44002-AC9
Flow and Strength of Cohesive Granular Materials
We have developed a fluorescent imaging technique to examine gravity driven dense granular flow. Using an index matched interstitial liquid allows us to image particles in the bulk and away from side walls. In a paper published during the past grant year cycle in Physical Review Letters, we showed that the velocity correlations in uniform dense granular flows are remarkably similar to the hydrodynamic response of an elastic hard-sphere liquid. Further we showed that the correlations are different at the boundaries. All in all, our observations appear to indicate that hydrodynamic approaches to modeling granular flow will be fruitful for dry granular flows, which is a first step towards understanding wet granular flows which are more complex because of the presence of the surrounding liquid.
The next stage in the development of the technique is to incorporate a second fluid which acts as a bond between particles and thus making the system cohesive. Earlier, in the grant cycle we demonstrated that addition of liquid to granular matter in fact decreases the force required to keep wet granular material flowing. Surprisingly it was found that the cohesive force introduced, because of the presence of the liquid bridges between particles, is more than offset by the lubrication forces introduced between particles by the presence of the liquid. This result was also discussed briefly in the previous year’s nugget and has been published in a Physical Review article during the past year grant cycle. However, a satisfactory explanation of this effect is lacking because we do not have a current understanding of the actual duration of contact times between particles as they flow past each other when undergoing shear. The next stage in the development of our imaging technique will allow us to image the liquid bridges, and its shape changes to answer precisely this question.
