# The Sticky Physics of Van der Waals Forces

Geckos are amazing creatures.  They scamper up walls, scuttle along ceilings and hang upside down on polished glass surfaces.  However, the secret of their amazing climbing ability remained a mystery until relatively recently.  The secret lies in weak intermolecular forces, described by Van der Waals in 1873.

Johannes Diderik van der Waals was born in Leiden in The Netherlands in 1837.  Despite the constraints of his working class education, Van der Waals rose up to become the first Physics professor of the University of Amsterdam in 1877.  He went on to win the Nobel Prize of Physics in 1910, for his work on the equation of state for gases and liquids.

The van der Waals equation is based on a modification of the ideal gas law.  It approximates the behaviour of real fluids, taking into account the nonzero size of molecules and the attraction between them.

The equation may be used as a PVT equation for compressible fluids (polymers).  In this case, the specific volume changes are small and it can be simplified as:

$(p+A)(V-B) = CT$,

where p indicates the pressure, V is the specific volume, T is the temperature, and A, B, C are parameters.

The van der Waals force (or van der Waals’ interaction) is the sum of the attractive or repulsive forces between molecules (or parts of the molecule) other than those due to covalent bonds or the electrostatic interaction of ions with one another or with neutral molecules or charged molecules.

# Opposites Attract

All intermolecular forces are Van der Waals forces.  They are not true bonds in the sense of sharing or transferring electrons, but are weaker attractive forces.  These forces include dipole-dipole forces, hydrogen bonding, and ionic interactions:

• Dipole-dipole forces exist between polar regions of different molecules.  The presence of a dipole means that the molecule has a partially positive end and a partially negative end.  Opposite partial charges attract each other, whereas like partial charges repel.  Biological systems utilise a special type of dipole-dipole force, known as hydrogen bonding.
• Hydrogen bonding involves hydrogen.  The hydrogen atom must be bonded to either an oxygen atom or a nitrogen atom.  Hydrogen bonding is significantly stronger than a “normal” dipole-dipole force and is very much stronger than London dispersion forces (very weak and short-lived attractions between molecules that arise due to the nucleus of one atom attracting the electron cloud of another atom).  Hydrogen bonding may be either intra-molecular or intermolecular.
• Ionic interactions may serve as intermolecular or intra-molecular forces in biological systems.  In some cases, these may involve metal cations, such as Na+, or anions, such as Cl.  In many cases, the cation is an ammonium ion from an amino group.  The anion may be from a carboxylic acid.  Oppositely charged ions attract each other strongly.

# Geckos: Superheroes of the Natural World

Geckos, spiders and the comic-book hero Spiderman seem to defy gravity by scurrying rapidly along smooth walls and ceilings.

The toes of a gecko have attracted many researchers’ attention, since they can stick to a wide variety of surfaces, without the need of liquid or surface tension.

The secret of this colourful lizard’s adhesion turns out to be relatively weak intermolecular forces that draw materials together any time they get close.

The underside of a gecko’s foot looks like a tyre tread and is covered in millions of microscopic hairs.  Each hair splits into hundreds of tips just 200 billionths of a metre wide.  It is suggested that this adhesion was due to van der Waals forces, a very weak intermolecular forces, between the finely divided bristles and the surfaces.

These van der Waals forces explain how a gecko can support its own body weight on just one finger, and a single gecko hair can lift the weight of an ant.  To un-stick, the gecko pulls its foot away at a different angle.

# Rise of the Gecko-Bots

Prof Kellar Autumn and Mark Cutkosky compared natural and polymer-based synthetic gecko hairs using a machine that simulated gecko climbing.  Both synthetic and natural materials could be re-used up to 30,000 times without losing their stickiness.

In 2010, the Standford University researchers developed the aptly-named Stickybot, a Gecko-like robot, climbs vertical surfaces.  Its controllability is based on geometry and physics, not chemistry.  Van der Waals force allow the gecko to hang and support its entire weight on only one toe, when it places the other toe on the glass or pulls it back.  Cutkosky said that the robot toes can be called a one-way adhesive, which stick only when the pull is in one direction.

To cling to any surface, the Stickybot III uses its special feet.  The feet draw inspiration from a gecko and have tiny hairs on them, which are almost 5 times smaller than that of human hair.  The robot also has a long tail that reduces the weight load on each of its sticky foot, making the climb easier.

Several institutes have been developing robots that can climb walls.  Some scientists envisage “geckobots” being used to search for survivors in a burning building or disaster zone, to explore the rocky terrain of Mars, or even as toys.

# Harnessing Van der Waals Forces with the “Gecko” Tape

The effort to uncover the mechanisms behind gecko climbing has already yielded synthetic material that sticks in the same way.  Besides the adhesive properties, two other things are needed for demonstrating how the gecko’s toes (or a futuristic Spiderman suit) work: easy detachment from a surface after it has stuck and self-cleaning.

Stick, peel and re-stick a piece of existing adhesive tape several times and it quickly loses its clingy properties.  Compared with generic sticky tape, gecko tape has no “visco-elastic” adhesive to dry out.  As a result, it remains attached for longer without leaving residue.  Synthetic adhesives work best on glass.  Rough or uneven surfaces remain more of a challenge.

Gecko adhesives could yield transformative applications in robotics, industry, sports and clothing.

Researchers  from the Zoological Institute at the University of Kiel, Germany have produced a tape that bonds to a target surface using the same principal that allows geckos and insects to walk on ceilings.  The team led by Stanislav Gorb managed to mimic the setae – tiny hairs with flattened tips that cover the feet and legs of geckos and climbing insects.  On contact, the tips of the setae splay out, maximising their surface area and providing the traction and adhesion necessary to move up walls or across ceilings.

The tape is created from silicon.  Approximately twice as hard to remove as conventional adhesive tape, it leaves no sticky residue.  The picture above shows a researcher hanging from the ceiling on a 20 cm x 20 cm square of the product.  The “gecko tape” can also be attached, removed and re-attached thousands of times, and can even be applied underwater.

### Medicine is one target area for the application of these adhesives.

The technology could help develop easily removable, high-tech and durable bandages, or gripping surfaces for instruments designed for use in delicate surgery.  Since the mechanism works in the wet, it might even be used to support implants within the human body.

German firm Binder launched a silicone-based bio-inspired adhesive on the market in 2012.  The company has explored medical applications and another technology for use in the manufacturing of electronics circuit boards.

However, production costs are high and the gecko-type adhesive is unlikely to replace conventional sticky tape just yet.

But one possible use always comes up in a cocktail party science discussion: could they allow humans to scale walls like Spider-Man?

# Everybody Gets One…

Physicist and engineer Nicola Pugno, from Turin Polytechnic (Italy) calculated that a person wearing gloves and boots made of carbon nanotubes and structured to mimic gecko feet could indeed cling safely to a wall or a ceiling.

The suit would have to work on every kind of surface and for long periods of time.  But Prof Pugno said in 2007: “We are not very far, in my opinion, from a kind of Spider-Man suit.”

Professor Metin Sitti of Carnegie Mellon University, thinks the idea is “not impossible”.  “Selecting a lightweight person and applying the gecko adhesive to many parts of the suit (not just the feet and hands) would improve the chances of success”, he explains.

Another example of physicists seeking inspiration in Nature for new design principles.

Personally, I think a handy window cleaning innovation ought to be in the works…