ARTICLESPUBLISHED ONLINE: 21 MARCH 2010 | DOI: 10.1038/NMAT2724

Irreversible nanogel formation in surfactantsolutions by microporous flowMukund Vasudevan1, Eric Buse2, Donglai Lu3, Hare Krishna4, Ramki Kalyanaraman5,6, Amy Q. Shen3,Bamin Khomami5 and Radhakrishna Sureshkumar7,8*

Self-assembly of surfactant molecules into micelles of various shapes and forms has been extensively used to synthesizesoft nanomaterials. Translucent solutions containing rod-like surfactant micelles can self-organize under flow to formviscoelastic gels. This flow-induced structure (FIS) formation has excited much fundamental research and pragmatic interestas a cost-effective manufacturing route for active nanomaterials. However, its practical impact has been very limitedbecause all reported FIS transitions are reversible because the gel disintegrates soon after flow stoppage. We present anew microfluidics-assisted robust laminar-flow process, which allows for the generation of extension rates many orders ofmagnitude greater than is realizable in conventional devices, to produce purely flow-induced permanent nanogels. Cryogenictransmission electron microscopy imaging of the gel reveals a partially aligned micelle network. The critical flow rate for gelformation is consistent with the Turner–Cates fusion mechanism, proposed originally to explain reversible FIS formation inrod-like micelle solutions.

Reversible FIS formation in macroscopic flows has beenreported 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 networksor entanglements at equilibrium1–23. Until now, such experimentshave been conducted in shear flow generated by devices such asconcentric cylinders or a cone-and-plate arrangement. The fluidcontained within the narrow gap between the cylinders (the coneand the plate) is sheared by the rotation of the outer cylinder(the cone). The solutions show Newtonian rheology at low shearrates, that is, the viscosity of the solution is independent of theshear rate or equivalently the uniform velocity gradient in thedevice γ̇ . However, at a critical shear rate γ̇c a phase transitionoccurs as a result of FIS formation. This is characterized by anabrupt increase in the solution viscosity, referred to as shearthickening. FISs are qualitatively different from simple micellaraggregates, as they show pronounced anisotropy, as gleaned fromoptical birefringence measurements, and are very sensitive tofactors such as the type and concentration of the surfactant andcounter-ion, the temperature and the presence of added salts2,3,6,8–12.Several surfactants, including cetyl-trimethyl ammonium bromide(CTAB), are known to undergo this structure transition in thepresence of salts such as sodium salicylate1–3,8,19–21 (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 inshear flow indicated the evolution of FISs that revealed theemergence of rod-like micelles aligned along the flow direction,with no appreciable correlation in the spacing between them4,5.

1Mineral Processing R&D, Cytec Industries Inc., Stamford, Connecticut 06902, USA, 2Department of Mechanical, Aerospace and Structural Engineering,Washington University, St Louis, Missouri 63130, USA, 3Department of Mechanical Engineering, University of Washington, Seattle, Washington 98195,USA, 4Department of Physics, Washington University, St Louis, Missouri 63130, USA, 5Department of Chemical and Biomolecular Engineering, Universityof Tennessee, Knoxville, Tennessee 37996, USA, 6Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996,USA, 7Department of Biomedical and Chemical Engineering, Department of Physics, Syracuse University, Syracuse, New York 13244, USA, 8Department ofEnergy, 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 spatialand temporal integrity of a gel, in shear-flow experiments ina Couette cell4. The presence of gel-like structures imparts anelastic character to the otherwise non-elastic Newtonian solution;hence, positive elastic normal stresses have been observed onlywhen FISs are present2,3,6,7,16,22. The discontinuity in normal stressesat the interface between FISs and the surrounding Newtoniansolution induces an elastic interfacial instability that manifests astemporal fluctuations in the shear as well as normal stresses inthis regime19. NMR imaging has revealed that FIS formation isaccompanied by shear banding20,21, and cryogenic transmissionelectron microscopy (cryo-TEM) showed FISs to be entangledmicellar networks with pores of the order of several tens ofnanometres23–26. Such structures could have served as templatesin nanomanufacturing applications had they been irreversible. Thetemplating aspect has been explored27–30 by using orthosilicates toreactively coat shear-induced micelles of the CTAB/NaSal system.However, these reactions occur over several hours and have tobe 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 producestable and irreversible FISs.

We first identified the critical parameters for FIS formation,using a conventional cone-and-plate rheometer. This informationwas used in the design of themicrofluidic device. The shear viscosityas a function of shear rate of the two CTAB/NaSal surfactant–saltsolutions used in this study is given in Fig. 1a. The surfactantconcentrations (CD) used were 0.003M and 0.05M, with the molarratio of salt to surfactant R = 1 and 0.28, respectively. For theconcentrations considered, small spherical and stiff cylindrical

436 NATURE MATERIALS | VOL 9 | MAY 2010 | www.nature.com/naturematerials

© 2010 Macmillan Publishers Limited. All rights reserved.

