Chemical Science

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aBiopolymeres Interactions Assemblages, IN

France. E-mail: anne-laure.fameau@nantes

240675083bDepartment of Chemical & Biomolecul

University, Raleigh, North Carolina 27695

Fax: +1 9195153465; Tel: +1 9195134318

† Electronic supplementary informationtransition temperature of 12-HSA, as welstability, water fraction evolution, and solESI also contains UV-Vis absorbance datand Black Pearl� 880 from Cabot. See DO

Cite this: Chem. Sci., 2013, 4, 3874

Received 25th June 2013Accepted 8th August 2013

DOI: 10.1039/c3sc51774h

www.rsc.org/chemicalscience

3874 | Chem. Sci., 2013, 4, 3874–388

Multi-stimuli responsive foams combining particles andself-assembling fatty acids†

Anne-Laure Fameau,*a Stephanie Lamb and Orlin D. Velev*b

Foams that respond to external stimuli are a new type of highly functional soft matter and an emerging

research subject in the field of materials and colloid science. Herein, we report a new way to control

foam stability using light irradiation. This unconventional approach involves a combination of the

thermal response of a fatty acid and the ability of carbon black to convert light stimulus to heat. This

simple system features both high foamability and high foam stability, but can easily be destroyed by

light. Upon exposure to UV or solar illumination, the carbon black particles absorb light causing an

increase in temperature inside the foam lamella. This results in a transition of the fatty acid assemblies

from tubes, which stabilize the foam films, to micelles, leading to rapid foam destabilization. The foam

destabilization rate under UV irradiation was correlated to foam liquid fraction, fatty acid concentration,

as well as carbon black concentration. Our approach for the formulation of photo-thermo-responsive

foams could be easily generalized and extended to magnetic particles, giving rise to the first foams

exhibiting thermo-photo-magneto-tuneable stability. Such multi-stimuli responsive systems can find

applications in diverse industries requiring highly stable systems which can be destroyed on-demand.

Introduction

Foams are thermodynamically metastable dispersions of twoimmiscible phases (gas/water) that separate with time. To formand stabilize foams, amphiphilic molecules or particles areadded to the system. Applications for foams are found in a widerange of large-scale products and processes, ranging from food todetergents, from oil recovery to the production of macroporousmaterials.1,2 Foam stability is an important parameter in manyapplications, and recent research progress has resulted in thecreation of very stable foams3–8 as well as in understanding themechanisms which lead to foam stability.5–7,9–13 For applications,such as recovering radioactive materials, chemical decontami-nation or washing, both the formation of stable foams as well ascontrolled destabilization – the ability to rapidly destroy thefoams on demand – are required.14 However, foam destabiliza-tion typically requires the use of chemical agents, which areharmful to the environment.15 In the current thrust towards

RA, Rue de la Geraudiere, 44316 Nantes,

.inra.fr; Fax: +33 240675084; Tel: +33

ar Engineering, North Carolina State

-7905, USA. E-mail: odvelev@ncsu.edu;

(ESI) available: SANS data verifyingl as data pertaining to long term foamar response of 12-HSA/CBP foams. Thea and TEM images of Monarch� 800I: 10.1039/c3sc51774h

1

sustainable development, the on-demand breakage of foams byexternal stimuli instead of by chemical defoamers could reducethe use of large quantities of polluting agents.

There is abundant literature on responsive surfactants,polymers, emulsions and gels;16–18 however, few studies exist ontriggering foam stability by external stimuli.8,15,19–22 The use oflight and temperature as stimuli for controlling just the foam-ability of a foamulsion was reported recently by Salonen et al.22

Foams, which could be manipulated using external elds, werealso reported recently. Magnetically responsive foams, whichexhibit excellent stability in the absence of a eld, but can berapidly destroyed on demand with the application of athreshold magnetic eld were described by Lam et al.20 At thesame time, the rst foams exhibiting temperature-tunablestability were designed using thermally-responsive fatty acidassemblies.8 Other thermoresponsive systems, such as foa-mulsions, have also been formed using methycellulose/tannicacid complexes.21 Compared to other stimuli such as tempera-ture or magnetic eld, light offers signicant advantages in thatit can be directed precisely at a location of interest withoutphysical contact.23 Photoresponsive foams were achieved in arecent study using a photosensitive azobenzene-based surfac-tant.15 The authors demonstrated the possibility of light-induced foam destabilization, but their system could beimpractical for common applications, as azobenzene surfac-tants may be unsuitable for the environment, and are typicallynot commercially available.

