Retraction dynamics of viscous drops and sheets depend on the relative magnitude of the viscous force over the inertia-capillary force. The dynamics are more complicated in the case of viscoplastic drops/sheets because the yield stress of the fluid also comes into play. The retraction of slender viscoplastic drops and sheets depends on the relative magnitude of the yield stress over the capillary stress. Depending on its relative magnitude, the yield stress can completely resist the retraction. In this study, the retraction of viscoplastic drops and sheets has been investigated theoretically neglecting the effect of the surrounding medium. Using long-wave theory we derive the retraction criteria of slender drops (axisymmetric) and sheets (two-dimensional) for yield stress fluids. Direct numerical simulations are also performed by solving the complete momentum conservation equations. A good agreement is found between the numerical results and the proposed retraction criteria.

粘性液滴和薄片的回缩动力学取决于粘性力相对于惯性毛细管力的相对大小。在粘塑性液滴/片材的情况下，动力学更加复杂，因为流体的屈服应力也会发挥作用。细长的粘塑性液滴和薄片的收缩取决于屈服应力相对于毛细管应力的相对大小。取决于其相对大小，屈服应力可以完全抵抗收缩。在这项研究中，从理论上忽略了周围介质的影响，对粘塑性液滴和薄片的收缩进行了研究。使用长波理论，我们得出了屈服应力流体的细长液滴（轴对称）和薄板（二维）的收缩标准。通过求解完整的动量守恒方程，也可以进行直接数值模拟。在数值结果和建议的回缩标准之间找到了很好的一致性。