NATURE MATERIALS DOI: 10.1038/NMAT2724 ARTICLES

100 300 5000.002

0.006

0.01

100 300 5000.002

0.006

0.01

b

100 300 5000.002

0.006

0.01

100 300 5000.002

0.006

0.01

101 102

Shear rate (s¬1)

100 103

0.006

0.010

0.002

app

(Pa

s)η

Time (s)

101 102 103

2.5

1.5

0.5

(Pa)

τ

a b

Figure 1 | Shear-induced structure formation in macroscopic flow. a, Log–log plot of apparent viscosity ηapp versus shear rate for CTAB/NaSal solutionswith CD=0.003 M, R= 1 (filled squares), and CD=0.05 M, R=0.28 (filled triangles), where R is the molar ratio of salt to surfactant concentration.Dotted vertical lines denote the respective critical shear rate γ̇c required for shear-induced structure formation. b, Log–log plot of shear stress as a functionof time for γ̇ = 100 s−1 (filled square), 150 s−1 (filled diamond), 200 s−1 (filled triangle) and 300 s−1 (filled circle). The respective time periods t∗ requiredfor stress to increase and reach the plateau value is denoted by the dotted vertical lines. The product γ̇ t∗ is the critical strain γc required for shear-inducedstructure formation, which was found to be constant for a given concentration of surfactant–salt solution.

micelles of typical length≈20 nm with diameter≈5 nm are knownto form at equilibrium3,31. As reported in Fig. 1a, under shearflow and at low shear rates, the apparent viscosity ηapp, definedas the ratio of the average shear stress to the shear rate applied,remained almost constant, and the solutions showed Newtonianbehaviour. As the shear rate exceeded the critical shear rate γ̇c(≈10 s−1 for CD = 0.003M, R= 1, and ≈100 s−1 for CD = 0.05M,R = 0.28), a relatively large increase in viscosity was observedin both of the solutions because of FIS formation. For γ̇ � γ̇cthe structures disentangled under strong flow and, consequently,the viscosity decreased. At high γ̇ values, we observed foaming,and centrifugal forces threw the fluid out of the rheometer.Hence, rheological characterization of these solutions was limitedto shear rates≤1,000 s−1.

The inception time required for FIS formation decreased as γ̇was increased. This is illustrated in Fig. 1b, where, for a solutionwith CD= 0.05M and R= 0.28, we applied shear rates of 100, 150,200 and 300 s−1. On the basis of the time required to achieve therespective stress plateau value t∗, denoted by the dotted lines inthe figure, we obtained the critical strain γc = γ̇ct . Irrespective ofthe magnitude of γ̇c applied, γc was found to be almost constant≈8,500 for this solution. Similarly, γc≈4,000 forCD=0.003MwithR=1. Hence, FIS formation occurs when the applied strain exceedsa critical value for shear rates ≥ γ̇c. As reported in our earlier work(see Fig. 7 in ref. 3), when the strain applied on the solution washeld constant or the shear rate was stepped down to zero, the stressdecreased rapidly and abruptly at first, followed by a slower stressrelaxation process. Overall, the relaxation process occurs in a fewseconds. Hence, FIS formation is a reversible process.

The mechanisms of reversible FIS formation/disintegration arenot precisely understood at a molecular level; however, bothcontinuum-level models and molecular simulations of micelledynamics are slowly emerging32–38. As described by Cates ina pioneering paper, micelles are equilibrium polymers; that is,they constantly undergo scission and recombination even atequilibrium39. Typically, the timescale of scission/recombinationreactions is a fraction of a millisecond to a few milliseconds. Thisis smaller than the flow timescale, or equivalently the inverse shearrate, which in macroscopic flow experiments is of the order of10ms or larger. Under shear flow, flow alignment and tumblingin the vorticity direction of rod-like/worm-like micelles favourinter-micelle collisions that can lead to fusion of micelle segmentsto form longer rods/worms, which can eventually entangle. Theseentanglements may be considered as soft attractive bonds thatpreserve the integrity of the FIS against thermal fluctuations as well

as short-range electrostatic repulsion37. When the flow is stopped,structure relaxation aided by thermal fluctuations and electrostaticrepulsion could lead to disentanglement of the network.

