in which relative compressibility κ = κ p κ f and density ρ = ρ p ρ f reflect the ratios between particle (.) p and fluid (.) f properties. The resonance frequency of our device that we used for excitation was around 2 MHz, leading to a viscous boundary layer thickness of ∼ 0.4 µm in water. To approximate the acoustic contrast factor of E. coli, we took the buoyant density of E. coli analysed by Baldwin et al. and the acoustic impedance of cells derived by Olofsson et al. and ended up with a value of Φ ≈ 0.07. Thus, the critical radius for cells in our system is ∼ 0.95 µm. Since E. coli cells are very close to that critical radius (approx. 2 µm in length, 1 µm in diameter) it is essential to account for acoustic streaming effects to obtain trapping capabilities.