Microfluidic rheology of methylcellulose solutions in hyperbolic contractions and the effect of salt in shear and extensional flows

Methylcellulose solutions are known to form microfibrils at elevated temperatures or in the presence of salt. The fibrils have a significant impact on the solution's rheological properties. Here, the shear and extensional properties of methylcellulose solutions with added salt are measured using hyperbolic microfluidic channels, allowing for new characterization at lower molecular weights and higher shear and strain rates that are difficult to access by macroscale rheology studies. 1 and 2 wt% methylcellulose solutions with molecular weight of 150 kg mol −1 with NaCl content between 0 to 5 wt% have been characterized. All solutions were found to be shear thinning, with power law thinning behavior at shear rates above 100 s −1 . The addition of NaCl up to 5 wt% had only small effects on shear viscosity at the shear rates probed (100 s −1 and 10 000 s −1 ). Extensional viscosities as low as 0.02 Pa s were also measured. Unlike the results for shear viscosity, the addition of 5 wt% NaCl caused significant changes in extensional viscosity, increasing by up to 10 times, depending on extension rate. Additionally, all solutions tested showed apparent extensional thinning in the high strain rate regime (>100 s −1 ), which has not been reported in other studies of methylcellulose solutions. These findings may provide insight for those using methylcellulose solutions in process designs involving extensional flows over a wide range of strain rates. The addition of NaCl to methylcellulose solutions increases the extensional viscosity, which can be measured quantitatively using hyperbolic microfluidic contractions. Fluid stresses during flow can be visualized via birefringence.