Imagine if we have a suspension of poly-disperse rigid spherical particles (~ 5 micron, 10-30 % RSD) which are being pressure filtered into narrow tubes. Ideally one should have random distribution of particle sizes in axial and radial directions. Interestingly some workers report that smaller particles are found near the walls of the filtered bed and sometimes there is little particle size segregation i.e. the different particle sizes are somewhat homogeneously distributed in the filtered bed so formed under pressure.
I am sure this phenomenon must be well known in fluid mechanics and suspension rheology. Could anyone point out the process by which smaller particles can collect near the tube walls rather than form a homogeneous distribution in a packed bed formed from pressure filtration? Assume no sedimentation because that will show axial particle size distribution. I am more interested in the radial distribution of particle sizes because the filtration is fast enough to prevent settling. Thanks
This segregation phenomenon, migration as a result of shear-induced diffusion, in a microfiltration process is described in the following paper:
A. M. C. van Dinther • C. G. P. H. Schroe¨n • A. Imhof • H. M. Vollebregt • R. M. Boom, Flow-induced particle migration in microchannels for improved microfiltration processes, Microfluid Nanofluid (2013) 15:451–465. DOI 10.1007/s10404-013-1158-0.
This segregation phenomenon, migration as a result of shear-induced diffusion, in a microfiltration process is described in the following paper:
A. M. C. van Dinther • C. G. P. H. Schroe¨n • A. Imhof • H. M. Vollebregt • R. M. Boom, Flow-induced particle migration in microchannels for improved microfiltration processes, Microfluid Nanofluid (2013) 15:451–465. DOI 10.1007/s10404-013-1158-0.
Thank you for the paper. I will search more about the suggested terms.
I do not know well this problem but may be dominated by fluid velocity field and differential velocity between particles and fluid. Similar effect occurs when trying to pack spheres in a cylinder by dropping the mixture some some height.
The fluid will have zero velocity near the wall and max in the centre. Due to the terminal velocity large particles will move faster in the flow direction while some smaller move slower along the wall.
I suggest you use keywords "hydrodynamic chromatography" in a Google Scholar search. Joe Goddard
as others, chromatography and shear induced segragation as well as depltion phenomenon are possible origins depending on flow rate and particle to pore size ratio. Sometimes the straining may be present also
I think hydrodynamic chromatography is a good analogy but here we should keep in mind that the capillary is being packed with polydisperse particles which are pretty heavy like silica unlike molecules. Mere diffusion is almost impossible or even Brownian motion. So if we imagine a suspension traveling in a capillary (so the silica bed is building up with time), what phenomenon can cause the smaller particles to be found near the capillary walls? Thanks.
I believe this has little to do with diffusion. The simplest picture is that large particles are selectively excluded from the region of slow-moving fluid near the wall. In other words, it is a steric effect combined with non-uniform (Poiseulle) axial fluid velocity. Of course, for dense suspensions the effect may be diminished by particle interactions. You now seem to wish to add gravity and transient effects, and I have no simple answer to offer for such ambiguous and ever-changing definitions of the problem.
Dear Dr. Goddard, I agree it has nothing to do with diffusion of particles as the particles micron sized as I had mentioned earlier. Your idea of larger particles being selectivity excluded near the walls because of non-uniform flow is interesting. To the best of my knowledge only two groups in the world are studying this problem and their latest version was that we don't know why it happens. I was curious if engineers are aware of this issue or had a solution. Indeed the problem of uniformly packing a capillary with spherical particles (silica, polymer etc.) is one of the most challenging problems (with more than 30 years of efforts) in the chromatographic literature. The idea is very simple on paper. One makes a suspension of particles and the suspension is pressure filtered at high pressures (3000 to 30,000 psi) into the capillary which has a frit at the end. This results in a packed bed to spheres in the capillary. Since in reality, the particles are never monodisperse, the same group found that packed capillaries which gave poor flow profiles (broad Gaussian peaks, instead of narrow peaks) are the ones which have smaller particles around the walls. They used confocal laser microscopy to study the wall regions. See this type of work for example:
http://www.sciencedirect.com/science/article/pii/S0021967312015828
in fact we need more information on the system: permeability, porosity, chemistry, particle size and chemistry, physico-chemical conditions and flow conditions
Dr. Omari, I am not doing this work as I saw this in a conference. I was interested in the particle segregation phenomenon around the walls. However I can describe typical the conditions above: Silica (1.7 micron) is dispersed in an organic solvent such as acetone or methanol (~ 10% wt/vol) concentration. The suspension is filled in a chamber which is connected to an empty capillary (~ 75 micron, 50-100 cm long) . One end of a chamber is connected to a constant pressure pump. The chamber is pressurized upto 30,000 psi which forces the suspension to filter into a capillary, which eventually forms a packed bed. This bed is later used for chromatography. Please see
http://cosmoscience.org/archives/2011/Difficulties%20in%20the%20Synthesis%20and%20Packing%20_Laura%20Blue.pdf
- Badges
- Science topic
- Similar topics
- Engineering
- Chemical Engineering