Index of content:
Volume 129, Issue 4, April 2011
- NONLINEAR ACOUSTICS 
Measurement of material nonlinearity using surface acoustic wave parametric interaction and laser ultrasonics129(2011); http://dx.doi.org/10.1121/1.3560945View Description Hide Description
A dual frequency mixing technique has been developed for measuringvelocity changes caused by material nonlinearity. The technique is based on the parametric interaction between two surface acoustic waves(SAWs): The low frequency pump SAW generated by a transducer and the high frequency probe SAW generated and detected using laser ultrasonics. The pump SAW stresses the material under the probe SAW. The stress (typically <5 MPa) is controlled by varying the timing between the pump and probe waves. The nonlinear interaction is measured as a phase modulation of the probe SAW and equated to a velocity change. The velocity–stress relationship is used as a measure of material nonlinearity. Experiments were conducted to observe the pump–probe interaction by changing the pump frequency and compare the nonlinear response of aluminum and fused silica. Experiments showed these two materials had opposite nonlinear responses, consistent with previously published data. The technique could be applied to life-time predictions of engineered components by measuring changes in nonlinear response caused by fatigue.
“Compression-only” behavior: A second-order nonlinear response of ultrasound contrast agent microbubbles129(2011); http://dx.doi.org/10.1121/1.3505116View Description Hide Description
Oscillating phospholipid-coated ultrasound contrast agent microbubbles display a so-called “compression-only” behavior, where it is observed that the bubbles compress efficiently while their expansion is suppressed. Here, a theoretical understanding of the source of this nonlinear behavior is provided through a weakly nonlinear analysis of the shell bucklingmodel proposed by Marmottant et al. [J. Acoust. Soc. Am. 118, 3499–3505 (2005)]. It is shown that the radial dynamics of the bubble can be considered as a superposition of a linear response at the fundamental driving frequency and a second-order nonlinear low-frequency response that describes the negative offset of the mean bubble radius. The analytical solution deduced from the weakly nonlinear analysis shows that the compression-only behavior results from a rapid change of the shell elasticity with bubble radius. In addition, the radial dynamics of single phospholipid-coated microbubbles was recorded as a function of both the amplitude and the frequency of the driving pressure pulse. The comparison between the experimental data and the theory shows that the magnitude of compression-only behavior is mainly determined by the initial phospholipids concentration on the bubble surface, which slightly varies from bubble to bubble.