Principles of Barnacle Adhesion Flashcards
(43 cards)
- Ways of sticking to surfaces:-
- Mechanical
- Suction
- Viscosity
- Capillarity
- Friction
What is adhesion?
“The action or process of adhering to a surface or object”
Why is adhesion more common in the marine environment?
Unlike on land, marine animals can be sessile because food is brought to them, waste is taken away and gametes dispersed.
This saves energy, but their attachment must be strong as development causes mortality.
Synthetic adhesives
- sticking underwater
- does not require clean surfaces
Mechanical interlocking
Mechanical interlocking - lock and key type adhesion to the surface (although electrostatic interactions are also required…).
• At the micro-scale, most materials are rough.
• The glue must be able to spread on the surface.
• The glue must be able to enter the roughness features.
• The glue must then harden.
• All of these processes present challenges underwater, as we will see!!
Mechanisms of adhesion:
2. Suction & Stefan adhesion
Neither of these is really ‘adhesion’ mechanisms per se, in that there are no attractive forces. Rather, they are physical processes that interfere with removal.
Suction
- Requires an elastic cup which is initially contracted and then expanded to form a zone of reduced pressure.
- Not attractive.
- Theoretical maximum resistance of 1 atm.
- Very weak in ‘shear’. Easy to slide off.
Stefan adhesion
- Resistance to viscous flow
- No zone of reduced pressure
- Not attraction
- Attachment pad - viscosity of medium and speed the pad is being moved
Capillary tubing
- Depends on the surface energy (determined by the chemistry) of the surface and the surface energy (called surface tension…) of the liquid.
- If the tube has more ‘energy’ (is more reactive) than the liquid, the liquid will ‘prefer’ to stick to the glass, rather than to itself, so it is drawn up the tube. Glass has a lot of free energy compared to water.
- Can also draw surfaces together, but liquid must have lower energy than the surface for this to work.
- But like suction, capillary is very weak in shear.
Friction
Octopus (among other things) - manage to attach very strongly using, ostensibly, a suction mechanism. - stronger adhesion than 1 atm
- This supernatural feat, and many others, can be ascribed to a frictional contribution to adhesion.
- In order to break a suction bond, the seal must be compromised – usually by pulling inwards.
- If this can be prevented, the suction force is theoretically limitless.
- Slip of the perimeter seal is governed by friction….
Many different unrelated organisms have developed similar ways in overcoming adhesion problems - give the example of tree frogs.
- Tree frogs secrete a liquid adhesive (mucous) from their soft toe-pads, producing a capillary effect
- Capillary adhesion is weak in the ‘shear’ direction, however, so frogs must also use boundary-layer friction between their cuticle and the surface to prevent slip.
- Toe pads subdivided into little grippers
Many different unrelated organisms have developed similar ways in overcoming adhesion problems - give the example of bush cricket.
The tarsi of the cricket Tettigonia viridissima also have smooth flexible pads that are used for capillary attachment.
A liquid (cuticular oil) is also secreted from between the lamellae of these pads, mediating adhesion in a similar way to tree-frogs.
Microscopic droplets of water in the cuticular oil form an emulsion that stiffens under strain. The more water, the more resistance to shear.
This ‘non-Newtonian fluid’ hardens ONLY when a force is applied!
J. R. Soc. Interface., 2006; 3:689-697
Challenges od adhesion mechanisms for marine organisms
- Surface energy
- Surface hydration
- Surface contamination
- Surface roughness
Describe surface energy
- Surface energy is a measure of the ‘reactivity’ of a surface; how capable it is of forming new bonds with materials it comes into contact with.
- Hydrophilic - high energy - love water - water spreads out across the surface.
- Hydrophobic - low energy - hate water - water forms droplets
- Superhydrophobic - specific surface texture means water barely even comes in contact with it.
Challenges - surface energy
Glue gets outcompeted by water for contact with the substrate
what is true in air does not translate underwater….
- A hydrophilic surface, which would be optimal for adhesion in air, can be very difficult to contact underwater due to strongly bound water – a hydration layer.
- Ironically, therefore, organisms may be expected to adhere better to low-energy surfaces underwater better than they can to high-energy surfaces.
