Birds are active matter!
A classic example of an active matter system is a flock of birds. A flock is composed of thousands of individual birds (such as Starlings shown in the video) which act autonomously. Each bird is consuming its own on-board energy supply in order to move. Despite the fact that each bird is able to fly independent of the other birds, the collection of birds maintains a coherent, amorphous shape. Notice that a flock executes turning maneuvers in unison. |
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Ciliary fields like the one shown on this paramecium are also found in your airway. They assist the removal of mucus from the lungs by acting as a conveyor belt.
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Birds are not the only active matter around. All of life as we know it is active matter. Biological stuff runs on and is built by energy consuming particles working in unison. If you were to spill open one of the 100,000-billion cells which make up your body, you would find a stew of proteins, fibers, membranes, and genetic material. Many of these components consume the body’s fuel source, adenosine triphosphate (ATP), in order replicate, store information, or even move. Because of this active bio-machinery, our muscles can contract, our skin can heal, and you can cough up mucus. Check out this video which shows cilia on a paramecium. Similar ciliary fields exist in our trachea in order to remove mucus and contaminants from our lungs.
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E coli bacteria can exhibit collective motion. They align and even swarm in dense suspensions. Video: Matthew Copeland, University of Wisconsin, Madison via Youtube.
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Volvox is a colony of algae which can hydrodynamically couple with neighbors resulting in a waltz-like dance. Video: Raymond Goldstein Lab via Youtube.
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The most insightful experiment is one which you can control. Biological systems contain the inherent complexity of life and are therefore difficult to tune, test, and tweak in the laboratory setting. In order to research active matter physics, some researchers have been looking for model active matter systems which can be easily experimentally studied. This means that the system should be robustly repeatable, highly tunable, and live long enough for the duration of an experiment. One such approach is to make synthetic active matter systems in the lab.
One possibility has been explored by researchers at Harvard University by making self-propelled machines out of vibrating motors and toothbrush bristles (called Bristlebot). These experiments have been studying the collective dynamics of swarms of the robotic Bristlebots and exemplify how very simple interactions (robots bumping into eachother) can lead to very complicated collective behavior. Another approach pursued by researchers at NYU is to create tiny, self-propelled particles which, upon the addition of a fuel source, zip around and bump into one another. When the particles collide, they begin to crystallize and form a new, dense phase far different than the non-motile particles. |
Bristlebot is a tiny machine made out of a vibrating motor and a toothbrush like bristle. Turn the motor on and the machine zips around. It is a model system to explore collective dynamics in active matter.
Researchers from Paul Chaikin's lab at NYU have made tiny, micron sized particles which can propel themselves through a catalytic chemical reaction. When moving, the particles undergo a phase transition which is very different from the passive case.
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