A compound droplet deforming in a constricted tube widely appears in drug delivery and microfluidic devices. In such a constriction, an indentation can present at the trailing surface of the droplet. However, this aspect has not been fully investigated and understood so far. This study focuses on the effects of some dimensionless parameters on the negative curvature, i.e., indentation, at the trailing surface of a compound droplet moving through a constricted tube. The presence of the constriction at the middle of the tube length enhances the droplet indentation. Numerical results were obtained for the capillary number Ca (varied in range of 0.1–1.0), the inner-to-outer droplet radius ratio R₂₁ (varied in range of 0.2–0.9), the droplet-to-tube radius ratio R₁₀ (varied in range of 0.2–0.9), the inner-to-outer interfacial tension coefficient ratio σ₂₁ (varied in range of 0.1–6.4), and the normalized depth of the constriction d/R (varied in range of 0.0–0.8). The results reveal that the most influencing factor is Ca, increasing its value leads to the increment of the maximum indentation at the trailing surface of the inner and outer droplets. The indentation is also increased with increasing the value of R₁₀ and d/R. In contrast, increasing R₂₁ results in a decrease in the indentation at the trailing surface of the outer droplet. When increasing σ₂₁, the indentation at the trailing surface of the inner one is quickly suppressed, while the outer droplet is minorly affected. We also point out the patterns of the trailing surface of the inner and outer droplets and their transitions from one to the other patterns in the diagrams based on these parameters.

在狭窄的管中变形的化合物液滴广泛出现在药物输送和微流体装置中。在这样的收缩中，在液滴的后表面上会出现凹痕。但是，到目前为止，尚未对此方面进行全面研究和理解。这项研究集中在一些无量纲参数对负压曲率的影响，即负压曲率，该负压曲率是在通过压缩管的复合液滴尾随表面的。在管长度的中间出现缩颈会增强液滴的压痕。获得了以下数值结果：毛细管数Ca（在0.1-1.0范围内变化），内外液滴半径比R ₂₁（在0.2-0.9范围内变化），液滴与管径比R₁₀（在0.2-0.9范围内变化），内到外界面张力系数比σ ₂₁（在0.1-6.4范围内变化），和收缩的归一化深度d / R（在0.0-0.85的范围内变化0.8）。结果表明，影响最大的因素是Ca，其值的增加导致内部和外部液滴的后表面处最大压痕的增加。压痕也随着R ₁₀和d / R的值增加而增加。相反，增加R ₂₁导致在外部液滴的后表面处的压痕减小。当增加σ ₂₁，内层尾表面的压痕被迅速抑制，而外层液滴受到的影响较小。我们还根据这些参数指出了内部和外部液滴的尾随表面的图案，以及它们在图中从一个到另一个图案的过渡。