Turner and Cates presented an interesting theoretical scenariofor the formation of highly flow-aligned structures from rod-likemicelles in extensional flow40. Their hypothesis was that flowalignment and fusion of rods can lead to longer micelles. As therotational (angular) diffusion timescale TD of the rods increasesmarkedlywith increasing rod length L, longer rodswill be convectedmore strongly by the flow and, at a critical extension rate E ,which is of the order 1/TD, this positive feedback mechanism willlead to gelation. In other words, a phase transition can occurwhen the so-called Peclet number Pe ≡ ETD, is of the order of 1.For a dilute solution of rods, 1/TD ∼ kBT/(ηL3), where kB,T ,ηdenote the Boltzmann constant, absolute temperature and solutionviscosity, respectively41. For an average rod length of 20 nm in asolution of viscosity 1mPa s (dilute aqueous solution) at roomtemperature, TD ∼ 10−6 s, suggesting that a flow deformationrate of ∼106 s−1 or larger is required to induce FIS formation.In conventional rheometric devices, such high deformation ratescannot be achieved with the present state of the art because ofpractical limitations, for example, centrifugal and/or inertial forcescould cause flow instabilities, which make the interpretation ofthe data very difficult. These considerations motivated experimentsdiscussed below, in which an extension-dominated flow field,generated in amicrofluidic porousmedium, is applied rapidly to thesame micelle solutions that showed reversible structure formationin shear flow (Fig. 1).

To explore whether rapid straining in an extension-dominatedflow could result in the formation of irreversible FISs, a micro-porous flow was designed. The typical microchannel height Hwas 100 µm and the channel width W varied from 50 to 500 µmin different regions of the device. To achieve the large strainvalues required for FIS formation, we made numerous smallerchannels by inserting polydispersed (20–100 µm) glass particles(Polysciences) inside the microchannel. A small amount of theseparticles was added before injecting the CTAB/NaSal solution. Thechannels were designed with a constriction to hold the particles,as shown in Fig. 2a. Under flow, these particles accumulated nearthe constriction and formed a pseudo-packed bed. Image anal-ysis revealed the presence of pore sizes much smaller than theaverage particle size, as the polydispersed particles formed a non-homogeneous packed bed. Hence, the flow area within the poreswas significantly smaller than the flow area of the microchannel.We estimated the deformation rate S as well as the strain γ using

NATURE MATERIALS | VOL 9 | MAY 2010 | www.nature.com/naturematerials 437© 2010 Macmillan Publishers Limited. All rights reserved.

ARTICLES NATURE MATERIALS DOI: 10.1038/NMAT2724

100 µm 100 µm

10 s 1,000 s

a b c

e f

h i

g

d

Figure 2 | Irreversible gelation in microporous medium flow. a–d, Montage of shear-induced gel formation in a microchannel with W=H= 100 µm.Particles 20–50 µm in size form a pseudo-packed bed at the constriction. The surfactant solution with CD=0.003 M and R= 1 is injected from the right ata flow rate of 5 ml h−1. Flow-induced gels form in the pores and exit the bed. e, The gels, once formed, have a stable interface with the surrounding solutionand flow downstream. The flow rate was 2 ml h−1. f, Gel formation from a surfactant solution with CD=0.003 M and R= 1 in a 500 µm channel at a flowrate of 1.5 ml h−1. In this case, a sufficient number of 50–100-µm particles is added to jam the channel to create a microporous medium flow. g, The elasticnature of the gel phase is evident from the observation that a particle that accidentally escapes the porous bed is tethered by a gel filament. The arrows ina–g indicate the flow direction. The flow rate was 5 ml h−1. h,i, The gel trapped in the channel is observed to be stable after the flow is stopped (flow ratebefore stopping was 3 ml h−1). Images were taken 10 s (h) and 1,000 s (i) after cessation of flow. No disintegration of the gels has been observed forseveral months of storage.

the Kozeny–Karman relationship42:

S=150Q (1−ε)2

D2p H ε3

and (1)

γ =150WL (1−ε)2

D2p ε

3(2)

Here Q is the flow rate, Dp is the particle diameter, and L is thelength of the porous bed with porosity ε. Experiments were carriedout at flow rates ranging from 0.01ml h−1, which is the minimumpumping rate of the syringe pump, to 7ml h−1, which correspondsto the maximum sustainable pressure drop for the system: higherpressure drops resulted in fluid leakage from the input lines andoccasional channel rupture. Note that equation (2) implies thatthe strain in the flow device, which is given by the product ofthe deformation rate and the residence time, is independent ofthe flow rate, as with increasing flow rate the deformation rateincreases linearly whereas the residence time decreases linearly(equation (1)). For Q< 1ml h−1, no gel formation was observedeven after very long times (h) of operation. However, forQ rangingfrom 1ml h−1 to 7ml h−1, which corresponds to a deformationrate in the range from 9.3× 105 s−1 to 6.5× 106 s−1, robust gelformation was observed almost instantaneously after the particlesformed a packed bed at the constriction, as illustrated by theflow visualization images in Fig. 2a–d. We further note that inthe absence of the particles, the maximum extension rate in

the microfluidic device is easily estimated as the gradient ofthe average velocity. This, for a flow rate of 5ml h−1, yields anextension rate of approximately 5,000 s−1, which is much smallerthan the critical extension rate estimate for micelles with lengthsof 20 nm. This explains why in the absence of particles, no gelformation was observed.

