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Physics of Biology

The role of physics in biological diversity



Cavitation is a phase change from a liquid to a gas, akin to boiling.  But while boiling is the result of raising temperature, cavitation occurs with decreases of pressure.  Cavitation generates very short lived bubbles that collapse back in on themselves so quickly that they emit shock waves that are powerful enough to erode holes in metal.  How have fast moving aquatic organisms dealt with cavitation?

Mantis shrimp appear to not only avoid cavitation despite moving at over 30 m/s, but also may harness cavitaion to help fracture hard bodied prey.  In this video (30,000 fps), a smithii mantis shrimp smashes a snail shell.  Notice the flash of light and bubble at impact.  That is cavitation.  Most often, we only see cavitation at mantis shrimp impacts, and not before when we would expect it given how fast they are moving through the water (30 m/s). Why?


 Ninjabot: a physical model of the mantis shrimp strike 

To study when cavitation forms in these complex fluid dynamic conditions, I built Ninjabot, a physical model of the extremely fast mantis shrimp (Stomatopoda).

Ninjabot rotates a to-scale appendage within the environmental conditions and close to the kinematic range of mantis shrimp’s rotating strike. Ninjabot is an adjustable mechanism that can repeatedly vary independent properties relevant to fast aquatic motions to help isolate their individual effects. Ninjabot’s appendage can reach speeds of 30 m/s at accelerations of 3.2 × 10^4 m/s^2 making Ninjabot the fastest biomimetic robot to date. The first study with Ninjabot explored the kinematic predictors of cavitation onset in non-uniform conditions.

It turns our that it is rather easy to cavitate when moving at these speeds. Here cavitation forming on a mantis shrimp appendage rotated by Ninjabot at 26 m/s.  Filmed at 30,000 fps.  Why does cavitation easily form when an appendage is moved with Ninjabot but not on the animals?

What good is power?

Power amplification is associated with extreme biological performance. Organisms that drive motion though the release of elastic energy jump higher and move faster than those that drive motion through muscle alone.  Why?  In a recent paper in Science, we explored the limitations of power amplification and the conditions in which it improves performance.

What makes a good spring?

Springs are commonly treated as idealized objects that return all the energy stored in them.  But, like any actuator,  springs face limitations. Understanding the conditions in which different spring material and geometric properties limit spring efficiency is crucial to understanding how different species tune power amplified systems. In collaboration a Sheila Patek and Al Crosby, we’ve been exploring questions like: How does a ‘good’ spring for powering a low-force, high-velocity motion differ from one driving high-force, low-velocity motions?   Under what conditions is spring efficiency prioritized over resilience?

Geometric Latches

The fastest biological motions are often triggered by the release of a latch that has held in check stored elastic energy.  But some organisms amplify power without a physical latch.  It has been proposed that the relative positions of different portions of the body in relation to gravity create a ‘geometric’ latch.  Using simple musculoskeletal models, we’ve been exploring morphological variations that result in ‘geometric’ latches.


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