The researchers speculate the force that allows spiders to climb glass and hang on ceilings is something known as the van der Waals force.
This form of attraction, based on the positive and negative charges of individual molecules, acts only when molecules of opposite charges are within a few nanometers of one another. The triangular-tipped setules on spiders' feet are perfectly designed to take advantage of the van der Waals force because they form hundreds of thousands of flexible contact points.
Because there are many small contact points, spiders can adjust the number of contacts needed for different surfaces, whether vertical, horizontal, smooth or rough. Though the total van der Waals force on the spider's feet is strong, it is really just the sum of many small attractive forces on each setule. That makes moving its foot easy; the spider just lifts each setule one at a time, rather than trying to lift all at once.
And unlike many types of glue, the van der Waals force is not affected by the surface or the surrounding environment. This allows for an unusually high degree of adhesion on wet or oily surfaces. Like spiders, insects have evolved with their own climbing strategies, including claws and clamp-like devices on their feet. In addition, insects secrete an oily liquid that gives them extra adhesion. Insects and other gravity-defying creatures have special pads on their feet to increase their contact with the surfaces they scale.
Insects have two of these pads on the ends of each of their feet, called pulvilli pads. They are flat and broad. The pulvilli pads are covered in tiny hairs called setae.
These setae are hollow and curved at the top, which led scientists to initially think they helped the fly grip the surface, like tiny hooks. Spiders and geckos have setae on their foot pads too and it is know that their setae actually form weak bonds with the surfaces on which they climb.
These weak bonds are called Van Der Waal bonds and are miniscule in magnitude. However, when the forces between setae and surface are combined they create a force large enough to suspend the creature in an upside-down position. The force generated by the gecko as it hangs upside-down on a glass surface could actually hold times its body weight. Due to these strong combined forces, geckos use dry-adhesion to stick to the surfaces they scale.
Insects however appear to rely on wet adhesion too. They secrete a substance through the hollow setae that creates a film between the pulvilli pads and the surface. This liquid is composed of special sugars and oils and may act as a natural glue, helping to stick the insect to the surface it is climbing. The substance may help in a more indirect way through something called capillary adhesion; the same principle that applies when a glass with a wet base sticks to a smooth surface.
Other studies actually suggest that this liquid layer is not there to help to stick the insect to the surface, but rather to help it unstick itself. Insects have two claws at the end of each foot, as well as the pulvilli pads.
It is thought that these claws play a part in unsticking the foot once it is time to move on. The claws likely push against the surface, thereby helping to release the pads and leave the insect-free to move its foot.
So could humans ever create the technology to mimic this gravity-defying movement? Fell, associate professor in the department of entomology at Virginia Polytechnic and State University, adds some other details: "The segments, or tarsi, at the end of insect legs possess clawlike structures that help the insect hold on to different types of surfaces. These tarsal claws are used to grip the tiny irregularities on rough surfaces.
But in some cases, insects do make use of a kind of adhesion. If the surface is smooth, the insect can hold on using the adhesive action of hairs located on sticky pads known as the arolia or pulvilli on the tarsi.
The pads typically contain numerous hairs that secrete an oily substance that causes the tips of the hairs to adhere to the surface. This substance provides the traction and stickiness that allows insects to hold on to smooth surfaces, such as glass. Sign up for our email newsletter. Already a subscriber?
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