In numerical simulations, the air chamber length L has been increased from 250 to 1000 µm with steps of 50 µm. Figure ) indicates that the strongest average streaming velocity can be expected for L = 500 µm. At this air chamber length, the surrounding PDMS seems to compress the embedded air in a way that supports the vibrational mode of the resonating thin wall. However, further investigations are needed to evaluate the influence of the air chamber length properly. Additionally, we cannot fully exclude other factors such as large scale deformations of the PDMS device due to overall changes in the stability of the features.