Human sperm have shown an astonishing ability to navigate through highly viscous fluids, seemingly defying major principles of physics, particularly Newton’s third law of motion. A research team led by Kenta Ishimoto, a mathematical scientist at Kyoto University, delved into how these microscopic swimmers maneuver through substances that should impede their movement. Their findings, published in October 2023 in the journal PRX Life, reveal insights that challenge long-standing scientific understanding.
Newton’s third law states that “for every action, there is an equal and opposite reaction.” This principle describes a symmetry in nature where opposing forces counteract one another. For instance, when two equal-sized marbles collide, they transfer their momentum based on this law. Yet, the study highlights that not all natural systems adhere strictly to these principles.
The researchers focused on “non-reciprocal interactions” observed in chaotic systems, such as those involving flocks of birds or swimming sperm. These motile agents exhibit unique movement patterns that allow them to bypass the constraints of equal and opposite forces. Unlike inanimate objects, living organisms like sperm generate their own energy, propelling themselves through their surroundings and disrupting the balance of forces.
In their investigation, Ishimoto and his colleagues analyzed experimental data from human sperm and modeled the movements of Chlamydomonas, a type of green algae. Both organisms utilize flexible, wave-like flagella to swim. Typically, in highly viscous fluids, a flagellum’s energy would dissipate, significantly hindering movement. Surprisingly, the study found that sperm and algal cells could move efficiently despite these conditions.
The research identified a characteristic termed “odd elasticity,” which allows these flexible appendages to navigate through viscous substances without losing significant energy to the fluid. However, this property alone did not fully account for the propulsion created by the flagella’s wave-like motion. To address this, the researchers introduced a new term: “odd elastic modulus,” describing the internal mechanics of the flagella.
The team concluded, “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.” This research not only challenges existing physics principles but also has potential implications for technology. The findings could inform the design of small, self-assembling robots that mimic biological materials, advancing the understanding of collective behavior in living systems.
This study marks a significant step forward in the exploration of microscopic organisms and their interactions with physical laws, opening new avenues for both scientific inquiry and technological innovation.
