In this paper, the formation and breakup process of compound pendant drops (CPDs, pendant drops with smaller drops or bubbles in them) at the tip of a glass capillary and its effect on upstream velocity fluctuation are experimentally investigated. The formation process of an air/water compound drop from a CPD consists of four main stages. First, an air plug in the capillary flows into the small liquid pendant drop to initialize a small CPD. Next, a liquid slug flows into the CPD, and the liquid in the CPD accumulates. Subsequently, an air plug flows into the CPD, and it coalesces with the existing air bubble in the CPD. The accumulation and coalescence stages repeat, until the CPD reaches a critical weight, then the CPD filly breaks up to produce a compound drop. For the air/SDS-solution system, the bubbles in the CPDs do not coalesce, and the contact line of the CPDs initially climbs along the capillary and then moves downwards with the growth of the CPDs. The upstream velocity fluctuates during the periodical formation and breakup of the CPD due to Laplace pressure variation at the tip of the glass capillary. By adding surfactant into water, the fluctuation of the upstream velocity decreases. The size distribution of the compound drops produced by the breakup of CPDs is quantified, and the results show that the current system is able to produce monodisperse compound drops.
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