Droplet-based microfluidics system has become a new platform in the field of synthesis and analysis. The main research of microdroplets technology concentrates on the formation of micrometer-sized droplets and to manipulate them, including separation, consolidation, mixing, sorting, capture and assembly of droplets . Many sorting methods have been reported, which typically include hydrodynamic sorting , electric field sorting , surface acoustic wave sorting  and micro valve sorting .
SAW is a kind of elastic sound wave which only propagates on the solid surface. Many articles have been reported to achieve micro droplets sorting using SAW technology . However, there has been a concern in this technology. When the micro droplets flow through the SAW field, the SAW radiation force should only apply on the targeted liquid droplets but not on the adjacent droplets. As a result, the control of spacing between adjacent droplets is required so that only one droplet is in the SAW field at a time.
Flow focusing device was used to generate micro droplets. The continuous phase is fluorinated liquid FC-40 and the dispersed phase is deionized water. When they are focused on a cross structure, dispersed phase under the squeeze and shearing force of the continuous phase will break up to form droplets. Droplets are then poured into the flow channel, using the principle of fluid dynamics to control the spacing between droplets. Finally the droplets flow into the channel through the SAW field to achieve droplets sorting. The schematic drawing of SAW based micro fluidic sorting test setup is shown in Fig.1.
When the adjacent droplets flow through a steady fluid flow, there is a relationship between the distance change between adjacent droplets and the velocity change of droplets. Based on this, two kinds of flow channel are designed. One is the tapered flow channel, giving a change to the flow rate by changing the cross-sectional area. The other is a “T” shaped fluid channel. The volume flow rate of an inlet can be changed to control the flow rate of the droplets. The end of the flow channel is designed into two branches with different flow resistances for droplets sorting.
Fig.2 (a) shows the FEA simulation of the flow field of tapered flow channel design, in which the spacing between droplets is controlled by the flow velocity change induced by channel structure. Fig.2 (b) shows the flow field in the T shape channel, where the spacing between droplets is regulated by adjusting the flow rate of the two inlets. In the experiment, a high-speed video camera was used to measure the droplets velocity and droplet distance in different position. The injection flow rate is changed to obtain different spacing between droplets. As shown in Fig.3 and Fig.4, the results match well with the simulation analysis and the value of spacing ratio with and without regulation (X2/X1) has a linear relationship with the volume flow rate of the regulation fluid.