Scientists Study Self-Propulsion to Design Tiny, Nano-Scale Swimming Machines

When you have something as tiny as bacteria and spermatozoa, movement becomes a whole new challenge. These tiny organisms are subjected to relatively small inertial forces compared to the viscous forces exerted by the surrounding liquid. In other words: moving is hard. Now, scientists have taken a closer look at self-propulsion in a bid to potentially design tiny, nano-scale swimming machines.

In order to learn a little bit more about the forces at work on a nanoscale, the researchers focused on two joined spheres of different radii-dubbed a dumbbell-rotating in a model fluid. The scientists used a simulation to study the effect of small-scale inertial force on the dumbbell's propulsion. Then, they compared this with the results from theoretical calculations describing locomotion.

It turns out that this dumbbell actually can't self-propel in a pure Newtonian fluid, which is a model fluid whose viscosity does not change with its flow rate, in the absence of inertia. This is due to the fact that if a dumbbell rotating in the counter-clockwise direction propels upwards in the absence of inertia, it would have to move downwards when rotating in the counter-clockwise direction. This would cause the propulsion to occur in the same direction, which would mean the dumbbell couldn't propel.

That's not all, either. The researchers also found that a rotating dumbbell propels with the large sphere due to inertial forces in the fluid and the small sphere ahead in a pure viscoelastic fluid. This means that the scientists could derive the ultimate dumbbell geometry for a self-propelling small-scale swimmer.

The findings could help researchers design self-propelled micro- and nanoscale artificial swimming machines. These machines, in turn, could be used in medical applications to help treat patients by targeting specific areas and allowing treatments to be more mobile within the body.

The findings are published in The European Physical Journal E.