Very Theoretical Fluidics/Particle Question

Nothing to support this, but my feeling is that such a separation would only occur with small tilt angles and a slow, smooth see-saw action.

For appreciable angles (say > ±10°) and frequencies >0.5Hz (or abrupt see-saw action of say >0.05Hz) I'd think that the sloshing (to use a technical term) of the fluid moving from end to end of the tube would ensure a decent mix.

This would also depend on how much (if any) air space there is in the tube.

I don't see the proposed separating mechanism here. The three components are in a even distribution going through the pumping mechanism of a heart. This rocking action has the same amount of agitation as the heart. Let me explain my confusion by asking a question, if the RBC aggregate in the center, will the WBC or PLT be at the cap or the bottom end and why? Now I can see a centrifuge or a screen or filter paper making a separation. But the point of this tool is to keep the fluid slowly moving. I believe this will prevent and not promote separation from happening

It would have nothing to do with being more numerous or middle-sized, it would be down to SG and viscosity of the different components. Do you have this data?

If it was a problem, wouldn't a couple of quick shakes with the hand when removed from the device take care of it?

If the blood was going to separate out into it's individual components, I would think that the heaviest would tend to settle along the entire bottom of the tube, with the lightest being along the top.

Since plasma is clear, can you separate out the other cells and platelets individually?

My thinking is, that you could start with plasma in the tube and do a separate experiment with each of the other components, individually, with dye added, this would give you a visual indication of how each one is being dispersed.

IF the pivot point was actually near the top level of the fluid (i.e., if there was a point in the fluid, near the top surface, that did not experience any rotary motion) AND if there was a small volume of very low density material that would tend to float to the top of the fluid, THEN this low-density material might tend to settle at this point. [It would have to be a small volume of low-density material, otherwise it would experience the mixing motion.]

I envision turning the vials slowly end-over-end; slow enough to prevent sloshing and potential entrainment of air into the blood.

I don't know the relative influences of particle size vs density in causing differential settling, but I suspect the coworker had a point. Interesting question.

If anything settles in the sloshing action I would imagine that it be the smallest cells, platelets.

If the analogy can be made, think barrel laying on it's side containing water, large pebbles, medium pebbles, and small pebbles. If you rock the barrel from end to end the smallest pebbles will tend to fall through the gaps of the larger ones and settle to form the bottom layer.

Given the pivot is well below the center of mass of the fluid, the section above (colored red, will be under going an arcuate movement, whilst waves caused by the rocking will be producing progressive orbits in the transferring mass.

It would only be possible to achieve 'non mixing' by relating the rocking frequency to the fluids natural frequency, so nulling wave formation, and placing the pivot point 'mid depth in the fluid (when horizontal and stationary)

The problem is nowhere near as simple as thinking about particles moving in a fluid. If the proteins that make up these living cells were released, then one can get protein separation happening. But depending on the severity of the agitation, one will get completely different responses. As the latter paper describes, severe agitation likely causes the platelet cells to rupture and clotting begins. In contrast a gentile agitation reduces "a debris build up" implying that less and not more separation is happening. Don't forget though, this is blood we're talking about that has many different proteins in suspension beyond the three cells and the plasma.

You bring up a point I neglected (did think I'd need) to add to the story.

The blood is collected in a tube with EDTA - an anti-coagulent. In our case with blood cell counters, we need to inhibit the clotting.

To another point above - once the tube was removed from the rocker, a simple once or twice inversion would negate the 'pool' of RBCs in the middle. That was my co-workers ultimate end.

To elaborate on his story, when we analyze the sample the tube was un-capped and was presented to the sample probe of the analyzer. He thought that if the tip of the probe happened to be placed into the pool the end results would show a false increase in the RBC value. Of course instrument malfunction could cause the elevated value. But he was certain the instrument was working well and 'thinking outside the box'.

Thanks all for the great discussion! I think my friend would be proud! ss

the first answer was the correct answer, though he did not break it down, this is simple fluid dynamics, once you hit anything even close to ten degrees tilt at the speed a rocker runs at the lower amount of the fluid rolls or creates a wave against the end of the tube creating the mixing effect, due to the continual movement the contents never have a chance to settle so the mixing is very comprehensive.

We know the method is very efficient and concerns unfounded, why? any non mixed component would clot and there would be visible small debris within the tube or bag, in addition to this clotting is supported by exposure to oxygen which is why it does not clot in our bodies, yet does so on the skin after a cut, even blood from haemophiliacs clots on the skin.

As for the particle theory, you are imagining the these particles separate by weight and size, they do not or we would have collections of them in our lower body portions and slow moving blood areas, Vein and arteries move blood at different speeds, capillaries are your ultimate proof of failure in the particle theory, capillary blood is not pumped at all, so separation by a sedimentation process/weight particle separation would occur at this point.

so the answer to your question is no, it is not possible

Rocking agitators seem to have been around for well over 30 years. Oscillation and turbulent flow seem to be very effective at keeping certain mixtures uniform. This method would have been abandoned long ago if it didn't actually work for this application.