PHAR 305 Rheology Lecture 4 PDF

PHAR 305 (PHARMACEUTICAL TECHNOLOGY II) Rheology Assoc. Prof. Dr. Muhammad Sohail FACULTY OF PHARMACY CYPRUS INTERNATIONAL UNIVERSITY Rheology… The term rheology refers to the science of flow properties of matter, and viscosity is the resistance offered when one pa...

PHAR 305 (PHARMACEUTICAL TECHNOLOGY II) Rheology Assoc. Prof. Dr. Muhammad Sohail FACULTY OF PHARMACY CYPRUS INTERNATIONAL UNIVERSITY Rheology… The term rheology refers to the science of flow properties of matter, and viscosity is the resistance offered when one part of the liquid flows past another. Therefore, principles of rheology play an important role in liquid, and semiliquid, and semisolid dosage forms such as solution, suspension, emulsion, cream, pastes, etc. The manufacturer of medicinal and cosmetic creams, pastes, and lotions must be able to pour the products with an acceptable consistency and smoothness for each batch. Rheology is also involved in the mixing and flow of materials, their packaging into containers, and their removal prior to use. Rheology… The rheology of a particular product, which can range in consistency from fluid to semisolid, can affect the acceptability of the product to the patient, its physical stability, and even its bioavailability by affecting the rate of drug absorption from the gastrointestinal tract and dosage forms. Basic Terminologies: Viscosity Viscosity tends to prevent a fluid from flowing when subjected to an applied force. The tenacity with which a moving layer of fluid drags adjacent layers of fluid along with it determines its viscosity. In other words, viscosity describes the internal friction of a moving fluid. A fluid with high viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion. The reciprocal of viscosity is called fluidity and is given the symbol φ, that is φ = 1/η. Rheology of Fluids Materials are classified into two general types depending upon their flow properties: Newtonian fluids Non-Newtonian fluids Newtonian systems (fluids) The force required for one layer of a liquid to slip past another layer with a given velocity depends directly on the viscosity of the liquid and on the areas of layers exposed to each other and inversely on the distance separating the two surfaces. The difference in velocity (dv) between two planes of a liquid separated by an infinitesimal distance (dx) is the velocity gradient or a rate of shear (dv/dx). The force per unit area (F/A) needed to bring about this flow is known as the shearing stress. The higher the viscosity of a liquid, the greater is the shearing stress (i.e., force/unit area) needed to yield a certain rate of shear. Newtonian systems (fluids)… When the relationship between the shear stress and rate of shear is directly proportioned, it is described as a Newtonian flow. F/A = ηdv/dx where F/A is the shearing stress (dyne.cm−2); dv/dx is the rate of shear (sec−1), and η is the coefficient of viscosity (dynes.sec.cm−2), usually referred to simply as viscosity. η =F/A/dv/dx η =dynes.cm -2/sec -1 Non-Newtonian systems (fluids) Plastic Flow Pseudoplastic Flow Dilatant Flow Plastic Flow: Example Pseudoplastic Flow This type of flow is generally exhibited by long chain or linear polymers, e.g., methylcellulose, sodium carboxymethylcellulose, and tragacanth. At rest, the linear polymers or macromolecules are dispersed at random in the dispersion medium. As shearing stress is applied, the macromolecules become aligned with the long axis parallel to the direction of flow. With this ordered alignment the molecules pass one another with less frictional resistance, and the viscosity is decreased. Pseudoplastic Flow… If the shearing stress is decreased, the orientation of the macromolecules becomes more random, and greater frictional resistance to the flow is reflected in an increased viscosity. Since only a molecular alignment is involved, there is no lag time. This type of fluid displays decreasing viscosity with increasing shear rate. Probably the most common of the non-Newtonian fluids, pseudoplastics include paints, emulsions, and dispersions of many types. This type of flow behavior is sometimes called shear-thinning. Pseudoplastic Flow… Dilatant Flow This type of flow exhibits an increase in viscosity as the shearing stress is increased and is exhibited by polyphasic systems, for example, concentrated suspensions and zinc oxide paste. The behavior of a dilatant curve is explained by postulating that at rest, a minimum volume of dispersion medium, which diminishes the resistance to flow, surrounds each of the particles. When shearing is applied, the particles are rearranged and the dispersion medium is displaced from around the particles, which produces increased friction between particles and increased resistance to flow. Dilatancy is also referred to as shear-thickening flow behavior. Dilatant Flow… A good example of a dilatant fluid is a suspension of corn starch in water. If such a suspension is compressed quickly by hand, the suspension will turn almost solid. If releasing the pressure, the suspension will flow freely again. Dilatant Flow THANK YOU FOR LISTENING