Bimetallic Microswimmers Speed Up in Confining Channels

Chang Liu, Chao Zhou, Wei Wang, and H. P. Zhang

Phys. Rev. Lett. 117, 198001 – Published 3 November 2016

Abstract

Synthetic microswimmers are envisioned to be useful in numerous applications, many of which occur in tightly confined spaces. It is therefore important to understand how confinement influences swimmer dynamics. Here we study the motility of bimetallic microswimmers in linear and curved channels. Our experiments show swimmer velocities increase, up to 5 times, with the degree of confinement, and the relative velocity increase depends weakly on the fuel concentration and ionic strength in solution. Experimental results are reproduced in a numerical model which attributes the swimmer velocity increase to electrostatic and electrohydrodynamic boundary effects. Our work not only helps to elucidate the confinement effect of phoretic swimmers, but also suggests that spatial confinement may be used as an effective control method for them.

Received 15 June 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.198001

Physics Subject Headings (PhySH)

Electrophoresis

Colloids Low Reynolds number swimmers Microswimmers

Finite-element method Optical microscopy

Polymers & Soft Matter

Authors & Affiliations

Chang Liu¹, Chao Zhou², Wei Wang^2,3, and H. P. Zhang^1,4,*

¹Department of Physics and Astronomy and Institute of Natural Sciences, Shanghai Jiao Tong University, Shanghai 200240, China
²School of Material Science and Engineering, Harbin Institute of Technology, Shenzhen Graduate School, Shenzhen 518055, China
³Center for Soft and Living Matter, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea
⁴Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China

^*To whom correspondence should be addressed. hepeng_zhang@sjtu.edu.cn

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Vol. 117, Iss. 19 — 4 November 2016

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Images

Figure 1
Swimmer dynamics in linear channels. (a) Illustration of the self-electrophoretic propulsion mechanism. Orange and blue represent high and low electric potentials, respectively. The electric field $\vec{E}$ points from the anode end to the cathode, where the anode corresponds to the Au end in AuRu swimmers and the Rh end in AuRh swimmers. The white arrow indicates the direction of motion of the swimmer. (b) Scanning electron image of linear channels with a length $L$ , width $w$ , and height $h$ . In (c), a typical AuRu swimmer trajectory is plotted on an optical image of channels; each dot represents an instantaneous position (separated by $1 / 30 \sec$ in time) of the swimmer and is color coded by the normalized swimmer velocity $V / V_{\circ}$ (see text). In (d), the normalized swimmer velocity $V / V_{\circ}$ is plotted versus the channel width in the main frame and versus the channel cross-section area in the inset. (e) and (f) Normalized swimmer velocity versus the concentrations of $H_{2} O_{2}$ and ${NaNO}_{3}$ . Swimmer velocity outside channels, $V_{\circ}$ , is plotted in the insets. All linear channels have the same length of $L = 16 μ m$ . Solutions with 15% $H_{2} O_{2}$ are used to produce data in (c),(d), and (f). ${NaNO}_{3}$ is only used in experiments in (f). See supporting videos S1 to S4 in the Supplemental Material [48] for swimmer motion in different channels.
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Figure 2
(a) Schematic diagram of the model (not to scale). A cylindrical coordinate frame ( $r$ , $z$ ) is defined with the origin located at the center of the swimmer; the local normal direction on the swimmer and channel surfaces is denoted as $\vec{n}$ . Dimensions of the swimmer and channel are defined. The anode and cathode of the swimmer is shown in yellow and gray, respectively. (b)–(c) Electric potential and fluid flow around a stalled swimmer ( $V = 0$ ) in channels. Lines in (b) and (c) represent the electric field and stream lines, respectively. To produce results in Fig. 2(c), we set the swimmer zeta potential to zero ( $ζ_{s} = 0$ ) so that only electro-osmotic flow on the channel surface (not on the swimmer surface) is generated. (d)–(e) Normalized swimmer velocity versus the channel width $d_{c}$ and the reaction flux $J$ . The legend of (d) shows the channel zeta potential used for each data set. Default values (see text) are used for simulation parameters that are not specified in (b)–(e).
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Figure 3
Normalized swimmer velocity $V / V_{\circ}$ increases with the swimmer length. Swimmer velocities inside and outside channels are shown in the inset. Numerical and experimental data are shown by open and solid symbols, respectively. Experimental data are measured with AuRh swimmers in linear channels with $w = 1.2$ and $h = 2 μ m$ ; Solutions with 15% $H_{2} O_{2}$ are used. Default values (see text) are used for simulation parameters except $J = {6 \times 10}^{- 6} mol / m^{2} / s$ and $d_{c} = 2 .3 μ m$ .
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