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Acoustic streaming is the time-independent fluid velocity generated when a sound field is applied to a fluid. A steady flow is created by the fluid absorbing the oscillating acoustic energy. The magnitude of this streaming is a function of the sound frequency and the acoustic intensity. Acoustic streaming supplements the bulk fluid velocity to further assist particle removal and to increase the rate of surface chemistry refresh.
Two sources create streaming velocities in acoustic streaming: the transducer and vibrating bubbles. First, a surface vibrating parallel to itself (the transducer/resonator assembly) generates a pressure wave. At megasonic frequencies, this wave motion propagates through the fluid in a direction perpendicular to the resonator face plane. The wave propagation is analogous to a beam of light. Any surface exposed to the beam is impacted by the wave velocity. Surfaces that are shadowed experience much lower (if any) velocity from this acoustic streaming component. This shear velocity scales with the acoustic power (or intensity) applied to the transducer, commonly known as the forward power parameter of megasonic operation.
The second component of acoustic streaming is induced by the velocity gradient near a bubble under megasonic vibration. Megasonic cavitation creates these vibrating submicron bubbles. This velocity component scales with both the frequency and intensity of the sonic field. While the magnitude of this component is less than the velocity generated by the transducer, these bubbles also form within the boundary layer. Therefore, the velocity associated with their vibration is effective in dislodging particles that are not exposed to the higher flow velocities outside the boundary layer.
The primary effect of acoustic streaming is the strong localized flow of cleaning solution, whose shear force is a primary particle removal agent. The forces of acoustic streaming dislodge particles from the surface. Acoustic streaming also increases the transport of detached particles away from the surface and into the bulk flow to prevent reattachment. Boundary layer thickness also decreases by this phenomenon to expose more particles to fluid shear forces. Additionally, acoustic streaming increases the refresh rate of chemistry at the surface of the substrate. The combination of these actions explains how megasonic energy boosts cleaning and increases the supply of fresh chemistry to the surface.
The magnitude of acoustic streaming from megasonic energy is great enough to visibly lift the surface of a bath. When a megasonic transducer is operating, the acoustic streaming will push the surface of the bath liquid upwards. This visible lifting of the surface is evidence of the power of acoustic streaming.