The limits of dry powder dispersion
Written by: Paul Kippax
Beyond discussions of the relative merits of different dry powder dispersion systems for particle size analysis, a topic in which I have more than a passing interest, there lies the more fundamental question of whether there are physical limits to dry powder dispersion.
In laser diffraction analysis dry dispersion is the preferred sample preparation method – its quick, simple and there is no requirement for dispersant. But for some materials the challenge of dispersion without particle damage seems insurmountable. Is it, or do we simply need better technology?
Malvern has a long-standing relationship with the Institute of Particle Science and Engineering at Leeds University in the UK so it’s great to be able to use this blog post to highlight some recent collaborative work with Dr Graham Calvert on this very topic.
Can I use dry dispersion?
Graham Calvert’s work suggests the answer to this question depends, as you might expect on the design of your disperser – so it’s clearly important to pay some attention to this during instrument selection. However, his research also indicates that some powders do have properties that make dry dispersion especially tough – to the point of being impractical, that there are physical limits to dry dispersion.
In the published study the flow function of a number of powders was derived from shear test data. As its name suggests this involves shearing one powder face against another, to measure cohesivity. A low flow function suggests a cohesive material that flows poorly while higher figures are associated with free-flowing material.
The data show a good correlation between flow function and dispersion efficiency. Free-flowing materials such as glass beads disperse easily under pretty much all conditions. Once you enter the cohesive region though things become much more difficult.
A rock and a hard place?
With certain types of material you ramp up dispersive air pressure to input more energy and dispersion efficiency rises – BUT you get to the end of your ability to add more power – max pressure, best dispersion geometry – and you still only have a dispersion efficiency of say 85% (relative to the benchmark of full dispersion as measured by wet analysis). In the study this is how calcium carbonate samples (Durcal) behave.
These materials are just too cohesive for dry dispersion to be practical.
With other materials you see a peak in dispersion efficiency as dispersion pressure increases. But, the peak is, say, 90%. So you don’t reach full dispersion and increasing pressure just takes you further away.
These are materials that are both cohesive and friable. Increasing pressure increases both dispersion and particle breakage so you never reach the point you want to work at.
Turning to wet dispersion
When you encounter materials such as these you need to turn to wet dispersion. It may not be as easy as dry but there is lots of guidance to help and with some materials you simply don’t have a choice. Some samples just need the gentle dispersive action that only wet measurement can provide.