The design of artificial swimmers and other motion-controlled objects often involves asymmetrical or charged objects. G. Loget and A. Kuhn, from the University of Bordeaux, used the polarization of conducting materials in an external electric field to operate their swimmers. Bipolar electrochemical reactions at the surfaces of the objects results in the asymmetric production of gas bubbles, which propels the objects.
(a) A stainless-steel ball in an external electric field in aqueous H2SO4: water splitting induces gas bubble formation (H2 on the cathode, on the left, and on the anode, on the right). The scale bar is 250 μm. (b) A glassy carbon sphere in a polydimethylsiloxane microchannel is propelled in an aqueous solution of HCl and HQ, under the influence of an external field. The scale bar is 100 μm. Reproduced with permission from Nat. Com-mun. 2 (2011), DOI: 10.1038/ncomms1550.© 2011 Nature Publishing Group.
As reported in the November 15, 2011 online edition of Nature Communications (DOI: 10.1038/ncomms1550), the researchers achieved linear and rotational motions of millimeter-sized conducting objects, due to gas bubble formation. Placing the swimmer between an anode and a cathode induces a potential difference between two opposite sides of the swimmer and electrochemical reactions such as water splitting result in differential gas bubble formation on the object surface. Other electrochemical reactions could also be used to enhance the asymmetry of the propulsion and therefore produce higher speeds. The bubble propulsion was also shown to be more efficient for larger objects where larger potential differences across the surfaces and smaller viscosity effects combine and increase the propulsion.
Macroscopic rotors were also developed based on a similar principle, where exposure of only one small conducting part on each blade causes asymmetrical bubble production. Vertical rotors can also take advantage of the buoyancy of bubbles, pushing the rotor blades up and reaching speeds of 0.70 rpm. Tuning the adherence of the bubbles to the sails then allows optimization of the rotation speed.
