MODEL FORVISCO-ELASTICITY : MODEL FORVISCO-ELASTICITY Mr. Nitin M. Kadam
M.Pharm [Pharmaceutics]
[email protected]
Slide 2: Non newtonian systems - contineous or steady shear rheometry.
Oscilatory and creep measurement are also of considerable importance for investigating the properties of semisolid drug products, foods, cosmetics, etc.
The materials are classified as a Viscoelastic materials... INTRODUCTION
Slide 3: Materials Shows Viscoelastic Property Systems studied in pharmacy belong to this class are as follows,
Creams, Lotions, ointments, suppositories, suspension,
Colloidal dispersing, emulsifying, suspending agents ,
Biological Materials like Blood, sputum, and cervical fluid
Slide 4: Deformation of these materials can be divided to two components:
Elastic component – Hooke’s law
Viscous component – Newton’s law
Deformation of such materials combination of Hooke’s law and Newton’s law.
Slide 5: The behaviour of solids [elastic] were given by
Hooke’s law:
E= F/g E= Elastic modulus , g = strain
h= viscosity, F= Shear stress
G= Shear rate The behaviour of Newtonian fluids were given by Newton’s Law: Hooke’s law & Newton’s Law h = F/G Where,
Slide 6: Methods that used to predict the behaviour of visco-elasticity. They consist of a combination of elastic behaviour and viscous behaviour.
Two basic elements that been used in this model:
Viscous dashpots with viscosity h which follows Newton’s law.
Elastic spring with modulus which follows Hooke’s law Mechanical Model
Slide 7: The common mechanical model that use to explain the viscoelastic phenomena are:
Maxwell
Spring and dashpot align in series
Voigt
Spring and dashpot align in parallel
Standard linear solid
Maxwell model and Voigt align in parallel.
Slide 8: Maxwell model consist of spring and dashpot in series and developed to explain the mechanical behaviour.
On the application of stress, the total strain is the sum of strain in spring & dashpot. Elastic spring Viscous dashpot Maxwell Model
Slide 9: Can also known as the Kelvin model.
It consists of a spring and dashpot in parallel.
The compliance of a viscoelastic material following this model is given as a function of time, t, by,
J= J∞ (1-e-t/T) Voigt Model Viscous dashpot Elastic spring Strain is expressed in as a deformation or compliance,J.
Slide 10: Maxwell model can accurately predict the phenomenon stress relaxation
Voigt Model can accurately predict the creep phenomenon
Standard linear solid model was developed to combined the Maxwell and Voigt model to describe both creep & stress relaxation
e.g. Wool fat behaviour at 30°C. [Creep curve] Standard linear solid Model
Slide 11: Oscillatory testing
Does not disturb structure.
Sample is subjected to oscillatory driving force in an apparatus known as Rheogoniometer.
S.S produced results in SR praportional to the surface velocity of the material.
The viscoelastic behaviour of materials obtained by oscillatory shear measurements can be analyzed by an extension of the Maxwell spring-and –dashpot model.
Slide 12: Write a note on Viscoelasticity
Gives a brief description how the chosen mechanical model can be used to estimate the creep or stress relaxation behavior for semisolid materials? Example of the exams question
Thank you : Thank you