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Shear-induced defect formation in a nonionic lamellar phase.

B Medronho1, M Rodrigues, M G Miguel

  • 1Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal. bfmedronho@portugalmail.pt

Langmuir : the ACS Journal of Surfaces and Colloids
|April 9, 2010
PubMed
Summary

Shear flow induces reversible defects in lamellar phases, shifting behavior from viscous to solid-like. Temperature influences defect rigidity, while shear rate controls defect density, impacting material properties.

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Area of Science:

  • Soft Matter Physics
  • Materials Science
  • Rheology

Background:

  • Lamellar phases are complex fluids with ordered structures.
  • Understanding their response to external stimuli like shear flow is crucial for material applications.

Purpose of the Study:

  • To investigate the impact of shear flow on the structure and rheological properties of nonionic oriented lamellar phases.
  • To elucidate the relationship between shear-induced structural defects and macroscopic material behavior.

Main Methods:

  • Deuterium (2H) Nuclear Magnetic Resonance (NMR) spectroscopy was employed to probe structural changes.
  • Rheological measurements, including elastic modulus and viscosity, were performed under varying shear rates and temperatures.

Main Results:

  • Shear flow induces reversible structural defects in lamellar phases, leading to a transition from viscous to solid-like behavior.
  • Elastic modulus increases reversibly with preshear rate, and temperature controls defect rigidity while shear rate controls defect density.
  • The lamellar phase exhibits shear-thinning behavior (eta ~ gamma^n, n ~ -0.4), consistent with theoretical models.

Conclusions:

  • Shear-induced structural defects are a key feature of lamellar phase rheology, acting as a pretransition to multilamellar vesicle formation.
  • The findings provide insights into the fundamental mechanisms governing the mechanical response of ordered soft materials.