Microstructure and rheology of a flow-induced structured phase in wormlike micellar solutions

Abstract

Surfactant molecules can self-assemble into various morphologies under proper combinations of ionic strength, temperature, and flow conditions. At equilibrium, wormlike micelles can transition from entangled to branched and multiconnected structures with increasing salt concentration. Under certain flow conditions, micellar structural transitions follow different trajectories. In this work, we consider the flow of two semidilute wormlike micellar solutions through microposts, focusing on their microstructural and rheological evolutions. Both solutions contain cetyltrimethylammonium bromide and sodium salicylate. One is weakly viscoelastic and shear thickening, whereas the other is strongly viscoelastic and shear thinning. When subjected to strain rates of ∼10 3 s −1 and strains of ∼10 3 , we observe the formation of a stable flow-induced structured phase (FISP), with entangled, branched, and multiconnected micellar bundles, as evidenced by electron microscopy. The high stretching and flow alignment in the microposts enhance the flexibility and lower the bending modulus of the wormlike micelles. As flexible micelles flow through the microposts, it becomes energetically favorable to minimize the number of end caps while concurrently promoting the formation of cross-links. The presence of spatial confinement and extensional flow also enhances entropic fluctuations, lowering the energy barrier between states, thus increasing transition frequencies between states and enabling FISP formation. Whereas the rheological properties (zero-shear viscosity, plateau modulus, and stress relaxation time) of the shear-thickening precursor are smaller than those of the FISP, those of the shear-thinning precursor are several times larger than those of the FISP. This rheological property variation stems from differences in the structural evolution from the precursor to the FISP. microfluidics | microrheology | mesh size S urfactant molecules in aqueous solutions can self-assemble into different structures, such as spherical micelles, cylindrical micelles, lamellar phases, and vesicles (1). The morphology of these self-assembled structures is influenced by surfactant concentration, temperature, external additives (e.g., cosurfactants or salts), and flow conditions. Cylindrical micelles in the presence of inorganic or organic salts can self-assemble into large, flexible, and elongated wormlike micelles that exhibit viscoelastic properties (2-4). The ionic strength of the salt screens the electrostatic repulsions between the charged surfactant head groups, promoting cylindrical micellar growth (1-3). At higher salt concentrations, the entangled linear micelles can transition to branched and multiconnected micellar networks, as evidenced by rheological measurements, light scattering techniques, and electron microcopy (EM) imaging (5-18). Additionally, wormlike micellar solutions are known to exhibit a variety of interesting phenomena, some of which are shear banding (19-21), shear thickening (22, 23), shear-induced transitions and insta...

Rheological characterizations of wormlike micellar solutions containing cationic surfactant and anionic hydrotropic salt

“…Since semidilute micellar solutions usually possess weak viscoelasticity, they are more desirable for oil recovery and encapsulation related applications with higher mobility. Moreover, cationic surfactant solutions with organic salts in the semidilute regime have shown not only interesting rheological behaviors [Ouchi et al (2007); Shikata et al (1987); Vasudevan et al (2008)] but also exhibited unique capabilities in forming irreversible nanostructured phases [Cardiel et al (2013);Cardiel et al (2014b); Dubash et al (2011);Vasudevan et al (2010)] for encapsulation and sensing applications [Cardiel et al (2014a); Lu et al (2010)]. Motivated by the temperature sensitive nature of SHNC and the advantage of semidilute wormlike micellar solutions, this work focuses on the rheological behavior of aqueous CTAB/SHNC mixtures in the semidilute regime, in which micelles are positively charged and isotropically distributed.…”

Topology, length scales, and energetics of surfactant micelles

“…It has long been known that the emerging superstructures can range from spherical and elongated cylindrical to very long, flexible wormlike micelles with or without branches [1][2][3][4][5][6][7][8][9] and topologically rich knotted structures [7,8]. The diversity in microstructure and rheological properties make micellar solutions beneficial to numerous applications [10] as hydrofracking fluids in oil industry, turbulent friction drag reducing agents [8], thickening agents in consumer products, drug carriers in targeted delivery [11], and templates to create functional nanofluids with tunable mechanical or optical properties [12][13][14].…”

Electro-conductive porous scaffold with single-walled carbon nanotubes in wormlike micellar networks

“…This irreversible gelation originates from a combination of the high rates of strain (_ $ 5000 s À1 ) and from the extensional characteristics of the flow. Cardiel et al [26] extended this study and reported the formation of FISP from both ionic shear thinning and shear thickening micellar solutions when subjected to strain rates $ 10 3 s À1 and strains $ 10 3 . The FISP consists of entangled, branched, and multiconnected micellar bundles, evidenced by electron microscopy imaging.…”

“…The smaller gap size (higher shear rates) in the micropost array can facilitate scaffold formation with smaller pore sizes, therefore enhancing the connectivity of SWCNTs in the FISP-SWCNTs. However, we only employed the flow rate of 15 lm in this work to keep the experimental conditions consistent with our previous work to form plain FISP [26] for comparison purposes. The gap size between the micropost arrays was maintained at $15 lm with height $75 lm.…”