FYFD

Archer fish are well-known for their ability to hit aerial targets with perfectly aimed jets of water, as we’ve discussed previously. But a new study shows they use a similar technique to form underwater jets that help them uncover food. The researchers found that the fish altered the timing of their jet formation based on the type of substrate -- fine sand, course sand, or mud -- that the food pellet was hidden in. A great next step in this research would be using a technique like particle image velociometry (PIV) to measure the flow field directly and see to what extent the fish’s actions are altering the jet they produce. (Image and research credit: J. Dewenter et al.; GIF source: freshphotons)

Archer fish have a remarkable superpower. When hunting, they target insects above the water and knock them down with a precision strike from a jet of water they spit out. As previous research has shown, the archer fish packs an impressive punch by carefully modulating the water jet so that its tail travels faster and catches up to the front of the jet just as it strikes its target. Even more impressively, the archer fish can make this perfect strike on targets at different distances, which requires the fish to make significant adjustments to each jet. As this video from Deep Look discusses, the archer fish’s impressive hunting hints that it may have greater intelligence than we thought possible, given a comparison of its brain to ours. (Video credit: Deep Look)

Inside Science has a new documentary all about fluid dynamics! It features interviews with five researchers about current work ranging from the physics of surfing to the spreading of diseases. Penguins, sharks, archer fish, 3D printing, and influenza all make an appearance (seriously, fluid dynamics has everything, guys). If you’d like to learn more about some of these topics, I’ve touched on several of them before, including icing, penguin physics, shark skin, archer fish, and disease transmission via droplets.  (Video credit: Inside Science/AIP)

Archer fish hunt by shooting jets of water at their prey to knock them into the water where the fish can eat them. Previous research showed that the archer fish's projectile jet is pulsed such that the water released at a later time has a greater velocity. This makes the jet bunch up so that a ball of liquid hits the prey with greater force than the jet would otherwise. A recently released paper shows that the archer fish actively adjust their liquid jets in order to strike targets at different distances while maintaining this bunching effect. To control the jets, the fish adjust both how long they jet and what speed they impart to the fluid by changing how they open and close their mouths. (VIdeo credit: Nature; research credit: P. Gerullis and S. Schuster; via phys.org; submitted by @jchawner)

The archer fish hunts by shooting a jet of water at insects in the leaves above and knocking them into the water. How the fish achieve this feat has been a matter of contention.  A study of high-speed video of the archer's shot shows that fluid dynamics are key.  The fish releases a pulsed liquid jet, imparting greater velocity to the tail of the jet than the head.  As a result, the tail tends to catch up to the head and increase the jet's mass on impact while decreasing the duration of impact.  Simultaneously, the jet tends to break down into droplets via the Rayleigh-Plateau instability caused by surface tension.  Surface tension's power to hold the water in droplets combined with the inertial effects of the pulsed jet create a ball of fluid that strikes the archer's prey with more than five times the power than vertebrate muscles alone can impart. For more on archer fish, check out this video and the original research paper by A. Vailati et a. (Photo credits: Scott Linstead and BBC; submitted by Stuart R)