Time-domain observations of fast and slow ultrasonic group velocities in microsphere suspensions

In dispersive media, propagating wave packets can exhibit abnormal group velocities that differ substantially from the phase velocity of the dominant mode comprising the pulse. Recently, both arbitrarily large and negative acoustic group velocities were shown to exist in an aqueous suspension of polymer microspheres, using a broadband measurement technique. In this talk, we report on the direct time-domain observation of abnormal group velocities in microsphere suspensions. These dispersion-related propagation effects include slow group velocities, where the envelope speed is reduced by as much as 25%, relative to the phase velocity, and fast group velocities, which can be arbitrarily large or negative. The role of signal bandwidth in creating pulses in which such group velocities are manifested is discussed. The practical limits of slowing the group velocity in microsphere suspensions is also explored.