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초록

Rising bubbles in viscoelastic liquids exhibit distinct shape transitions and velocity discontinuities that are often interpreted through hoop-stress relaxation once the flow is fully developed. How the initial stress field is established and how it governs early morphology, however, remains poorly understood. We examine the early-stage evolution and show that the transient buildup of elastic stress in the polymeric ring, rather than its relaxation, controls the initial deformation. By varying the polymeric viscosity ratio, relaxation time, and surface tension, we span Weissenberg and Capillary numbers of Wi∈[5.72,572] and Ca∈[3.43,68.6]. The results show that stress accumulation introduces a finite delay between deformation and viscoelastic feedback and thereby preconditions the subsequent deformation pathway. Stronger elastic effects induce lateral stretching and transient flattening before axial elongation or tail development; under some conditions, the early-stage influence of viscoelasticity opposes trends inferred from quasi-steady analyses. The computed stress and conformation fields reveal localized elastic-stress accumulation near the frontal interface and circumferential stretching that forms a ring-like structure, whereas stress development near the trailing interface is delayed. These findings show that bubble topology is governed not only by stress relaxation but also by the preceding transient buildup of elastic stress. The results establish a direct link between viscoelastic parameters and early-stage morphological evolution and provide a basis for predicting how the initial stress field shapes later dynamics.

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