We report here a simple system stabilized using a “green”surfactant, which has both high foamability and foam stability,

This journal is ª The Royal Society of Chemistry 2013

Fig. 1 Photograph of a freshly-made foam and microscope images corre-sponding to the foam head and liquid drained out of the foam. (a) Micrograph offresh foam showing thick foam films containing 12-HSA tubes and CBP. (b) 12-HSA tubes and carbon black particles in solution. The scale bar represents 50 mm.

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but which can easily be destroyed by light. The concept intro-duced in this study is to combine particles that are goodabsorbers of UV irradiation with thermally responsive fatty acidmolecular self-assemblies in order to produce photo-thermor-esponsive foams. Metallic particles are intense light absorbers,with absorption resulting in a rise in temperature at the particlesurface.24–27 This photothermal conversion process has been oflarge interest for many applications in biomedicine, such asdrug release and bioimaging.28,29 Carbon-based particles havealso been shown to give rise to strong photothermal heatingeffects, and for this reason have been used for solar energycollection.27,30

In this study, carbon black particles are mixed with a ther-moresponsive fatty acid – 12-hydroxystearic acid (12-HSA).31–33

12-HSA is an inexpensive molecular surfactant available in largequantities and at low cost, derived by the hydrogenation of asustainable material – ricinoleic acid from castor plants.Ultrastable foams have previously been obtained with self-assembled 12-HSA tubes of micron size that are adsorbed at theair/water interface and jammed within the foam liquid chan-nels.8,34 Researchers also showed that upon heating, the tubestransform into micelles due to a change in temperature. This inturn changes the packing parameter of the fatty acid assembliesinside the foam lamella, leading to complete destruction of thefoam; as micelles with a nanometric size cannot efficientlyblock the gravitational drainage of liquid from the foam, or theoccurrence of coalescence and coarsening between bubbles.8

We report here how the stability of a foam system containingcarbon black particles and thermoresponsive 12-HSA tubes canbe manipulated using UV irradiation. To gain insight on themechanisms of light-induced destabilization, we varied thefatty acid concentration, carbon black particle concentration, aswell as the initial water fraction in the foam. To generalize ourapproach, we used other particles known to be good absorbersof light radiation, such as metallic particles, and explored foamresponse to both light and magnetic elds.

Results and discussionFoaming properties before and aer UV light irradiation

The system studied is a mixture of carbon black particles (CBP)at 1 g L�1 and micron-sized 12-HSA tubes at a concentration of10 g L�1 with a temperature-dependent transition from tubes tomicelles at 45 �C as determined by Small Angle Neutron Scat-tering (SANS) (Fig. SI1a†). A large volume of foam can beproduced from this mixture by hand-shaking, demonstratingthe high foamability of this system (Fig. 1). At room tempera-ture, foam volume was evaluated by visual inspection over timein order to determine if the presence of CBP affected thestability of the 12-HSA foam. Foams containing both 12-HSAand CBP were shown to be stable for several months (Fig. SI2†).This result demonstrates that the presence of particles in the 12-HSA tube solution does not modify the stability of the 12-HSAfoams, which comes from the presence of a high quantity offatty acid tubes jammed in the foam liquid channels.8 To testwhether the addition of CBP affected the response of 12-HSAfoam to thermal stimulus, we observed the response of foam

This journal is ª The Royal Society of Chemistry 2013

samples exposed to temperature conditions above and belowthe transition temperature which induces a change in packingparameter of 12-HSA tubes in the foam lamella. Foams placedin an oven at T � 40 �C, which is lower than the transitiontemperature, remained stable. When placed in an environmentabove the transition temperature of 45 �C, the foam wasdestabilized in few minutes as the nanometer-sized micellesresulting from 12-HSA re-assembly at T > Ttransition cannotsuppress gravitational drainage and interbubble gas diffusion.8

Thus, foams containing 12-HSA and CBP are still thermores-ponsive. In addition, when samples were irradiated with UVlight from the top, a very rapid foam destabilization wasobserved in just a few seconds (Fig. 2). The same foam con-taining only 12-HSA tubes remained stable even aer 420seconds of UV exposure. Therefore, the presence of CBP isessential for UV-induced foam destabilization.

