Frogs capture prey using shear-thinning saliva that spreads over insects when the tongue hits and then thickens and sticks when the tongue retracts – according to researchers in the US. In combination with the tongue's unique material properties, this two-phase, viscoelastic fluid makes the tongue extremely sticky, allowing frogs to capture and swallow prey heavier than themselves in the blink of an eye. The research could lead to the development of new types of adhesives and material-handling technologies, say the scientists.

Frogs can capture flying insects at astonishing speeds with a flick of their whip-like tongues. But it is not just lightweight insects that they can grab. Research has shown that a frog tongue can pull up to 1.4 times the frog's body weight. And frogs have been recorded capturing larger animals such as mice and birds.

At the start of the latest study, Alexis Noel, at the Georgia Institute of Technology in Atlanta, and colleagues, filmed common leopard frogs, Rana pipiens and other species capturing crickets with a high-speed camera at 1400 frames per second. They found that a leopard-frog's tongue can capture an insect in less than 0.07 s – five times faster than humans can blink.

Honey trap

The team's calculations show that when the tongue is retracting, the force on the insect can reach 12 times that of gravity. The tongue is able to adhere to prey under such forces because it is extremely soft and viscoelastic, and coated in a non-Newtonian, shear thinning saliva, according to the researchers. Shear thinning is the property of some fluids whereby a shear force on the fluid reduces its viscosity. At low shear rates the saliva is very thick and more viscous than honey. But when subjected to high shear forces, for example when the tongue is accelerating in to prey, the saliva thins, becoming around 50 times less viscous, the researchers found.

"During prey impact, the saliva experiences high shear rates, resulting in the saliva becoming thin and liquidy, penetrating insect cracks," explains Noel. "During insect retraction, the saliva experiences low shear rates, firming up and maintaining grip on the insect."

"Frog saliva is much like paint, another shear-thinning fluid," says Noel. "Paint is easy to spread on walls with a brush. Once the brush is removed, the paint then remains firmly adhered to the wall. This is because paint viscosity changes with applied shear rate."

Soft material

The researchers also found that the frog tongue is one of the softest known biological materials. It is as soft as brain tissue and 10 times softer than the human tongue. The extreme softness allows the tongue to deform and wrap around the prey during impact, creating a large contact area, aiding capture and adhesion.

The tongue's softness and viscoelastic nature also helps it maintain contact with the insect as it retracts back into the mouth. According to the researchers, the tongue is highly dampened and as the insect is yanked towards the frog it acts like a shock absorber, storing energy in its soft tissue and reducing separation forces between saliva and insect. Noel uses the analogy of a bungee cord. "If the tongue were stiffer, it would be like a human jumping off a bridge with a stiff rope wrapped around the ankle."

Once the insect is inside the frog's mouth the shear thinning saliva comes in to play again. The frog retracts its eyeballs into the mouth cavity to push the insect down its throat. This motion produces a shearing force parallel to the tongue that is high enough to turn the saliva thin and watery, and the insect is released and swallowed. The two-phase saliva helps in all phases of prey capture: low viscosity assists during impact and release, while high viscosity assists in prey adhesion.

Reversible adhesives

The researchers believe that these mechanisms could inspire the design of synthetic reversible adhesives for high-speed applications. Noel told Physics World that she could imagine such an adhesive "being used for a fast object collection mechanism in drones" or as a way to grab delicate objects off a conveyer belt in a manufacturing plant.

Pascal Damman of the University of Mons in Belgium told Physics World: "This study confirms what we showed in our work on chameleons, the combination of elastic deformation of the tongue together with the viscous mucus ensure efficient prey capture. I'm however surprised to see that the adhesion force observed for the frogs are much smaller than the adhesion strength observed for chameleons."

The study is described in Royal Society Interface.