The conditions reported in Fig. 2a–d are Dp ≈ 30 µm, poros-ity ε = 0.5 (based on two-dimensional image analysis) andQ = 5ml h−1, which give S ∼ 106 s−1. Hence, γ ≈ 7,000 can beaccomplished in approximately 1ms. Note that this deformationrate is several orders of magnitude greater than can be realized inthe conventional shear-flow experiments and satisfies the criticalcondition prescribed by the Turner–Cates mechanism discussedabove; that is, the Peclet number is ∼1. Hence, as the solutionenters the pores, it experiences a sudden and steep increase inthe deformation rate and strain by several orders of magnitudeand forms a gel phase through a Turner–Cates-like mechanism.Thus, as seen in Fig. 2a–d, a gel-like phase is observed to comeout of the pores. As the packing is not ideal, a few large gapsbetween the channel and the particles allow some of the incomingfluid to pass through without being sufficiently strained. As aresult, the gels have a very distinct phase separation with thesurrounding fluid, as seen in Fig. 2e. Moreover, a well-definedinterface between the gel and the surfactant solution is observeddownstream of the microporous packing. As long as the flow isimposed, the gel is continuously formed and flows downstream.Gel formation was verified with different channels using differentsizes of particles. Phase transition in a larger channel of 500 µm

438 NATURE MATERIALS | VOL 9 | MAY 2010 | www.nature.com/naturematerials

© 2010 Macmillan Publishers Limited. All rights reserved.

NATURE MATERIALS DOI: 10.1038/NMAT2724 ARTICLES

Figure 3 | Gel phase suspended in clear fluid. Photograph showing the gelphase (white, right panel shows magnified image) suspended in clear fluid,all of which constituted the entire effluent from the microfluidic experimentdiscussed in the text. The sample shown has been stored under normallaboratory conditions for approximately 20 months after the originalexperiment. The gels were obtained from a microfluidic device, such asthose shown in Fig. 2a, at a flow rate of 2 ml h−1. The system here isCTAB/NaSal, with a CTAB concentration = 0.003 M, R= 1. The whiteregion at the bottom of the test tube in the left panel is an artefact ofreflected light during flash photography.

with 50–100-µm-sized particles is shown in Fig. 2f. In this case,a sufficient number of particles was added so that they jammedthe microchannel, and gel formation was observed soon afterthis. The gel phase is ‘sticky’ or elastic, as seen from Fig. 2g,where a particle that accidentally escapes from the constrictionis tethered by a strand of the gel. Robustness of gel formationwas also tested for other concentrations of CTAB and NaSal(CD = 0.05M, R= 0.24–0.34), as well as other surfactants such ascetyl pyridinium chloride (CD= 0.1Mwith 0.06MNaSal) and SDS(CD = 0.02–0.04M with counter-ion tetradecyl trimethylamineoxide 0.06M–0.08M). All of these surfactants showed irreversiblegel formation by microporous flow.

In the case of gels formed in microfluidic channels, even afterflow stoppage, the gels remained intact over time. Figure 2h and idepict in situ images of the CTAB/NaSal gel (CD= 0.003M, R= 1)in a 150 µm channel soon after the flow was stopped (10 s) andafter 1,000 s respectively. As illustrated in Fig. 3, gels were foundto be intact even several months after production. This is anextraordinary result considering the fact that typically FISs formedin conventional rheometers under shear flow could not be sustainedeven for a few seconds after the flow is stopped. A salient featureof FIS formation in pure shear flow is the presence of slow kinetictimescales, which translates to the existence of a finite inceptiontime43–45. In contrast, the experiments reported here for extension-dominated flow have very low or little latency, that is, the gelformation is practically instantaneous as the solution is passedthrough the porous medium.

To further verify the hypothesis that extensional flow canlead to the formation of permanent gels, we explored whethera sudden contraction flow, (which creates an infinite extensionrate at the contraction plane), such as the one realized by asyringe and needle, can lead to the formation of a gel phase. Asshown in Supplementary Figure S1, we observed that gel formationdoes occur, by subjecting the micelle solution to contractionflow (see the Methods section for more details). This experimentfurther corroborates the mechanism of permanent gelation basedon extension-induced micelle alignment and end-to-end fusion.However, the amount of gel that can be formed by this arrangementis extremely small. Hence, the use of particles to form an in situpacked bed is a technique that offers practical potential for a robustand scalable materials manufacturing process.

3

1

200 nm

4 2

Figure 4 | Cryo-TEM image of nanogel. Cryo-TEM image of theCTAB/NaSal gel, where the CTAB concentration=0.003 M, R= 1. Theimage indicates partially aligned zigzag micelle regions. The numbers 1–3illustrate the end-to-end length, width and spacing between the micellarregions respectively and 4 shows a folded hole-like region.

