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Granular Materials

Granular Materials are just collections of macroscopic particles. They include food products such as rice and corn, building materials such as sand and gravel. Their applications include transporting seeds and grains in food processing industry, processing powers and pills in pharmaceutical industry, and handling building materials in Civil Engineering. Granular materials are also close related to Geology because they provide a laboratory possibility to study the cause of earthquake and formation of sand dunes,etc. However, the technology for handling and controlling granular materials is still poorly developed. Two notorious examples are flow blockage in a hopper and separation of materials when people try to mix them together in pharmaceutical industry.

From a physical point of view, there is still no existing theory that can systemically explain the behavior of granular materials.The difficulty lies in their unique nature: the inelastic interaction among particles, ordinary temperature playing no role and the lack of a separation of scale for temporal and spatial average. For example, traditional statistical mechanics does not apply here because inelastic interaction will make the granular system settle down immediately after the energy input. Thus they cannot explore phase space anymore to achieve a statical average. Also, the formation of cluster, induced by inelastic interactions, is one of the key reasons that why ordinary fluid mechanics cannot apply to Granular Flow.

Reference:

Photoelasticity Technique

One of the exciting technique that has been used in our group is the photoelasticity technique. By making the particles out of photoelastic materials, we can actually view the stress(unit: force/area) pattern inside those particles. The principle of photoelasticy is illustrated in the following picture:

The characteristic feature of a photoelasticity materials is that they exhibit birefringence under applied stress. The principal planes of optical symmetry for the birefringent material coincide with the eigenvectors of the stress tensor. The photoelastic sample to be analyzed is then placed between two crossed circular polarizer (a circular polarizer is a combination of a linear polarizer and a quarter-wave plate). The light from a light source passes through a circular polarizer and enters the sample. Light traveling through a stressed sample splits into two components depending on the polarziation, each traveling with different velocity due to different index of refraction. The two components then coming out the sample with a phase difference, which is proportional to the difference of two principle indices of refraction. Therefore, at different points of the sample, the phase difference will be different because of different local stress. The two components will then be recombined with another circular polarizer and show visible stress information as the below picture:


Hopper Project

Introduction

Granular flow, which concerns the collective motion of macroscopic particles, is an important frontier of nonequilibrium physics, with many engineering applications. Of the many questions currently being studies related to granular flow, I will be studying the question of "jamming", which concerns how and why a granular flow comes to a halt. The notorious problem of flow jamming in a hopper is industrially important and still needs understanding from the basic physics viewpoint.

One of major challenges to understand the behavior of granular flow and its transition to jamming is the lack constitutive relation. Traditional fluid mechanics cannot apply to granular flow due to the nature of inelastic interaction among granular particles and the lack of separation of scales between microscopic and macroscopic scale. Previous studies focus on jamming in a hopper have only obtained conclusion about the statistics of the jamming probability.

I have been involved in the 2D hopper project since March,2008.Our work concerns jamming and flow in a 2-Dimensional hopper. Compared to the previous studies mentioned about, our work is trying to build a connection between jamming phenomena and particle-scale dynamics. We have two major goals: we want to predict when the flow stops in a hopper and we want to know how this can be explained by the internal particle dynamics, using the method of particle tracking and photolelastic stress images.


Apparatus



Pictures


(3)Resource for Investigating Wield Jamming Problem
sepher's occasional wield jamming picture (1 out of 10):
SepherWeirdJam1.jpg         SepherWeirdJam2.jpg         GoodForceChain.jpg
my wield jamming picture (3 or 4 out of 10?):
MyWeirdJam1.jpg         MyWeirdJam2.jpg         MyWeirdJam3.jpg

(4) Resource for "Advanced Optics" Course,taught by Jungsung Kim
system decription: systemdecripitionjunyao1.doc        
Pictures for testing the noise in camera:
Darkcurrent noise:
darkcurrent_0.jpg         darkcurrent_1.jpg         darkcurrent_2.jpg

Supposed to be the same image, but they are different due to camera noise:
Run1O2.8I45_0000.jpg         Run1O2.8I45_0001.jpg         difbetweenpic0andpic1.jpg

Pictures for testing how much light is enough for certain strain/stress to show up.
lesslightNoStress.jpg         morelightYesStress.jpg        

Video


JunyaoTangHopperFlowGSNP.avi
Orginal Video taken by High-Speed Camera (500 frames/second):       
Run3O3I45_Orginal640x480.wmv

Through processing high-speed video, we can track the movement of particles that eventually form the jamming arch, and mark them with different color by computer, so that we can visualize how these "arch particles" come togther to stop the granular flow. As we can see, these arch particles are from highly disparate regions with no obvious correlation to each other:


veloctiy spatial field video:        
Run7O3I45vsftime15.avi
Sepher's Force Chain Break Video:         sepherforcechainbreak23mm45deg.mpg

Publication


Resource

Group Thesis Resource:

TrushThesis.pdf
Academic Resource: Group Thesis Resource:

TrushThesis.pdf


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