I am using Sorvall Legend RT+ centrifuge with 500 ml centrifuge bottle instead of tube. I need to determine centrifuging time by stokes law but I cant understand how I will find the distance from the surface of the liquid in the centrifuge tube to the center of the axis of the centrifuge and distance from the particles in the centrifuge tube to the center of the axis of the centrifuge.
Remember that further away from the axis of rotation the RCF increases. The RCF in a centrifuge is proportional to the distance from the centre of rotation (which you can measure directly) and proportional to the square of the speed of rotation. Stoke's equation assumes that particles are spherical which most particles are not. This is covered in Chapter 1 of Preparative Centrifugation A Practical Approach that I edited some years ago
Dear Sir,
Thanks for your suggestion. But I am not sure how I will calculate the centrifuging time from RCF value and I have no copy of the book. I can calculate the time from the paper I attached in my first question. I need to determine only the centrifuging time for a rotational speed of 4000rpm.
Thanks again for giving your valuable time
waiting for your suggestion
Calculation of RCF can be done using the formula:
RCF = 11.18xRx(RPM/1000)2 where R is in centimetres.
Usually you work with RCFav which is the distance from the centre to half way down the bottle or tube. As a guide use the following: 1000g for 10min for cells, nuclei and flocculated precipitates, 10,000g for 10min for larger cell organelles such as mitochondria and peroxisomes and also bacteria, 100,000g for 60min for smaller organelles such as ribosomes
D=((18η ln (Rf /Ro ) )/((ρp - ρf )ω^2 ))^(1/2)/t^(1/2)
Where η is the viscosity of the fluid
ρf is the density of the fluid
ρP is the density of the particle
Ro is the starting radius of rotation
Rf is the ending radius of rotation
ω is the rotational speed (radians/second)
t is arrival time at Rf (location of detector)
From here you can calculate the diameter of the particle or the time it takes to settle the particles in the centrifuge tube.
Hi @ Grytan Sarkar, please have you found a way around your question? - I am also searching answers for a similar question. if so, would you mind kindly sharing? Thank you.
Hello @Anthony, Please see the link
Article Preparation of Wyoming bentonite nanoparticles
@Asim Biswas Can you pl share the link for this formula? where did you get this?
@Grytan Sarkar: I think density difference shouldn't be squared, otherwise the formula becomes dimensionally incorrect.
Maybe it is obvoius, but I was wondering if you can really use stokes law (as I see it is shown everywhere), I mean that formula without any changes, because of how the ecuation was deduced. I read the deduction, but didn't do the deduction for this particular case myself, maybe I should...I figured you can, because now in this system the gravitational force is the centrifugal force on the particle and the fricction and "flotation" are acting on the other direction, this vectors are perpendicular to the ones when the system is not under centrifugal force.
I figured that if you use that formula R1 is the radius of the centrifuge tube that is closer to the axis of rotation and that R2 is the radius that is further away. The diference between them will be the diameter of the tube? ?
My tubes are inclined in the rotor, so it will be a little different...
Thank you very much for your comments.
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