The Structures And Dynamics Of Colloidal Systems Under Pinning Disorders

Confinement is an important way to manipulate colloidal systems in soft condensed physics. There are many types of confinements, and more and more attention has been focused on the so called random pinning confinement in recent years. The results from computer simulations have predicted that the dynamics of colloidal systems may slow down under random pinning confinement, the system may even transform from liquid to glass state with the increase of pinning fraction. But there is still little experimental results on confinements in colloidal systems. In this paper, the classical hard-sphere colloidal system is employed and the confinement is achieved by adding pre-defined random pinning particles. We try to understand how the microscopic structures and dynamics of the colloidal glass systems evolve under this random pinning confinement. The motions of the colloidal particles can be observed through a modern optical microscopy. A serial of video data from colloidal glass samples of different pinning fractions are taken by a video microscopy. Using particle tracking technology, we can process the video to get the positions of all the particles in real time, and then we can calculate the structural and dynamical parameters of the systems. With the increase of pinning particles, the peaks and shapes of pair correlation function do not change significantly, which indicates that the disordered structure of colloidal glass is preserved. The mean square displacement, on the other hand, decreases with increasing pinning, signaling the slowing down of dynamics. In addition, the overlap function decays slower, and the relaxation time grows monotonically as a function of pinning fractions, which indicated that the motion of the non-pinning particles are hindered by the pinning particles. The decrease of four point susceptibility demonstrates that the dynamic heterogeneity decayed. This is further supported by the shrinking of cooperative rearrangement regions, which we attribute to the confining effects of the pinning particles.