Reactive Momentum Transfer Contributes to the Self-Propulsion of Janus Particles

We report simulations of a spherical Janus particle undergoing exothermic surface reactions around one pole only. Our model excludes self-phoretic transport by design. Nevertheless, net motion occurs from direct momentum transfer between solvent and colloid, with speed scaling as the square root of the energy released during the reaction. We find that such propulsion is dominated by the system’s short-time response, when neither the time dependence of the flow around the colloid nor the solvent compressibility can be ignored. Our simulations agree reasonably well with previous experiments.

Figure 1

(a) An active colloid densely covered with “frozen” DPD particles. A pair of solvent particles (black) in the vicinity of the active zone (grey) experiences a change of their velocities according to Eq. (1) at the time of reaction. (b) Average momentum transferred to the colloid from the fluid as function of square root of energy release. The line is a linear fit.

Figure 2

A transient profile of the velocity (black curve) and the total displacement (blue curve) of an active colloid ($R=1.36$) as function of time ($t$) from the time of reaction ($t^{*}$). The velocity decay and the total displacement after the reaction impulse are compared to Stokes friction (black dashed curve) and its time integral (blue dashed curve). The red curve shows the result of low pass filtering (LPF).

Figure 3

(a) $⟨\mathrm{\Delta }{r}^2⟩/2\mathrm{\Delta }t$ as function of $\mathrm{\Delta }t$ for various reaction frequencies. (b) Drift velocity in $z$ direction as function of reaction frequency (linear fit shown as dashed line). The average velocity and the standard deviations are extracted from total of 15 000 reactions. In both parts, $E_r=39.6k_{B}T$.