As a result of their irreversibility and stability, the gels couldbe collected in vials, after exiting the channel, and independentlycharacterized. As explained earlier, both the gel and the surfactantsolution came out of the pores. We characterized the gel andthe surfactant solution using cryo-TEM: see the Methods sectionfor the sample preparation procedure. A typical cryo-TEM imageis shown in Fig. 4. The image shows that the gel consists of anetwork of partially aligned micellar regions (white domains in thefigure). Specifically, analysis based on 30 samples yielded an averageend-to-end micelle length of 176± 13 nm, and the average widthand average inter-micelle spacing were found to be approximately28±1 nm and 41±6 nm, respectively. The average hole dimensionwas found to be 27± 4 nm. The feed solutions did not show anyorder, which indicates that the gel phase is qualitatively differentfrom the micelle solution.

The irreversibility and permanence of the phase transition isintriguing, and has opened up an entirely new dimension inthe study of flow-induced assembly of surfactant micelles, which,until now, has produced only reversible transitions. The Turner–Cates mechanism per se does not imply the transition to beirreversible40. Hence, molecular and mesoscopic simulations arerequired to understand the mechanisms by which rapid strainingcould quench the micelle aggregates into a thermodynamicallystable state or a kinetically trapped metastable state. A recent studyconducted by Raghavan and co-workers46 indicates that certaincationic surfactants, in the presence of certain organic counter-ions with aromatic character, have strong birefringence on shear.Importantly, the birefringent structures persist for hours after shearcessation, even though there was no permanent phase transition. Ithas been shown that progressively increasing shear in a lyotropiclamellar phase can lead to multiple phase transitions, yieldingthe formation of stable multilamellar vesicles with short- and/orlong-range order47–49. It is not apparent whether such mechanismscould apply to the dilute rod-like systems studied here. It is plausiblethat pre-shearing in the Poiseuille flow section of the channel couldcreate aggregates that further undergo a secondary phase transitionunder stronger (extension-dominated) flow in the porous medium.In addition, the presence of aromatic salicylate salts46 during thephase transition might lead to freezing of the microstructures into

NATURE MATERIALS | VOL 9 | MAY 2010 | www.nature.com/naturematerials 439© 2010 Macmillan Publishers Limited. All rights reserved.

ARTICLES NATURE MATERIALS DOI: 10.1038/NMAT2724

a partially aligned state, as demonstrated by the cryo-TEM image,which could result in the FIS transition to a kinetically trappedmetastable state. Furthermore, if the charge distribution on themicelles is not uniform, soft attractive interactions resulting fromelectrostatic dipoles could contribute to stabilizing such partiallyaligned aggregates, that is, the nearest-neighbour interactionsprovide a ‘caging’ effect, such that the metastable structure is stableto thermal perturbations.

The principal result of this study is that permanent nano-structured gels can be formed by the application of flow in theabsence of further external chemical stimulus. The observation callsfor theoretical and experimental studies aimed at understanding theeffect of flow type and deformation rate on the non-equilibriumself-assembly of a ubiquitous class of molecules (surfactants)used in applications ranging from drug delivery to enhancedoil recovery. The technique uses dilute surfactant–salt solutions,thereby efficiently using the material, and is easily scaled-upbecause the principles of porous medium process design are wellestablished. Furthermore, the microfluidics-assisted route could beused to circumvent processing challenges in other areas, such asthe production of molecular composites built on a nanofibre ornanotube scaffolds50 or for conjugation of nanofibres with activemolecules51. The nanogel could also be functionalized to synthesizenew mesoporous active materials for sensing, cellular delivery,catalysis and photonic applications.

MethodsIn this study, two aqueous solutions of surfactant CTAB (Fluka) with added saltNaSal (SigmaAldrich) were prepared using nanopure water with a resistance greaterthan 17.9M�. The solutions had CTAB concentrations CD = 0.003M and 0.05M,respectively, with molar ratios of salt to surfactant R= 1 and 0.28, respectively. Ashaker was used to mix the contents, after which the solutions were equilibratedfor three days before being used for analysis. The samples were stored at roomtemperature without direct exposure to sunlight.

Rheological experiments were carried out using a TA Instruments AR 2000constant stress and strain rheometer, using a cone and plate geometry with adiameter of 60mm (cone angle 0◦59′49′′, truncation 27 µm). The torque rangeof the rheometer was 0.1–200 µNm, and the minimum stress measurable was0.0008 Pa. The temperature was controlled at 24 ◦C by a Peltier plate. A solventtrap was used to minimize evaporation. At the end of each experiment, thePeltier plate was flushed with water to remove the sample, and then dried andcleaned using isopropyl alcohol (Sigma Aldrich) before adding a new sample.Extreme care was taken to ensure reproducibility, and all results reported wereaveraged over three runs.

