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Flocking rods in a sea of beads: swarms through physical interactions

Many living creatures, such as birds, sheep, and fish, make coherent flocks or swarms. Flocking animals travel together, coordinating their speed and turns in an often visually striking manner. This can have benefits for the animals – flocking birds can use aerodynamics to fly more efficiently, sheep can move together as a group to evade predators, and fish can use collective sensing to find preferred locations in their environment. Flocks emerge in biological systems because animals try to follow their neighbors.

But how about non-living things? Can they spontaneously form swarms without any biological motive?

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The Origin of Random Forces Inside Cells

Place yourself in a bumper car at a carnival waiting to bump into your friends. Soon enough you hear the small engine of your bumper car start and you begin to move around, bumping into anyone in your way. While the motion of your car is mostly controlled by the steering wheel, random events—like fluctuations in the motor power, your car hitting small bumps on the floor, and other cars hitting you—can affect the motion as well. What if I told you that a cell and its parts function in a similar way? Just as your car is powered by electricity, molecular motors—bio-molecules that can convert chemical energy into mechanical work—power the movement of living organisms by generating forces. In order to produce these forces, molecular motors depend on an organic molecule called ATP.

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From errant to coherent motion

Have you ever seen those wide shapes moving in the sky at dawn, made of thousands of starlings, or the swarms of fish swimming in the ocean (see Figure 1)? The ability to organize and move in groups without a leader is called collective motion and has been observed at various spatial scales in the living world, from birds to locusts, cells, and bacteria.

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Scaling up biology

The difference between a bacterium and a whale are huge, and not just their size. However, there are hidden scaling laws underlying all living things. These scaling laws are found to be due to the fractal-like nutrient distribution systems. Here, we review how to derive the scaling law for metabolic rate with organism mass, illustrating its generality and ubiquity.

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“Precise” Polymers Promote Fast Proton Transport

One of the greatest challenges of making proton exchange membrane fuel cells is designing the membrane after which they are named. Engineers would love to have a membrane that transports protons quickly and efficiently. Although there are polymeric materials (e.g. Nafion) that can do this fairly well, there is still a need for faster transport. This week’s paper investigates the role of a polymer’s “precise” structure in facilitating fast proton transport.

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Not Just Spinning Their Gears: Extracting Useful Work from Bacterial Swarms

Imagine you and your friends are trapped by a super-villain in a cage. There is a giant gear with a diameter half the length of a football field in the center. The only way to open the cage door, get out, and stop the villain’s evil plans will be to rotate this gear by one full revolution. This is a daunting task for one person — but if you have enough friends, you can grab the gear’s teeth and push it together to escape. An analogous task is faced by flocks of tiny bacteria in today’s two featured papers. In “Bacterial ratchet motors”, Di Leonardo and colleagues discuss the mechanics of bacteria pushing a single gear, and in “Swimming bacteria power microscopic gears”, Sokolov and colleagues discuss how bacteria can interact to power more than one gear.

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The Lutetium Project: combining research, arts and outreach on YouTube

What does a physicist study? If you ask this question to the general public, you’re likely to hear back either about the extremely small — quantum physics, particle physics — or the extremely large — general relativity or cosmology. Indeed, those are probably the most visible fields of physics, having been depicted in Hollywood movies and TV series, and being prominently featured on the cover of popular science books and magazines.

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Seeing Inside Sand: Visualizing Force Chains with Photoelastic Disks

As their name suggests, so-called “granular materials” are made up of “grains” — small (but macroscopic) pieces of sand, glass beads, coffee grounds, or almost any other solid you can think of. Granular materials can flow like a liquid (like sand in an hourglass), resist deformation like a solid (like the sand under your feet at the beach), or quickly transition between these states (like pebbles in a rockslide).

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Putting the controversy over atomic-molecular theory to rest

There are many things that we “know” about the world around us. We know that the Earth revolves around the Sun, that gravity makes things fall downward, and that the apparently empty space around us is actually filled with the air that we breathe. We take for granted that these things are true. But how often do we consider whether we have seen evidence that supports these truths instead of trusting our sources of scientific knowledge?