Pharmaceutical Drugs and Dosage: Rheology - Review questions answers
Review questions
12.1 Indicate
which statement is TRUE and which one is FALSE.
A. Pseudoplastic flow
is shear-thinning type and dilatant is shear-thickening type.
B. Flocculated systems
exhibit negative thixotropy, while defloccu-lated system with more than 50% by
volume of solid dispersed particles exhibits dilatant flow behavior.
12.2 Which of
the following is the most desired behavior in high-concentration suspension
formulations sold in a bottle for use by the patient? Explain why.
A. Pseudoplastic flow
B. Dilatant flow
C. Thixotropic flow
D. Antithixotropic flow
E. Newtonian flow
12.3 Define
Newton’s law of flow and draw the diagrams to illustrate the effect of shear
stress on the rate of flow and viscosity of fluids obeying Newton’s law. Draw
flow diagrams to illustrate the effect of shear stress on the rate of flow and
viscosity for three types of non-Newtonian fluids.
12.4 Define thixotropy and draw a hysteresis loop to explain the
thixo-tropic phenomenon. Explain why thixotropic phenomenon is desir-able for
pharmaceutical formulations.
Answers:
12.1 A. True
B.
True
12.2 According
to Newton’s law of flow, the rate of flow (D)
is directly proportional
to the applied stress (τ). That is, τ = ή · D, where ή is the viscosity. Simple
fluids, which obey this relationship, are referred to as Newtonian fluids and the fluids which deviate are known as non-Newtonian fluids (refer to Figure 11.1).
12.3 Thixotropy
is the property of non-Newtonian pseudoplastic fluids to show a time-dependent
change in viscosity. Many gels and col-loids are thixotropic materials,
exhibiting a stable form at rest but becoming fluid when agitated. Thixotropic
flow is a reversible gel–sol gel transformation. On setting, a network gel
forms and provides a rigid matrix that will stabilize suspensions and gels.
When sheared by simple shaking, the matrix relaxes and forms a solution with
the characteristics of a liquid dosage form for ease of use. On standing, the
particles collide, flocculation occurs, and the gel is reformed. The shearing
force on the injection as it is pushed through the needle ensures that it is
fluid when injected; however, the rapid resumption of the gel structure
prevents excessive spreading in the tissues, and consequently, a more compact
depot is produced than with the non-thixotropic suspensions (refer to Figure
11.2).
Related Topics
TH 2019 - 2023 pharmacy180.com; Developed by Therithal info.