Scientists found sperm in violation of Newton’s third law

Scientists found sperm in violation of Newton’s third law

Human sperm use their whip-like tails to penetrate viscous liquids, seemingly in defiance of Newton’s third law of motion, according to a new study describing the movement of these germ cells, as well as unicellular algae.

Kenta Ishimoto, a mathematical scientist at Kyoto University, and his colleagues studied the non-reciprocal interactions of sperm and other microscopic biological “swimmers” to understand how they slide through substances that, in theory, should resist their movement.

When Newton created his famous laws of motion in 1686, he tried to explain the relationship between a physical object and the forces acting on it by means of a few simple principles that, as it turned out, were not always applicable to microscopic cells writhing in a sticky liquid

Newton’s third law can be formulated as follows: “For every action there is an equal and opposite reaction.” It means a certain symmetry in nature, when opposite forces act on each other. In the simplest example, two balls of the same size, having collided while rolling on the ground, will transfer their force to each other and bounce, guided by this law.

However, nature is chaotic, and all physical systems are connected by such symmetries. The so-called non-reciprocal interactions [non-reciprocal interactions] are found in uncontrolled systems consisting of flocks of birds, particles in liquid, and floating spermatozoa.

These mobile agents move in such a way that they exhibit asymmetric interaction with the surrounding fluids, exploiting a loophole in the work of equal and oppositely directed forces, which allows to bypass Newton’s third law.

Ishimoto and his colleagues analyzed experimental data from human spermatozoa and modeled the movement of the green alga Chlamydomonas. Both swim using thin, bent flagella that protrude from the cell body and change shape or deform to propel the cell forward.

Highly viscous liquids tend to dissipate the energy of the flagellum, preventing the sperm or unicellular algae from moving at all. However, elastic flagella can somehow move these cells without causing a response from the environment.

The researchers discovered that the tails of the sperm and the flagella of the algae have what the scientists say is a “strange elasticity” that allows these flexible appendages to move without losing much energy in the surrounding fluid.

However, this property of strange elasticity does not fully explain the driving force of the wave-like movement of the flagella. Therefore, based on the simulations, the researchers derived a new term – the strange modulus of elasticity – to describe the internal mechanics of the flagella.

“From simple solvable models to biological flagellar waveforms for chlamydomonas and spermatozoa, we have studied the strange elastic modulus to decipher nonlocal, nonreciprocal internal interactions within the material,” the researchers conclude.

The obtained results can help in the development of small self-assembling robots that mimic living materials, and modeling techniques can be used to better understand the principles of collective behavior, the team adds.

Related posts