Microfluidic filament thinning of aqueous, fibrillar methylcellulose solutions

Methylcellulose (MC), a methoxy-substituted cellulose ether, is widely used as a rheology modifier, binder, and water-retention agent in a variety of food, pharmaceutical, construction, and consumer applications. While soluble in water at low temperatures, MC reversibly transitions to a turbid hydrogel upon heating or upon the addition of NaCl, due to the formation of a fibrillar network. These complex MC solutions and gels experience a range of flow conditions in industrial processes, including shear and extensional flows. While the shear rheological behavior has been well characterized for many MC solutions, the extensional rheological behavior is often more challenging to characterize, particularly for solutions with lower molecular weights, relaxation times, and viscosities. Filament stretching using a flow-focusing microfluidic device is a promising method to resolve extensional properties of lower molecular weight and lower viscosity polymeric solutions, and it is used here to characterize MC solutions at varying NaCl concentrations. The flow-driven apparent extensional viscosity can be calculated from transient filament diameter thinning behavior of 1 wt% MC with a molecular weight of 150 kg/mol. The apparent extensional viscosity increased as the concentration of NaCl increased, from 0.947 ± 0.005 Pa s to 15.1 ± 0.6 Pa s between 0 and 8 wt% NaCl, respectively. The increase in apparent extensional viscosity is attributed to the presence of fibrils in the MC solutions containing NaCl annealed at room temperature, as demonstrated with cryogenic transmission electron microscopy. The study of the extensional behavior of this commercially relevant polymer should enable new ways to process MC, such as fiber spinning and extrusion.