11/02 - 11/06/2015 NCPA Physical Ultrasonics at the 170th ASA Meeting.

Sunethra Dayavansha, Ukesh Koju, Nazanin Omidi, Cecille Labuda and Joel Mobley, members of the Ultrasonics research group attended the 170th Acoustical Society of America in Jacksonville Florida.  Joel Mobley chaired the Physical Acoustics Metamaterials I and II sessions.  Ukesh Koju gave a talk "Propagation of pulsed ultrasonics fields in a band gap of a two dimensional phononic crystal" in the Physical Acoustics Metamaterials section.  Sunethra Dayavansha gave a talk "Temperature dependence of shear wave speed in a viscoelastic wormlike micellar fluid" in the Biomedical Acoustics Complex Media section.

Temperature dependence of shear wave speed in a viscoelastic wormlike micellar fluid. E. G. Sunethra K. Dayavansha, Cecille Labuda and Joseph R. Gladden. J. Acoust. Soc. Am. 138, 1796 (2015); http://dx.doi.org/10.1121/1.4933697 Wormlike micellar fluids are viscoelastic and can support shear waves. Phase transitions of the micellar aggregates are temperature dependent and can manifest as sharp changes in the shear wave speed as a function of temperature. In this work, the variation of shear speed with temperature of 200 mM CTAB/NaSal micellar fluid in a 5:3 ratio was studied. The dependence of shear wave speed on time between fluid synthesis and measurement was also investigated. Shear wave propagation through the fluid was observed as a time varying birefringence pattern by using a high speed camera and crossed polarizers and shear speed was calculated by edge tracking and wavelength measurement techniques. A gradual increase in shear wave speed was observed in the temperature range 20—40 °C. A phase transition was observed to occur between 6 and 7 °C. There was no evidence of variation of shear wave speed with time. The implications of the shear wave speed variation over a wide temperature range will be discussed.

Propagation of pulsed ultrasonic fields in a band gap of a two dimensional phononic crystal. Ukesh Koju and Joel Mobley. J. Acoust. Soc. Am. 138, 1751 (2015); http://dx.doi.org/10.1121/1.4933531 A band gap in the transmission spectrum of a finite two dimensional phononic crystal is examined in the time domain using pulsed ultrasonic fields. The phononic crystal consists of a hexagonal array of copper cylinders (r = 1.19 mm) in an aqueous matrix with a lattice constant of 2.9 mm. Measurements of the transmission properties of the sample are performed using ultrasonic wave groups of various center frequencies and bandwidths. Among the band gaps in the low-MHz range, we concentrate on the gap from 1.48 MHz—1.70 MHz. The phase velocity, group velocity, and attenuation coefficient spectra are determined and compared with expectations.