Vibrated granular matter
- Granular matter is defined as a large collection of particles the size of which is larger than one micron so that Brownian motion is negligible. Its behavior has been studied at least since the days of Charles-Augustin de Coulomb (1736-1806), who originally stated his law of friction for granular materials. In the physics community interest in granular media started to grow considerably around 1990, driven by the fast-growing performances of computer simulations. Since then the number of publications in this field has surged enormously. Because of the dissipative nature of particle collisions, in order to maintain a steady flow or a dynamic steady state, energy has to be fed constantly into a granular system. In lab experiments this is often done by applying a sinusoidal horizontal or vertical oscillation to the container. One of the aims of this work was to study effects of the combined action of both forms of agitation. In the presented experiments vertical and horizontal oscillations were superposed such that every point of the support followed a circular trajectory. By choosing a ring-shaped container geometry, the long-time dynamics of a closed, mass conserving system devoid of disturbances from the influx and outpouring of grains could be studied. This setup was used to examine spatially extended surface wave patterns of a granular bed. Standing waves oscillating at half the forcing frequency were observed within a certain range of the driving acceleration. The dominant wavelength of the pattern was measured for various forcing frequencies at constant amplitude. These waves are not stationary, but drift with a velocity equal to the transport velocity of the granular material, determined by means of a tracer particle. At higher forcing strength localized period doubling waves arise. These traveling solitary wave packets are accompanied by a locally increased particle density. The length and velocity of the granular wave pulse were measured as a function of the amount of material in the container. Inspired by traffic flow models that explain the spontaneous appearance of pulses – “phantom jams” - out of initially homogeneous flow a simple continuum model for the material distribution was developed. Based on the measured granular transport velocity as a function of the bed thickness, it captures the essence of the experimental findings. Furthermore the fluidization of a monolayer of circularly vibrated glass beads was studied. At peak forcing accelerations within a certain interval a solid-like and a gas-like domain coexist. The solid fraction decreases with increasing acceleration and shows hysteresis. Complementary to the experimental studies a molecular dynamics simulation was used to extract local granular temperature, basically defined as the variance of the particle velocity distribution, and number density. It was found that the number density in the solid phase is several times that in the gas, while the temperature is orders of magnitude lower. To investigate the transition of a crystalline particle packing to a fully fluidized state a separate setup was used. Particles were confined to two dimensions in order to keep them visible at all times. With the help of a high speed camera all particles could then be traced. The vibration was restricted to the vertical direction. The experiment was designed flexible enough to allow an easy variation of driving parameters and the use of particles of various sizes. An initially close packed granular bed was exposed to sinusoidal container oscillations with gradually increasing amplitude. At first the particles close to the free surface become mobile. When a critical value of the forcing strength is reached the remaining crystal suddenly breaks up and the bed fluidizes completely. This transition leads to discontinuous changes in the density distribution and in the root mean square displacement of the individual particles. Likewise the vertical center of mass coordinate increases by leaps and bounds at the transition. It turns out that the maximum container velocity v is the crucial driving parameter determining the state of a fully fluidized system. For particles of various sizes the transition to full fluidization occurs at the same value of v^2/gd, where d is the particle diameter and g is the gravitational acceleration.