However, one hurdle hampering these devices is the fact that they are constantly subject to Brownian fluctuations; in other words, they are jiggled around by collisions with the molecules in the surrounding fluid. As a result, their direction of propulsion is randomized every few seconds, making directional migration impossible.
“However, during self-propulsion, the motors create chemical gradients and flow fields in the fluids that surround them,” says Jaideep Katuri, lead author of the study. “These, in turn, are affected by their environment. We thought that we could maybe take advantage of the interactions that result from this to design a directional response.”
The researchers surmised that the surrounding topographical features, together with the micromotors’ interactions with these features, could be used to obtain a directional flow of particles – but only if these features were properly designed. “We needed to somehow achieve asymmetric topography of the surfaces on which the motors swim,” says Jaideep. “This asymmetry ensures that the particles find it easier to move in one direction over the other, and helps us achieve a robust particle flow that would be impossible on a homogeneous surface.”
To do this, Josep Samitier’s Nanobioengineering group were able to help. Using their expertise in micropatterning, they engineered the surface topography to have angled ‘teeth’ that make them behave much like a ratchet – allowing motion in one direction only.
“As well as resulting in directional trajectories and oriented particle flows, we’ve seen that these ‘ratchets’ could also be used to to concentrate the micromotors in confined spaces,” adds Jaideep.
This ability to control the behavior of normally diffusive systems opens new possibilities in microfluidics, lab-on-a-chip applications and eventually for the drug and other delivery or pick-up applications of these formerly random swimmers.
Jaideep Katuri, David Caballero, Raphael Voituriez, Josep Samitier, Samuel Sanchez (2018). Directed Flow of Micromotors through Alignment Interactions with Micropatterned Ratchets. ACS Nano, epub ahead of print