For the microfluidic experiments, soft lithography was used to etch amicrochannel pattern on to a silicon wafer52. A polydimethylsiloxane (PDMS)mixture was prepared with a 10:1 w/w ratio of silicone elastomer base Sylgard 184to its curing agent (Dow Corning), poured over the pattern in a Petri dish, heatedat 60 ◦C for an hour to harden the PDMS, and then carefully cut. Holes were drilledat the channel inlet and outlet. A clean, dry glass slide and the patterned side of thePDMS were placed in a plasma chamber at 300mtorr for a minute to increase theirsurface energy and facilitate covalent bonding, after which they were tightly pressedagainst each other to form the microfluidic device. A small quantity of polydisperse(20–100 µm) glass particles (Polysciences) was inserted into the channel holes bymeans of forceps. Subsequently, polyethylene tubes (BD) were then inserted intothe holes. Medical-grade syringes with a 1ml volume (BD) and 27 G 1/2 needles(BD) were used to inject the surfactant solution into the microchannel by means ofa Harvard Apparatus syringe pump. To eliminate contamination from the lubricantoil used in the syringes, all syringes and needles were sonicated in isopropyl alcohol(Sigma Aldrich), rinsed thoroughly with nanopure water, and dried at 120 ◦C for aday before being used in the experiments. A Leica DM IRB inverted fluorescencemicroscope with a high-speed Photron camera (60 frames per second) was used toobserve the flow within the microchannel.

In experiments exploring permanent gel formation in a syringe and needle,a clear shear thinning surfactant solution of CTAB/NaSal (0.1M/0.06M) wasinjected from the syringe needle into a microchannel containing the same clearsolution. The Supplementary Fig. S1 shows the formation of threads of a gel-likephase issuing forth from the needle. The threads were found to be stable evenafter flow stoppage. Cetyl pyridinium chloride/NaSal (0.1M/0.06M) solutionalso produced a similar result. Furthermore, gel formation was found to occurby using this set-up for glass and plastic syringes, indicating that materialsspecificity is not involved.

In preparation for cryo-TEM measurements, the solution containing the gelsample was centrifuged (in a model 225 Centrific Centrifuge, Fisher Scientific)

at 2,000 rpm for 10min, after which the gel phase was carefully transferred to aglass flask for further characterization. Sample preparation for cryo-TEM analysisinvolved dipping the TEM grid directly into the gel sample for 5 s to mount a smallamount of gel on to the grid, and then storing the grid in a liquid nitrogen filledglass Dewar for 5min before mounting in the cryo-TEM holder. The sample wasthen transferred to a cryo-holder cooledwith liquid nitrogen at−180 ◦C.

Cryo-TEM measurements were made using an FEI Tecnai F20 transmissionelectron microscope at an accelerating voltage of 200 kV under a low electron dose.The statistical analysis of the gel morphology was carried out by estimating thedesired linear dimensions from the scale bar inside the TEM images. Open-sourceimage analysis software (Image J) was used for this purpose. Several such imageswere analysed to obtain at least 30 measurements for the statistical variables, witheach image taken from different regions of the gel. The morphology is characterizedusing the four measures depicted in Fig. 4: the end-to-end length of a contiguousmicelle region, the width of the micellar region, the spacing between alignedregions and the linear dimension of folded hole-like structures. The reported valueis the average of all these measurements.

Received 16 December 2009; accepted 11 February 2010;published online 21 March 2010

References1. Rehage, H., Wunderlich, I. & Hoffman, H. Shear induced phase transitions in

dilute aqueous surfactant solutions. Prog. Colloid. Polym. Sci. 72, 51–59 (1986).2. Wunderlich, I., Hoffmann, H. & Rehage, H. Flow birefringence and

rheological measurements on shear induced micellar structures. Rheol. Acta 26,532–542 (1987).

3. Vasudeven, M., Shen, A., Khomami, B. & Sureshkumar, R. Self-similarshear-thickening behaviour in CTAB/NaSal surfactant solutions. J. Rheol. 52,527–550 (2008).

4. Liu, C. & Pine, D. J. Shear induced gelation and fracture in micellar solutions.Phys. Rev. Lett. 77, 2121–2124 (1996).

5. Hu, Y. T., Boltenhagen, P. & Pine, D. J. Shear thickening in low concentrationsolutions of wormlike micelles I. Direct visualization of transient behaviourand phase transitions. J. Rheol. 42, 1185–1208 (1998).

6. Hu, Y. & Matthys, E. F. The effects of salts on the rheological characteristicsof a drag-reducing cationic surfactant solution with shear-induced micellarstructures. Rheol. Acta 35, 470–480 (1996).

7. Hu, Y. & Matthys, E. F. Characterization of micellar structure dynamics for adrag reducing surfactant solution under shear: Normal stress studies and flowgeometry effects. Rheol. Acta 34, 450–460 (1995).

8. Cressely, R. & Hartmann, V. Rheological behaviour and shear thickeningexhibited by aqueous CTAB micellar solutions. Eur. Phys. J. B 6, 57–62 (1998).

9. Cappelaere, E., Cressely, R., Makhloufi, R. & Decruppe, J. P. Temperature andflow-induced viscosity transitions for CTAB surfactant solutions. Rheol. Acta33, 431–437 (1994).

