Optical Control Of The Individual And Collective Motion Of Janus Micromotors

Micromotors,also known as colloidal motors or swimming micro-nanorobots,are a class of microparticles that can move autonomously by consuming energy stored in their environment.They have been widely applied in various fields such as drug delivery,environmental remediation,and micro/nano manipulation.To improve the working efficiency of micromotors in the above applications,it is crucial to control their motion behavior at both individual and collective levels in time and space.Optical control of micromotors has attracted much attention due to its advantages such as remote,programmable,and multidimensional control.At the individual level,the speed,direction,and trajectory of micromotors have been controlled by light.However,current optical control strategies are limited to single-mode motion with low precision.At the collective level,the emergence of swarming motion waves among oscillating micromotors has the potential to achieve swarm cooperation of micromotors,but its mechanism remains unclear.In addition,the control of the propagation of the motion waves has not yet been achieved.Therefore,the goal and content of this thesis is to control the motion modes of micromotors by regulating light on-off and their trajectories by introducing structured light and optoelectronic tweezer technology.In addition,this thesis reveals the underlying mechanism of motion waves in oscillating micromotors through a combination of experimental measurements,numerical simulations,and theories.Furthermore,structured light technology has been applied to control the initiation and propagation of waves.The main results of this thesis are as follows:In terms of understanding and controlling the individual motion of micromotors,we first prepared Janus Ag-Si O₂ oscillating motors.Based on the effect of p H on the oscillation frequency of individual motors and local p H profile around individual motors,a qualitative explanation of the relationship between the periodic motion of Janus oscillating micromotors and chemical reactions on the micromotors surface is provided.We have discovered“dark pulse”of oscillating motors after switching light off.According to the effect of light intensity,illumination time,H₂O₂ concentration,KCl concentration on the intensity of dark pulse,we have revealed that the mechanism of the dark pulse is the self-phoresis resulting from the Ag-catalyzed decomposition of H₂O₂.Based on the nature of the oscillating motor’s dark pulse,we have realized different motion modes such as intrinsic oscillation（f_s=0）,triggered oscillation（0.1 Hz<f_s<6 Hz）,continuous motion（6 Hz<f_s<30 Hz）,and semi-intrinsic oscillation（30 Hz<f_s<1 k Hz）by changing the frequency of the light switching.In addition to controlling motion modes of micromotors,we have achieved real-time control of the individual trajectories of micromotors with high precision by combining structured light technology and the optoelectronic tweezer effect.We found that the micromotors tended to be confined to and moved along the boundary of the light patterns.To understand the physical mechanism of such a confinement effect,the distribution of electric fields was simulated by COMSOL software,which showed that the electric field intensity was strongest at the boundary.The theoretical calculation confirmed that a micromotor experienced positive dielectrophoresis.Based on the simulation and calculation results,we concluded that this confinement effect of the micromotor resulted from the light-induced dielectrophoresis.Based on the confinement effect,the motion trajectories and direction of the micromotor were controlled by static light patterns,and the spiral motion modes and long-distance transport of the micromotor were achieved by dynamic light patterns.In order to understand and control the collective behavior of micromotors,we have revealed the underlying mechanism of motion waves among Janus Ag-PMMA oscillating micromotors by combining the measurement of local p H by fluorescence probes with a reaction-diffusion model.By measuring the change of p H values in space and time during the propagation of motion waves,we found that the oscillating micromotors generated OH^-chemical waves.A reaction-diffusion model was established to show that oscillating micromotors form a reaction-diffusion system that spontaneously generates chemical waves.Based on this understanding,we further elucidated the nature of the motion waves based on colloidal interface theory,which showed that the sequential motion of the micromotors resulted from the motor’s response to chemical waves.In addition,we proposed that at low population densities,the micromotor was driven by self-phoresis,which exhibited independent ballistic waves;whereas at high population densities,the micromotor was driven by neutral diffusion-osmotic flows,which exhibited swarming waves.This proposed mechanism explains the two types of motion waves at different population densities.Based on the understanding of the motion wave,we used structured light technology to control the initiation,propagation and annihilation of motion waves.Since the oscillation reaction on the micromotor surface occurs only in the illuminated area,the light patterns can control the propagation range of the motion waves.By increasing the autocatalytic reaction on the motor surface after attenuating the light intensity,a motion wave was triggered and the wave speed was increased in the area where the light intensity was reduced.Based on these two principles,the path,direction,and shape of a motion wave were controlled by modulating the shapes of light patterns.On the other hand,the wave frequency was controlled by modulating the switching frequency of light patterns（0.08 Hz≤f_s≤0.25 Hz）.The wave velocity（10¹～10³μm/s）was controlled by modulating the distribution of light intensities.