Particles Propelled Like Rockets
Take a sphere coated half with gold and half with platinum, immerse it in hydrogen peroxide, and it will start to “swim.” The motion of such a so-called Janus particle was thought to arise solely from asymmetric chemical reactions between the fluid and the sphere’s two hemispheres. These asymmetric reactions set up fluid flows that propel the particle (see Focus: Asymmetry Helps Tiny Rods Swim Upstream). Now Daan Frenkel at the University of Cambridge, UK, and colleagues predict a second propulsion mechanism: a rocket-like thrust from the chemical reaction itself.
Frenkel and his colleague Wilson Poon at the University of Edinburgh, UK, were discussing how Janus particles swim when Poon commented that the fuels most often used to propel the particles are also used to propel rockets. Intrigued by the observation, the duo wondered whether the chemical reactions that occur at the surface of Janus particles might directly impart momentum to the particles in a rocket-like manner. Along with other colleagues, they set up computer simulations to find out.
In their simulations, they consider Janus particles where only one hemisphere interacts chemically with the fluid. In that case, they find that indeed the chemical reaction—the decomposition of hydrogen peroxide into water and oxygen—can directly impart momentum to the particle, with the thrust leading to particle speeds comparable to those seen in experiments. Following this result, the researchers say that the momentum from the reaction must be considered when studying the motion of real Janus particles where fluid-flow propulsion also occurs. They also say that scientists could exploit this propulsion mechanism to alter a particle’s speed by changing the particle’s shape or by repositioning its reactive patches.
This research is published in Physical Review Letters.
–Katherine Wright
Katherine Wright is a Senior Editor for Physics.