椭球与圆球混合胶体体系的玻璃化转变

Glass transition in binary mixture of colloidal ellipsoids and spheres

以椭球与圆球混合的胶体体系为研究对象,通过增加体系的面积分数,从实验上研究了混合体系发生玻璃化转变过程中结构和动力学行为的演变规律.在结构方面,通过计算和分析径向分布函数、泰森多边形以及取向序参量,发现椭球可以有效地抑制圆球结晶,整个体系在结构上始终保持无序.在动力学方面,通过计算体系的均方位移和自散射函数,发现随着面积分数的增加,体系的动力学明显变慢,弛豫时间在接近模耦合理论预测的玻璃化转变点快速增大并发散.通过考察快速粒子参与的协同重排行为,发现协同重排区域形状、大小和位置都与椭球的存在密切关联.

The nature of glass and glass transition are considered to be one of the most fundamental research problems in condensed matter physics. Colloidal suspension provides a novel model system for studying glass and glass transition,since the structures and dynamics of a colloidal system can be quantitatively probed by video microscopy. Traditional systems for studying glass transition typically are single-component systems composed of either isotropic or anisotropic colloidal particles. Recently, glass transition of mixture of isotropic and anisotropic colloids has attracted great attention,such as the observation of rotational glass and translational glass, and the establishment of the two-step glass transition.Similarly, computer simulations have also shown that mixture of isotropic and anisotropic colloidal particles could manifest interesting, new glassy behaviors. However, the experimental study of the glass transition in such a colloidal mixture is still rare. In this paper, we experimentally investigate the glass transition of a binary mixture of colloidal ellipsoids and spheres. The colloidal spheres are polystyrene microspheres with a diameter of 1.6 μm, and the ellipsoids are prepared by physically stretching from polystyrene microspheres of 2.5 μm in diameter. The major and minor axes of the as-prepared ellipsoid are 2.0 μm and 1.2 μm, respectively. The mixture is confined between two glass slides to make a quasi-two-dimensional sample. To prevent the mixture from crystallizing, the mixing ratio of ellipsoids and spheres is chosen to be 1/4 in number, which is similar to the mixing ratio used in the classical Kob-Anderson model of binary sphere mixture. We systemically increase the area fraction of colloidal mixture to drive the glass transition. We then employ bright-field video microscopy to record the motion of the particles in the colloidal suspension at a single particle level, and the trajectories of individual particles are obtained by standard particle tracking algorithm. Through the analysis of radial distribution function, Voronoi diagram and local order parameter, we find that the ellipsoids can effectively inhibit the spheres from crystalizing, and the structure of the system remains disordered when increasing the area fraction. For dynamics, mean square displacement and self-intermediate scattering function are calculated. We find that the dynamic process of the system slows down substantially when increasing the area fraction, and the relaxation time of the system increases rapidly and diverges close to the glass transition point predicted by the mode coupling theory. Moreover, we analyze the fast particles that participate in cooperative rearrangement regions（CRRs） in the system, and find that the shapes, sizes and positions of CRRs are closely related to the locations of the ellipsoids in the system.