This week's paper is about frog saliva. It is in the Journal of the Royal Society Interface lead authored by Alexis Noel at the Georgia Institute of Technology. A non-Newtonian fluid is a fluid with a non-linear relationship between its sheer stress and shear rate. Another explanation is that non-Newtonian fluids "change their viscosity or flow behavior under stress". Ketchup is the classic example of a non-Newtonian fluid, but others include blood, honey, cream, and the infamous oobleck. This paper adds frog-saliva to the list.
The authors used a combination of high-speed video of frogs and toads catching insects, and biomechanic studies of the physics and chemistry of frog tongues and saliva. They have some great quotes such as "we laboriously collected 0.3 ml of saliva from 15 leopard frogs and placed it into a cone and plate rheometer". They show that when frog saliva hits the prey insect its viscosity decreases, allowing it to spread over the insect and into all the nooks and crannies of the insect exoskeleton. This drop in viscosity is achieved by the speed of the frog's tongue, which creates very fast saliva sheer rates. To swallow the insect, the frog retracts its eyeballs (yes! really!) which again creates a sheer force parallel to the tongue, pushing the insect off the frog's tongue and down its throat. But how does the frog keep the insect stuck to its tongue as it flips its tongue back in its mouth? During this retraction, the force is pulling perpendicular to the tongue at which the viscosity of the saliva increases so it stiffens, holding tight to the insect. A non-Newtonian fluid!