Institut Charles Sadron News

A recent study in collaboration with Martin In (L2C, Montpellier) has shed new light on the complex flow behavior of gemini surfactants, a class of surfactants made up of two or more surfactant-like moieties connected by spacer groups. The study of these molecules was pioneered by Raoul Zana at ICS (CRM). These systems are particularly interesting in soft matter research due to their similarities with living polymers and polyelectrolytes, exhibiting dramatic changes in viscosity with concentration.

Researchers found that in bulk solutions, the viscosity of gemini surfactants can increase by as much as seven orders of magnitude simply by increasing concentration within the semidilute regime. However, this dramatic thickening doesn't behave the same way at the surface. Microrheology (tracking microparticles at the air–solution interface and in the volume) measurements show that the surface shear viscosity does not mirror the bulk behavior. Surprisingly, in some cases, it even appears to become negative, a counterintuitive result that calls for deeper investigation.

By comparing measurements from both traditional (macroscopic) and particle-scale (microscopic) rheology, the study highlights pronounced differences in how these materials flow, especially near interfaces. These discrepancies seem to arise from depletion effects affecting both the air–solution and particle–solution boundaries.

This work emphasizes the importance of considering interfacial behavior when studying complex fluids.

Article :

Microrheology of gemini surfactants at interfaces and in solutions in the dilute and semidilute regimes, Soft Matter, 2024,20, 8835-8844.

Mayssa Medfai,  Antonio Stocco, Christophe Blanc, Maurizio Nobili and Martin In

Figure. Wormlike Gemini surfactants in the semidilute regime shown on the cover of the book edited by R. Zana