Sub-Newtonian coalescence in polymeric fluids

When a liquid drop makes initial contact with any surface, an unbalanced surface tension force drives the contact line, causing spreading. For Newtonian liquids, either liquid inertia or viscosity dictates these early regimes of spreading, albeit with different power-law behaviors of the evolution of the dynamic spreading radius. In this work, we investigate the early regimes of spreading for yield-stress liquids. We conducted spreading experiments with hydrogels and blood with varying degrees of yield stress. We observe that for yield-stress liquids, the early regime of spreading is primarily dictated by their high shear rate viscosity. For yield-stress liquids with low values of high shear rate viscosity, the spreading dynamics mimics that of Newtonian liquids like water, i.e., an inertia-capillary regime exhibited by a power-law evolution of spreading radius with exponent 1/2. With increasing high shear rate viscosity, we observe that a deceptively similar, although slower, power-law spreading regime is obeyed. The observed regime is in fact a viscous-capillary where viscous dissipation dominates over inertia. The present findings can provide valuable insights into how to efficiently control moving contact lines of biomaterial inks, which often exhibit yield-stress behavior and operate at high print speeds, to achieve desired print resolution.

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Elasto-viscoplastic spreading: From plastocapillarity to elastocapillarity

França,

Jalaal,

Oishi

2024

Phys. Rev. Research

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We study the spreading of elasto-viscoplastic (EVP) droplets under surface tension effects. The non-Newtonian material flows like a viscoelastic liquid above the yield stress and behaves like a viscoelastic solid below it. Hence, the droplet initially flows under surface tension forces but eventually reaches a final equilibrium shape when the stress everywhere inside the droplet falls below the resisting rheological stresses. We use numerical simulations and combine volume-of-fluid (VOF) method and an EVP constitutive model to systematically study the dynamics of spreading and the final shape of the droplets. The spreading process explored in this study finds applications in coating, droplet-based inkjet printing, and 3D printing, where complex fluids such as paints, thermoplastic filaments, or bio-inks are deposited onto surfaces. Additionally, the computational framework enables the study of a wide range of multiphase interfacial phenomena, from elastocapillarity to plastocapillarity. Published by the American Physical Society 2024

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Sub-Newtonian coalescence in polymeric fluids

Abstract: Complex fluids show deviations from their Newtonian counterparts in terms of droplet coalescence. The figure shows transition of such coalescence kinetics from Newtonian into the sub-Newtonian regime.

Cited by 4 publications

References 42 publications

Insights into bubble–droplet interactions in high-viscoelastic evaporating polymer droplets

Insights into bubble–droplet interactions in high-viscoelastic evaporating polymer droplets

Rapid Spreading of Yield-Stress Liquids

Elasto-viscoplastic spreading: From plastocapillarity to elastocapillarity

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