capillarity depends on good wetting of the surfaces and a low contact angle between surfaces. Force is proportional to both contact area (Laplace) and circumference (surface tension). Stefan adhesion describes the situation where a viscous fluid fills the space between two plates and, due to its viscosity, resists the flow that accomanies the separation of the plates.

Tree frogs adhere by capillarity. The wetting agent frogs use is a coating of smooth cells on the pads, augmented by mucus. Numerous mucus glands provide a supply of liquid over entire pad for good wetting of surface, even on low surface energy substrates (like waxy leaves). Canal-like separations between cells drain excess mucus. Frog mucus is a good wet adhesive: viscous, long chain, high molecular weight polymer.

Approximate adhesion force in tree frogs: 14.3 g/cm2

In Ingo Scholz, W. Jon P. Barnes, Joanna M. Smith and Werner Baumgartner, (2009) "Ultrastructure and physical properties of an adhesive surface, the toe pad epithelium of the tree frog, Litoria caerulea White" by in Journal of Experimental Biology 212, 155-162 in particular at page 159, the image at AFM shows the channels between the pillars and the surface of the top of a pillar. The surface on the top seems to have a texture within hunderd nanometers, it means a roughness able to trap air and to be hydrophobic. converselt the vertical wall of the pillar seems to have a different texture that can be morre hydrophillic.

For hydrophobic conditions due to surface roughness we can refer to the following paper: S. Moulinet, and D. Bartolo,(2007),Life and death of a fakir droplet: Impalement transitions on superhydrophobic surfaces, THE EUROPEAN PHYSICAL JOURNAL E 24, 251260

FORCES: CAPILLARY AND SHEAR

Gualtiero put in the file some ideas about shear force and on the mechanics that should govern sherar force. My idea is that adhesive (capillary) force generate the normal force that generate static friction (via Coulomb's friction). A proper design of both pillars and gaps can be done, in particular the following elements seem interesting:

- channel gap: it does NOT affect the capillary force exerted by each channel; actually the force is due to the two menisci formed at right and left side of the channel but does not depend on the liquid quantity. --> the force is proportional to the Lenght of the channels .

- pattern: some pattern have a higher ratio (Lenght of the channel/Surface of the pillar). The higher is the ratio the higher is the force. There is a very big difference in such ratio among square, centered square and exagonal pattern and also differences with the pillars' shape.

- different functions: probably different areas should have different functions such as for example different wettability. in detail: the space between the pillars (whose lenght is h-e, e<<1) is responsible for the function "store liquid", the region at the top of the pillars (namely between h-e and h) is responsible for the meniscus formation, the upper surface could be responsible for "preventing water motion" and for providing contact surface for exerting shear stress.

DESIGN: PILLARS' SHAPE, PATTERN, ETC..

DESIGN Consequences: gap --> store liquid, exert surface tension force --> the gap can be small, in order to provide an higher force its lenght should be high, the frog use an "exagonal pattern" for that reason. From my calculation an exagonal pattern of cylindric pillars could exert a force 1.15 times greater than a square pattern (cylindrers remainig the same).

Passing from circular pillars to poligonal pillars (squares and exagons) the force can reach 1.27 times that obtained for a square pattern of cylinders and 1.1 times that obtained for exagonal pattern of cylinders. See the attached file pattern.pdf and check it, please.

In particular, the pattern with staggered square block pillars can better prevent crack propagation in the meniscus. The design is directional (it means it works prevalently in one direction) and not so powerfull as the exagonal, but it seems to be able to work.

MANUFACTURING

MANUFACTURING (by Gualtiero)

In order to reduce the gap while continuing using the present manufacturing technology I suggest the following scheme.

The process consists of 5 phases:

1) ELASTIC SUBSTRATUM. create a thin layer of blue rubber, you can use a milled square frame, you put a drop of liquid silicon blue rubber on it, spread out by using a blade that slip over the frame --> you obtain a flat rubber square, leave it there for one hour until it is dry.

2) MASK. drill (by laser, drill or end mill) a sheet of plastic or steel and create a series of through holes, deburr the mask. Note: by using SU8 technology we can also obtain an exagonal pattern with exagonal pillars.

