Driven by the need to prepare monodisperse compound droplets, the formation mechanism of compound droplets was comprehensively investigated. With increasing poly(vinyl alcohol) (PVA) concentration in the W2 phase, the formation mechanism of inner W1 droplet is not affected while the behavior of the O phase in the W2 phase is different. The W1/O compound droplets can form stably in an inner squeezing - outer dripping regime, but the structure of the W1/O compound droplets are affected by the formation time matching between inner W1 droplet and W1/O compound droplets, which influences the stability of the compound droplets. Moreover, the formation process of the W1/O compound droplet is composed of cone recoiling, neck formation, neck developing, neck thinning and neck pinch-off. The formation time of the W1/O compound droplet is mainly determined the neck formation stage. The higher interfacial tension is unfavorable to the neck formation at the initial stages, but it makes the Laplace pressure difference increasing, which promotes the thinning of the neck in the neck pinch-off stage. The results provide more in-depth insights of the effects of surfactants on the formation of compound droplets, benefiting for preparing monodisperse and stable compound droplets.
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