Dynamics of Compound Elastoviscoplastic Droplets in Shear Flow: Deformation, Yielding, and Stability

This study investigates the deformation and yielding of compound droplets that contain a Newtonian core surrounded by an elastoviscoplastic shell in uniform shear flow. Numerical simulations combine a Volume-of-Fluid interface tracker with the Saramito constitutive model. We examine how the Plastic number (Pl), Weber number (We), and Deborah number (De) influence droplet dynamics. A larger inner-to-outer radius ratio (R * ) alters the stress distribution and amplifies shell elongation. Yield stress, measured by Pl, drives transitions between fluid-like and solid-like responses, while elasticity, captured by De, stabilises unyielded zones. Droplets are fully yielded at Pl ≤ 0.4 and remain rigid at Pl ≥ 8. Core.shell coupling reduces deformation relative to a single-phase droplet, even as We increases. A regime map defines fully yielded, partly yielded, and unyielded states in the Pl–De plane. These insights support the design of stable emulsions, targeted drug carriers, and complex-fluid processes.