Rheology in Pharmacy

Rheology

• The science that explains how liquids flow and how solids deform
• Viscosity is the resistance of liquids to flow
• Applications in Pharmacy: Formulation and analysis of emulsions, pastes, suppositories, and tablet coating
• Applications in Pharmacy: Consistency and smoothness of medicinal and cosmetic creams, pastes and lotions
• Applications in Pharmacy: Involved in mixing and flow of materials, their packaging into containers and their removal prior to use
• Applications in Pharmacy: Rheological properties influence the selection of process equipment

Newtonian Systems

• Liquids that follow Newton’s Law of Flow
• Involves shearing stress (F), rate of shearing (G), and viscosity (η)
• Newton's Law of Flow: Shearing stress is directly proportional to the rate of shear

Non-Newtonian Systems

• Liquids that don’t follow Newton’s Law of Flow
• Three classes of non-newtonian liquids:
  • Plastic
  • Pseudoplastic
  • Dilatant

Plastic Flow

• Flow curves don’t pass through the origin
• The material does not flow until a certain "yield value" of shear stress is exceeded
• Viscosity decreases with increasing shear stress

Pseudoplastic Flow

• Viscosity decreases with increasing shear stress
• The rheogram starts at the origin
• The rheogram results from a shearing action on linear polymers
• Viscosity decreases with increasing shear stress because alignment of long axes in the direction of flow reduces internal resistance

Dilatant Flow

• Viscosity increases with increasing shear stress
• The dilatant flow exhibits by heavy suspensions
• At rest, particles are closely packed
• The amount of vehicle in the suspension permits particles to move relative to one another at low shear rates
• As shear stress increases, the bulk system expands and leads to an increase in the interparticle void volume
• The amount of vehicle becomes insufficient to fill the voids, thus increasing the resistance to flow

Thixotropy

• Isothermal and slow recovery, on standing of a material, of a consistency lost through shearing.
• The shear rate of a thixotropic material is increased in a constant manner and then decreased at the same rate resulting in a hysteresis loop.

Thixotropy in Formulation

• A desired property in liquid pharmaceutical systems that should have high consistency in the container and pour and spread easily.

Rheogram

• A graphical presentation of shear rate (G) and shear stress (F)
• Also known as consistency curve and flow curve
• The slope of the line represents fluidity

Viscosity

• The force required to produce a velocity of 1cm/sec between two parallel planes of liquid 1 cm2 in area and separated by a distance of 1cm.
• The unit of viscosity is "poise"

Fluidity

• The reciprocal of viscosity

Modern Viscometers

• Equipment used to determine the viscosity of liquids and semisolids

Explore the fundamental concepts of rheology, focusing on how liquids flow and solids deform. This quiz covers Newtonian and Non-Newtonian systems, viscosity, and their applications in the pharmaceutical industry, such as in the formulation of emulsions and creams. Test your knowledge on these critical principles and their real-world implications.