diffusion time of a solid sphere of radiusris given by
trot¼4 pr^3 Zs
kBTð 6 : 7 ÞwhereZsis the viscosity of the solvent. The relation also applies to spherical
random coils, withr&rg. For elongated particles, the rotation time is
longer than predicted by Eq. (6.7). Iftrot 41 =shear rate, orientation will
readily occur. If shear rate is very small, the rotary Brownian motion will be
overriding, no orientation will occur, and the viscosity will be higher. The
extent of the viscosity difference depends on the deviation from sphericity of
FIGURE6.6 Dependence of the apparent viscosity Za of xanthan solutions
(concentration indicated) on shear rate; low ionic strength. (Approximate results
from various sources.)