Study On The Motion Characteristics Of Particle Groups In Light-induced Micro-nanofluid Droplets

The precise control of microfluidics is a new kind of technology that has attracted more and more attention in recent years.It is a new model to use light to control the movement of microdroplets and to induce the transport of particles in microdroplets.It has the advantages of non-contact control,easy adjustment of wavelength and power,high spatial and temporal resolution,which has a wide application prospect in the fields of micro-scale mass transfer,heat transfer,micro-mixing,and water monitoring.Optical manipulation technology can be realized by utilizing:?1?direct optical force?optical radiation pressure and optical tweezers?;?2?optical modulation electric drive?photoelectric wetting and optical controlled electroosmosis?;?3?light-induced capillary force?wettability gradient and Marangoni effect?.Compared with the former two methods,the latter method does not need special optical equipment or complicated micromachining steps.However,the capillary force produced by the wettability gradient is very small,and the contact angle of the droplet is so small to overcome the effect of interface tension.Therefore,only a very few special liquids can move in the microchannel,and there are still shortcomings such as space limitation,short displacement distance,and slow speed.At present,most researches on the movement of microdroplets by light and inducing the transport of particles in microdroplets are based on the experimental observations.For the theoretical and simulation research under the action of these different forms of forces,there is only the mechanical model of the light drive under a single force,and there is no unified mechanical model and analytical solution method.Therefore,it is difficult to study the movement of light-induced droplet motion under various forms of interaction.Therefore,it is necessary to establish corresponding mechanical models and analytical solutions to study the movement of light-induced droplet motion under different forms of force.Based on the relevant needs,this paper establishes the corresponding physical model and analytical solution method,which can not only describe and analytically solve the microscale flow induced by light in different forms of force,but also contribute to the dynamic information of micro-nano particles and explain the relevant physical laws.It also provides a good guide for the theoretical analysis of particle movement accurately.In this paper,the basic principle of low Reynolds number flow in micro and nanoscale is firstly introduced,and the basic physical characteristics and motion law of micro and nanoscale particles are analyzed based on the motion equation.Secondly,the experiments on the light-induced motion behavior of micro and nanoparticles are carried out respectively.The movement behavior of particles is recorded and analyzed by a high-speed camera and particle image velocimetry?PIV?technology.Thirdly,based on the movement's different behavior of particles of different sizes,the main parameters affecting the movement of particles are analyzed and the movement characteristics of optical induced micro and nanoparticles are qualitatively analyzed and explained.The main conclusions are as follows:?1?Particles in micro and nano-size exhibit different light-induced motion behaviors under different light fields.The frequency of incident light and the number density of particles are important factors that affect the motion of particles induced by light.The smaller the wavelength or the higher the frequency of incident light,the greater the number density of particles,and the faster intense the light-induced particle movement;Conversely,the larger the wavelength of the incident light or the smaller the frequency,the smaller the particle number density,the smoother the light-induced movement.?2?Under the light irradiation,ZnIn₂S₄ particles?d=3?m?with low number density and micron-scale mainly exhibit two-dimensional horizontal diffusion motion,while at high number density,they gather and form three-dimensional eddy current motion.However,Fe₃O₄ particles?d=300 nm?with low number density have no obvious motion under light induction,and it also exhibits three-dimensional eddy current motion under high number density.?3?The diffusion motion of low number density particles may be related to the change of the surface Zeta potential under the light irradiation,while the eddy current movement of high number density particles is related to the heat absorption of particles.The heat absorption effect leads to the change of the surface temperature of droplets,produces a huge gradient of surface tension,which induces the Marangoni effect,and further induces the particles to form eddy current motion.?4?Based on the experimental study of the above two kinds of particle motion characteristics,the corresponding physical model of light-induced motion behavior is established,and the two experimental phenomena are quantitatively analyzed based on numerical simulation and theoretical analysis.The results of the theoretical solution and numerical simulation are compared with the experimental results,and the distribution of the flow field obtained by theory and simulation is consistent with the experimental results,which proves the validity of the quantitative theoretical analysis and mechanical model.Our work provides theoretical support for the efficient motion of light-induced micro and nanoparticles and the precise regulation of flow behavior in the optical microfluidic system,which is of great significance for the design and optimization of micro-scale mass transfer,heat transfer,and micro-mixing devices.