Wormlike Micelle Solutions

   Several aqueous surfactant solutions are known to contain very long, flexible self-assembled wormlike micelles.  These wormlike micelles are similar to polymers in that they are quite flexible (typical persistence lengths of ~ 20 nm versus diameters of ~ 5 nm) and they exhibit contour lengths on the order of microns.  These so-called equilibrium or living polymers are different from classical polymers in that they are constantly breaking and recombining and, therefore, do not exhibit a quenched distribution of lengths or molar masses.  This ability to break and recombine profoundly affects the dynamical behavior of these systems.  Stress relaxation in entangled classical polymer solutions is well described via a reptation mechanism in which the polymer chains diffuse along their contour path or tube until they escape at which point the imposed stress is completely relaxed.  While equilibrium polymers or wormlike micelles can also relax stress via curvilinear diffusion or reptation, their ability to break and recombine provides another route for stress relaxation.  The so-called reptation time is the time a polymer chain requires to diffuse along its contour length thereby escaping from the stressed initial tube.  When the equilibrium polymer or wormlike micelle breaking/recombination time is much longer than the wormlike micelle reptation time, stress relaxation is dominated by the reptation process and the stress relaxation behavior of equilibrium polymers should be essentially the same as for classical polymers.  However, when the wormlike micelle breaking/recombination time is much shorter than the reptation time, the breaking/recombination kinetics dominate the stress relaxation process leading to monoexponential or Maxwellian stress relaxation behavior.

    Our group's studies to date have focused on three wormlike micelle systems: cetyltrimethylammonium bromide (CTAB)/sodium salicylate (NaSal), CTAB/potassium bromide (KBr), and cetylpyridinium chloride (CpCl)/NaSal/sucrose solutions.  Complete multiangle laser light scattering (MALLS), dynamic light scattering (DLS), diffusing wave spectroscopy probe motion and mechanical rheometry measurements are either completed or in progress for these systems.  The MALLS measurements provide a measure of the static correlation length and more importantly the zero wavevector structure factor which is a measure of the osmotic compressibility present in these systems.  The DLS measurements provide the hydrodynamic correlation length (osmotic relaxations) as well as establishing the importance of longer-time concentration fluctuation relaxation modes.  The DWS probe motion measurements provide a measure of the bulk moduli (see Brownian motion in viscoelastic media).  The mechanical rheometry measurements provide the traditional framework with which our other measurements can be compared in the hopes of understanding just what the so-called microrheology techniques actually measure.

Current and Past Group Members

    S. Amin, S.J. Dees ('99-'01), T.W. Kermis ('69-'00), S. Thomason ('00),  K.P. Rufener ('96-'98), R.M. van Zanten ('99-'00)
 

Current and Past Collaborators

    P. van der Schoot, D. Wirtz ('96-'98)

Publications to Date

• S. Amin, T.W. Kermis, R.M. van Zanten, S.J. Dees & J.H. van Zanten, "Concentration Fluctuations in CTAB/NaSal Solutions", Langmuir 17, 8055-8061 (2001).
• J.H. van Zanten & K.P. Rufener, "Brownian Motion in a Single Relaxation Time Maxwell Fluid", Phys. Rev. E 62, 5389-5396 (2000).