We then set out to investigate why manipulation in the kHz range is possible and aim to understand the influence of PDMS, channel geometries, and the embedding of the IDTs in our unique design. We conjecture that the mechanism of acoustoinjection in the kHz frequency domain is caused by the transmission of acoustic energy at the interfaces between PDMS, the liquid-metal IDTs, and liquids within the flow channels. The acoustic transmissivity at the LiNbO3-wafer to PDMS interface is related to both the density, ρ, and speed of sound, c, in the components resulting in the acoustic impedance: ρ1 c1 / ρ2 c2. As an example, the interface between water and air acts as a nearly perfect mirror, with a transmissivity of only a few hundredths of a percent, especially in the case of high frequencies (>500 Hz) due to the large difference in density and speed of sound. To overcome this effect, transmission of an acoustic pressure wave in a medium of similar density and speed of sound is required. Since PDMS has a speed of sound of 1070 m s -1 and a density of 0.965 g cm -3 , similar properties to water (1500 m s -1 and 1 g cm -1 ), a transmissivity of around 68 % of the initial intensity at the interface between both is expected at the injection spot. Therefore, embedding IDTs with the same height as the flow channels within same PDMS slab enables efficient transmission of acoustic pressure waves from the IDT aperture towards the focal region where we observe an impact on the passing droplet. Further, we assume that embedding the IDTs into PDMS decreases the eigenfrequency of the entire system.