Velocity Fluctuations in a Slowly Sheared Bubble Raft

A surprising feature of flow in slowly sheared model foam (bubble raft) is a measured discontinuity in the rate of strain as a function of position such that part of the system is ``flowing'' and the rest is undergoing ``elastic'' deformations [J. Lauridsen, G. Chanan, and M. Dennin, Phys. Rev. Lett. v. 93, 018303 (2004)]. Detailed measurements of the distribution of nonlinear bubble rearrangements have been reported in connection with this discontinuity. In this paper, measurements of the fluctuations in velocity under the same conditions are reported. The fluctuations are characterized by the second and third moments of the velocity distribution. A surprising feature is the qualitative behavior of these moments as a function of position in the system, especially across the discontinuity in rate of strain. In addition, the measured dependence of the second moment of the velocity fluctuations on rate of strain is compared with predictions of simulations of the bubble model and reasonable agreement is found.