The formation mechanisms of S/W/O compound droplets in a horizontal T-junction device was investigated. According to the capillary number, C-a, of a continuous phase and the ratio, R, given by R = Q(main)/Q(side), the formation mechanisms which are the double-encapsulation, squeezing regime, dripping-like regime, and the jetting-like regime can be distinguished. In the squeezing regime, the lag of the PS shell relative to the W phase results in a different squeezing stage, in which the neck thickness decreases firstly and then increases because of the limited deformation of the W/O interface by the PS shell. After the PS shell totally intrudes into the main channel, the neck thickness decreases again. In the dripping-like regime, the gap distance decreases linearly with the entry of the PS shell into the main channel. Therefore, compared with W/O droplets in dripping regime, the dominant force that breaks off the neck remains to be the pressure drop force, and not the shearing force, because of the comparatively large diameter of the PS shell to that of the channel. A high C-a and low R contribute to the formation of the jetting-like regime, in which the inertia force and shearing force contribute to the formation of a long tip because of the large flow rates in the main and side channel. In addition, regimes in the flow pattern diagram dependent on C-a and R are obtained. (C) 2017 Elsevier Ltd. All rights reserved.