NUCLEAR-MAGNETIC-RESONANCE IMAGING OF APPARENT SLIP EFFECTS IN XANTHAN SOLUTIONS

Nuclear magnetic resonance imaging has been used to investigate the fl ow of 0.2% aqueous solutions of xanthan gum. Apparent slip was observe d in solutions made from the material supplied by UNAM but not in that supplied by Aldrich or Kelco. The apparent slip velocity was a consta nt fraction of the maximal velocity for a given contraction ratio. The apparent slip velocity also appeared to be independent of L/D but was strongly influenced by the wall stress, the observed apparent slip ve locities being comparable with those determined using the Mooney analy sis on capillary flows. After exposure to the action of a syringe pump , which reduced the mean molecular weight of the xanthan, the UNAM xan than solution no longer exhibited apparent slip. Slip appears to be a function of molecular weight, possibly through sensitivity to the aspe ct ratio of the molecule. (C) 1996 Society of Rheology.