Research On Optical/Magnetic Composite Field Control Method For Micromotors And Their Collective Behaviors

As a new type of device,micro/nanomotors can realize its autonomic motion by utilizing the energy provided by the external environment to complete the target task at the micro scale,so they have great potential in different applications such as biomedicine,environmental remediation,etc.Compared with individual micro/nanomotor,micro/nanomotor swarms formed by a large number of micro/nanomotors through local interaction often exhibit complex collective behaviors and cooperative capability that micromotors do not possess.The configuration of swarm has great influence on its application,such as ribbon-like configuration,chain-like configuration and so on.However,the reported ribbon micromotor swarms that were powered by the physical field required intricate control strategies.The driving conditions and experimental devices which the chemically driven micromotor swarms require are relatively simple,thus attracting great attention from researchers.However,the reported chemically powered micromotor swarms are constructed by self-generated electric field,ultrasonic field,etc.Micromotors in swarm can only realize the isotropic self-organized aggregation and form swarms with circular or nearly circular structure,leading to limitations for the control of swarm configuration.In response to the above problems,we propose to develop magnetic photochemically driven micromotor,and use it as an individual to construct ribbon-like micromotor swarms by using the magnetic assembly behavior under static magnetic field.Firstly,we developed MB@TiO₂ micromotor with both magnetic responsiveness and positive phototactic behavior by coating a layer of TiO₂ on the surface of MB through ultrasonic-assisted sol-gel method.The results show that MB@TiO₂ micromotor is subjected to asymmetric photocatalytic reactions on the irradiated and shady surfaces,resulting in the uneven distribution of the reaction products O₂ and H⁺to form a concentration gradient on both sides of the micromotor,leading the MB@TiO₂ micromotors to realize positive phototactic motion toward the light source.Benefiting from its positive phototactic motion behavior,the MB@TiO₂ micromotor can easily overcome its own gravity to navigate in complex terrain or move in the water far away from the substrate.Secondly,we use the magnetic interaction among the MB@TiO₂ micromotors to construct ribbon-like photochemically powered MB@TiO₂ micromotor swarms,and study the collective behaviors of the swarms.The results show that under the composite control of ultraviolet light and magnetic field,the MB@TiO₂ micromotors not only reversibly assemble to form ribbon-like MB@TiO₂ micromotor swarms,but also realize positive phototaxis motion.The motion velocity of micromotor swarms increases with the increase of H₂O₂ concentration and light intensity in the environment,and the phototaxis motion direction of micromotor swarms can be controlled by adjusting their orientation.Compared to individual micromotor,the ribbon-like MB@TiO₂ micromotor swarms exhibit high robustness similar to the swarm robots and regular straight motion direction.Based on the above reversible assembly and positive phototaxis motion behaviors,the ribbon-like MB@TiO₂ micromotor swarms can not only adapt to different complex environment through reconstruction of their configuration and orientation,but also utlize its reversible ribbon-like swarm structure to accomplish controllable cooperative manipulation and"cleaning"of cargoes with a volume much larger than indivial micromotor in open or restricted area.In summary,we have developed magnetically MB@TiO₂ micromotors as well as their ribbon-like swarms with positive phototaxis,developed control strategy based on optical/magnetic composite field,and demonstrated its cooperative cargo manipulation.The research results not only provide new ideas for simple construction and manipulation of micromotor swarms,but also are expected to provide new technical solution for micro/nano manipulation,pollutant cleaning and micro surgery.