Thank you for the response Mr. Abdali. I don't think it is a density problem. The same wax mixture crystallizes perfectly in a vial even at room temperature. When i try to emulate it in the rheometer there is this problem of incomplete crystallization.

One possible explanation is given below as to why the wax mixture does not crystallize completely when it is held between the two plates of a rheometer at low temperatures when, on the other hand, it crystalizes perfectly even at room temperatures in a vial.

Undisturbed nuclei would be needed to promote crystallization or the network formation.  This is available when the mixture is sitting in a vial.  On the other hand, when it is trapped between two plates of a rheometer, the crystal growth or network formation starts to build-up stresses within the material as it is in a confined space.  Since no deformation can takes place due to the confinement, stress relaxation begins within the constrained material and this perturbs the nuclei thereby stunting crystal growth and preventing complete crystallization. .  Further, during the stress relaxation process, the crystals (at least those that are partially formed) will tend to move away from the confining surface, thereby leaving the oil layer closer to the plate surface.

Mathematically, it can be shown how the two competing mechanisms of stress relaxation and crystallization with diametrically opposite outcomes are such that neither of the two can sustain themselves within a constrained material.  The following paper shows the mathematical formulation to argue out the case for physical hardening of an asphalt binder.  Some useful hints can be got from this paper as the present situation is somewhat similar though not exactly the same.

Aroon Shenoy, Stress relaxation can perturb and prevent physical hardening in a constrained binder at low temperatures, Road Materials and Pavement Design, Vol. 3(1), pp. 87-94 (2002).

What is your geometry made from?  If it is aluminum, or other high surface energy metal, than that might provide the driving force for your phase separation.  Do you have access to plates made of a different material?  Or, you might try increasing the gap diameter.  That might at least give you some improved kinetic stability.  If surface energy is responsible, then there might be some dissolved moisture (emulsified) in the oil that is playing a role.  Try drying out your oil before the measurements and see if that helps.

Sashikumar,

I think Aroon is right. The confinement can hinder crystal growth. Have you tried using a very small autotension during the measurement. This would adjust the gap and avoid /minimize confinement. The autotension is basically a small normal force you apply on the sample, so that when and if the sample expands  (or contracts) the upper plate moves up or down to retain the set normal force. This normal force needs to be small enough that it is smaller than the forces exerted by the sample during crystallization. So, the rheometer would need to be very sensitive to such small normal forces. This might minimize the confinement aspect. Although, I would not expect complete elimination of the confinement effects; so there would be some amount of oil-wax separation. Let me know if this works.

I think its caused by the temperature difference within the sample, so you need to either

reduce the gap if PP geometry is used or some sort of insulation to the upper geometry.

I do work on 300 microns  gap and its fine.

I hope this will help