Anomalous decrease in lamellar spacing by shear flow in a nonionic surfactant/water system

Area of Science:

• Soft matter physics
• Materials science
• Physical chemistry

Background:

• Investigating the structural response of lamellar phases in hepta(oxyethylene glycol)-n-hexadecyl ether (C16E7)/water systems to applied shear flow.
• Utilizing small-angle neutron scattering (SANS) to probe structural changes at varying shear rates (10⁻³–30 s⁻¹) and C16E7 concentrations (40–55 wt %) at 70°C.

Discussion:

• Observed a significant and discontinuous decrease in lamellar repeat distance (d) with increasing shear rate (0.1–1 s⁻¹), down to 40% of the resting state.
• Noted that 'd' increases after reaching a sharp minimum (d*) at higher shear rates, a phenomenon consistent across different lamellar orientations.
• Found that the minimum repeat distance (d*) remains constant (~5 nm) irrespective of C16E7 concentration, aligning with bilayer thickness from X-ray scattering.

Key Insights:

• Shear flow induces water layer exclusion within the lamellar phase.
• The system segregates into distinct surfactant-rich and water-rich regions under shear, without reaching macroscopic phase separation.
• The observed structural changes are strongly linked to the dynamic behavior of water layers under shear stress.

Outlook:

• Further research could explore the precise mechanism of water expulsion and the dynamics of the segregated regions.
• Investigating other surfactant systems and flow conditions will elucidate the generalizability of these findings.
• Understanding these shear-induced structural transitions is crucial for applications involving surfactant-based materials in flow environments.