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LEARNING OBJECTIVES

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After completing this chapter, the reader should be able to:

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  • ► Define the terms Newtonian, plastic, and pseudoplastic flow.

  • ► Discuss examples of Newtonian rheological behavior, with special emphasis on Newtonian pharmaceutical systems.

  • ► Calculate the viscosity of Newtonian solutions by using the appropriate units.

  • ► Identify the units and magnitude of viscosity values of common pharmaceutical liquids.

  • ► Explain the role of temperature in reducing the viscosity of Newtonian fluids.

  • ► Determine the Newtonian viscosity of a pharmaceutical using capillary viscometers and the appropriate experimental methodology and calculations.

  • ► Discuss the non-Newtonian rheology of pharmaceutical systems.

  • ► Explain the significance and calculate the plastic viscosity and yield values of plastic rheograms.

  • ► Interpret pseudoplastic and dilatant rheograms and identify shear-thinning and shear-thickening behaviors.

  • ► Discuss the importance of thixotropy in pharmaceutical systems.

  • ► Explain the significance of rheology in biological and pharmaceutical systems.

  • ► Identify and discuss the properties of viscosity-inducing agents and their significance in pharmaceutical products.

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INTRODUCTION

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The term rheology is derived from the Greek words rheo, “to flow,” and logos, “science.” Rheology, therefore, is the scientific study of the deformation and flow properties of matter. Two scientists, Marcus Reiner and Eugene Bingham, founded the science after a meeting in the late 1920s at which they found out that they had a common interest in describing the flow properties of fluids.1

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Rheology can be applied to solid, liquid, and gaseous states of matter. From a rheological perspective, solids are classified as being completely resistant to deformation, liquids as being less resistant, and gases as being completely nonresistant. For pharmaceutical applications, however, the discussion of rheology in this chapter focuses primarily on liquids and semisolids (e.g., creams, ointments, and gels). In addition to medical and pharmaceutical applications, rheology is important in many other industries. For example, the rheological property of lubricating fluids such as engine oil is the main determinant of their functional value. The deformation and flow properties of cement, concrete, and paints are important in the construction industry. In the food industry, the rheological properties of ingredients and final products are essential for quality control. Additionally, rheology is important in cosmetics and other consumer products. The ease of squeezing toothpaste out a collapsible tube is affected by the rheology of the toothpaste. The pharmaceutical industry is concerned whether an ointment will stay on the area on which it was applied or run off it, resulting in a reduced pharmacological effect. Moreover, the industries are concerned with the rheological properties of their materials because the rheological properties will affect the processing, the manufacturing, and the characteristics of final product. For example, a powder that does not flow well may not mix homogenously with other powders or produce tablets or capsules with variable weights. Medicine is concerned with the rheology of biological fluids because a change in the rheological properties of blood, saliva, ...

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