Shear Induced Micellar Structures

Abstract

Mixed solutions of the anionic surfactant Sodiumdodecylsulfate (SDS) and the zwitterionic Tetradecyldimethylaminoxid (TDMAO) were studied under shear in a Couette‐type apparatus. For shear rates exceeding a critical shear gradient γc the solutions show birefringence which approaches a constant value ΔnMAX with increasing shear rate. At the same time properties like the shear viscosity and the normal stress difference increase dramatically. A structural modification must thus have taken place for γ > γc: a shear‐induced structure (SIS) has developed. – The structure of the micelles in the solution at rest was determined by SANS‐, electric birefringence‐ and viscosity experiments for different mixing ratios, concentrations and ionic strength. The solutions under shear were studied by SANS, flow‐birefringence and rheological measurements. – At rest small, rodlike micelles which consist of mixed surfactants are present. The length of the micelles are smaller than their mean distance and the viscosity is low in this case. The dimensions of the micelles were determined from the positions of the correlation peaks in the SANS‐patterns which are due to repulsive forces between the charged aggregates. At a mixing ratio of 9:1 (TDMAO:SDS; C = 100 mM) the solutions are viscoelastic and no SIS is detected. The SIS is thus formed only in solutions with surfactant concentrations which are below the overlap concentration. At γc – at which the SIS is beginning to form – the product of γc and τROT, the rotational relaxation time of the rods, is much smaller than one. – As soon as a critical shear gradient is reached these former isotropic micellar solutions show birefringence, an extinction angle of zero degrees, normal stress difference and an increased viscosity. Former isotropic SANS‐patterns deform and peaks show the appearence of a highly ordered structure in equilibrium with unordered micelles. All micelles taking part in the SIS are completely aligned in the direction of flow. Formation of the SIS is highly dependent on concentration, mixing ratio and ionic strength. Finally, two proposals for the nature of SIS are presented: A shear induced phase transition into domains of (pseudo)nematic phases seems most likely to us and is consistent with our experimental results. But also a pearl‐string like array in domains of oriented micelles is conceivable.

Photorheological Effects in Aqueous Micellar Tetramethylammoniumhydrogen-2-dodecyl Malonate via Photodimerization of Acridizinium Bromide

“…This behavior is reminiscent of observations in multiphase fluids (16) and of elastic instabilities in couette flow of certain polymer solutions (17,18). The former effects, ascribed to tumbling of the director in liquid crystal fluids, are in keeping with the view of liquid crystal domains in SIS (11,12), the latter effects with long polymer-like surfactant aggregates (13). Stable oscillations, however, require periodically changing processes, such as build-up and destruction of SIS, that are concerted in the entire volume investigated.…”

Rheology of Wormlike Micelles: Equilibrium Properties and Shear Banding Transitions

“…More recently, rheo-optical techniques were applied with success to the study of wormlike micelles [2,14,117,161,[169][170][171][172]. Experimental set-ups designed for studying complex fluids generally utilize a Couette cell as a shearing device (Figure 14).…”

“…The vector u(s) is tangent to the micellar axis and s denotes the curvilinear length. The bending free energy per unit of s is proportional to the product κC 2 , where κ is the bending modulus of the chain and C its curvature. For surfactants, Ben-Shaul and coworkers have shown that this free energy has a molecular origin [95].…”

“…The distribution in length c(L) is broad and given by Eq. (2). c(L)dL denotes the number density of chains of length comprised between L and L + dL.…”

“…In the general context of complex fluids, wormlike micelles have received considerable attention from theoreticians and experimentalists during the past decade [1][2][3][4][5][6][7][8]. One reason for this interest is due to their remarkable rheological properties.…”