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).