This movie shows the ant from Fig. 5A, a single ant digitally isolated from the network (Movie 2 of 10.1242/jeb.093021). - Secrets of ant rafts revealed Architecture of flash-frozen ant assemblages offers inspiration for robot designers. Nature 11 June 2014 doi:10.1038/nature.2014.15400 nature/news/secrets-of-ant-rafts-revealed-1.15400 To negotiate floods and cross streams, fire ants band together — literally — linking together to form rafts and bridges in a feat of social cooperation and biophysics. Now, engineers have made a close study1 of the ants architectural technique, pointing the way towards new approaches for robot designers and materials scientists. To understand the properties of the ant structures, David Hu, a mechanical engineer at the Georgia Institute of Technology in Atlanta, sought to observe not just the surface of the ant clumps but the structure and joints underneath. First, Hu and his team collected ant colonies — shovelling them, dirt and all, into buckets. After separating out the ants from the dirt, they then put 100 or so ants into a cup and swirled, causing the ants to form into a ball (no water necessary -- they come together almost like dough). The researchers then froze the ball with liquid nitrogen so they could examine it in a micro-computed-tomography scanner to come up with a 3-D picture. But the heat of the scanner melted the ball into a heap of dead ants. After months of experimenting with techniques to keep it together, lead author Paul Foster, now at the University of Michigan, found an unlikely source of inspiration in crack cocaine — specifically, in a method of vaporizing the drug to inhale it. “We did the same process — not with crack, but glue,” says Hu, adding that the authors decided against calling it the ‘crack-pipe method’ in their paper. The researchers heated the glue in an aluminium pot over a flame, with the frozen ant ball suspended on mesh above. The glue vapour rose and lightly coated the ants. Hu and his team found that the ants had grabbed hold of one another with adhesive pads on their legs, which they stretched out to create pockets of air. They also tended to orient themselves perpendicularly to one another, distributing their weight and creating a light, buoyant structure. The formation seems to take advantage of the ants’ different sizes, with smaller ants slotting neatly in between larger ones to add more connections. Each ant averaged 14 connections to fellow ants. The study is published today in the Journal of Experimental Biology (1). Radhika Nagpal, who creates biologically inspired robots at Harvard University in Cambridge, Massachusetts, says that Hu’s ants could make great models for modular robots. “There’s lots of interesting outcomes of this work,” she says. “Imagine robots that need to construct a barrier or patch a hole during a disaster response.” Rather than building one perfect robot, she notes, designers are increasingly exploring building a “colony of simple robots that use their bodies and the connections between them to build new structures.” Most projects in this vein have used geometric robots with precise connections. But ants do not create a perfect lattice, suggesting a sloppier, more organic approach in which robot shapes are varied and irregular and connections between them are inexact, Nagpal says. Hu thinks that the properties of ant structures might not only inform the design of robot swarms, but also the design of ‘smart’ materials that assemble themselves in response to temperature, light or other variables. Hu is working on getting larger ant structures — recognizably distinct as bridges, rafts and other forms — into a bigger scanner to begin detailing the properties of the different functional shapes. And once they are frozen and coated in glue, they will last forever, Hu says. “One day,” he jokes, “we will have a miniature museum of ant structures.” - How fire ant architects connect to build balls J Exp Biol 217, 2029. June 15, 2014 doi: 10.1242/jeb.108787 jeb.biologists.org/content/217/12/2029.1.full For red fire ants (Solenopsis invicta), rain gently drumming on the ground is the trigger for a mass exodus. Streaming from their nest as the water levels rise, the ants rapidly assemble and grip onto their nearest neighbours, forming a raft to carry them to safety. What is more miraculous is that each individual ant is denser than water and in danger of sinking if submerged. However, the ants dont just draw the line at constructing rafts: they routinely form bivouacs, assemble towers and even coalesce into droplets when swished in a cup. ‘You can consider them as both a fluid and a solid’, explains David Hu from the Georgia Institute of Technology, USA, who is most interested in the ants because they are large enough for him to begin to find out how they interact to pull off these remarkable engineering feats. Hu teamed