Multiphysics simulation of acoustic trapping and streaming in a TinyLev acoustic levitator for the assembly of artificial cells

Computational models of the TinyLev acoustic field were developed in COMSOL Multiphysics (v5.4). The TinyLev structure (STL file) was imported for 3D simulation. Acoustic field distribution within the trapping region was solved with the Pressure Acoustics, Frequency Domain interface, which solves the Helmholtz equation in the frequency domain: ∇^2 p1 + k_eq^2 p1 = 0, where p1 is total acoustic pressure, k_eq is the wave number, ρ is density, and c is speed of sound. No background pressure field was added; thus p1 = p (pressure variable within the interface). Droplet trapping of a 2 mm aqueous droplet in air was modeled using the Particle Tracing for Fluid Flow interface to couple the acoustic field from the pressure acoustics model with particle motion. For acoustic streaming simulations, a 2D geometry representing the ACDC droplet domain and sub-compartments was prepared. First-order acoustic fields (p1, u1, T1) were solved using the Thermoviscous Acoustics, Frequency Domain interface. The boundaries of the ACDC droplet in this model generate streaming. The Laminar Flow interface was then added to compute the time-averaged net (streaming) flow driven by first-order fields using: (2) ∇ · (ρ0 u2) = −∇ · ⟨ρ1 u1⟩ (3) ∇ · u2 = ∇ · ⟨ρ0 (u1 u1⟩ where subscript 1 denotes first-order acoustic fields and subscript 2 denotes the streaming flow. First-order terms (right-hand side) were introduced into the Laminar Flow model as weak contributions in domains and boundaries. Stokes drift contributions were included at boundaries responsible for streaming, using: (4) u2 = −⟨(ξ1 ∇) u1⟩. Particles of 5 µm diameter were released within specified cores using the Particle Tracing for Fluid Flow interface, and their trajectories were analyzed under acoustic streaming. The tension on the levitated droplet network induced by acoustic streaming was computed as the integral of the product of fluidic shear rate and fluid viscosity, based on models shown in Fig. S12. Computational fluid dynamics modeling was also performed in COMSOL Multiphysics using a microfluidic module as described previously (ref. 75).