Chapter 2
Theoretical Modeling of Viscoelastic Phases M . E. Cates
Downloaded by PENNSYLVANIA STATE UNIV on December 10, 2015 | http://pubs.acs.org Publication Date: December 9, 1994 | doi: 10.1021/bk-1994-0578.ch002
Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE, England
Viscoelastic surfactant phases usually contain long, semiflexible mi cellar aggregates; these behave as reversibly breakable polymers. A "reptation-reaction" model, which couples the diffusive disentangle ment of the micelles to the kinetic equations describing their reversible breakdown, explains many recent experimental observations. Work on the linear viscoelastic spectrum is reviewed, with an emphasis on ex tracting quantitative structural and kinetic data from rheological exper iments. This can be done by analysing small departures from Maxwell behavior in the spectrum. A rheological theory of nonlinear response in steady shear is also described. The microscopic constitutive equation displays a flow instability, which leads to a plateau in the shear stress under increasing shear rate, and large normal stresses.
Wormlike micelles In many aqueous surfactant solutions, very long, semiflexible micellar aggre gates are formed (1-4)- Typical systems involve ionic surfactants in the pres ence of added salt, though some nonionic surfactants show similar phases. The overlap threshold of the micelles can be very low (a few percent by volume); at higher concentrations the resulting phase is viscoelastic, and strongly resem bles a semidilute polymer solution. For example, the osmotic pressure Π and high-frequency shear modulus (plateau modulus) G
0l
fraction φ approximately as