3) in order to reduce the pitch between the pillars we can adopt also a pre-stretch of the solidified ELASTIC SUBSTRATUM (1) as shown in the pdf file.

4) PILLARS MANUFACTURING. Pour a drop of new blue liquid rubber on the mask, turn it down and press it against the ELATIC SUBSTRATUM (1) the excess of rubber should flow out from the holes.

5) ELIMINATE THE EXCESS. By using a blade remove the excess of rubber from the upper part of the mask. Let the silicon drys.

6) REMOVE FIRST THE MASK AND THEN THE STRESS. when the mask is removed the pillars have a distance P, if you now remove the stress the distance decreases as well.

MANUFACTURING (by Mark)

Still another option might be to make a positive on the Haas. I have an idea about doing this:

1. Make a sparse checkerboard array of hexagonal pockets. The pockets are quite small (as big as we are willing to let our pillars be) and also reasonably shallow so we can use a small tool.

2. Fill the pockets with some temporary silicone and let it harden.

3. Now machine the alternate checkerboard of pockets, in between the ones we have previously machined and filled.

4. Remove the silicone from the first set of pockets. We now have a (quite fragile) positive mold onto which silicone can be poured to make the desired structure.

The reason for doing this is that with the Haas we can make pockets very close to each other, with very thin walls between the pockets, if the walls are not free-standing. By making first the "odd" elements in the matrix and then temporarily filling them, we provide support to the walls.

MANUFACTURING (by Mark 2)

If this seems like it will be too fragile, we can try the idea of embedding a sacrificial film to make the channels.

This could be done simply by embedding thin sheets of teflon or something like that in a stiff elastomer and then pulling them out.

EXPERIMENTS

1) the first set of experiments is oriented to understand some possible adhesive mechanisms in robots climbing a wet surface and

2) validate the results of the force as calculated by analytical models

3) use a coloured liquid (water+ink) to determine the shape of the meniscus in different condition of adhesion, geometry of the pillars and their arrangement.

EXTRA) if possible it could be interesting the analysis of the fingerprints left on a vertical glass by a real frog. The idea is that the frog should leave some traces (fingerprints) of its toe pad. The fingerprints should have both microscopic and nanoscopic information about the pattern. I attach a pdf file possible_experiments.pdf concerning points 1-2-3 and extra.

PRESENTATIONS

  • January 15, 2010: WetAdhesionPresentation_011510.ppt:Kimberly Harrison's Group Presentation which includes information about prototype testing and the design of a flexure based force sensor


Aerovironment Wet Gripper Project


The aim of this project was to design a gripper which used a wet adhesive to quickly attach a light, flying object to a vertical or sloped surface. Here we describe the results of some of our investigations.

Notes on Specific Wet Curing Adhesives

Tile Bond Roof Adhesive: http://www.bestmaterials.com/detail.aspx?ID=14828
  • tack-free in 5-15min
  • full cure 4 hours (accelerated by humidity)
  • shelf life 1 year
  • substrates include roof tiles

Sonoclastic Clear 25: http://www.bestmaterials.com/detail.aspx?ID=11225

  • tack-free in 25-30min
  • full cure 14 days (accelerated by humidity)
  • shelf life 1 year
  • substrates include clay tile, concrete, masonry, aluminum, wood, vinyl

Everbuild Stixall Combined building adhesive and sealant: http://www.powertools2u.co.uk/Sealants-and-Adhesives/Everbuild/Everbuild-Stixall-Combined-building-adhesive-and-sealant-290ml.htm

Glue Name Tack Free Full Cure Shelf Life Viscosity Substrates
Tile Bond Roof Adhesive 5-15min 4 hours 1 year ? roof tile
Sonoclastic Clear 25 25-30min 14 days 1 year ~16,000 poise clay tile, concrete, masonry, aluminum, wood, vinyl
Elmer's Rubber Cement ? ? ? ? ?
Best Test White Rubber Cement ? ? ? ? ?
Lexel Clear SuperElastic? Sealant ? ? ? ? ?
LubriMatic? White Lithium Grease ? ? ? ? ?
3M Super 77 Multipurpose Adhesive ? ? ? ? paper, fabric, foam, plastic, metal, wood, and more!

