Thermo-rheological properties of xanthan solutions: from shear thinning to elasto-viscoplastic behavior

The thermo-rheological behavior of xanthan solutions with concentrations spanning a wide range is investigated experimentally. After carefully identifying four distinct regimes of concentration we focused on highly concentrated xanthan solutions. By combining several rheological techniques, it is shown for the first time that such solutions belong to the broad class of elasto-viscoplastic materials by exhibiting both a yield stress and elasticity that manifests around the solid-fluid transition. The soft and weakly entangled structure responsible for the elasto-viscoplastic rheological behavior is controlled by two factors:imposed stress, temperature. Consequently, concentrated solutions of xanthan may yield to either imposed stress or temperature. The systematic analysis of the elasticity mediate solid-fluid transition at various operating temperatures revealed the presence of a novel state termed as "molten solid". A clear relationship between the rheological states and the molecular states (native, denaturated, re-naturated) is established. Past a critical concentration, aqueous solutions of xanthan gum exhibit an elasto-viscoplastic rheological behavior.