10. Hartmann, V. & Cressely, R. Influence of sodium salicylate on the rheologicalbehaviour of an aqueous CTAB solution. Colloids Surf. 121, 151–162 (1997).

11. Hartmann, V.&Cressely, R.Occurrence of shear thickening in aqueousmicellarsolutions of CTAB with some added organic counterions. Colloid Polym. Sci.276, 169–175 (1998).

12. Hartmann, V. & Cressely, R. Shear thickening of an aqueous micellar solutionof cetyltrimethylammonium bromide and sodium tosylate. J. Phys. II 7,1087–1098 (1997).

13. Hartmann, V. & Cressely, R. Simple salts effects on the characteristics of theshear thickening exhibited by an aqueous micellar solution of CTAB/NaSal.Europhys. Lett. 40, 691–696 (1997).

14. Kim, W. & Yang, S. Effects of sodium salicylate on the microstructure of anaqueous micellar solution and its rheological responses. J. Colloid Interface. Sci.232, 225–234 (2000).

15. Wheeler, E. K., Fischer, P. & Fuller, G. G. Time-periodic flow induced structuresand instabilities in a viscoelastic surfactant solution. J. Non-Newton. Fluid 75,193–208 (1998).

16. Nowak, M. Elastic properties of a dilute surfactant solution in the shearinduced state. Rheol. Acta 40, 366–372 (2001).

17. Nowak, M. Shear induced phase separation in cationic surfactant solutionsaround a rotating sphere. Rheol. Acta 37, 336–344 (1998).

18. Prötzl, B. & Springer, J. Light scattering experiments on shear inducedstructures of micellar solutions. J. Colloid Interface Sci. 190, 327–333 (1997).

19. Bandyopadhyaya, R., Basappa, G. & Sood, A. K. Observation of chaoticdynamics in dilute sheared aqueous solutions of CTAT. Phys. Rev. Lett. 84,2022–2025 (2000).

20. Britton, M. M. & Callaghan, P. T. Shear banding instability in wormlikemicellar solutions. Eur. Phys. J. B 7, 237–249 (1999).

21. Britton, M. M. & Callaghan, P. T. Two-phase shear band structures at uniformstress. Phys. Rev. Lett. 78, 4930–4933 (1997).

22. Rehage, H. & Hoffmann, H. Viscoelastic surfactant solutions: Model systemsfor rheological research.Mol. Phys. 74, 933–973 (1991).

23. Keller, S. L., Boltenhagen, P., Pine, D. & Zasadzinski, J. A. Direct observationof shear-induced structures in wormlike micellar solutions by freeze-fractureelectron microscopy. Phys. Rev. Lett. 80, 2725–2728 (1998).

440 NATURE MATERIALS | VOL 9 | MAY 2010 | www.nature.com/naturematerials

© 2010 Macmillan Publishers Limited. All rights reserved.

NATURE MATERIALS DOI: 10.1038/NMAT2724 ARTICLES24. Shikata, T., Sakaiguchi, Y., Uragami, H., Tamura, A. & Hirata, H. Enormously

elongated cationic surfactant micelle formed in CTAB-aromatic additivesystems. J. Colloid Interface Sci. 119, 291–293 (1987).

25. Lin, Z. et al. Influence of surfactant concentration and counterion tosurfactant ratio on rheology of wormlike micelles. J. Colloid Interface Sci. 239,543–554 (2001).

26. Lin, Z., Cai, J. J., Scriven, L. E. & Davis, H. T. Spherical-to-wormlike micelletransition in CTAB solutions. J. Phys. Chem. 98, 5984–5993 (1994).

27. Kim, W. & Yang, S. Flow-induced silica structure during in situ gelation ofwormy micellar solutions. Langmuir 16, 4761–4765 (2000).

28. Kim,W.&Yang, S. Flow-inducedmicrostructure in aqueous cationic surfactantsolution in the presence of structure-enhancing additives. J. Chem. Eng. Jpn 34,227–231 (2001).

29. Kim,W. & Yang, S. Helical mesostructured tubules from Taylor vortex-assistedsurfactant templates. Adv. Mater. 13, 1191–1195 (2001).

30. Kim, W. & Yang, S. Preparation of mesoporous materials from theflow-induced microstructure in aqueous surfactant solutions. Chem. Mater.12, 3227–3235 (2000).

31. Garg, G., Hassan, P. A. & Kulshreshtha, S. K. Dynamic light scattering studiesof rod-like micelles in dilute and semi-dilute regime. Colloids Surf. A 275,161–167 (2006).

32. Macías, E. R. et al. On the shear thickening flow of dilute CTAT worm-likemicellar solutions. J. Rheol. 47, 643–658 (2003).

33. Fielding, S. M. & Olmsted, P. D. Kinetics of the shear banding instability.Phys. Rev. E 68, 036313 (2003).

34. Fielding, S. M. & Olmsted, P. D. Spatio-temporal oscillations and rheochaos ina simple model of shear banding. Phys. Rev. Lett. 92, 084502 (2004).

