Suspension, Paste and Powder flow - Prospects of a Unified Approach

Publication Reference: 
12-26
Author Last Name: 
Melrose
Authors: 
John R Melrose
Report Type: 
SAR
Research Area: 
Wet Systems
Publication Year: 
1999
Publication Month: 
04
Country: 
United Kingdom

This review concerns systems that contain rigid particles packed to high concentrations. Either dry powders or in a mixture with a fluid phase. Colloids (sub- micron particles, fluid saturated) at one extreme of length scale and dry powders (larger than 100 FM, no fluid) at the other extreme. Suspensions and pastes are a broad range of materials lying between these extremes. Often these are highly poly- disperse, in a visco-elastic matrix and sometimes of three phases. Scientists in soil- mechanics, chemical engineering, physics and physical chemistry research these systems. Each bring their own approach and there is a need for unification of understanding.

The scope of the report is on the flow and mechanical properties of such systems in regimes where inertia can be neglected. In the context of dry powders this will be limited too systems in either static equilibrium or in quasi-static flow. In the context of colloids and suspensions this will be limited to dense and concentrated systems.

We lack tools to relate particle shape, polydispersity, Ph, ionic strength and polymeric components in suspensions to their bulk flow properties. Despite the evident fact that we have learnt much as to how to process and design such materials, technology would greatly benefit from such tools. Particle base simulations will be shown to provide a potentially powerful route to this. The report will review both techniques (Section 4) and results (Section 5) for flow curves in concentrated colloids. Many microscopic models of inter-particle motion, structure and kinetics exist for colloids and suspensions, but with little validation or observational support. The review will point out that simulations of colloids in shear flow are giving many new insights, in particular as to the nature of states at high shear rates. Section 7 will discuss issues and prospects for future simulations.

By reviewing certain aspects of suspensions, paste and powders together the report will try to identify those features which are common due to their particulate nature and which made lead to a unified understanding. This is done in section 6.

In recent years a new debate has emerged on the basic physics of force transmission in dry powders. In part computer simulations have prompted this. The debate is reviewed. Many new ideas and new arguments are being floated and some of these do hold prospects for a unified approach. Partly in response to this report, the author has added to this debate by showing features of force transmission common between simulations of granular media and colloids.

A more precise study has begun of the transition between a visco-elastic liquid and visco-elastic solid on increasing packing. Understanding this is identified as a key step in any unified view. Recent results are reviewed and questions set.

Flow instabilities such as shear banding, paste fracture, and shear thickening are common in suspensions and paste, but little is understood. The report will identify issues and recent work.

Given the increasing role of particle based simulations the report will review in section 7 the uses and prospects for these.