ARTICLES PUBLISHED ONLINE: 21 MARCH 2010 | DOI: 10.1038/NMAT2724 Irreversible nanogel formation in surfactant solutions by microporous flow Mukund Vasudevan 1 , Eric Buse 2 , Donglai Lu 3 , Hare Krishna 4 , Ramki Kalyanaraman 5,6 , Amy Q. Shen 3 , Bamin Khomami 5 and Radhakrishna Sureshkumar 7,8 * Self-assembly of surfactant molecules into micelles of various shapes and forms has been extensively used to synthesize soft nanomaterials. Translucent solutions containing rod-like surfactant micelles can self-organize under flow to form viscoelastic gels. This flow-induced structure (FIS) formation has excited much fundamental research and pragmatic interest as a cost-effective manufacturing route for active nanomaterials. However, its practical impact has been very limited because all reported FIS transitions are reversible because the gel disintegrates soon after flow stoppage. We present a new microfluidics-assisted robust laminar-flow process, which allows for the generation of extension rates many orders of magnitude greater than is realizable in conventional devices, to produce purely flow-induced permanent nanogels. Cryogenic transmission electron microscopy imaging of the gel reveals a partially aligned micelle network. The critical flow rate for gel formation is consistent with the Turner–Cates fusion mechanism, proposed originally to explain reversible FIS formation in rod-like micelle solutions. R eversible FIS formation in macroscopic flows has been reported in surfactant solutions containing rod-like or worm- like cylindrical micelles in the presence of certain counter- ions at a surfactant concentration below the ‘overlap threshold’, that is, the surfactant concentration required to form networks or entanglements at equilibrium 1–23 . Until now, such experiments have been conducted in shear flow generated by devices such as concentric cylinders or a cone-and-plate arrangement. The fluid contained within the narrow gap between the cylinders (the cone and the plate) is sheared by the rotation of the outer cylinder (the cone). The solutions show Newtonian rheology at low shear rates, that is, the viscosity of the solution is independent of the shear rate or equivalently the uniform velocity gradient in the device ˙ γ . However, at a critical shear rate ˙ γ c a phase transition occurs as a result of FIS formation. This is characterized by an abrupt increase in the solution viscosity, referred to as shear thickening. FISs are qualitatively different from simple micellar aggregates, as they show pronounced anisotropy, as gleaned from optical birefringence measurements, and are very sensitive to factors such as the type and concentration of the surfactant and counter-ion, the temperature and the presence of added salts 2,3,6,8–12 . Several surfactants, including cetyl-trimethyl ammonium bromide (CTAB), are known to undergo this structure transition in the presence of salts such as sodium salicylate 1–3,8,19–21 (NaSal). Overall, shear thickening requires favourable inter-micelle interactions, which can be abetted by the addition of salts. Light-scattering experiments on the CTAB/NaSal system in shear flow indicated the evolution of FISs that revealed the emergence of rod-like micelles aligned along the flow direction, with no appreciable correlation in the spacing between them 4,5 . 1 Mineral Processing R&D, Cytec Industries Inc., Stamford, Connecticut 06902, USA, 2 Department of Mechanical, Aerospace and Structural Engineering, Washington University, St Louis, Missouri 63130, USA, 3 Department of Mechanical Engineering, University of Washington, Seattle, Washington 98195, USA, 4 Department of Physics, Washington University, St Louis, Missouri 63130, USA, 5 Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA, 6 Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA, 7 Department of Biomedical and Chemical Engineering, Department of Physics, Syracuse University, Syracuse, New York 13244, USA, 8 Department of Energy, Environmental and Chemical Engineering, Washington University, St Louis, Missouri 63130, USA. *e-mail:rsureshk@syr.edu. They also observed FISs as comb-like structures, with the spatial and temporal integrity of a gel, in shear-flow experiments in a Couette cell 4 . The presence of gel-like structures imparts an elastic character to the otherwise non-elastic Newtonian solution; hence, positive elastic normal stresses have been observed only when FISs are present 2,3,6,7,16,22 . The discontinuity in normal stresses at the interface between FISs and the surrounding Newtonian solution induces an elastic interfacial instability that manifests as temporal fluctuations in the shear as well as normal stresses in this regime 19 . NMR imaging has revealed that FIS formation is accompanied by shear banding 20,21 , and cryogenic transmission electron microscopy (cryo-TEM) showed FISs to be entangled micellar networks with pores of the order of several tens of nanometres 23–26 . Such structures could have served as templates in nanomanufacturing applications had they been irreversible. The templating aspect has been explored 27–30 by using orthosilicates to reactively coat shear-induced micelles of the CTAB/NaSal system. However, these reactions occur over several hours and have to be carried out under carefully controlled conditions. In short, conventional instruments do not allow the easy capture of FISs. Here, we present a robust process using microfluidics to produce stable and irreversible FISs. We first identified the critical parameters for FIS formation, using a conventional cone-and-plate rheometer. This information was used in the design of the microfluidic device. The shear viscosity as a function of shear rate of the two CTAB/NaSal surfactant–salt solutions used in this study is given in Fig. 1a. The surfactant concentrations (C D ) used were 0.003 M and 0.05 M, with the molar ratio of salt to surfactant R = 1 and 0.28, respectively. For the concentrations considered, small spherical and stiff cylindrical 436 NATURE MATERIALS | VOL 9 | MAY 2010 | www.nature.com/naturematerials © 2010 Macmillan Publishers Limited. All rights reserved.