Temperature and Shear Stress Effect on Reological Properties of Oil-Disperse Systems

About the authors:

Lyudmila P. Semikhina, Dr. Sci. (Phys.-Math.), Professor, Institute of Physics and Technology, University of Tyumen; semihina@mail.ru

Anna M. Pashnina, Postgraduate Student, Institute of Physics and Technology, University of Tyumen; dolphfindiva-1989@bk.ru

Irina V. Kovaleva, Postgraduate Student, Institute of Physics and Technology, University of Tyumen; irishechka_72@mail.ru

Dmitry V. Semikhin, Cand. Phys.-Math. Sci., Associate Professor, Department Information Systems, University of Tyumen; assist@inbox.ru

Abstract:

The research of viscosity of oil dispersed systems within temperature range 20-70°C and shear rates was investigated using a rotary viscometer “Brookfield DV-II+Pro” on the example of seven oils samples of Russian fields. According to the experimental data, the activation energy of viscous flow (E), and the entropy changes (∆S) were calculated from Eyring — Fraenkel equation. It was found that an increase in the shear rate leads to a gradual disappearance of difference between the values of E₁ and E₂ at low (T < T*) and high (T > T*) oil temperatures, where T* ≈ 40-50°C is phase transition temperature close to the paraffin melting temperature found in the oils. This is the basis of the theory that the effect which happens when the oils are heated to T = T*, can also be obtained mechanically at T < T*, if the oil is subjected to shear deformations at high shear rates. The theory was confirmed by an independent method for measuring the particle sizes of nanoaggregates in oil with the use of a laser analyzer Zetatrac at T < T* and T > T* before and after the action of shear deformations on it's thin layer (2.1 mm) inside the measuring cell of the rotary viscometer.

It was established for the first time that a phase transition at a temperature T* causes a multiple decrease in the particle sizes of the nanoaggregates in oils, a similar effect can be achieved at T < T*, if the oil is subjected to shear deformations. The magnitude of the shear stress τ*  ≈  10 Pa, at which the particles of the nanoaggregates in the oils are destroyed, is estimated.

It is shown that, according to the experiment, the decrease in oil viscosity with increasing shear rate is caused by the growth of entropy due to the destruction of coagulation structures and particles of the disperse phase of oil dispersed systems.

Moreover, the effect of the entropy changes is greater than when the energy of activation for viscous flow increases due to the destruction of nanoaggregates in oils.

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