Fluid agitation techniques, such as ultrasonication and shear homogenization, are often used to break down soft particle agglomerates (i.e., particle clusters in a shallow secondary minimum) by introducing enough energy into the colloidal dispersion.
Is it possible to use similar (or any, for that matter) techniques to break down hard particle aggregates (i.e., particle clusters in the deep primary minimum)?
Strictly speaking, particle agglomeration is a reversible process, whereas aggregation is supposed to be irreversible.
Assume solid sub-micron particles dispersed in water.
Omar Z. Sharaf
By normal convention, agglomerates are loose and aggregates are hard. There is confusion in the literature on these terms so ISO is tending to prefer 'cluster'.
Extended high-powered (e.g. 600 W) sonication is really ultrasonic milling and the tip erodes and contaminates the sample. See:
Ultrasound, cavitation, and the singing kettle http://tinyurl.com/olueohz
In a powder with small primary size particles (< 0.5 microns typically) then solid solid diffusion gives solid bridges in the product. See:
Adhesion and cohesion http://tinyurl.com/zwb2wlh
and:
Controlling powder bulk density by optimizing particle size and shape distribution
https://www.brainshark.com/malvern/vu?pi=271602698&b=1&tx=120486&c1=22
Dispersion and nanotechnology
https://www.brainshark.com/malvern/vu?pi=zHSznNgprz5chz0
Plus the attached.
The only possible (and difficult) route is to dissolve the solid bridges with a technique called stress corrosion milling. Slides 55 onward in the last webinar above.
More background than suggestion...
The attractive force between particles is elastic which means if you try to separate them by "pulling" along the line perpendicular to the surfaces, when you "relax", the particles will come together again (unless diffusion is strong enough). Tangential hear is required to separate them in a non-elastic way. An analogy is to separate two magnets. Try pulling them apart along the same axis as the attraction without any sideways movement - very difficult. But apply sideways movement (e.g., twist) and they come apart readily. For particles, the shear force will be determined in part by the surface area of the particle exposed to the fluid. This is why aggregates can be reduced in size but clusters of just a very few particles (e.g, doublets) are difficult.
(I've probably used poor terminology but I hope the idea is clear.)
Breaking particle aggregates is close to breaking individual particles. So in principle the theoretical answer is yes. The problem in practical terms that the smaller the particles or the aggregates the more difficult it is to apply high enough shear force for breaking or eroding. Because thermodynamically decreasing interfacial area is favoured you may end-up with larger particle aggregates. Milling e.g. quartz for extended duration will lead to a decrease in surface area after a first increase.
- Badges
- Science topic
- Similar topics
- Engineering
- Chemical Engineering
- Membranes