Stabilization of elastoviscoplastic flow via gas-assisted interfacial slip

The suppression of interfacial instabilities in elastoviscoplastic fluids remains a crucial challenge in confined flow physics. In this work, numerical simulations are performed on the extrusion of elastoviscoplastic polymer flow to study the suppressing mechanism of gas assistance on interfacial instabilities. The Saramito–Herschel–Bulkley model is used to characterize the complex rheological behavior of elastoviscoplastic polymer. An in-depth comparative analysis of the extrusion of an elastoviscoplastic polymer flow with and without gas assistance is carried out, aiming to reveal the effects of gas assistance in enhancing surface stability. Furthermore, the effects of normalized gas inlet velocity (U/V) and Bingham number (Bi) on the deformation and internal stress of the polymer are studied. The results show that the auxiliary gas establishes a continuous lubricating layer, which transforms the wall slip boundary into a gas-slip condition, thereby homogenizing shear transmission and suppressing the onset of sharkskin instability, and there exists an optimal gas inlet velocity condition. In addition, the Bingham number (Bi) governs the transition between elastic and yield-dominated flow regimes, where higher Bi enhances local yielding and interfacial disruption, resulting in exacerbated surface fluctuations, intensified sharkskin defects, increased surface roughness, higher internal stress, and larger regions with high values of the trace of the stress tensor. The gas layer effectively mitigates this by regulating near-wall stress continuity. This work benefits a deeper understanding of sharkskin instability of elastoviscoplastic polymers, thereby providing guidance to gas-assisted material extrusion additive manufacturing.