Scientists Unveil the Physics Behind the Unique Motions of Sperm and Green Algae
Discover how the wriggling motions of sperm cells and green algae defy Newton’s third law of motion, according to a recent study by Kenta Ishimoto and colleagues.
The rendant sympathy of Newton’s Laws
Sir Isaac Newton’s three laws of motion lay the foundations of classical mechanics. His third law, stating that "for every action, there is an equal and opposite reaction," seems to hold true in our macroscopic world. However, as Kenta Ishimoto and his team found, microscopic cells like sperm and green algae exhibit intricacies that challenge this symmetry.
Sperm and Flagella: A Viscoelastic Tale
Sperm cells, with their whip-like tails, can propel themselves through viscous fluids, seemingly bypassing Newton’s third law. Scientists have long been baffled by how these cells can move efficiently through seemingly sticky environments. The key, it turns out, lies in the unique properties of their flagella.
The Breakdown of Symmetry in Microscopic Motions
Unlike macroscopic objects, the microscopic world is governed by different principles. Sperm cells and green algae, using their flagella (thin, bendy appendages), display asymmetric interactions with their surroundings. These interactions include generating their own energy, which is added to the system with each movement, pushing the system far from equilibrium and rendering Newton’s laws less applicable.
Scanning electron micrograph of a sperm cell in a fallopian tube. (Science Photo Library/Canva)
Odd Elasticity and Nonlocal Interaction
In their study, published in October 2023, Ishimoto and colleagues found that the tails of sperm and the flagella of green algae possess an odd elasticity. This property allows these appendages to whip about without losing much energy to the surrounding fluid.
The Discovery of Odd Elastic Modulus
Further investigation into the propulsion mechanism led the researchers to derive a new term, an odd elastic modulus, describing the internal mechanics of flagella. This term helps explain the nonlocal, nonreciprocal interactions within the material that enable these cells to wiggle their way through their environment.
Applications in Robotics and Biomedicine
The researchers conclude their study by highlighting the potential applications of their findings. The understanding of flagella’s odd-bending modulus can help in designing small, self-assembling robots that mimic living materials. Their modeling methods could also improve our understanding of collective behavior.
Conclusion
This study underscores the fascinating complexity of the microscopic world. As our understanding of these principles deepens, we can expect to see significant advancements in various fields, from robotics to biomedicine.
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