Effect of shear rate, temperature, and salts on the viscosity and viscoelasticity of semi‐dilute and entangled xanthan gum

Xanthan gum, a naturally‐occurring, high‐molecular‐weight, anionic polyelectrolyte, exhibits significant drag‐reducing properties at low concentrations in water, suggesting promising applications in geothermal networks. The flow behavior of xanthan solutions are explored, specifically at concentrations in the semi‐dilute (1000 ppm by mass) and entangled (4000 ppm) regimes as well as in both water and various salt solutions across a temperature range of 5–85°C. The viscosity and viscoelastic properties of xanthan solutions examine relationships between polyelectrolyte concentration, temperature, and salt concentration. Viscosity decreases by two orders of magnitude in 4000 ppm xanthan solutions in deionized water and with 50 mM NaCl (in the high salt limit) as the temperature increases from 5 to 85°C. Next, the addition of salts affects viscosity differently in the two concentration regimes. Specifically, at 25°C, the zero‐shear rate viscosity upon salt addition decreases by 63% for 1000 ppm solutions (from 0.54 to 0.2 Pa s) but increases by 280% for 4000 ppm solutions (from 28 to 107 Pa s). At 1000 ppm, salt ions cause chain collapse, reducing viscosity; At 4000 ppm, entanglements led to structural changes, resulting in higher viscosity. Furthermore, three salts commonly found in geothermal waters, namely NaCl, Na2CO3, and NaHCO3, exhibited similar changes in viscosity and shear thinning behavior in the entangled regime across temperatures from 5 to 85°C. Viscosity decreases as a function of shear rate for 4000 ppm xanthan in water (closed symbols) or 50 mM NaCl (open symbols). Open symbols are larger than closed symbols at both 5 and 85 degrees Celsuis.