Study On Motion Mode Of Assemble Particles Loaded By Micromotors And Motion Analysis Of Single Particle

In the past decade or so,the field of microfluidics has developed rapidly,with a large number of new flow phenomena emerging.The study of flow mechanisms is also exciting and has become an important branch of fluid dynamics.As the scale of flow characteristics decreases,the inertial force rapidly decays,which is often ignored in microflow problems.Research has shown that introducing significant inertial forces can help achieve efficient motion of micromotors and precise control of microparticles.Generating significant inertial forces implies that microflows cannot operate in the usual low Reynolds number regime,and instead require the establishment of flow fields with characteristic velocities of ～1 m/s and characteristic timescales of ～1 μs at the microscale.Experimental studies have shown that microbubbles can collapse within a few microseconds,with a velocity of ～1 m/s,making them an alternative type of background flow field.Due to the difficulty in reliably controlling the interaction between bubbles and particles using conventional microcavitation techniques,this study utilizes a bubble driven micromotor’s asymmetric surface catalytic decomposition reaction to obtain controllable and periodically triggered bubbles,ensuring the repeatability of the experiment.By manipulating the directional motion of the micromotor through magnetic guidance,the interaction between bubbles and particles at the near gas-liquid free surface is achieved,revealing the interaction law between the self-driving combination(micromotor+microbubble+loaded particle).This article constructs a three-dimensional magnetic control platform to control the directional motion of the micromotor and change the motion speed.A magnetically controllable Pt-Ni-HGMs bubble driven micromotor(JM)was prepared,and the concentration of hydrogen peroxide in the experimental environment was calibrated using bubble growth time ～O(10 ms)and collapse time ～O(10 μs).The time domain asymmetry and spatial asymmetry caused by boundary constraints enable the micromotor to break through the flow constraints at low Re numbers and generate rapid motion.By equipping a programmable controller on a magnetic control platform,a two-dimensional software interface is developed to adjust the direction of motion of the micromotor in real time.A detailed study was conducted on the directional motion,rotational motion,and changes in motion mode of the micromotor in the horizontal direction.Real time display of external magnetic field direction and specific parameters through the software output interface,observation of micromotor movement using a microscope,storage and recording through software,and experimental results obtained through image and data analysis.Secondly,guided by the direction of the magnetic field,the bubble micromotor loads particles to form different assemble of motion modes.When the micromotor approaches the target particle,by adjusting the magnetic field direction to align the microbubble with the target particle,the self-driving combination of different motion modes(micromotor+microbubble+loaded particle)is established by quickly changing the motion direction of the micromotor.The motion direction and cumulative net displacement of the assemble mode with different size ratios are recorded.When the size of the loaded particle is larger than that of the micromotor,the jet generated by bubble collapse points to the larger loaded particle side,and the assemble moves towards the particle side.This is so-called “Pusher Mode”.When the size of the loaded particle is smaller than that of the micromotor,the bubble collapses and produces a jet towards the motor side.The micromotor pulls the particle forward,which is so-called the “Puller Mode”;When the dimensions of two particles are equivalent,their cumulative net displacement is zero.This is a so-called “Anchor Mode” similar to a anchor action of ship.Based on this,a series of phase diagrams of dimensionless size zones for different motion modes in experiments are presented,as well as the response relationship between the velocity of the combination and the size of the loaded particle,and the relevant factors affecting the experimental process are analyzed.Finally,based on the analysis of the mean velocity of the assemble particle,further research was conducted on the motion of loaded particles in the bulk solution under the impact of bubble collapse jet.Due to the high background flow velocity generated by the collapse of bubbles,it can break the microscale swimming theory provided by the scallop theorem and introduce inertial effects in microscale flow.After the bubble collapses,the loaded particles will generate a certain movement in the bulk solution.By using a high-speed camera with a frame rate of 450,000 fps,it can be observed that there are three stages of the loaded particle movement after the bubble collapses.As the bubble collapses,the central area of the bubble is a low pressure area,and the loaded particles first move towards the low-pressure area,generating a negative velocity;Afterwards,as the jet generates,the loading particle velocity becomes positive in a very short period of time,and in the subsequent stage,it continuously decays with the action of the jet.Provide solutions related to instantaneous velocity and relaxation time by loading the motion equation of particles,and fit them with experimental results.Record the motion displacement and instantaneous velocity variation curves of particles with different sizes/densities over time.