Did you know that human sperm can swim through thick fluids in a unique way? A recent study by Kenta Ishimoto, a mathematical scientist from Kyoto University, found that sperm and other tiny swimmers can move through viscous substances even when you’d expect them to struggle. This discovery challenges Newton’s third law, which states that every action has an equal and opposite reaction.
Newton developed his laws of motion in 1686, laying the foundation for how we understand movement and forces. But nature is more complicated than his laws suggest, especially at microscopic levels. For example, when sperm swim, they don’t always follow the same rules that larger objects, like marbles, do when they collide. Instead, they display “non-reciprocal interactions” where their movements don’t create equal reactions in the fluids they navigate.
This behavior can be seen in various biological systems, not just sperm. Scientists have noted similar patterns in flocks of birds and swimming particles. These swimmers move in unexpected ways, bending traditional physics rules because they generate their own energy.
In their October 2023 study, Ishimoto and his team looked at how both human sperm and green algae called *Chlamydomonas* swim. Both types have tails, or flagella, that change shape to push them through their environments. Surprisingly, these flagella, with a property called “odd elasticity,” allow sperm and algae to move efficiently without losing much energy.
Typically, if something swims in a thick fluid, it would lose energy, making it hard to move. But the unique flexibility of their tails helps them navigate even the stickiest of substances. The researchers introduced a new concept, the “odd elastic modulus,” to explain how flagella achieve their propulsion. This term helps us understand the mechanics of these tiny swimmers more clearly.
Understanding how these cells swim could lead to exciting advancements. Experts suggest that applying these principles could help design small robots that imitate living organisms. The research may also shed light on collective behavior in different systems, which is crucial in fields like robotics and biology.
This study not only reveals new insights about microscopic motion but also encourages us to rethink the laws that govern movement in nature. The findings were published in the journal PRX Life.
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