Temporal measurements of surfactant squeeze-out from a surface using magnetically levitated liquid bridges

A cylindrical liquid bridge, laden with surfactant and constrained at the two ends by circular rods, was levitated against gravity in a magnetic field gradient. On axially translating one of the support rods to decrease the bridgeʹs length, the axial electric resistance of the bridge was found to decrease. The relaxation time τ of the resistance as it approached its equilibrium value was measured as a function of surfactant concentration. Below the critical micelle concentration (CMC), we observed τ∼1 s; above the CMC τ∼1.6 s. The results are discussed in terms of capillary wave damping and surfactant squeeze-out from the surface.