Aging Process Of The Bond Between Colloidal Particles Measured Using Laser Tweezers

The colloid has significant relation with our daily life. To study the characters of colloids is of great importance on both the theory and the applications. Because the characters of colloidal are closely related to the characters of small colloidal particles, we can gain an insight into the macroscopic characters of colloid by studying the behavior of individual small colloidal particles. However, due to the lack of appropriate instruments, the experimental research of the colloid was limited largely on the macroscopic characters for a long time.Optical tweezers is an instrument to trap and manipulate small particles utilizing the radiation pressure of light. Since it manipulates particles without mechanical contact, it has very little influence on trapped particles, and can precisely measure the small force between sub piconewton to hundreds of piconewtons. Due to this characteristic, it can be used to investigate the characters of trapped particles without interference, and measure the particle interaction. Therefore, optical tweezers was successfully applied in the investigation of colloid, soon after its invention.Colloidal gels are out-of-equilibrium systems and usually undergo an aging process upon gelation. During the aging process the properties of the system, especially the rheological properties, frequently evolve with time. Because the rheological properties of colloidal gels are essential for many of technological applications, the aging process of the gel has been one of the focuses of the investigation.Aging processes is generally understood as the result of restructuring of the gel network because of the limited strength of bonds between particles. Recent study shows that the time-dependent interaction between particles can directly affect the aging process and the time-dependent bonding itself plays an important role in the aging process. This observation was obtained from macroscopic experiments. However the method can not ensure there is not any change of the network structure. In order to eliminate the influence of the network structure it is necessary to directly explore the aging of the bond in the particle scales, which will give a microscopic hint to understand the aging mechanism.In this article, linear chains of polystyrene particles, the simplest form of aggregates, are assembled and then the aging behavior of the chain is studied by monitoring the time evolution of its bending elasticity using laser tweezers. The result from this microscopic method shows that the bond rigidity between particles evolves approximately linearly with time. And a sintering model is proposed to explain the main mechanism of the aging process. It is claimed that the microscopic method as we used is useful for getting fundamental experimental data to understand better the mechanism of the aging process.During the work of studying the aging process of colloidal gels from particle level, we use optical tweezers to measure the interaction force of particles. Transverse trapping efficiency of optical tweezers is important in the force measurement experiment. For improving the transverse trapping efficiency, we propose a simple scheme in which the Gaussian beam does not fully cover the aperture of the objective. Both experiment and theoretical simulation qualitatively demonstrate the scheme. And the results will be useful for design of optical tweezers.