Influence of Asymmetry and Driving Forces on the Propulsion of Bubble-Propelled Catalytic Micromotors

Abstract

Bubble-propelled catalytic micromotors have recently been attracting much attention. A bubble-propulsion mechanism has the advantage of producing a stronger force and higher speed than other mechanisms for catalytic micromotors, but the nature of the fluctuated bubble generation process affects the motions of the micromotors, making it difficult to control their motions. Thus, understanding of the influence of fluctuating bubble propulsion on the motions of catalytic micromotors is important in exploiting the advantages of bubble-propelled micromotors. Here, we report experimental demonstrations of the bubble-propelled motions of propeller-shaped micromotors and numerical analyses of the influence of fluctuating bubble propulsion on the motions of propeller-shaped micromotors. We found that motions such as trochoid-like motion and circular motion emerged depending on the magnitude or symmetricity of fluctuations in the bubble-propulsion process. We hope that those results will help in the construction and application of sophisticated bubble-propelled micromotors in the future.

Keywords: active matter; bubble propulsion; complex-shaped multi-compartmental microgels; complex-shaped multi-compartmental microparticles; self-propelled micromotors.

Figure 1

Schematic illustrations of the propeller-shaped micromotor and synthesizing methods. (a) 3D sketch of the micromotor; (b) 2D sketch of the micromotor with diameter d ~140 μm and an angle of propeller θ ~50°; (c) Schematic illustrations of centrifuge-based droplet-shooting device (CDSD); (d) Synthesizing diagram of the propeller-shaped micromotors. A cross sectional image of the glass capillary (I), the spherical microparticles (II) and the propeller-shaped micromotors with PtNPs (III) are shown; (e,f) Designs of a propeller-shaped micromotor used in our experiments and numerical simulation with a single catalytic site (e) and with double catalytic sites (f).

Figure 2

Experimental results of bubble propulsion of the propeller-shaped micromotors with a single catalytic site in the H₂O₂ solution. (a) A microscope image of a propeller-shaped micromotor with a single catalytic site; (b) Time series from t = 5.6 s to t = 8.0 s of a propeller-shaped micromotor with a single catalytic site propelled by bubbles; (c) Schematic illustration of the trajectory of (b); (d) The whole trajectory of the micromotor in (b); Cyan arrow: t = 0 s; magenta arrow: t = 10 s; (e) Trajectory of another micromotor. The notation of arrows is the same as in (d); (f) The time variation of |r(t)|. Black solid line: For trajectory of (d); red dashed line: For trajectory of (e); (g) The time variation of $\mathsf{\phi }\left(t\right)$. The notation of each line is the same as in (f).

Figure 3

Experimental results of bubble propulsion of the propeller-shaped micromotors with double catalytic sites in the H₂O₂ solution. (a) A microscope image of a propeller-shaped micromotor with double catalytic sites; (b) Time series from t = 7.8 s to t = 9.4 s of a propeller-shaped micromotor with double catalytic sites propelled by bubbles; (c) Schematic illustration of the trajectory of (b); (d) The whole trajectory of the micromotor in (b); Cyan arrow: t = 0 s; magenta arrow: t = 10 s; (e) Trajectory of another micromotor. The notation of arrows is the same as in (d); (f) The time variation of |r(t)|. Black solid line: For trajectory of (d); red dashed line: For trajectory of (e); (g) The time variation of $\mathsf{\phi }\left(t\right)$. The notation of each line is the same as in (f).

Figure 4

Numerical analyses of bubble propulsion of the propeller-shaped micromotors with a single catalytic site. (a–d) Calculated trajectories of the micromotor in each condition of $\hat{\mathsf{\sigma }}$_side1 and $\hat{\mathsf{\sigma }}$_arc1. $\hat{\mathsf{\sigma }}$_side1 = $\hat{\mathsf{\sigma }}$_arc1 = 0 (a); $\hat{\mathsf{\sigma }}$_side1 = $\hat{\mathsf{\sigma }}$_arc1 = 1 (b); $\hat{\mathsf{\sigma }}$_side1 = $\hat{\mathsf{\sigma }}$_arc1 = 5 (c); $\hat{\mathsf{\sigma }}$_side1 = $\hat{\mathsf{\sigma }}$_arc1 = 10 (d); Cyan arrow: t = 0 s; magenta arrow: t = 100 s; (e) The time variation of |r(t)|. Black line: For trajectory of (a); red line: For trajectory of (b); green line: For trajectory of (c); blue line: For trajectory of (d); (f) The time variation of $\mathsf{\phi }\left(t\right)$. The notation of each line is the same as in (e).

Figure 5

Numerical analyses of bubble propulsion of the propeller-shaped micromotors with double catalytic sites. (a–e) Calculated trajectories of the micromotor in each condition of Σ = ($\hat{\mathsf{\sigma }}$_side1,$\hat{\mathsf{\sigma }}$_arc1,$\hat{\mathsf{\sigma }}$_side2,$\hat{\mathsf{\sigma }}$_arc2). Red lines/curves: the sides and the arcs that generate a pushing force with large fluctuation. Blue lines/curves: the sides and the arcs that generate a pushing force with small fluctuation. Σ = (1, 1, 1, 1) (symmetric) (a); Σ = (1, 10, 1, 1) (asymmetric) (b); Σ = (10, 1, 1, 1) (asymmetric) (c); Σ = (1, 10, 1, 10) (symmetric) (d); Σ = (10, 1, 10, 1) (symmetric) (e); Cyan arrow: t = 0 s; magenta arrow: t = 100 s; (f) The time variation of |r(t)|. Black line: For trajectory of (a); blue line: For trajectory of (b); purple line: For trajectory of (c); green line: For trajectory of (d); orange line: For trajectory of (e); (g) The time variation of $\mathsf{\phi }\left(t\right)$. The notation of each line is same as (f); (h) The time variation of mean square displacement (MSD) $G_{\mathrm{MSD}}\left(t\right)$. The notation of each line is same as (f).