up with Paul Foster and Nathan Mlot to investigate how balls of living fire ants self-assemble (p. 2089). Gently swirling 110 ants in a beaker to form a sphere, the team then swiftly froze the structure in liquid nitrogen and coated it in Super Glue™ vapour to preserve the minute contacts within, ready for Angela Lin to visualise the structures in a CT scanner. ‘With the CT scan we can focus on individual ants and see how they are connected to their neighbours’, explains Hu, who adds that processing the images could only be partially automated because it is hard to tell where one ant ends and another begins. However, after months of painstaking scrutiny, Foster and Hu discovered that on average, each ant participated in 14 contacts – reaching out with all six legs to grip neighbours and receiving eight contacts back to its body – although large ants participated in as many as 20 contacts and the smallest ants participating in only eight. ‘It turns out that 99% of the legs are connected to another ant and there are no free loaders’, says Hu, who admits that he was impressed by the high degree of connectivity. Next Foster digitally removed all of the limb connections so that he could take a closer look at the ways that the ants bodies packed together, and he was amazed to see that instead of clustering together in parallel, like grains of rice in a jar, the ants had actively oriented their bodies perpendicular to each other. ‘They have to be alive to do that,’ says Hu, adding, ‘It requires some intelligence, and suggests that somehow they sense their relative orientation.’ The duo also analysed how closely the ants bodies packed together and realized that the smaller ants were packing in to fill the gaps between the larger ants to increase the number of contacts. They also noticed that the ants were actively pushing on each other, using their legs like tiny jacks to increase the distance between neighbours and reduce the density of the ball. Hu explains that by introducing air pockets between their bodies, the ants increase their water repellency and buoyancy, which is why their rafts are so effective. Finally, Hu and Foster took a closer look at the contacts made by individual ants with a scanning electron microscope and saw that the insects rarely used their mandibles to grip on to other ants. Instead they mainly used their legs, holding on with hooks on their feet and the sticky pads that allow them to walk on vertical surfaces. So, having discovered how fire ants self-assemble to form light but stable structures, Hu is keen to know how they react to reinforce weak points in structures where ant architecture could fail. References 1. Foster, P. C., Mlot, N. J., Lin, A. & Hu, D. L. J. Exp. Biol. 217, 2089–2100 (2014). Fire ants actively control spacing and orientation within self-assemblages J Exp Biol 217, 2089-2100. June 15, 2014, doi: 10.1242/jeb.093021 jeb.biologists.org/content/217/12/2089.abstract Abstract To overcome obstacles and survive harsh environments, fire ants link their bodies together to form self-assemblages such as rafts, bridges and bivouacs. Such structures are examples of self-assembling and self-healing materials, as ants can quickly create and break links with one another in response to changes in their environment. Because ants are opaque, the arrangement of the ants within these three-dimensional networks was previously unknown. In this experimental study, we applied micro-scale computed tomography, or micro-CT, to visualize the connectivity, arrangement and orientation of ants within an assemblage. We identified active and geometric mechanisms that ants use to obtain favorable packing properties with respect to well-studied packing of inert objects such as cylinders. Ants use their legs to push against their neighbors, doubling their spacing relative to random packing of cylinders. These legs also permit active control of their orientation, an ability ants use to arrange themselves perpendicularly rather than in parallel. Lastly, we found an important role of ant polymorphism in promoting self-aggregation: a large distribution of ant sizes permits small ants to fit between the legs of larger ants, a phenomenon that increases the number of average connections per ant. These combined mechanisms lead to low packing fraction and high connectivity, which increase raft buoyancy and strength during flash floods. Supplementary Material Video jeb.biologists.org/content/suppl/2014/06/05/217.12.2089.DC1/JEB093021.pdf Movie 1. This movie simultaneously shows CT scan cross-sections and the resulting 3D rendering reconstructed from those sections. Movie 2. This movie shows the ant from Fig. 5A, a single ant digitally isolated from the network. Video Abstract jeb.biologists.org/content/suppl/2014/06/11/217.12.2089.DC2/JEB093021_Video_Abstract.pdf
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