Microswimmers boosted by surrounding solvent

To more efficiently deliver an anticancer drug to malignant cells in the body, some researchers have employed artificial microswimmers. Such particles propel themselves through a solvent by generating a chemical, electric, or thermal gradient, often via a reaction at their surfaces (see the article by Jeffrey Moran and Jonathan Posner, Physics Today, May 2019, page 44 ). Some of the most studied artificial swimmers are Janus particles, which have two distinct surfaces that can easily generate the asymmetry needed for motion (see the Quick Study by Steve Granick, Shan Jiang, and Qian Chen, Physics Today, July 2009, page 68 ).

Now Daan Frenkel and Shaltiel Eloul from the University of Cambridge and Wilson Poon from the University of Edinburgh are proposing a new mechanism that can move a Janus particle. Their simulations reveal that solvent molecules can act as rocket propellant: Upon contact with the Janus particle’s surface, the solvent reacts on one side of the particle and imparts momentum, producing a net displacement. To model the propulsion mechanism, the researchers considered reactions in a dissipative fluid that contains Janus particles and conserves the system’s total momentum.

The results indicate that the average momentum transferred to a Janus particle by the fluid is a function of the energy released during the exothermic reaction. The square-root dependence distinguishes the rocket propulsion movement from other mechanisms associated with a temperature or diffusion gradient. The researchers used their simulations to estimate the speed of a 2-µm-diameter polymer Janus particle that is half coated with platinum and immersed in a hydrogen peroxide solution. The results agree with observations, so the authors conclude that the momentum from exothermic reactions contributes to the speed measured in Janus particles, although it is not the sole mechanism at work. (S. Eloul et al., Phys. Rev. Lett. 124, 188001, 2020 .)