Theoretical And Experimental Research On Dynamics Of Multiple Microspheres In Evanescent Wave Traps

The interactions,between light and microscopic particles in dispersed system,involve energy and momentum transfers.By optical trapping,we can observe the dynamics of single particles easily,which is very helpful for the study of their behaviors and the interactions between them.One common situation is the optical binding of multiple simultaneously illuminated particles,where the optical forces not only arise directly from the incident light field,but also stro ngly rely on the field modification due to light scattering.These objects may interact mutually,and consequently,organize themselves into specific configurations.Although the features of optical forces have been widely exploited in single particle manipulations,the mechanical effects of light scattering between multiple particles were ignored between multiple particles for a long time.Quantitative study on the dynamics of multiple particles is not well developed.Meanwhile although evanescent wave traps have natural superiority for particle organizations in optical binding,there is no quantitative study on the behaviors of the particles in dense structures formed in these traps.Therefore,a more complete research would not only enhance the understanding of the theories,but also explore possible applications.Based on this,one-dimensional and two-dimensional dense configurations of multiple SiO2 micro-spheres are obtained in evanescent wave traps.After position tracking,dynamics of single particles in the structures and hydrodynamics between them in the chains are analyzed.The research work mainly includes the following aspects:1.Dynamics simulations of single Brownian particles.Common methods of stiffness calibration are generally compared.The dynamics of single Brownian particles are simulated in two ways of velocity and position iteration,respectively,using Monte-Carlo and finite difference methods.After the stiffness values are calibrated with equipartition theorem method for the simulated position fluctuations,the inaccuracies of two ways are found in the same level,however,the way of position iteration requires much fewer time than the other way.Meanwhile,other simulations are also employed,such as the influences of the inappropriate axis system on the position correlation functions,and dynamics analysis of Brownian particles with external torques.2.Evanescent wave trap system.Dual counter-propagating evanescent optical fields and double pairs of dual counter-propagating evanescent optical fields are generated at a dielectric interface,and microspheres are observed to organize themselves into different specific dynamic configurations respectively.3.Simultaneous position tracking of multiple spheres.The diffraction features of optical microscope are introduced,and digital images are simulated for single point source in different SNR(signal to noise ratio).The regimes of single particle tracking are compared,and the present local maximum regime is developed based on the observed features of the particle chains.The developed regime can reach the same accuracy but save much time in al.4.Dynamics of single micro-sphere in one-dimensional chains.The SiO2 microspheres in diameters of 1um are observed to organize themselves into one-dimensional chains with small separations in counter-propagating evanescent fields.After position tracking of the particles,average position separations between adjacent particles in the chains are compared in different conditions.The chain can be modeled as N spheres connected with N-1 Gaussian springs,and every particle is taken as one probe in its ambient optical trapping potentials,where the stiffness of the traps can be obtained from the spring constants of the connections.The strength of these connections is characterized by measuring the Brownian position fluctuations of the optically bound microspheres under different conditions,and the dependence of the strength on incident laser power and number of particles in the chain is determined.The minimum power required for stable chain formation is predicted,which is found to accord with the experimental phenomena.5.Dynamics of one-dimensional chains.The features are compared between chains of colloidal particles and polymer chains,and Rouse/Zimm models for polymer chain models are introduced.Due to the difficulties in solving the kinetic equations for chains of N particles,independent modes with weighted position vectors are used to describe the kinetic states of the chains.Tensors for descriptions of hydrodynamics interactions between particles are introduced,and kinetic equation in general matrix form is shown.Eigen-method is shown and validated by the dynamics simulations of the chains conforming to Rouse/Zimm models.The modes are obtained using the eigen-method,and decay features of the auto-correlation functions are analyzed and explored.6.Dynamics of single spheres in two-dimensional configurations.The position fluctuations of single Brownian particles are observed in the demonstrated self-organized square dynamic lattice in four evanescent optical fields at a dielectric interface.After the subtraction of the center-of-mass motions,and the transformations of the axis system,the distribution features of the microspheres are shown.The stiffness of the optical potential is calibrated using the equipartition theorem method,and the results have shown equivalently effective confining potentials around the particles in the lattice.