Quantitative NMR velocity imaging of a main-chain liquid crystalline polymer flowing through an abrupt contraction

The flow of isotropic and liquid crystalline (LC) hydroxypropylcellulose (H PC) aqueous solutions into an abrupt axisymmetric contraction has been quan titatively measured by pulsed-field-gradient NMR techniques. Steady-state a xial velocity profiles, acquired upstream of the contraction, reveal a larg e contraction entry length for the LC solution. This entry flow field exist s over an order of magnitude change in flow rate and is attributed to elast icity that is associated with polydomain liquid crystallinity. Pronounced, off-centerline velocity maxima (in an axisymmetric flow field) were present upstream of the contraction, in the entry flow region. Apparently, a more viscous and elastic core of fluid was present along the centerline; this fl uid resisted elongational strain more than the fluid closer to the walls. Q uantitative velocity profiles were extracted from displacement distribution s and corrected for elongational dispersion. The isotropic solution velocit y profiles matched those obtained from viscoelastic simulations using an ap proximate Doi-Edwards model, parameterized with independent rheological dat a.