Introduction to the special issue on the theory and applications of acoustofluidics.

Acoustofluidics is a burgeoning field that applies ultrasound to micro-scale to nano-scale fluidic systems. The discovery of the ability to effectively manipulate fluids and particles at small scales has yielded results that are superior to other approaches and has been built into a diverse range of research. Recasting the fundamentals of acoustics from the past to include new phenomena observed in recent years has allowed acoustical systems to impact new areas, such as drug delivery, diagnostics, and enhanced chemical processes. The contributions in this special issue address a diverse range of research topics in acoustofluidics. Topics include acoustic streaming, flows induced by bubbles, manipulation of particles using acoustic radiation forces, fluid and structural interactions, and contributions suggesting a natural limit to the particle velocity, the ability to deliver molecules to human immune T cells, and microdroplet generation via nozzle-based acoustic atomization.