| Date | 15th, Jul 2022 |
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Their swirling, clustering behavior might someday inform the design of self-assembling robotic swarms.
In its earliest stages, long before it sprouts its signature appendages, a starfish embryo resembles a tiny bead, spinning through the water like a miniature ball bearing.
MIT scientists have found that starfish embryos spontaneously swim together at the surface to form large crystal-like structures that collectively ripple and rotate for relatively long periods of time before dissolving as embryos mature.
Jiggling crystals
To understand what might be triggering embryos to assemble like crystals, the team first studied a single embryo’s flow field, or the way in which water flows around the embryo. To do this, they placed a single starfish embryo in water, then added much smaller beads to the mix, and took images of the beads as they flowed around the embryo at the water’s surface.
Based on the beads’ direction and flow, the researchers could map the flow field around the embryo. They found that the cilia on the embryo’s surface beat in such a way that they spun the embryo in a particular direction and created whirlpools on either side of the embryo that then drew in the smaller beads.
Mietke, a postdoc in Dunkel’s applied mathematics group at MIT, worked this flow field from a single embryo into a simulation of many embryos, and ran the simulation forward to see how they would behave. The model produced the same crystal structures that the team observed in its experiments, confirming that the embryos’ crystallizing behavior was most likely a result of their hydrodynamic interactions and chirality.
In their experiments, the team also observed that once a crystal structure had formed, it persisted for days, and during this time spontaneous ripples began to propagate across the crystal.
“We could see this crystal rotating and jiggling over a very long time, which was absolutely unexpected,” she says. “You would expect these ripples to die out quickly, because water is viscous and would dampen these oscillations. This told us the system has some sort of odd elastic behavior.”
The spontaneous, long-lasting ripples may result from interactions between the individual embryos, which spin against each other like interlocking gears. With thousands of gears spinning in crystal formation, the many individual spins could set off a larger, collective motion across the entire structure.
The researchers are now investigating whether other organisms such as sea urchins exhibit similar crystalline behavior. They also explore how this self-assembling structure could be replicated in robotic systems.
“You can play with this design principle of interactions and build something like a robotic swarm that can actually do work on the environment,” she says.
Written by Jennifer Chu
