Jets of discontinuously shear thickening suspension

Abstract

In the present work, jets of discontinuously shear thickening (DST) suspension emerging from a vertical pipe have been experimentally studied. The suspension flows through the pipe under gravity at nearly constant flow rate and the stress field in the jet is mostly extensional with the stress levels governed by gravitational forces. The dynamics of the jet was recorded with a high-speed camera at 300 fps, then the recorded videos were discretized into pictures and carefully analyzed with a written and executed program in MATLAB, based on image processing. The suspension is a concentrated mixture of calcium carbonate (CC) microparticles dispersed in water at 68% volume fraction with addition of different concentrations of polyamide (PA) microfibers of 700μm x 15 μm in size. The CC suspension solely exhibits strong DST behavior characterized by a viscosity divergence above a critical stress. At a given flow rate, depending on the jet length (distance between the pipe outlet and a point where the jet hits the experimental table), different behaviors are observed varying between stable viscous thread to coiling instability and elastic instabilities with periodic rupturing of the jet ascribed to achievement of the critical stress of the DST onset under extensional deformations. Addition of fibers mainly stabilizes the jets and this effect is still not fully understood and perhaps depends on the flow-induced orientation of fibers under extensional flow, increasing extensional viscosity. In this communication we will discuss in detail different regimes and present experimental correlations between the fiber concentration and dynamic parameters of the jet instability (wavelengths, lateral oscillation amplitudes, etc.)