 |
 |
|---|---|
 |
 |
 |
 |
Here, I've included some images and data from the granular piston experiment. In short, we confine plastic disks to a tall narrow geometry where the walls are fixed and we slowly raise the floor. The disks resist the motion because of friction between the sidewalls and between neighboring disks. In this particular set of experiments, we use 700 monodisperse disks.
We use photoelastic disks and view the experiment through crossed polarizers. Whenever the disk is under stress, it rotates the polarization of light and thus appears bright. The local stress is a function of the gradient2 of the image and can be calibrated.
Below are sequences of images that were taken with a video camera and frame grabber. In the following table of 6 images we show the initial buildup of stress and the failure -- which is clearly evident in the left hand corner. The images are taken at 0, 51, 102, 152, 160.5, & 162.5 seconds, respectively. What should be obvious is that the stress builds up slowly and is periodically released in short bursts (or failures.).
|
|
|---|---|
|
|
|
|
 |
 |
|---|
The pressure profile as a function of depth can be calculated for each image by calculating the pressure withing a narrow sliding window. Doing so for each image and assigning an intensity (black-blue-green-red), corresponding to the local stress, and plotting this as a function of time gives the following space-time diagram:
time Pressure appears as intensity |
One thing that can be seen is the banding that appears from the HCP lattice and the propagation of stress chains along the horizontal axis of each picture. Also, this structure stays largely intact while the piston is in motion. A second thing to note are the discontinuities about 1/4 and 3/8 of the way across the image. These are the "failures" shown above -- literally where there is a large buildup of stress and a sudden release.
 Duke Physics |
 CNCS |
|---|
