Yielding and Flow of Monodisperse Emulsions

We have measured the yield transition of monodisperse emulsions as the volume fraction, φ, and droplet radius,a, are varied. We study the crossover from the perturbative shear regime, which reflects the linear viscoelastic properties, to the steady shear regime, which reflects nonlinear, plastic flow. For small oscillatory strains of peak amplitude γ, the peak stress, τ, is linearly proportional to γ. As the strain is increased, the stress becomes nonlinear in γ at the yield strain, γy. The φ dependence of γyis independent ofaand exhibits a minimum near the critical volume fraction, φc≈ 0.635, associated with the random close packing of monodisperse spheres. We show that the yield stress, τy, increases dramatically as the volume fraction increases above φc; τyalso scales with the Laplace pressure, σ/a, where σ is the interfacial tension. For comparison, we also determine the steady shear stress over a wide range of strain rates, γ. Below φ ≈ 0.70, the flow is homogeneous throughout the sample, while for higher φ, the emulsion fractures resulting in highly inhomogeneous flow along the fracture plane. Above φ ≈ 0.58, the steady shear stress exhibits a low strain rate plateau which corresponds with the yield stress measured with the oscillatory technique. Moreover, τyexhibits a robust power law dependence on γ with exponents decreasing with φ, varying from 23 to 12. Below φ ≈ 0.58, associated with the colloidal glass transition, the plateau stress disappears entirely, suggesting that the equilibrium glassy dynamics are important in identifying the onset of the yield behavior.