Difference in structural changes of surfactant aggregates near solid surface under shear flow versus those in the...

Area of Science:

• Soft matter physics
• Surface science
• Materials chemistry

Background:

• Nonionic surfactants like pentaethylene glycol monododecyl ether (C12E5) form multi-lamellar vesicles in water.
• Shear application typically induces buckling instability in surfactant layers.
• Confinement effects near solid substrates can alter membrane fluctuations, but near-surface structural changes are understudied.

Purpose of the Study:

• To investigate structural changes in surfactant aggregates near a solid substrate under shear.
• To understand the influence of shear rate ramping on near-surface structures.

Main Methods:

• Neutron reflectometry (NR) was employed to analyze structural changes.
• Controlled shear rates were applied to surfactant solutions confined between solid substrates.

Main Results:

• Observed shear thickening at low shear rates and shear thinning at high shear rates, consistent with bulk behavior.
• Detected a discontinuous change in lamellar structure and surfactant condensation near the substrate.
• Demonstrated that shear rate ramping speed governs shear-induced structuring.

Conclusions:

• The study provides the first experimental evidence of shear rate ramping speed controlling near-surface structuring of surfactant aggregates.
• Near-surface structural rearrangements differ from bulk behavior under shear.
• Neutron reflectometry is effective for probing interfacial phenomena in sheared soft matter.