Hi. I believe that you are asking about a gravitational collapse of the sample, which is different from physical ageing. This gravitational collapse is due to an activated process, meaning that the time-to-collapse depends exponentially on stress

t = K*exp(-sigma/sigma_0),

with K and sigma_0 constants and sigma some stress measure (e.g. shear stress). If the stress is cause by gravity, you have sigma proportional to g, so that

t = C*exp(-g/g_0)

with g_0 an unknown constant. In the gravitational field g_e of the earth, we have a time-to-collapse

t_e = C*exp(-g_e/g_0).

Taken together,

t/t_e = exp( (g_e - g)/g_0 )

or

g = g_e + g_0*ln(t_e/t)

In your case you want to observe gravitational collapse at a time scale t ~ 2 hours, while t_e ~ 1 year = 8760 hours, giving

g = g_e + g_0*8.4

As you see, there is an unknown constant g_0 (that depends on the material as well as the geometry and adhesion to the container wall). This unknown constant can only be determined if you somehow measure the time-to-collapse of the material as a function of the applied g.

Kind regards

standard lab centrifuges are not designed to determine sedimentation velocities.

You may operate them for a defined time to measure how fast sedimentation front has been moved. By stopping centrifuge at a defined time you likely will stir up the suspension making accurate determination impossible. Same might happen when you place the tubes back in vertical position. It will be more difficult if 2 sedimentation fronts will form due to having 'particles' and other particles used as fillers.

Better option would be to use an analytical centrifuge, e.g. LUMiSizer or Lumifuge.

Beside this you should be aware that by centrifugation you can only speed up separation caused by gravity, not slow destabilizing processes. In most cases acceleration applied will not be large enough to force e.g. coalescence.

Dear Stefan B Lindström ,

I hope you will help me about the experiments you showed in your answer.

First: in your description of the gravitational field of Earth you said that:

"the gravitational field of Earth and g is the applied field. The time-to-collapse due to g_e is: t_e = C*exp(-g_e/g_e) = C/e".

Why the exponential's denominator is still g_e and not g_0?

Second:

_ What type of experiments colud I try to measure the time-to-collapse of the material as a function of the applied g?

_ Doy you know some method to solve it?

_ What parameters I have to fix before doing some experiments?

_ I don't know the value of g_0, I only tried to see the collapse of my material with centrifugation with a repetition of some cycles, without any calculation of g, so I don't know if I have already test a time to collapse of an year o maybe of a greater period.

Kind Regards

Nice answers already from Stefan (hej!) and Titus, two experts. I'll just add that I have old, unpublished data (before Lumisizers existed) using a standard centrifuge with a swing out rotor and Turbiscan tubes. We (in fact my patient and excellent student) centrifuged, stopped, looked for collapse and centrifuged again. Using this method we found a scaling law relating time to collapse in the centrifuge with time to collapse on the shelf (1g). It would be great if Stefan's theory can be used to model this data: we'd finally have a reliable test for predicting shelf life of weakly flocculated colloidal dispersions.

maybe we should add to people reading this first time, discussion is about so called delayed separation - i.e. dispersion seemingly stable over a distinct time for what ever reasons start to collapse. I was also not aware about this when answering previously.

Alan, we might should repeat your measurements or similar with LUMiSizer.

@Titus I'm glad you said that! It would be a nice project - get in touch.