(503a) Internal Dynamic Modes and Rheology of Cellulose Nanofibrils with Salt

Cellulose nanofibrils (CNF) present unique opportunities for rheology modification in complex fluids, and can be used in composites to form aerogels, transparent films, and other advanced materials. Mediation of electrostatic interactions between the nanofibrils in aqueous suspensions is critical to determining the rheological properties of the fluids. Here we consider the effect of salt on the internal dynamics and rheology of dilute CNF suspensions. At sufficiently high concentrations, the neat suspensions are transparent yield-stress fluids which display strong shear thinning behavior. Salt addition produces an increase in gel modulus, with retention of optical clarity, and the onset of strain stiffening behavior. A systematic series of dynamic light scattering measurements reveal intriguing dynamics in neat suspensions. In very dilute CNF suspensions, salt slows diffusive dynamics, whereas at higher CNF concentrations, the addition of salt both accelerates primary diffusive dynamics and introduces stretched exponential relaxation. Interestingly, accelerated diffusive dynamics are observed in suspensions which stiffen with added salt. The acceleration of dynamics on introduction of attractive interactions parallels the behavior observed in dense repulsive colloidal glasses that display re-entrant behavior with increasing attraction strength at a constant volume fraction. Our combined rheological and light scattering study explores this analogy and provides insight for engineering complex fluids using CNF.