Capillaries-based co-flowing microfluidic devices were designed and then fabricated for the controlled production of water-in-oil-in-water (W/O/W) double-emulsion droplets, where polystyrene dissolved in fluobenzene was employed as the oil phase. Two configurations of the co-flowing devices were employed in the experiments. In the so-called one-step device, the inner capillary Lip locates at the same cross-section of the middle capillary Lip. Thus, the core drop and the shell drop depart from their capillaries ends simultaneously, forming a double-emulsion drop in one step. While in the so-called two step device, the inner capillary tip locates upstream to the middle capillary tip. The core drop and the shell drop break off from their respective capillaries ends successively, forming a double-emulsion drop in two steps. The effect of the fluids now rates on the double-emulsion droplets size as well as their formation mechanism was studied comprehensively in the proposed devices. Experimental results implied that slight difference in device configurations would lead to significant difference in droplet formation mechanisms and thus size-control laws. In the two-step device, the inner stream of fluid had little influence on the breakage and the outer diameter of the double-emulsion drops, while in the one-step device the effect was significant. With the produced double-emulsion droplets, millimeter-sized polymer capsules with single hollow cavity and ultra-thin shells were obtained by solidifying the oil layers of the compound drops using the solvent evaporation method. (C) 2013 Elsevier Ltd. All rights reserved.