Acoustic streaming

Sound produces rapid pressure and velocity oscillations in a fluid. In a real, viscous fluid, some acoustic momentum is transferred to the fluid and dissipated; the nonlinear effects of this oscillatory motion can produce a slower, time-averaged flow. This steady component is called acoustic streaming. It is distinct from the back-and-forth oscillation of fluid particles that constitutes the sound wave itself.

Streaming is governed by the acoustic field, fluid properties, boundaries, and device geometry. As a result, its speed, scale and shape vary widely: flows can appear as localized vortices, circulation cells or a jet with a return flow. Predicting the pattern in a device may require accounting for solid structures as well as the fluid channel.

Boundary-layer streaming develops where viscous effects are important near a solid wall. In classic descriptions, oscillatory motion near the boundary produces a steady flow there and can drive larger vortices in the adjacent fluid. Rayleigh streaming is a familiar form of this boundary-related circulation in standing-wave arrangements; Schlichting streaming refers to the flow within the thin oscillatory boundary layer. The resulting vortex orientation and strength depend on the specific geometry and acoustic conditions.

Eckart, or bulk, streaming arises when an acoustic wave loses energy as it propagates through the fluid. The transfer of momentum can drive a flow along the sound beam, often described as a jet. At small scales, boundary effects often have a strong influence, though bulk streaming can occur in suitable configurations. Oscillating gas bubbles create another local form, called microstreaming, with flows that depend on the bubble's oscillation mode and the surrounding conditions. These mechanisms can coexist, and the term does not imply one universal flow pattern.

In microfluidics, flows are often laminar and diffusion alone can make mixing slow. Acoustic streaming can stir fluid, mix neighbouring streams, transport droplets, or help pump small volumes without a conventional moving mechanical part. Surface acoustic waves can also generate flows in droplets and channels, enabling operations such as droplet displacement. These are research and engineering methods whose performance depends on device design and operating conditions.

Streaming can also be an unwanted side effect. In acoustic particle manipulation, the fluid drag from streaming may compete with the acoustic radiation force used to trap or aggregate particles. This interaction can set practical limits on which particle sizes can be manipulated. Conversely, designers may exploit streaming when circulation or mixing is desired. The same acoustic actuation can therefore help one task while complicating another.

Researchers observe streaming using tracer particles and flow-imaging methods such as particle image velocimetry, then compare measurements with fluid-dynamics models. Accurate predictions can be challenging because the pattern depends not only on the channel but also on acoustic attenuation, boundaries, transducers and the materials surrounding the fluid. Some commonly used theories assume a slow steady flow relative to the acoustic oscillations; this assumption may not hold in every modern microacoustic device.

Acoustic streaming should not be confused with cavitation itself. Stable oscillation of a bubble can produce bubble-associated microstreaming; violent bubble collapse is a separate phenomenon and should not be treated as an ordinary streaming mechanism. Experimental conclusions about streaming's effects on particles or biological samples are specific to the acoustic and fluid conditions used.