Elongational deformation of wormlike micellar solutions

Abstract

We have investigated the uniaxial elongation behavior of six different wormlike micelle systems covering a broad range of surfactant concentrations cs and salt/surfactant ratios R using the capillary breakup elongational rheometry (CaBER). In the fast-breaking limit (high cs and R), filament lifetime tfil is controlled by the equilibrium shear modulus G0 and the breakage time λbr obtained from small oscillatory shear according to tfil/G0∝λbr2/3 and relaxation time ratios λe/λs≈1 are found. When reptation dominates (high cs, low R) λe/λs<1 is observed similar as for solutions of covalently bound polymers. In this concentration regime, the micellar structure seems not to be affected by the strong elongational flow. In contrast, high filament lifetimes up to 1000 s and λe/λs values up to 10 are observed at low cs irrespective of R. This indicates the formation of elongation-induced structures (EISs). A minimum viscosity and a minimum initial diameter are required for creating EIS. Additional filament stretching experiments indicate that a critical total deformation has to be exceeded for structure build-up. Finally, our experiments reveal a distinct difference regarding the dependence between solutions of linear and branched micelles of filament lifetime on viscosity suggesting that CaBER is a versatile means to distinguish between these structures.

Rheology of branched wormlike micelles

“…Finally, elongational flow is found to induce structures at low surfactant concentrations whether the micelles are branched or linear. These studies demonstrate that extensional flow can discriminate between branched and linear, entangled micellar solutions, but not unambiguously due to the possibility of extensional-induced structures [52]. The nature of these extensionalinduced structures and their relationship to branching topology are at present, unresolved.…”

“…The mechanism for this reduction is proposed to be the same branch-sliding mechanism proposed for the reduction in the shear viscosity. Using similar methods, Sachsenheimer et al [52] also determined that extensive micellar branching greatly reduced the Trouton ratio to a value similar to that for Newtonian fluids. The authors highlight additional, significant differences between branched and unbranched, entangled WLM solutions in extensional flow.…”

Anomalous diffusion and stress relaxation in surfactant micelles

“…Further, depending upon the surfactant and salt concentrations, which in turn determine the microstructure, we observe normal, subdiffusive and superdiffusive motion of surfactants. Specifically, superdiffusive behavior is associated with branch sliding, breakage and recombination of micelle fragments as well as constraint release in entangled systems.Over the past decades, the structure, dynamics and mechanical properties of self-assembled aggregates of cationic surfactants have been studied extensively [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Self-assembly of cationic surfactant solutions can be controlled by manipulating the solvent-mediated electrostatic interactions among the surfactant molecules by altering the counter ion concentration.…”

“…i. Mechanisms and energetics of branching: Branched structures are formed in a cationic surfactant solution as the salinity is increased [8][9][10][11]. However, the physical mechanisms underlying their formation are unclear.…”

“…In this work, we have addressed the abovementioned questions via a coarse-grained molecular dynamics (CGMD) simulation of cationic surfactant micelle solutions of cetyl-trimethyl-ammonium-chloride (CTAC), in the presence of a binding organic salt sodium salicylate (NaSal), a widely used system in experimental studies [3][4][5]11].…”