Nanobot Propulsion Methods: Theory and Experiments

Robots are mechanical systems that perform tasks through a series of perception, cognition, and manipulation steps. Robots with dimensions between 10-9 -10-4 m are classified as micro/nanobots. These are often called micro/nanoswimmers (or micro/nanomotors) since most micro/nanobots are used in liquid environments. These tiny robots can be precisely guided to locations where human access is restricted. Consequently, micro/nanobots are widely applicable in different fields, including environmental cleaning, medicine, exploration, and military applications. Micro/nanobots (or micro/nanoswimmers) are often designed differently from their macro-sized counterparts for two reasons. First, while macro-sized robots typically use onboard batteries, micro/nanobots generally use external energy sources, such as chemical, magnetic, electric, acoustic, and optical energy, due to the current technological restriction of battery miniaturization. Second, micro/nanobots must create a propulsive force differently than macro-sized robots. Unlike macro-sized robots, micro/nanobots swimming in a Newtonian liquid by reciprocal motion show zero net displacement since the drag force exerted on objects dominates over the inertial force at such small scales. Methods to create propulsive forces in micro/nanobots can be inspired by small-scale living entities. For example, microorganisms create propulsive forces by using non-reciprocal motion, such as spermatozoon and bacterial flagellum that use undulatory and cork-screw locomotion, respectively. The purpose of this dissertation is to design and fabricate micro/nanobots and study their propulsion mechanisms. Four micro/nanobots are newly designed by tuning properties of micro/nanobots, such as geometry, material, physical, and chemical properties. Furthermore, their propulsion mechanisms are understood for efficient and controlled propulsion by fully characterizing their resultant locomotion in Newtonian fluids. In the first chapter, general knowledge of micro/nanobots, including definition and theory of micro/nanobots, is introduced. In the following chapters (chapter 2-4), the state of the art of micro/nanobots actuated by different external energies is thoroughly reviewed, especially focusing on the effect of the design of micro/nanobots on their locomotion and propulsion mechanisms in fluids. Newly developed micro/nanobots are introduced in each chapter, consisting of subsections addressing motivation, experimental methods, results and discussion, and conclusion. Chapter 2 reports catalytically driven core−shell nanowires. The investigated catalytic locomotion in H2O2 solution shows that, unlike conventional bimetallic nanowires that are self-electroosmotically propelled, our core−shell nanowires show both a noticeable decrease in rotational diffusivity and increase in motor speed with increasing nanowire length. Numerical modeling based on self-electroosmosis attributes decreases in rotational diffusivity to the formation of toroidal vortices at the nanowire tail, but fails to explain the speed increase with length. To reconcile this inconsistency, we propose a combined mechanism of self-diffusiophoresis and electroosmosis based on the oxygen gradient produced by catalytic shells. The possible contribution of diffusiophoresis to an otherwise well-established electroosmotic mechanism sheds light on future designs of nanobots, at the same time highlighting the complex nature of nanoscale propulsion. Chapter 3 reports multi-wavelength light-responsive Janus microbots consisting of black TiO2 microspheres asymmetrically coated with a thin Au layer. Conventional photocatalytic micro/nanobots are limited to the use of specific wavelengths of light due to their narrow light absorption spectrum, which limits their effectiveness for applications in biomedicine and environmental remediation. Unlike conventional robots, our microbots are propelled by light, both in H2O2 solutions and in pure H2O over a broad range of wavelengths including UV, blue, cyan, green, and red light. An analysis of the motion of the robots shows that the robot speed decreases with increasing wavelength, which has not been previously realized. A significant increase in robot speed is observed when exploiting the entire visible light spectrum (>400 nm), suggesting a potential use of solar energy, which contains a great portion of visible light. Finally, stop−go motion is also demonstrated by controlling the visible light illumination, a necessary feature for the steerability of micro/ and nanomachines. Chapter 4 reports two types of newly developed nanobots that are magnetically actuated. First, nanowire-based magnetic surface walkers are reported. The nanowires are made of hard-magnetic CoPt alloy synthesized by means of template-assisted galvanostatic electrodeposition. The hard-magnetic behavior of the nanowires allows their alignment to be programmed with an applied magnetic field, as they can retain their magnetization direction after pre-magnetizing them. Their propulsion mechanism can be changed as a function of the applied rotating magnetic field frequency. By engineering the macroscopic magnetization, the locomotion mechanism of the nanowires is set to tumbling, precession, or rolling depending on the frequency of an applied rotating magnetic field. In addition, vortices were found by tracking polystyrene microbeads trapped around the CoPt nanowire when they were propelled by precession or rolling motion. Second, magnetically driven multilink nanowires are reported. The employed manufacturing process enables tuning the geometrical and material properties of nanowires and, therefore, allow resembling the shape and swimming strategies of spermatozoon. The resultant structure comprises an elastic eukaryote-like polypyrrole tail and rigid magnetic nickel links connected by flexible polymer bilayer hinges. Under a planar oscillatory magnetic field, the multilink nanowires display planar undulations. The multilink design exhibits an increase in swimming efficiency with increasing numbers of hinges in the structure. Annex 1 covers important yet overlooked aspects in the fabrication of nanowires observed during the fabrication process of nanobots. We examine the junction quality of electrochemically grown segmented nanowires. We particularly focus on the Au-Co system to illustrate this aspect. Annex 2 provides a strategy to fabricate supported anodic aluminum oxide (AAO) membranes, which served as templates to grow long nanowires with modulated diameters. This approach was also employed to fabricate core-shell nanobot reported in chapter 2