Streaming flows within ultrasonic levitators

doi:10.1063/5.0238857

Streaming flows within ultrasonic levitators

2024-11-18

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DOI：10.1063/5.0238857

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The streaming flows inside a single-axis ultrasonic levitator are experimentally investigated using particle imaging velocimetry (PIV). Detailed quantification of the flow motion within the levitator is provided for both scenarios, with and without a suspended droplet. In the case of suspended droplets, both water and ethanol are studied to investigate the varying external streaming around droplets with different volatilities. The quantified PIV results reveal two acoustically induced jet flows, originating from the transducer and reflector, within the empty levitator in the absence of a suspended droplet. These flows, identified as Eckart streaming, dominate the external streaming around less-volatile water droplets by directly interacting with their surfaces. For highly volatile ethanol droplets, the Stefan flow, which carries ethanol vapor away from the droplet surface, is clearly observed. This flow counteracts the motion of Eckart streaming within the levitator, preventing it from reaching the surface of the ethanol droplet. This observation confirms the presence of Stefan flow around a highly volatile droplet in the ultrasonic levitator. The interaction between Stefan flow and Eckart streaming determines the external streaming patterns of the ethanol droplet, which significantly differ from those around water droplets. Additionally, boundary-driven acoustic streaming near the droplet surface, also known as Rayleigh-Schlichting streaming, can only develop in the presence of a strong Stefan flow leaving from the highly volatile ethanol droplets. In contrast, without the protection of this robust Stefan flow, the boundary-driven streaming around less volatile water droplets are swept away by Eckart streaming within the levitator. This study emphasizes the importance of considering both Eckart streaming and Stefan flow when modeling the transport phenomena of droplets suspended in single-axis ultrasonic levitators. The differences in external streaming patterns between less volatile and highly volatile droplets may influence their corresponding heat and mass transfer processes in the levitator.

Keyword:

ACOUSTIC LEVITATION

DROPLET EVAPORATION

PARTICLES

STABILITY

PHYSICS

FIELD

Streaming flows within ultrasonic levitators

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