Self-Propelled Motion of Oil Droplets on Au Electrode during Sn Electrodeposition

INTRODUCTION

It was reported that macroscopic contact angles of an oil droplet put on an electrode surface decreased during electrodeposition of metals, such as Sn, Pb and Sb [1]. It was also reported that the difference in the contact angle between the front and rear sides of the droplet caused self-propelled lateral motion of the droplet [1]. This motion has attracted much interest from the viewpoint of the emergence of mechanical motion induced by chemical or electrochemical reactions.

Little has been reported about the factors affecting the velocity of the motion. It has only been reported that the velocity increases with a negative shift in the electrode potential [1]. In this work, we studied the factors affecting the velocity and the mechanism of the motion.

EXPERIMENTAL

The self-propelled motion of a nitrobenzene droplet on an Au ring electrode was observed during Sn electrodeposition (Sn²⁺ + 2e^- → Sn) in H₂SO₄ + 0.01 M SnSO₄. The volume of the droplet was about 1.0 μL. Figure 1 shows schematic of the experimental setup. In order to study the dependence of the velocity on the density of the aqueous electrolyte, K₂SO₄was added to the electrolyte to increase the density.

RESULTS and DISCUSSION

We studied the dependence of the velocity on the electrode potential when the concentration of H₂SO₄was 0.5 M. As shown in Figure 2, the droplet did not move, i.e., the velocity was zero, in the potential region above ca. -0.44 V, though the Sn electrodeposition occurred. On the other hand, the droplet moved in the potential region below ca. -0.44 V where both the Sn electrodeposition and hydrogen evolution reaction (HER) occurred. The velocity increased with a negative shift in the electrode potential.

In the potential region where the droplet moved, the Sn electrodeposition was limited by the mass-transport of Sn²⁺ions to the electrode surface, and hence the rate of electrodeposition was constant independently of the potential. We, therefore, think the velocity increase with a negative shift in the potential is probably attributed to the increase in the rate of the HER. Based on this idea, we suggest that the droplet motion is driven by the interaction between the Sn electrodeposition and the HER.

We also studied the dependence of the velocity on the density of the aqueous electrolyte. As shown in Figure 3, the velocity observed at -0.66 V vs. SHE increased as the concentration of K₂SO₄increased. That is, the velocity increased as the density increased. The buoyancy of the droplet increased with the increasing density, which probably led to the increase in the velocity.

This work is partially supported by the Research Institute for Science and Technology of Tokyo Denki University under Grants Q12E-02.

REFERENCES

 [1] S. Nakanishi, T. Nagai, D. Ihara, Y. Nakato, Chemphyschem, 9 (2008) 2302-2304.