Axial forces in capillary liquid bridges of polymer solutions

TL;DR

This study measures the axial forces in capillary liquid bridges of viscoelastic polymer solutions between particles, revealing how stretching rate and rheology influence rupture behavior and force scaling.

Contribution

It provides the first particle-scale force law for viscoelastic polymeric binders, incorporating effects of stretching rate, polymer concentration, and particle size.

Findings

Peak force increases with stretching rate due to viscous effects.

Force scales with a capillary number, and rupture distance with a Weissenberg number.

Viscoelastic effects delay rupture and alter force behavior.

Abstract

Liquid bridges form between particles during wet mixing with binders or by condensation due to ambient humidity. The consequences of capillary bridges can be quite drastic, creating macroscopic cohesion, as seen in sandcastles and in the formation of particulate agglomerates. Bulk effects in cohesive particles arise from forces generated by capillary bridges, so particle-scale measurements are needed to develop predictive models. Most existing studies at the particle scale assume Newtonian liquids. Yet many binders in industry and in the environment can exhibit viscoelastic behavior. In this study, we measure the axial force generated by liquid bridges of viscoelastic polymer solutions between two spherical beads during controlled uniaxial separation. We vary the polymer concentration, separation velocity, and particle size, and track the force as the bridge thins and ruptures. At quasi-static rates, the axial force remains dominated by capillarity and is not significantly affected by polymer rheology. However, increasing the stretching rate increases the peak force through viscous dissipation and promotes the formation of a viscoelastic filament, thereby delaying rupture. The peak axial forces collapse when rescaled by a capillary number and particle size, while the effective rupture distance collapses with a Weissenberg number. These results provide a simple first-order particle-scale force law for polymeric binders.