Physics works differently at the nanoscale, and this means that design principles that are familiar in the macro-scale world may not work when shrunk. A great example of this is the problem of how you would propel a nanoscale swimmer through water. To a human-scale swimmer, water resists forward motion by virtue of the fact that it has inertia. But on the nanoscale, it is the viscosity of water that is the dominant factor. To imagine what it feels like trying to swim at the nanoscale, you need to imagine being immersed in a vat of the most sticky molasses.
The mathematics of this situation is intriguing, and it’s been known for a while that any simple, back-and-forth motion won’t get you anywhere. Imagine a scallop, trying to swim by opening its shell slowly, and then shutting it suddenly. This strategy works fine in the macroscopic world, but on the nanoscale you can show that all the ground the scallop gains when it shuts its shell is lost again when it opens it, no matter how big the difference in speed between the forward and backward strokes. To get anywhere, you need some kind of non-reciprocal motion – a motion that looks different when time-reversed. In 2004, Ramin Golestanian and coworkers showed that three spheres joined together could make a nanoscale swimmer. Here’s an article about this work in Physical Review Focus, with a link to a neat animation; here’s another article in Technology Review: Teaching Nanotech to Swim.
This story has moved forward in two ways since this report. Earlier this year, Ramin Golestanian, together with Tannie Liverpool, from Leeds University, and Armand Ajdari, from ESPCI in Paris, analysed another way of propelling a nanoscale submarine. In this work, published in Physical Review Letters in June this year (abstract here, subscription required for full article), they considered a nanoscale vessel with an enzyme attached to the hull at one point. The enzyme catalyses a chemical reaction that produces a stream of reactants like a rocket’s exhaust. Like a rocket, this has the effect of propelling the vessel along, but the physics underlying the effect is quite different. It’s not the inertia of the exhaust that propels the vessel forward; instead it is the effect of the collisions of the reactant molecules as they undergo random, Brownian motion that have the effect of propelling our nanobot forward.
And today, Nature published an experimental report of a miniature swimmer (editor’s summary; full paper requires subscription) which illustrates some of these principles. In this work (from Bibette and coworkers, also at ESPCI, Paris), chains of magnetic nano-particles form a tail which wiggles when an oscillating magnetic field is applied, pulling a payload along.
Ramin has just joined us in the physics department at Sheffield, so I look forward to working with him to take some more steps on the road to a swimming nanobot.