Rubbing shear rate

Figure H1.1.4 A complete flow curve for a time-independent non-Newtonian fluid. r 0 and i , are the viscosities associated with the first and second Newtonian plateaus, respectively. Regions (1) and (2) correspond to viscosities relative to low shear rates induced by sedimentation and leveling, respectively. Regions (3) and (4) correspond to viscosities relative to the medium shear rates induced by pouring and pumping, respectively. Regions (5) and (6) correspond to viscosities relative to high shear rates by rubbing and spraying, respectively.

Lotions are less viscous than creams but still need to be sufficiently viscous to enhance suspension stability. Lotions also experience from low to high shear rates approximately 10-90 s 1 when being poured from a bottle, about 125 s 1 when being gently smeared on with fingers, and thousands of s 1 when being rubbed into the skin. Typical lotion bases include suspensions of cetostearyl alcohol or stearic acid particles. 

Fine powder PTFE is susceptible to shear damage, particularly above its transition point (19°C). Handling and transportation of the containers could easily subject the powder to sufficient shear rate to spoil it if the resin temperature is above its transition point. The phenomenon caWedfibrillation (Fig. 5.26) occurs when particles rub against each other, in which fibrils are pulled out of the surface of PTFE particles. Uncontrolled fibrillation must be prevented to insure good quality production from the powder. Premature fibrillation leads to the formation of lumps which cannot be broken up easily. 

Frictional heating n. (1) Heat evolved when two surfaces rub together. For a ffictional force F and relative velocity V, the rate of heat evolution is FV (SI J/s). (2) Viscous dissipation within liquids undergoing shear flow. At any point in a laminar-flowing liquid, the rate of viscous dissipation, per unit volume, is equal to the product of the shear rate times the shear stress, which is also equal to the product of the local viscosity and the square of the shear rate. Both these types of functional heating are important mechanisms of plastication in extruders and injection molders. 

Cone-and-plate viscometers of the type shown here are usually limited to fairly low shear rates. At higher shear rates, solutions tend to be flung from the gap by centrifugal force, and melts tend to ball up (like rubbing a finger over dry mbber cement). These problems can be overcome by enclosing the fluid around a biconical rotor, giving, in effect, two cone-and-plate viscometers back-to-back [6]. The flow curves earlier in this chapter were obtained with such a device. 

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