Transportation of Droplets within Two Substrates by Help of Surface Acoustic Wave

It is usually difficult to integrate all operation units of complicated biochemical analysis systems into a micro-fluidic substrate. A multi substrate micro-fluidic system (MSMS) is a good option, in which operation units can be integrated into two or more substrates. Thus, it is necessary to transport droplets between two substrates. In this paper, a new method for transporting droplets between two substrates has been proposed based on surface acoustic wave (SAW). It is composed of a 128°YX-LiNbO3 piezoelectric substrate (substrate 1) and an arc-shaped polymer. The arc-shaped polymer is seamlessly adhered to the piezoelectric substrate, and the lowest end is above another a substrate (substrate 2) with some distances. After a droplet in the substrate 1 is propelled to the arc-shaped polymer by SAW, it will slide off the polymer due to its gravity, and transported onto the substrate 2. Theoretical formulas of forces acted on the droplet have been deduced and the condition of the droplet sliding off the arc-shaped has also been obtained. Red dye solution droplets were used in the transporting experiments. Results show that the transportation of droplets is decided on curvature radius and hydrophobic property of the arc-shaped polymer and the droplet volumes. As applications, mixture operations have been implemented in substrate 2 after droplets have been transported from the substrate 1.