title: Development and optimization of acoustic bubble structures at high frequencies.
abstract: At high ultrasound frequencies, active bubble structures are difficult to capture due to the decrease in timescale per acoustic cycle and size of bubbles with increasing frequencies. However the current study demonstrates an association between the spatial distribution of visible bubbles and that of the active bubble structure established in the path of the propagating acoustic wave. By monitoring the occurrence of these visible bubbles, the development of active bubbles can be inferred for high frequencies. A series of still images depicting the formation of visible bubble structures suggest that a strong standing wave field exists at early stages of wave propagation and weakens by the increase in the attenuation of the acoustic wave, caused by the formation of large coalesced bubbles. This attenuation is clearly demonstrated by the occurrence of a force which causes bubbles to be driven toward the liquid surface and limit standing wave fields to near the surface. This force is explained in terms of the acoustic streaming and traveling wave force. It is found that a strong standing wave field is established at 168 kHz. At 448 kHz, large coalesced bubbles can significantly attenuate the acoustic pressure amplitude and weaken the standing wave field. When the frequency is increased to 726 kHz, acoustic streaming becomes significant and is the dominant force behind the disruption of the standing wave structure. The disruption of the standing wave structure can be minimized under certain pulse ON and OFF ratios.
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama ku, Nagoya 463-8560, Japan. judy.lee@aist.go.jp
authors.forename: Judy
authors.initials: J
authors.lastname: Lee
authors.forename: Muthupandian
authors.initials: M
authors.lastname: Ashokkumar
authors.forename: Kyuichi
authors.initials: K
authors.lastname: Yasui
authors.forename: Toru
authors.initials: T
authors.lastname: Tuziuti
authors.forename: Teruyuki
authors.initials: T
authors.lastname: Kozuka
authors.forename: Atsuya
authors.initials: A
authors.lastname: Towata
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Iida
mesh_terms: D000162:Acoustics; D005740:Gases / Q000737:chemistry*; D049449:Luminescence; D010316:Particle Size; D012967:Sodium Dodecyl Sulfate / Q000737:chemistry; D013499:Surface Properties
chemical_list: D005740:Gases; D012967:Sodium Dodecyl Sulfate