Segregation And Convection In Vibrated Granular Beds

Granular materials consist of a large number of solid particles that can move independently. Such matter is ubiquitous in the world around us and is closely bound up to our daily lives. Studies on them are related to the processes in industry, agriculture, construction, and pharmaceutical industry, and also to some disasters, such as landslide, mudflow, avalanche, and sandstorms. As such, it can be said that any advance in understanding the physics of granulars is bound to have a major economic impact.Granular materials have properties that are different from those commonly associated with either solids, liquids, or gases, and exhibit various unique behaviors due to their discreteness. They have been realized and utilized by people for a long time. Earlier in the 14th century, mariners have begun to make use of sandglass for timing. Only to recent years the properties of granular materials have been extensively studied, however. Particularly, the convection and"Brazil nut"segregation in vibrated granular systems have attracted great attention because of their importance in fundamental and practical research. Convection commonly occurs in vibrated granular beds, that the particles move downward along the container side wall and upward in the interior of the bed. The'Brazil nut'segregation is known as that a larger intruder particle embedded in a sea of vibrated small particles often rises and then stays on the top of the bed. In this thesis, we will give a skeleton for the studies reported in literatures on the convection and"Brazil nut"segregation in vibrated granular materials, and then introduce the experimental investigation we made on them in vertically vibrating narrow cylindrical containers.Our investigation indicates that the stainless steel spheres contained in a vertically vibrating narrow tube could be evolved into a state with convection or with no convection, depending on the size of sphere. When the diameter of the spheres is larger than 1 mm, the spheres are in a no convection state, while when the diameter is less than that, a convection state. It observed that the'Brazil nut'segregation only takes place in the bed with convection of particle. This indicates that convection of particle is a crucial mechanism leading to the size segregation occurring in vibrated granular beds. We investigate the segregation process through measuring the time the larger intruder particle rises from the container bottom to the free surface of the bed of small particles. Our experiments show that the segregation is controlled by the frequency and acceleration of applied vibration, however subjected to the effect of room temperature and air humidity. In addition, it is also found the existence of period-doubling and period-quadrupling motion of the bulk of the bed depresses the segregation, slowering the convection velocity and thus increasing the segregation time. The influence of such subharmonic motion on the segregation time is analyzed on the base of a completely inelastic bouncing ball model.