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> "small particles settle more slowly because of thermal
> zig-zagging".
This is not a quotation from what I wrote. In the last message
I agreed with you that zig-zagging is not an explanation. It is
part of a qualitative model which can be used to understand
settling of mud particles in a stationary container.
A smaller particle subjected to Brownian kicks (during its fall) will
usually need more time to reach the bottom than a larger particle for
which kicks are relatively less important. Thermal kicks have practically
no effect on particles whose masses exceed a certain limit, such as one gram.
What is wrong with this?
What kind of misconception I am going to implant by using this simple model.
This is not my model; I do not remember where I saw it for the first time.
The goal was to explain what happens in the centrifuge in
terms of what students already learned in seven chapters.
Explaining what happens in a stationary situation was
the first step. The next step was to introduce artificial gravity
which plays the essential role in rapid precipitation. The
model predicts (qualitatively) that precipitation is faster
when the angular velocity is larger.
I hope that somebody
can go one step further and compose a qualitative problem
based on what happens in the centrifuge. Is this possible?
Is this desirable? Is it worth trying?
Scenario: blood fractionation.
For non-colloids, sedimentation rate is proportional to
acceleration field. (Stokes. Svedberg.)
To get the desired rate we need XXX Gs.
Sample is held in the centrifuge at r=15cm.
Calculate the desired rotation rate in RPM.