RHEOLOGY Physical pharmaceutics-II notes for B.pharm 4th sem students

Physical pharmaceutics notes for B.pharm students

1. PHYSICAL PHARMACEUTICS-II BP403T UNIT-2 RHEOLOGY By- Himanshu rai

2. POINTS TO BE COVERED IN THIS TOPIC INTRODUCTION NEWTONIAN SYSTEM NON-NEWTONIAN SYSTEM THIXOTROPY AND ITS FORMULATION DETERMINATION OF VISCOSITY

3. INTRODUCTION The term “rheology” derived from the Greek word rheo (to flow) and “logos” (science), was suggested by Bingham and Crawford. Rheology is the science of the flow of material. It applies to the liquids, solids and semi-solids along with the substance which have complex micro structure such as suspension, muds,etc . Viscosity is an expression of the resistance of a fluid to flow, the higher the viscosity, the greater the resistance.

4. Importance of rheology in pharmacy Manufacturers of medicinal and cosmetic creams, pastes and lotions must be capable of producing products with acceptable consistency and smoothness and reproducing these qualities each time a new batch is prepared. It also involves in- Mixing and flow of materials Packaging into containers and removal prior to use. Wheather this is achieved by pouring from a bottle, extrusion from tubes, or passage through syringe needele .

5. Classification

6. NEWTONIAN SYSTEM A Newtonian fluid is defined as one with constant viscosity,with zero shear rate at zero shear stress, this law states that “ The shear stress in flowing fluid is directly proportional to the rate of shear”. Example of Newtonian fluids- Water, Alcohol, Mineral oil, Gasoline

7. NEWTONIAN’S LAW OF FLOW Newton's viscosity law's states that, the shear stress between adjacent fluid layers is proportional to the velocity gradients between the two layers. The ratio of shear stress to shear rate is a constant, for a given temperature and pressure, and is defined as the viscosity or coefficient of viscosity. F ∝ G or F= ƞ G Where F= Shear Force and G= rate of shear

8. VISCOSITY Viscosity is defined as the resistance in the flow of liquid. It is also defined as the internal friction between two layers of liquid which resists the flow of liquid. It is denoted by: (ƞ), also known as Viscosity coefficient. KINEMATIC VISCOSITY- It is defined as ratio of the absolute viscosity of the system (liquid) to the density of the liquid. It is dependent on the absolute viscosity & density of the system. It is measure of the resistive flow of the fluids under the influence of gravity. Denoted by: ν = μ / ρ Unit Stoke and Centistoke

9. FACTORS AFFECTING VISCOSITY Size of molecules Shape of the molecules Inter-molecular forces Temperature of the system

10. EFFECTS OF TEMPERATURE ON VISCOSITY Viscosity of a gas increases with increase of temperature. Viscosity of liquids decreases as the temperature is raised and the fluidity of a liquid, increases with temperature.

11. NEWTONIAN SYSTEM Newton was the first to study the flow properties of liquids in quantitative terms. Liquids that obey Newton’s law of flow are called as Newtonian fluids. F= nG

12. NON-NEWTONIAN SYSTEM The majority of fluid pharmaceutical preprations do not follow Newtons’s law of flow. These systems are referred to as non-Newtonian. Non-Newtonian behavior is generally exhibited by liquid and solid heterogenous dispersions. Example- Liquid suspensions, emulsions, colloidal solutions and ointments. Parts of non-Newtonian flow Plastic Pseudo plastic Diltant

13. Plastic The materials that exhibit plastic flow, such materials are known as Bingham bodies. The plastic flow curve does not pass through the origin & it intersects the shearing stress axis (or will if the straight part of the curve is extrapolated to the axis) at a particular point referred to as yield value . (f) . At stresses below the yield value, the substance acts as an elastic material. The slope of the rheogram is termed the mobility, analogous to fluidity in Newtonian systems, and its reciprocal is known as the plastic viscosity, U.

14. PSEUDO PLASTIC FLOW Many pharmaceutical products, including liquid dispersions of natural and synthetic gums (tragacanth, sodium alginate, methylcellulose. and sodium carboxymethyl cellulose) exhibit pseudoplastic flow. Pseudoplastic flow is typically exhibited by polymers in solution in contrast to plastic systems, which are composed of flocculated particles in suspension. The curve for a pseudoplastic material begins at the origin (or at least approaches it at low rates of shear). Therefore, there is no yield value.

15. DILATANT FLOW It is also called shear thickening system. The system exhibit enhance resistant to flow with increasing rate of shear. Certain suspensions with a high solids percentage of dispersed exhibit an in resistance to flow with increasing rates of shear . Such systems actually increase in volume when sheared & are called dilatant. Dilatant materials "shear thickening systems.“ When the stress is removed, a dilatant system returns to its original state of fluidity .

