Shear-induced sponge-to-lamellar transition in bicontinuous microemulsions evidenced by microfluidic-SANS

Shear flow applied to bicontinuous microemulsions is expected to induce a transition to lamellae via the suppression of surfactant monolayer fluctuations. Compared to the topologically analogous L₃ (sponge) phase, composed of surfactant bilayers, this transition is likely to occur at much higher shear rates.

We examine the flow response of a model bicontinuous microemulsion, D₂O/n-octane/C₁₀E₄ by coupling microfluidics with small-angle neutron scattering (SANS), attaining wall shear rates in excess of 10⁵ s^-1. The reduction of probed sample volumes down to ∼10 nL allows the spatial mapping of the structural and orientation changes within the microchannel, as a function of the flow field components.

With increasing flow rate, we observe a gradual increase in scattering anisotropy, accompanied by a decrease of the microemulsion domain size along the main flow orientation. A consistent description of the degree of anisotropy was obtained when considering the velocity gradient along the scattering plane perpendicular to the flow. We discuss the flow dependence of the effective bending rigidity, rationalizing a strong influence of shear on thermal membrane fluctuations. Assuming a similar shear dependence for the saddle splay modulus, the bicontinuous-to-lamellar transition can be attributed to the gradual disappearance of inter-lamellar passages.