Bubble driven catalytic micromotors
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This year (2014) we celebrate a decade anniversary of catalytic micromotors, in which they have come to represent one of the important technical advances, having shown promise in many important functions in biomedical and engineering fields such as sensing, detection, drug delivery, oil spill cleanup, etc. Catalytic nano-/ micromotors are structures that convert chemical energy present in the surrounding aqueous environment into mechanical work through a catalytic reaction induced by an asymmetrically placed catalyst. This dissertation focuses on the fundamental study of the motion mechanics of catalytic motors. Diffusiophoresis is the motion of motors due to diffusion of reaction entities, such as the fuel or by-products of the catalytic reaction. Small Janus catalytic motors i.e., spherical microbeads half coated with catalyst metal, utilize the diffusiophoresis mechanism. Introduction of a hydrophobic front surface in Janus catalytic has been observed to make them appreciably faster due to changes in reaction kinetics. The bubble propulsion mechanism is observed when the bubbles formed on the catalyst surface eject or burst. The motion of bubbles provides an opposing thrust to the motor. We have studied the motion of bubble propelled big Janus motors using a fast CCD camera. The formation of bubbles depends closely on the nucleation energy, which also is related to surface curvature. It is predicted that bubbles are easier to nucleate on a concave shaped surface than on a convex shaped surface. Thus, bubble propulsion can be easily seen in concave motors. The predictions were confirmed with nanoshell catalytic motors with catalyst coated inside the shell. Similarly, if the catalyst is coated in the inner surface of a tube, a tubular motor can be produced. We used graphene oxide nanosheets (GO) as templates and the stress effect in the multilayer of metal thin films to create microtubes. Finally, collective motion has been observed with 5-μm diameter Janus motors. These motors are too small for bubble propulsion to occur, but collectively they produce bubble and perform a synchronized motion. The collective motion is a result of Marangoni effect.