Motion Of The Micro-particles In Dispersion System Studied With Optical Tweezers

Since the dispersion system widely exists in nature, it is important to do research in this field. The properties of dispersion rely on the properties of small particles especially their dynamics behaviors, so the research on the motion behavior of micro-sized particles is a large application field of hydrodynamics in the research of dispersion system. However, it is difficult to investigate the dynamics of single particle experimentally for a long time due to the lack of appropriate instruments.Light carries both linear and angular momentum and can thus exert force and torques on matter. Optical tweezers exploit this fundamental property to trap objects in a potential well formed by tightly focused light. This technique allows the manipulation of microscopic objects without mechanical contact. Moreover, the optical forces on a micro-particle trapped in an optical trap can be accurately measured. Such optical tweezers have been used in the study of dynamics behaviors of colloidal particles.In this thesis, we use optical tweezers to study the motion of the particles in the dispersion system. The main subjects of this thesis are the diffusion and rotation of particles in the trap, manipulation of particles at the interfaces. The contents are also investigated related to the main subjects such as experimental setup and methods by experiment or simulation.One of the main features of particles in the dispersion system is Brownian motion which can be described by the diffusion coefficient. Optical tweezers is suitable for measuring the diffusion coefficient of particles due to its manipulation contactless. The researchers have developed several measurement methods for different research objects. In this thesis, the diffusion coefficients of micro-sized particles were investigated with several methods based on the nanometer optical tweezers system, and a new method with combination of high-low frequency power spectrum has been developed to measure the diffusion coefficient.Birefringent particles rotate when trapped in elliptically polarized light. The rotation rates of the particles have been investigated with the condition of the optical trap. When an infinity corrected oil-immersion objective is used for trapping, the maximum rotation rate of birefringent particles occurs close to the coverslip, and the rotation rate decreases dramatically as the trapped depth increases. The descease is due to the spherical aberration at the glass-water interface. In the thesis the spherical aberration is compensated by using a finite-distance-corrected objective to trap and rotate the birefringent particles, and the result shows that the trapped depth corresponding to the maximum rotation rate is moved to 50μm.One of the characteristic of optical tweezers is applicable to non-conventional geometries:thin films, interior of biological cell, membranes etc. Manipulation of the particles at the interfaces is the basis of optical tweezers applied to the interface system. However, it's different to manipulate the particles at the interfaces from manipulation of the particles in the solutions. We numerically investigated the transverse trapping forces on a dielectric sphere located at an oil/air-water interfaces with ray-optics model, and manipulated the micron-sized particles at an air-water interface. The results establish the base for the applications of optical tweezers to measure the interaction of colloidal particles at the interfaces.The polystyrene particle adhered to Hela cell was trapped by the optical tweezers to measure the rupture force between the particle and the cell. The process of bond rupture was observed with the help of detection photodiode. The result showed that the adhesion is a nonspecific adhesion due to a combination of electrostatic and steric effects.Experimental equipment is the basis for experimental research. This thesis is based on optical tweezers technology, so the optical stiffness the nanometer optical tweezers system is studied in the second chapter first. We measured the optical stiffness including the lateral stiffness and axial stiffness, and the axial displacement of the particles calibration with information entropy during the measurement process.Optical tweezers can trap multiple particles in single trap, and how to distinguish the number of particles in single optical trap was the key work in the experiment. A novel method based on laser scattering method to distinguish the numbers of nanometer-particles in single trap was presented in Chapter Seven. The number of particles in single trap was distinguished by measure the intensity of backscattering light with corresponding situation. In the thesis, the number of particles with diameters of 1μm,0.5μm,0.2μm,100 nm,73 nm was distinguished successfully. The method presented may find applications of optical tweezers in the nanometer dimension.