Abstract

 
We report results of a study aimed at ascertaining  how  particle cohesion  and pouring bias influence the final compact via the pouring process.  We  simulate  pouring by  two different methods for cohesive and non-cohesive particles.  In the cohesive case, a pre-compaction structure obtains composed of  a closed-packed phase; a low-density, disordered phase;  and  relatively large pores.  Upon compaction, we expect a stage of  particle accommodation to occur  at very low pressure.  (This stage has been extensively documented in experiments.) The final compact should  involve a closed-packed phase mixed with a relatively dense disordered phase.

Recent experimental work with cohesive powders (Naito et al., Powder Tech., 95, 214--219) shows that an extensive close-packed phase is indeed present  in the final compact. This  structure derives directly from the pouring process, and may therefore be interpreted as a relic of this process.   (We are developing a
 particle dynamics compaction program to further verify this result.)  In the non-cohesive case, the pre-compaction structure turns out to be  `polycrystalline', with low-density cubic `grains' separated by high-density  interfaces. Here, no accommodation phase should be present; compaction must involve particle deformation.  We  expect  the pre-compaction  structure to be preserved in the final compact. We are currently endeavoring to verify this conjecture.

We also address the influence of small biases in the direction of pouring.  Even a very small bias leads  to greatly magnified boundary effects in the cohesive case, whereas  the non-cohesive case remains  relatively unaffected. In conclusion, these preliminary results indicate that details of the pouring process strongly influence the obtained pre-compaction   structures. We  conjecture that the compact  is likely to preserve most of the features directly derived from the pouring process. Although this conjecture awaits verification, existing experimental results indicate that it is  correct in the case of cohesive particles.