The effect of air on sand near the jamming point
Static granular matter is traditionally studied in terms of contact forces between the particles only. Recent experimental evidence shows that this framework is insufficient to describe loosely packed fine granular matter: The influence of the interstitial air needs to be taken into account. This influence becomes particularly strong where the contact forces between the grains become vanishingly small, i.e., close to the jamming point. The objective of this project is to study in detail how the ambient, interstitial air influences the structure of loosely packed, static granular matter close to the jamming point. In particular, we want to clarify the following, questions:
- How does the sand bed respond to small disturbances and when does the ambient air pressure start to play a role? What causes the unexpectedly strong pressure dependence of the response of a granular bed to impacting objects?
- Are there structural changes in the sand bed as the pressure is reduced? Can air-fluidization facilitate the formation of extremely low granular packing densities, and if so, how? Is there any ageing in the pre-fluidized sand bed and if so, how does it depend on the ambient air pressure?
- Does air-fluidized sand close to the jamming point behave like an ordinary fluid? What happens to buoyancy and Archimedes’ law close to the jamming point?
In one of the first experiments we measure the pressure difference over and the air flow through the sand bed during the events caused by an impacting ball: splash and penetration, jet formation, and granular eruption. The used setup allows for fluidization of the sand bed to create a loose packing (41%). The experiments are done at different container pressures and different release heights.
Figure 1: Experimental setup
Info: Devaraj van der Meer