Deformation of a viscoelastic drop in planar extensional flows of a Newtonian fluid

This communication presents experimental data for steady and transient deformations of a purely elastic drop (PIB/PB Boger fluid) suspended in a Newtonian fluid (PDMS) undergoing a planar extensional flow produced by a four-roll mill. The viscoelastic effects alter the steady drop shape from being ellipsoidal to drop shapes with more blunt ends. The results reflect a balance between the direct tensile stress contribution of the viscoelastic fluid to the normal stress balance, and modifications of the viscous (i.e. Newtonian) stress and pressure due to viscoelastic changes in the flow. This is qualitatively consistent with the conclusion from recent simulation results [S. Ramaswamy, L.G. Leal, The deformation of a viscoelastic drop subjected to steady uniaxial extensional flow of a Newtonian fluid, Journal of Non-Newtonian Fluid Mechanics 85 (2–3) (1999) 127–163]. There is no overshoot of the drop deformation upon start-up of the flow, and a relative insensitivity to the Deborah number. However, when the external flow is turned off, there is a broad spectrum of relaxation times for the drop to return to a spherical shape, between the characteristic capillary timescale for a Newtonian fluid, aμ/σ, to the longer polymer relaxation time, τR.