The Individual Oscillation And Collective Behaviors Of Silver Based Micro/nano Motors

Micro/nanomotors are synthetic colloids that can move autonomously at the micron scale in an aqueous environment.As a kind of intelligent biomimetic material,they not only have the ability to transport cargos and detect chemicals at micro-scale,but also are excellent candidates for the research of active matter.Recently,photochemically powered,oscillatory micromotors are found to exhibit various nonlinear dynamics including periodic motion,synchronization,and wave propagation,all widely found in nature.As a kind of self-oscillatory,smart,and responsive material,micromotors can exhabit nonlinear behaviors without external signal.This type of oscillatory micromotor is a good model system that could help us understand similar nonlinear processes in nature and also offer a strategy for motor coordination.However,previous reports mainly foucused on describing phenomena,while systematic and deep analysis on the scientific laws behind the phenomena are still lacking.To systematically investigate the dynamics of oscillatory micromotors,we fabricated Ag based Janus micromotors of a uniform size distribution.We then studied the individual propulsion menchanism and the oscillatory dynamics through investigating the material transformation,surface chemical reaction dynamics,and near-field distribution.Synchronization of multiple micromotors and the propagation of motion waves across a large population were also quantitively characterized and qualitatively described by nonlinear methods.In addition,applications such as microparticles joining and signal transmission in micromotor system were also explored.Details of our findings are as follows:To understand the propulsion mechanism of PMMA-Ag oscillatory micromotors,we fabricated PMMA-Ag Cl micromotors and the speed decay of the motors was observed and demonstrated via frist order chemical reaction.Combined with the phase characterization before and after chemical reactions and the numerical simulation,we demonstrated the ionic self-diffusiophoresis of Ag Cl Janus micromotors.H⁺and Cl^-are generated at the Ag Cl surface via photodecomposition reaction and diffuse away at different speeds.Then,an electrical field around the motor pointing from the active side to the inert one is generated and drives an electroosmotic flow pointing in the same direction which causes the motor to move away from the active side.Based on production of solid Ag nanoparticles after reaction,application of microjoining was explored.PMMA-Ag Janus micromotors exhibited excellent periodic motion under Cl^-,H₂O₂,and UV light.Motor dynamics can be modulated by changing the environmental parameters.Based on the experimental results,we proposed a qualitative mechanism:Ag nanoparticles induce an autocatalytic decomposition of Ag Cl at the motor surface.This positive feedback causes a fast motion of micromotors via ionic self-diffusiophoresis.The Ag patches are then photoelectrochemically oxidized and induce a steady accumulation of Ag Cl on the motor surface.This process corresponds to the recovery stage.The Ag Cl patches gradually decompose into Ag nanoparticles,giving rise to the autocatalytic decomposition of Ag Cl again which triggers the next pulsation of the micromotors.Based on two modes of motor dynamics via temporal modulation of illumination,a specific series of continuous and oscillatory motion was achieved,and application of information coding was explored.Combining with acoustic manipulation and nonlinear methods,we studied three levels of synchronization of the oscillatory micromotors:among individual motors,in a cluster,and between two beating clusters.Oscillatory micromotors pulsate synchronously but still along their original direction while approaching each other within a certain distance.When motors aggregate into a cluster,a collective physical field dominates the periodic expansion and contraction of the cluster via the electroosmosis flow on the substrate while the motion of individual motor is suppressed.Floating beating clusters can be modulated by light intensity and ultrasonic driven voltage.Small clusters beat faster than big ones,and two clusters with different beating frequencies can also synchronize when they are close by.We also characterized and analyzed the order parameter for synchronization.Two types motion wave can be found in a large population of oscillatory micromotors.Stronger light and higher particle density increase the coupling strength among motors and promote the wave propagation.Waves propagate more stable and across a larger area under a low light intensity.Chemical wave modulates the motors in two ways:In a relatively low population density,micromotors become active upon a coming wave and move in their original direction.In this situation,the chemical wave propagates as a chemical signal carrier to activate the motors successively.In a relatively higher population density,the chemical wave gives rise to an electroosmotic flow on the substrate which drags the micromotors toward the wave front.Motion waves in oscillatory micromotors system can also propagate in a microchannel,or a light pattern,and can also trigger other type micromotors,suggesting the possibility applicaton of signal transmission in a micromotor system.As a new type of intelligent biomimetic material,oscillating micromotors can respond to the external environment,and act as a self-oscillating intelligent material unit.This thesis systematically investigated the individual motion and collective behaviors of oscillatory micromotors.The research on synchronization and wave propagation of oscillatory micromotors not only help us to understand similar nonlinear phenomena in nature,but also suggest that micromotors can communicate and coordinate with each other,offering a possible biomimetic strategy for the swarm control of micromachines.