Rheology-2part/ Thixotropy l Study Pharma l

Rheology

Thixotropy:

Definition:

Thixotropy is defined as an isothermal and comparatively slow recovery of the material consistency on standing when shear stress removed.

Thixotropy may be described as in the non-newtonian system, curve in the rheogram shows upcurve as well as down curve with respect to shear stress and rate of shear axis.

 Upcurve shows the breakdown of the floccules that sediment in the system and make suspended throughout in the system on applied shear stress (force) whereas down-curve shows reformation of floccules when shear stress removed but the reformation take some time to take its original state like initial one. This is known as thixotropy.

Mechanism:

- At rest:

At rest, system show friction to move without applying the shear stress because of the particles sediment in the system, results in increases in viscosity of the system.

- Applying shear stress;

When applying shear stress sediment particles start flow in the system or distributed throughout in the system.

- Removal of shear stress:

Now removing shear stress, distributed particles in the system start sediment (Sedimentation take place in very slow rate) and occupying their original state.

Measurement of Thixotropy:

Hysteresis loop formed by path followed upcurve and down-curve. The area under the hysteresis loop is consider for the measurement of thixotropy.

Measurement of thixotropic breakdown

1. Attempt to determine the structural breakdown with time at constant rate of shear. Represented by the given equation:

2. Attempt to determine the structural breakdown with time at increasing rate of shear. Represented by the given equation:

Thixotropy in formulation:

- Pharmaceutical product are having desirable high consistency in the container or even pour out and spread easily.

- Formulation regain its consistency on shaking and particles remain longer suspended in the formulation such as emulsion, ointment, paste, suspension, etc.

Measurement of Viscosity:

Measurement of viscosity depends on the choice of viscometer:

Capillary viscometer:

Capillary viscometer is used to determine viscosity of newtonian system. It is also known as Ostwald viscometer.

Principle:

It is used to determine the viscosity of test liquid by time required for test liquid pass between two points as flow by gravity through in vertical capillary tube.

Falling sphere viscometer:

Falling sphere viscometer is used to determine viscosity of non-newtonian system. It is also known as Hoeppler falling ball viscometer.

Principle:

A glass or steel ball roll down in a vertical tube that containing test liquid under gravity to pass between two point at desirable temperature.

Cup and bob viscometer:

Cup and bob viscometer is used to determine viscosity of non-newtonian system. It is of two types;

1. Searle type - In this, rotating bob and stationary cup.

2. Couette type - In this stationary bob and rotating cup.

Principle:

Cup and bob viscometer works on the principle of rotational viscometer. In this, sample sheared in the space between outer wall of the bob and inner wall of the cup by producing torque by rotating bob or cup.

Cone and plate viscometer:

Cone and plate viscometer is used to determine viscosity of non-newtonian system. It is also known as Ferranti-Shirley cone and plate viscometer.

Principle:

Cone and plate viscometer works on the principle of rotational viscometer. In which sample on the plate and touched with cone that rotating.

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