Department of Physics, Duke University
During graduate school, my main effort is spent in granular materials made of grains with a star shape. In general, highly elongated and/or concave grains can form free-standing columns or walls without external support, in constrast to the behavior of commonly seen convex grains (sand, rocks). This property is desire in material and architectural applications. I try to find structures that create this rigidty, and investigate the staibility of columns by changing particle properties or add external perturbations.
A dramatic illustration of particle shape effect in granular materials is the formation of free-standing walls and columns consisting of slender rods, staples, granular chains or star-shape particle (hexapod). Staples, for example, can act like hooks to form interlocking chains that resist tension. However, it is not clear how grains with shapes that do not support pair-wise interlocking, such as rod or hexapod, form rigid packings. Using both experiments and simulations, we show contact forces which exert substaintial torques on particles generate tensile stress that resist yielding and dilation. Our understanding of this mechanism intuitively explains why these columns can free-stand, and address the need to design granular materials withstanding large tension and having small Poisson ratio.
Aggregates of non-convex particles have shown to be particularly stable which makes them good candidates to design new lightweight and reversible structures. However, few is known about the fundamental reason of their stability. We just stick these packings into X-ray scanner! We (Jonathan Barés) develope a novel method to get the position and orientation of each particle as well as to detect their contact points. Measurement of the coordination numbers, statistics of the contact positions and local density evaluation for different packing configurations show a good agreement with the previous studies carried out at the global scale and permits to explain the main local mechanisms leading to stable structures.
The pullout of a spherical object buried in a silo of granular material is experimentally investigated in various column diameters with sand and rough glass beads. The boundaries are low-frictional cylinders (plastic tubes). As the object is initially buried at the bottom of the column, we gradually increase the pullout force right upon the failure of the material. The failure force depends on the pressure by material loaded above the object, which we call loading pressure, and also tube diameter. For the same loading pressure, small tube diameter can result in huge failure force comparing to large or infinite tube diameter. We also investigate the object motion after material failure using high-speed camera. This project is in collaboration with Dr. Payman Jalali.
A great challenge in experimental granular physics is to non-invasively measure the position and/or contact forces for all indiviual grains, as quick as possible. To achieve good results, delicated machinary such as X-ray, nuclear magnetic imaging or good index matching and laser sheet are required. Together with Stella Wang from North Carolina School of Science and Mathematics, we explore a very simple experimental setup that enables us to track hydrogel particles' motion in flowing water. Because the refraction indexes of the particles and the fluid are nearly matched, they are nearly invisible to our eyes. But they can trace out shadow rings when illuminated by a single parallel light source (e.g. single LED lamp), which not only reflect lateral positions, but also depths in one image.
Structures built from concave granular materials can be stable without external support and form free-standing structures. However, making them flow is now a new problem, which could limit their applications as a recycble construction materials. In the current experiments, we investigate the effect of vibration on destroying stable columns of hexapods. We find tall columns collapse faster than short ones.
Columns made of highly concave star-shape particles can freely stand and maintain stability, unlike columns of sand grains which collapse. We explore how stable these structures can be, and why they are stable. We measure column stability as a function of column height and diameter. We find inter-particle friction can increase column stability. We also explore different perturbations such as tilting, loading and mixing with beads.
Physics Department, Durham, NC 27705
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