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Topics: Science
Emily Brown
Topics: Science
Centuries after Isaac Newton came up with his laws of motion, scientists have been left shocked to realize that sperm seems to defy one of them.
The discovery that sperm can apparently defy one of the laws scientists have drawn on for so many years was recently made by Kenta Ishimoto, a mathematical scientist, and colleagues at Kyoto University.
The team investigated the sperm's defiance of Newton's third law of motion, which claims that 'for every action, there is an equal and opposite reaction'.
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The law suggests that opposing forces act against one another; for example, if two equal-sized marbles stuck one another, they would transfer their force and rebound.
When it comes to sperm, however, they manage to propel themselves through viscous fluids, rather than creating an equal and opposite reaction and being met with resistance.
Ishimoto and the team looked into these non-reciprocal interactions to try and figure out what was happening, studying experimental data on human sperm as well as modelling the motion of green algae, Chlamydomonas.
Both use flagella - the little tail bit on sperm - to drive the cells forward, but while highly viscous fluids would usually dissipate their energy, the flagella on sperm can continue to propel the cells forward.
In their research, the team found that sperm tails have an 'odd elasticity', which allows them to whip around without losing much energy to the fluid.
However, this didn't fully explain the propulsion caused by the wave-like motion of the flagella.
To help explain the process, the researches derived a new term, 'odd elastic modulus', to describe the internal mechanics of flagella.
As the flagella bent to respond to the liquid, they were able to avert the equal and opposite reaction and conserve energy.
Discussing their findings, the researchers said: "From solvable simple models to biological flagellar waveforms for Chlamydomonas and sperm cells, we studied the odd-bending modulus to decipher the nonlocal, nonreciprocal inner interactions within the material."
They continued: "Odd elasticity is not a generic term for activity in solids, but rather a well-defined physical mechanism that generates active forces in solids or in other systems in which a generalized elasticity can be defined without using an elastic potential."
The team then explained that their findings could have impacts in the real world by helping with the designs of small, self-assembling robots which mimic living materials, or to help better understand the underlying principles of collective behavior.