Fig. 1. Shear rate is demonstrated for Poiseuille flow through a needle. The shear rate here is the slope of the parabolic fluid velocity profile for a Newtonian fluid (i.e., a fluid with viscosity that is the same for any shear rate). A pseudoplastic fluid has a lower apparent viscosity for the shear rate near the wall of the needle and a higher apparent viscosity near the central channel of the needle. This results in a nearly pluglike flow for highly pseudoplastic fluids.