35. Fielding, S. M. & Olmsted, P. D. Nonlinear dynamics of an interface betweenshear bands. Phys. Rev. Lett. 96, 104502 (2006).

36. Boek, E. S., den Otter, W. K., Briels, W. J. & Iakovlev, D. Molecular-dynamicssimulation of amphiphilic bilayer membranes and wormlike micelles: Amulti-scale modelling approach to the design of viscoelastic surfactantsolutions. Phil. Trans. R. Soc. Lond. A 362, 1625–1638 (2004).

37. Padding, J. T., Boek, E. S. & Briels, W. J. Rheology of wormlike micellar fluidsfrom Brownian and molecular dynamics simulations. J. Phys. Condens. Matter17, S3347–S3353 (2005).

38. Huang, C. C., Xu, H. & Ryckaert, J. P. Kinetics and dynamics of wormlikemicelles under shear. Europhys. Lett. 81, 58002 (2008).

39. Cates, M. E. Reptation of living polymers: Dynamics of entangled polymersin presence of reversible chain-scission reactions. Macromolecules 20,2289–2296 (1987).

40. Turner, M. S. & Cates, M. E. Flow-induced phase transitions in rod-likemicelles. J. Phys. Condens. Matter 4, 3719–3741 (1992).

41. Doi, M. & Edwards, S. F. The Theory of Polymer Dynamics (Clarendon, 1986).42. McCabe, W. L., Smith, J. C. & Harriott, P. Unit Operations of Chemical

Engineering (McGraw-Hill, 1993).

43. Berret, J.-F., Gamez-Corrales, R., Lerouge, S. & Decruppe, J.-P.Shear-thickening transition in surfactant solutions: New experimental featuresfrom rheology and flow birefringence. Eur. Phys. J. E 2, 343–350 (2000).

44. Oelschlaeger, Cl., Waton, G., Candau, S. J. & Cates, M. E. Structural, kineticsand rheological properties of low ionic strength dilute solutions of a dimeric(gemini) surfactant. Langmuir 18, 7625–7271 (2002).

45. Oelschlaeger, Cl., Waton, G. & Candau, S. J. Rheological behaviour of locallycylindrical micelles in relation to their overall morphology. Langmuir 19,10495–10500 (2003).

46. Frounfelker, B. D., Kalur, G. C., Cipriano, B. H., Danino, D. & Raghavan, S. R.Persistence of birefringence in sheared solutions of wormlike micelles.Langmuir 25, 167–172 (2009).

47. Diat, O. & Roux, D. Preparation of monodisperse multilayer vesicles ofcontrolled size and high encapsulation ratio. J. Phys. II France 3, 9–14 (1993).

48. Diat, O., Roux, D. & Nallet, F. Effect of shear on a lyotropic lamellar phase.J. Phys. II France 3, 1427–1452 (1993).

49. Sierro, P. & Roux, D. Structure of a lyotropic lamellar phase under shear.Phys. Rev. Lett. 78, 1496–1499 (1997).

50. Pasquali, M. Swell properties and swift processing. Nature Mater. 3,509–510 (2004).

51. Joshi, A. et al. Nanotube-assisted protein deactivation. Nature Nanotech. 3,41–45 (2008).

52. Xia, Y. N. & Whitesides, G. M. Soft lithography. Annu. Rev. Mater. Sci. 28,153–184 (1998).

AcknowledgementsWe gratefully acknowledge NSF grants CBET 0404243 (A.Q.S., R.S.), CBET 0853735(R.S.), CBET CAREER 0645062 (A.Q.S.) and DMI CAREER 0449258 (R.K., H.K.) forsupport of this work. R.S. acknowledges insightful discussions with R. G. Larson andZ. Wang during his sabbatical at University of Michigan, Ann Arbor, and M. Cates andA. Morozov during his visit to University of Edinburgh.

Author contributionsM.V., R.S. and A.Q.S. planned and designed the microfluidic experiments. M.V. carriedout rheological characterization and microfluidic experiments with the help of E.B.,analysed and interpreted the data as well as prepared the manuscript under the guidanceof R.S., A.Q.S. and B.K. M.V. and H.K. conducted AFM characterization of the dried gelunder the guidance of R.K. D.L. carried out cryo-TEM imaging of the gel and interpretedthe data with guidance from A.Q.S. and R.S.

Additional informationThe authors declare no competing financial interests. Supplementary informationaccompanies this paper on www.nature.com/naturematerials. Reprints and permissionsinformation is available online at http://npg.nature.com/reprintsandpermissions.Correspondence and requests formaterials should be addressed to R.S.

NATURE MATERIALS | VOL 9 | MAY 2010 | www.nature.com/naturematerials 441© 2010 Macmillan Publishers Limited. All rights reserved.