16. THIXOTROPY Thixotropy is defined as the progressive decrease in viscosity with time for a constant applied shear stress followed by a gradual recovery when the stress is removed. Thixotropy is a time dependent shear thinning property. If the rate of shear is reduced once the desired maximum is reached, the down-curve would be superimposable on the up-curve. This is true for Newtonian systems. In case of non-Newtonian systems, the down-curve can be displaced relative to the up-curve. With shear-thinning systems (i.e., pseudoplastic), the down-curve is frequently displaced to the left of the up-curve, showing that the material has a lower consistency at any one rate of shear on the down-curve than it had on the up-curve. This indicates a breakdown of structure (and hence shear thinning) that does not reform immediately when stress is removed or reduced. This phenomenon is known as thixotropy.

17. APPLICATION OF THIXOTROPY Thixotropy is desirable property in liquid pharmaceutical systems. It has following application in pharmacy- A high consistency in the container yet pour are spread easily. It will regain consistency to maintain particles in suspended form. A well formulated suspension will not settle out readily in the container. It will become fluid on shaking and remain so long enough for a dose to be dispensed. It is also desirable with emulsion, lotion,cream , ointments and parental suspension to be used for Intramuscular Depot therapy.

18. NEGATIVE THIXOTROPY OR ANTI THIXOTROPY Rheopexy is phenomena in which a solution forms a gel more readily when shaken sheared than when allow to form the gel while the material is kept at rest. Exp- Magnesia magma, Clay suspension. Represents an increase rather then a decrease in consistency on the down curve. This increase in thickness or resistance to flow with increased time of shear. It is observed that when magnesia magma was alternately sheared at increasing and then decreasing rate of shear, the magma continuously thickened but at a decreasing rate, and it finally reached an equilibrium state in which further cycles of increasing-decreasing shear rates no longer increased the consistency of the material.

19. DETERMINATION OF VISCOSITY Newtonian system the rate of shear is directly proportional to the shearing stress. Therefore, single point viscometer the equipmentsthat work at the single rate of shear is sufficient. For evaluation of non-Newtonian fluids multipoint viscometer are required, because the apperent viscosity is to be determined at a several rate of shear to get entire consistency curve. Viscometer are used to determine viscosity.

20. CLASSIFICATION OF VISCOMETER

21. OSTWALD VISCOMETER (U-tube) The viscosity of a Newtonian liquid can be determined by measuring the time required for the liquid to pass between two marks as it flows by gravity through a vertical capillary tube known as an Ostwald viscometer. Ostwald viscometer is used to determine the viscosity of a Newtonian liquid. Both dynamic and kinematic viscosities can be obtained. Principle : When a liquid flows by gravity, the time required for the liquid to pass between two marks (E and F shown in Figure) through a vertical capillary tube is determined. Application- Used for formulation and evaluationof dispersion system such as colloids, suspension, emulsions.

22. FALLING SPHERE VISCOMETER This viscometer is based on the principle of Stoke’s Law. The sample & ball are placed in the inner glass tube & allowed to reach temperature equilibrium with the water in the surrounding constant temperature jacket. It is called as Hoeppler falling sphere viscometers . Procedure : The tube & jacket are then inverted, which effectively places the ball at the top of the inner glass tube. The time for the ball to fall between two marks is accurately measured & repeated several times. Application - a) Long time consuming. b) Viscometer is more accurate.

23. CUP AND BOB VISCOMETER This is a multipoint viscometer and belongs to the category of rotational viscometers. The sample is placed in the cup and the bob is placed in the cup up-to an appropriate height. Working : The test sample is place in space between cup and bob & allow to reach temperature equilibrium. A weight is place in hanger and record the time to make 100 rotations by bob, convert this data to rpm. This value represents the shear rate, same procedure repeated by increasing weight . The sample is accommodated between the gap of cup and bob. Cup or bob is made to rotate and the torque is measured by a spring or sensor in the drive of the bob. Ex. Stormer viscometer

24. CONE AND PLATE VISCOMETER It consist of a flat stationary plate and a wide angle rotating cone is placed centrally above it. The sample is placed at centre of stationary plate and then it is raised into the position under the cone. Sample is sheared in narrow gap between stationary plate and rotating cone. Advantage- a) Time saved in cleaning and filling. b) Temperature stabilization of the sample during a run. c) The rate of shear is constant throughout the entire sample being sheared. As a result, any change in plug flow is avoided.