- Some organisms settle with sticking to hydrophobic surfaces that they can contact easily. But some either remove water from the interface, or incorporate it into their glue, so that they can stick strongly to hydrophilic surfaces. •
Challenges: 2. Surface hydration
Removing water from the interface:
- • Mussels and barnacle larvae (cyprids) both use lipids to remove water from surfaces which, presumably, enables them to attach more easily to immersed hydrophobic surfaces.
Incorporating water into the adhesive:
• Water can be incorporated into adhesives in a process known as ‘complex coacervation’, mussels do this - Recognise that a high energy surface is the best to stick to, so remove the water
Challenges: 2. Surface hydration
Challenges: 3. Surface contamination
Research does not have a good handle on how organisms deal with surface contamination.
- Just as was the case for dealing with water, biofilm, for example, can be either incorporated into the glue, or removed from the surface.
- It seems that the tubeworm Hydroides may do the former, while mussels appear to brush away biofilm with their foot before attaching.
- Surface contamination also, inevitably, affects the surface energy and the wetting properties of the surface.
how does surface Surface roughness affect adhesion
• Generally speaking, rough surfaces have stronger adhesion (when mechanical interlocking is used).
however, whether the glue penetrates the texture depends on whether the surface is in the Cassie or Wenzel state…
which, in turn, depends on the surface energy of the material.
tend to find the cassie state on superhydrophobic surfaces that has been roughened in a certain way.
• If the flat material has a contact angle when its smooth of more >90o, then it may enter the Cassie state when rough it becomes more hydrophilic.
If the flat material has a contact angle of less than 90o, when it becomes rough it becomes more hydrophilic, wayter runs through nooks and cranny and spreads out.
What are the different states a water droplet can be in?
Water droplets on rugged hydrophobic surfaces typically exhibit one of the following two states:
(i) the Wenzel state in which water droplets are in full contact with the rugged surface (referred to as the wetted contact) or
(ii) the Cassie state in which water droplets are in contact with peaks of the rugged surface as well as the “air pockets” trapped between surface grooves (the composite contact)
Challenge 4 : surface roughness
- Remember, all of this changes underwater….
- A surface in the Cassie state may retain trapped air when placed under water which ‘may’ (the jury is out on this…) help prevent adhesion.
- If the air is not trapped, however, even a textured hydrophobic surface will enter the Wenzel state when placed underwater.
- This will allow entry of the adhesive into the texture and promote strong adhesion.
Challenges 5 reversibly
- describe tube feet
- Not many adhesive mechanisms are truly reversible.
- One that certainly is not, but looks like it is, is the tubefoot of the seastar….
- Involves a 3-gland system.
- Sfp1, (starfish foot protien 1) a large protein of 3853 aa, is the second most abundant constituent of the adhesive.
- Sfp1 is translated from a single mRNA and then cleaved into four subunits before being linked back together by disulphide bridges in a different combination - mature adhesive protein - not sure why it does this.
- Present in one of the glands and secreted and mixed with the contents of another one of these glands and produces a very strong teo component permandent adhesive bond.
- releases an enzyme from a third gald, digests the adhesive at the interface - brief permanent adhesion mechanims
Suction remora
The remora’s ‘sucker’ is a modified dorsal fin.
The fin is flattened into a pad and surrounded by a thick lip of connective tissue that creates the suction seal.
The lip encloses rows of plate-like structures called lamellae, from which rows of tooth-like spinules emerge for mechanical grip.
Suction Cephalopods
In the cephalopods, use of a liquid glue is an ancestral trait. More advanced cephalopods rely more on suction.
4 genera (Nautilus sp., Sepia sp., Euprymna sp. and Idiosepius sp.) use liquid adhesives.
One genus, Euprymna, also uses a deadhesive – i.e. produces one material to attach and another to detach!
Limpets
• Modern-day limpets also rely on suction for adhesion, using it for attachment during locomotion in addition to capillary/Stefan adhesion.
When exposed by the tide, however, limpets secrete a true adhesive hydrogel, containing:
o <97% water and several polar proteins,
o A 140 kDa glycoprotein complex for adhesion,
o Gives a tenacity of <500 kPa.
(all that’s known about marine gastropods)
snake and slug
• Similarly, the terrestrial slug Arion subfuscus produces a defensive secretion that is sticky and tough, despite being more than 95% water.
In tensile tests, the glue sustains an average peak stress of 101 kPa (it is strong!), and fails at an average strain of 9.5 (and stretchy!). How??
