If you were able to make a nanoscale submarine to fulfill the classic “Fantastic Voyage” scenario of swimming through the bloodstream, how would you power and steer it? As readers of my book “Soft Machines” will know, our intuitions are very unreliable guides to the environment in the wet nanoscale world, and the design principles that would be appropriate on the human scale simply won’t work on the nanoscale. Swimming is good example; on small scales water behaves, not as the free flowing liquid we are used to on the human scale; viscosity is much more important on small scales. To get a feel for what it would be like to try and swim on the nanoscale, one has to imagine trying to swim in the most viscous molasses. In my group we’ve been doing some experiments to demonstrate the realisation of one scheme to make a nanoscale object swim, the results of which are summarised in this preprint (PDF), “Self-motile colloidal particles: from directed propulsion to random walk”.
The brilliantly simple idea underlying these experiments was thought up by my colleague and co-author, Ramin Golestanian, together with his fellow theoretical physicists Tannie Liverpool and Armand Adjari, and was analysed theoretically in a recent paper in Physical Review Letters, “Propulsion of a Molecular Machine by Asymmetric Distribution of Reaction Products” (abstract here, subscription required for full paper). If one has a particle that has a patch of catalyst on one side, and that catalyst drives a reaction that produces more product molecules than it consumes in fuel molecules, then the particle will end up in a solution that is more concentrated on one side than the other. This leads to an osmotic pressure gradient, which in turn results in a force that pushes the particle along.
Jon Howse, a postdoc working in my group, has made an experimental system that realises this theoretical scheme. He coated micron-sized polystyrene particles, on one side only, with platinum. This catalyses the reaction by which hydrogen peroxide is broken down into water and oxygen. For every two hydrogen peroxide molecules that take part in the reaction, two water molecules and one oxygen molecule results. Using optical microscopy, he tracked the motion of particles in four different situations. In three of these situations – with control particles, uncoated with platinum, in both water and hydrogen peroxide solution, and with coated particles in hydrogen peroxide solution, he found identical results – the expected Brownian motion of a micron-sized particle. But when the coated particles were put in hydrogen peroxide, the particles clearly moved further and faster.
Detailed analysis of the particle motion showed that, in addition to the Brownian motion that all micro-size particles must be subject to, the propelled particles moved with a velocity that depended on the concentration of the hydrogen peroxide fuel – the more fuel that was present, the faster they went. But Brownian motion is still present, and it has an important effect even on the fastest propelled particles. Brownian motion makes particles rotate randomly as well as jiggle around, so the propelled particles don’t go in straight lines. In fact, at longer times the effect of the random rotation is to make the particles revert to a random walk, albeit one in which the step length is essentially the propulsion velocity multiplied by the characteristic time for rotational diffusion. This kind of motion has an interesting analogy to the kind of motion bacteria do when they are swimming. Bacteria, if they are trying to swim towards food, don’t simply swing the rudder round and propel themselves directly towards it. Like our particles, they are actually doing a kind of random walk in which stretches of straight-line motion are interrupted by episodes in which they change direction – this kind of motion has been called run and tumble motion. Counterintuitively, it seems that this is a better strategy for getting around in the nanoscale world, in which the random jostling of Brownian motion is unavoidable. What the bacteria do is change the length of time for which they are moving in a straight line according to whether they are getting closer to or further away from their food source. If we could do the same trick in our synthetic system, of changing the length of the run time, then that would suggest a strategy for steering our nanoscale submarines, as well as propelling them.