Human sperm cells and some microorganisms swim by deforming their bodies in a way that breaks Newtonâs third law of motion â and weâre closer to understanding how they do it. The findings could eventually inspire researchers to develop tiny robots that also violate this law as they swim.
Famously, Newtonâs third law can be summed up in the phrase âfor every action, there is an equal and opposite reactionâ. This means that as you push against a wall, the wall pushes back on you. âBut recently, physicists started to explore mechanics without Newtonâs third law,â says at Kyoto University in Japan. âHere, if you push a wall, it does not necessarily push back â it may escape away from you.â
Ishimoto and his colleagues wanted to study this property in tiny biological swimmers.
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They focused on human sperm cells and Chlamydomonas algae. Both swim using flagella â slender, hair-like filaments that stick out from the cellâs main body. Flagella are elastic, and can change shape to interact with the fluid surrounding the cell. This helps propel the cell forward in a so-called ânon-reciprocalâ way, meaning that they violate Newtonâs third law.
But the details of this process are still unclear. At the microscopic scale, researchers would usually expect the fluid to dissipate most of the cellâs energy. This should prevent it from travelling very far â or even at all â no matter how much it wiggles its elastic flagellum.
To work out how the cells manage to move despite this apparent obstacle, the researchers analysed the motion of sperm and algal cellsâ flagella as they swam. They found that these flagella have an unusual property, dubbed âoddâ elasticity, which allows them to wave without losing much energy to the surrounding fluid.
The researchers quantified the cellsâ odd elasticity and arrived at a number called the âodd elastic modulusâ. The higher this number, the more a flagellum can wave without the surrounding liquid suppressing its motion. This allows the cell to move forward non-reciprocally.
at Kyoto University, who also worked on the study, says calculating the odd elastic modulus for many different micro-swimmers could help scientists classify them and work out whether there are additional features that help them disobey Newtonâs third law.
At present, we donât know of all the microscopic process that help tiny swimmers defy this law of motion, says at the WrocĹaw University of Science and Technology in Poland. He says being able to calculate the odd elastic modulus and similar numbers could help create a âdictionaryâ of organisms that are capable of non-reciprocal movement.
Ishimoto says that the teamâs approach could also inform the design of artificial swimmers, helping researchers build small soft elastic robots that can violate Newtonâs third law.
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