Wormlike micelles (WLMs) are formed by reversible self-assembly of amphiphilic (e.g. surfactant) molecules usually with the aid of salt which acts to screen the electrostatic repulsion between the micelles to achieve giant structures. Their unique dynamics such as reversible scission give them interesting rheological properties which are sought in several industrial applications. It is therefore imperative to understand and predict these phenomena in WLMs using constitutive equations that incorporate reversible scission into the deformation dynamics of WLMs.
We compare the predictions of the Vasquez-Cook-McKinley (VCM) (2007) model which treats WLMs as Hookean dumbbells that break at half-length to form two shorter dumbbells, to an analogous Brownian dynamics (BD) simulation of the same physical model. We find a discrepancy between their predictions and trace it to the absence in the VCM model of the internal position of the nascent breakage point in the long micelle, which is needed to satisfy microscopic reversibility of breakage and fusion. We correct this deficiency in the VCM model by extending an ensemble-averaged bead-spring phase space model of Wiest et al. (1989) to include reversible scission of two-spring chains. The revision tracks the conformations of the two halves of the long micelle and transmits this information to the short micelles upon breakage and thereby recovers complete agreement with the BD results.
We extend the reversible scission kinetics to the slip-link tube model of Likhtman (2005) originally formulated for single polymer chains with entanglements. This facilitates the simulation of entangled WLMs and enables us to study the effects of entanglements on WLM rheology. We observe increased stresses for start-up shear flows in the entangled WLMs when breakage time was equal to reptation time. We propose that reversible scission acts as a means of constraint release which re-orientates chain segments in the velocity gradient direction and prevent retraction in the tube thereby causing increased stress. However, stress overshoot caused by the relaxation of the peak stress to a lower steady stress was observed in the fast-breaking regime. This suggests that reversible scission functions as both stress relaxation and constraint release mechanisms.
We then investigate strain hardening, a nonlinear rheological property. We explore different kinds of WLMs for strain hardening and systematically study their strain hardening dependence on salt concentration and temperature. By measuring stress relaxation following a step strain, we observe that strain hardening is prevalent over a temperature range of 15 - 25 C for a solution of cetyl trimethyl ammonium bromide (CTAB) with the added hydrotrope, sodium salicylate at hydrotrope-to-surfactant concentration ratios between 0.5 - 3.0. The extent of strain hardening upon nonlinear step-strain deformation varies non-monotonically as a function of salt-to-surfactant ratio for different temperatures. A transition from strain hardening to softening or linear response is observed at strains that are dependent on temperature and concentration.
Strain hardening was also observed in solutions of CTAB and hydrotrope, sodium 3-hydroxy-2-naphthoate. However, solutions of anionic sodium lauryl sulfate surfactants without hydrotrope but simple salt, sodium chloride strain softened, indicating that the hydrotrope is crucial to obtaining strain hardening in step strains. The results indicate a stress relaxation mechanism that is more complex than that of simple disentanglement and reversible scission, possibly involving strain-induced associations between micelles facilitated by hydrotropes that may act as physical crosslinkers.PHDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/145970/1/adamsaa_1.pd
