Relaxation of Dense Suspension

Abstract

Dense suspensions exhibit complex rheological behavior by behaving liquid-like at low shear stresses and solid-like at high shear stresses. The microscopic interactions between individual particles create non-Newtonian macroscopic behaviors by transitioning from hydrodynamic interactions to frictional contacts. As the particles are forced into frictional contact, the suspension’s viscosity discontinuously increases with respect to the shear rate, leading to solid-like characteristics. While much research has gone into how a suspension enters this solid-like state, little is known about how the suspension relaxes out of this stressed rheological state. To understand the relaxation behavior and its underlying physical mechanism, we investigate the relaxation of water-cornstarch mixtures at different cornstarch mass fractions. The relaxation of these cornstarch suspensions is explored by measuring the stress decay upon flow cessation with a rheometer and a texture analyzer, and by capturing the spreading dynamics of suspension drops upon the cessation of vibrations with a permanent magnet shaker. We show that the dense suspensions relax with two distinct timescales, and that both of these timescales are linearly dependent on the suspension viscosity in the stressed state.