Turning up the heat on wormlike micelles with a hydrotopic salt in microfluidics

Abstract

In equilibrium, wormlike micelles can transition from entangled to branched structures with increasing surfactant concentrations and ionic strength. Under flow conditions, structural transition of micellar solutions can follow very different trajectories. In this study we consider the flow of a semi-dilute wormlike micellar solution through an array of microposts, with focus on its rheological and microstructural evolutions. Specifically, the micellar solution (precursor) contains cationic surfactant cetyltrimethylammonium bromide (CTAB) and hydrotropic organic salt 3-hydroxynaphthalene-2-carboxylate (SHNC). We report the formation of a flow induced structured phase (FISP), with entangled, branched, and multi-connected micellar bundles, evidenced by electron microscopy and small-angle neutron scattering (SANS). By integrating gold-etched microheaters with the micropost design in a microfluidic device, we investigate the localized temperature effect on both the precursor and FISP, with complementary investigations from SANS. We observe that the FISP does not completely disintegrate at high temperatures, whereas, the precursor exhibits shortening of wormlike micelles as temperature increases. We also correlate the microstructure of both FISP and precursor with two point passive microrheology and bulk rheology characterizations.

Viscoelastic Synergy and Microstructure Formation in Aqueous Mixtures of Nonionic Hydrophilic Polymer and Charged Wormlike Surfactant Micelles

“…It was shown that at polymer concentrations 3–4.5 wt % corresponding to regime II the samples always contain spherical microdomains immersed in a matrix of densely packed wormlike surfactant chains often arranged parallel to each other, forming bundles and bundle-like loops (Figure ). Similarly arranged micellar chains were previously visualized in many wormlike micellar solutions ,− and in supramolecular polymers . Here we observe numerous microdomains included in such matrix.…”

Structural Evolution of Wormlike Micellar Fluids Formed by Erucyl Amidopropyl Betaine with Oil, Salts, and Surfactants

“…For sodium chloride, the addition of 0.2 wt % salt induced a slope of q –3 at low q . Such a slope at low values of q is often interpreted as accompanying phase separation, but it can also be indicative of the formation of a gel-like network with fractal characteristics . Notably here, the addition of 0.2 wt % NaCl did not induce obvious phase separation, with the sample remaining clear and optically isotropic, but the addition of 1.0 wt % NaCl did result in phase separation, wherein a dense gel-like EAPB-rich phase separated as a supernatant above an inviscid excess water phase.…”

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.…”