Theory of nonlinear acoustic forces acting on fluids and particles in microsystems

Acoustofluidics is the combined science of acoustics and microfluidics. In acoustofluidics, ultrasound acoustic waves are used to generate forces for manipulation of fluids and particles in networks of microchannels in a micro fluidic chip. The microscale systems are designed to enable fundamentally new capabilities in chemical, biomedical, or clinical studies of single cells and bioparticles.

This thesis, entitled Theory of nonlinear acoustic forces acting on fluids and particles in microsystems, advances the fundamental understanding of acoustofluidics by addressing the origin of the nonlinear acoustic forces acting on fluids and particles. Classical results in nonlinear acoustics for the non-dissipative acoustic radiation force acting on a particle or an interface, as well as the dissipative acoustic force densities driving acoustic streaming, are derived and discussed in terms of a single principal equation.

This principal equation relates the acoustic force density to the divergence in the acoustic (pseudo-)momentum-flux-density tensor, dependent only on the linear acoustic fields. Extending the principal equation to the case of an inhomogeneous fluid with variations in the continuous fluid parameters of density and compressibility, e.g., due to a solute concentration field, the thesis presents novel analytical results on the acoustic force density acting on inhomogeneous fluids in acoustic fields.

This inhomogeneity-induced acoustic force density is non-dissipative in origin, in contrast to the force densities driving acoustic streaming in a homogeneous fluid. It depends on the linear acoustic fields and the gradients in the fluid density and compressibility, and leads to a dynamic relocation and stabilization of inhomogeneities in acoustic fields, as demonstrated by simulations as well as experiments with aqueous solutions in an acoustofluidic glass-silicon microchip.

The ability of the acoustic force density to stabilize fluid inhomogeneities makes possible the development of a microfluidic analog to density-gradient centrifugation, called iso-acoustic focusing, which is demonstrated for acousto-mechanical phenotyping of single white blood cells and cancer cells in continuous flow.

The theory of the acoustic force density furthermore leads to the prediction of the possibility of using acoustic tweezers to actively manipulate miscible-fluid interfaces and concentration fields at the microscale. Experimental work in progress veries the validity of this prediction. Finally, boundary-driven acoustic streaming, historically limited to the study of homogeneous fluids, is brought into the realm of inhomogeneous fluids in a theoretical, numerical, and experimental study of acoustic streaming in inhomogeneous fluids.

The study presents the discovery and description of a new class of suppressed acoustic streaming flows resulting from the competition between the classical dissipative boundary-driven acoustic force density and the inhomogeneity-induced acoustic force density that stabilizes the inhomogeneity conguration, thereby suppressing the advective streaming flow.

The suppressed acoustic streaming flow evolves towards the well-known boundary-driven streaming pattern, as the acoustically stabilized inhomogeneity profile flattens by diffusion and advection on a one-minute time scale. More theoretical and experimental work is needed to fully understand the implications of the theory of acoustofluidics in inhomogeneous fluids presented in this thesis, and I can only hope that this thesis will inspire such research activities.