Notes from empirical testing of wet adhesives

- need a very viscous liquid to prevent air from coming through the pores of a porous solid like brick. Also need a non-zero thickness of this liquid-- this may be the more important thing.

Using a suction cup plus grease doesn't work, the suction cup has internal forces which causes the lip of the cup to push down against the brick while the middle of the suction cup lifts away from the brick. Once the lip of the suction cup gets close to the brick, air seeps in through the brick and the seal is lost.

Using a large flat piece of wood with lots of grease under it works pretty well, you can support 5 pounds in normal adhesion. It is very easy to slide it sideways, however. If you have only a thin layer of grease, the adhesion is less.

Using syrup or rubber cement or some other less-viscous fluid doesn't work well (with either the suction cup or the flat wood piece). It seeps into the brick, removing the layer of liquid between the brick and the wood/suction cup, and then the seal is weaker. Even right at first before it seeps in I think the seal is weaker -- I think there is some property of the liquid about how easily it cavitates or breaks the internal bonds of the liquid, and less-viscous liquids are worse at this. Also, I think with a non-viscous liquid it is a lot easier for it to be locally displaced, enabling e.g. one corner of the wood piece to temporarily touch down to the brick (due to transient forces, for example). Again, when it touches down, the seal is lost.

Testing a silicone suction cup on a flat, aluminum surface with syrup yielded max normal force of about 3-4 pounds. We think that a more uniform suction cup shape could yield higher forces.

Dual durometer suction cups (with higher stiffness near the center, and softer material near the outer edges) will yield better results than the all-stiff and all-soft suction cups.

We will investigate light-cure adhesives. One possible product is the 3M Light Cure Adhesive: http://solutions.3m.com/wps/portal/3M/en_WW/electronics/home/productsandservices/products/TapesAdhesives/LightCureAdhesive/ There are two types: UV cure and Visible-Light Cure. The UV-cure adhesive requires 0.5 seconds to cure, and the Visible-Light Cure requires 2-5 seconds. They offer both types of cure adhesives with varying viscosities. Shelf life is 6 months at room temperature, 12 months if you keep it in the refrigerator. We need more information on possible solvents for a release mechanism.

Super glue from ZAP is supposed to have a 12 month shelf life. ( http://supergluepds.com/DDefault.aspx?tabindex=1&tabid=4&cat=1 ). Another website ( http://www.stewmac.com/shop/Glues,_adhesives/Stewart-MacDonald_Super_Glues/3/Stewart-MacDonald_Super_Glues/Instructions/I-5180.html ) says:

Stewart-MacDonald Super Glues have a relatively long shelf life at room temperature: a bottle will be usable for up to a year, although the glue will thicken as the months go by. Once it is too thick for its original use, you can use it on other projects requiring a thicker glue.

Ultraviolet rays (sunshine) will reduce the shelf life of cyanoacrylates. High temperatures can deteriorate the original bonding strength of the adhesive.

Store in a cool dark location. Refrigeration at 40-50F (5-10C) is recommended for extended storage. Store partially-used bottles at room temperature: used bottles contain moist air from your workroom, which condenses and reduces shelf life. Always keep the bottles upright when not in use.

Accelerator needs no special storage, but avoid high temperatures and keep the cap tightly sealed. NEVER store adhesives with an accelerator.

Avoid moist or humid conditions. Replace cap tightly. Store in airtight conditions with a desiccant for best results.

This web site: http://www.glue-shop.com/info2.htm says you can get up to 7 years shelf life with superglues:

THE SHELF LIFE OF OUR CYANOACRYLATE ( SUPERGLUE ) ADHESIVES AT ROOM TEMPERATURE IS TYPICALLY 1-2 YEARS, BUT WHEN STORED IN A REFRIGERATOR ( BELOW 10 DEG C ) THE SHELF LIFE IS EXTENDED UP TO 7 YEARS!, AND IF STORED IN A FREEZER ( BELOW -5 DEG C ) THE SHELF LIFE IS VIRTUALLY INDEFINITE.

 
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