LAUSR

In this appendix, we derive an expression for the average scattering wave vector 〈 q 〉 when an ultrasonic wave from a spherically focusing transducer is incident on a cylindrical cell. The method is based on a ray approach, since this simplified approach leads naturally to a description in terms of scattering angles for the experimental geometry. We are interested in the possible values of ′ = −    q k k , where k  is the incident wave vector on a scatterer and ′  k is the scattered vector. Only some ( ) , ′   k k combinations are relevant due to the finite extent of the transducer. We found all such k  , ′  k by integrating over the surface of the transducer. For each point ( ) , ξ ζ on the transducer surface, two wave vectors L k  and S k  are possible results of the ray propagating from ( ) ζ ξ , in the direction of the focus into the interior of the sample, passing through the cell wall in either longitudinal (L) or shear (S) mode.