The CBP in the photoresponsive foams, are trapped insidethe matrix formed by the fatty acid tubes in the foam lamella(Fig. 1a). Under UV illumination, the CBPs absorb UV irradia-tion and act as photoresponsive heat converters. Snapshotsfrom videos taken with an infrared camera show that theaverage temperature at the destabilization front of the photo-responsive foam exceeds 45 �C aer 60 s, surpassing the tran-sition temperature at which tubes transition into micelles(Fig. 3). In the control foam, the temperature only increased by2 �C – from 25 �C to 27 �C – under the same dose of UV lightexposure. The increase of the local temperature in the CBP-containing foam channels to T > 45 �C is accompanied by thetransition of the 12-HSA assemblies from tubular structures,

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Fig. 2 Photographs of 12-HSA foam containing CBP (a) before illumination with UV light, (b) 20 s after UV light illumination, and (c) 1 min after UV light illumination –

during which time the sample is completely destroyed. Control foammade with 12-HSA at 10 g L�1 (d) before illumination with UV and (e) 7 minutes after illuminationwith UV light.

Fig. 3 Snapshots extracted from IR movies showing the temperature profiles of a 12-HSA/CBP foam sample contained in a glass vessel (a) before irradiation with UVlight, and (b) 60 seconds after UV light irradiation. (c) Control foam sample 7 minutes after UV light irradiation.

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which stabilize the foam channels, to micelles leading to veryfast foam destabilization (Fig. 4). This rapid transition of tubesinto micelles may also result in interfacial ows as well asdrastic changes to the shape of the lamellae and menisci in thefoam.8 We hypothesize that the combination of all theseparameters is responsible for the rapid lamellae rupture whichcauses foam destruction. Thus, a change in the packingparameter of the 12-HSA assemblies inside the foam due toparticle heating from light irradiation offers a versatile andsimple way to produce photoresponsive foams.

The extent of collapse experienced by the photoresponsive12-HSA foams can also be controlled. Halting the UV irradiationled to a stop in foam destruction. Indeed, the absence of UVlight and lack of photothermal conversion by CBP, leads to ahalt in the transition of fatty acid tubes to micelles. Thus, when

3876 | Chem. Sci., 2013, 4, 3874–3881

the system cools down to T < 45 �C the process is stopped. Werepeated cycles of foam production/destabilization upon UVirradiation up to ten times, and observed that foam properties(foamability and foam stability) remained unchanged. Thisresult demonstrates the complete reversibility of this foamsystem, which can be destabilized by UV irradiation multipletimes without chemical modication to the components in thesystem.

Effect of age and water fraction

The fraction of water in the foam is a crucial parameter fordetermining the structure of the foam as well as its response toexternal stimulus.15,35–37 In order to determine how foam ageand water fraction inuence the rate of foam destabilization

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Fig. 4 Schematic illustrating the mechanism of foam destabilization upon UV irradiation. (a) Fatty acid tubes and CBPs jam to form gel-like layers in the foam lamella.(b) Upon UV irradiation, the CBPs heat up, causing fatty acid tubes to transition into micelles. (c) This results in destabilization of the foam. This schematic is not to scale.In the experiments, the fatty acid tubes are �5–10 mm in length, and 600 nm in diameter.8,31 The carbon black particles are 20 nm in size, but form aggregates in thefoam varying from several hundred nanometers to several microns in size.

Fig. 5 (a) Evolution of the foam head volume as a function of UV illuminationtime for foams of different ages (different water fractions due to pre-drainagetimes). (b) Evolution of foam volume as a function of the illumination time forfoams containing different concentrations of 12-HSA with a fixed concentrationof Monarch� 800 at 1 g L�1. All samples were illuminated 10 minutes after foamformation. (c) Evolution of the foam volume as a function of illumination time forfoams with different concentrations of Monarch� 800 and a fixed 12-HSAconcentration of 10 g L�1. All samples were tested 6 minutes after foamformation.

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during UV irradiation, we rst measured the evolution of theaverage water volume fraction in the foams over time in theabsence of UV illumination at room temperature (Fig. SI3a†).These foams were formed from solutions containing 10 g L�1

12-HSA and a CBP concentration of 1 g L�1. Our data show thatthe foam entrapped a large amount of liquid during formation,starting with an average liquid fraction, 3, of 0.3, which corre-sponds to very wet foam (Fig. SI3a†). The foam drained in therst 30 minutes to 3 ¼ 0.13, but the rate of drainage decreasedaer this initial period of rapid foam evolution. Aer 24 hours,the water fraction decreased to 3 ¼ 0.1, which still correspondsto relatively wet foam.35,36,38,39 These fatty acid foams maintain ahigh liquid fraction because the 12-HSA tubes are jammedinside the foam channels, decreasing the rate of liquid drainageout of the foam.8We then recorded the foam volume decrease asa function of illumination time for foams at different stages ofthe aging process (Fig. 5a). The volume decrease of UV-illumi-nated freshly made foam, tested aer 4 minutes of drainage (3�0.19) was slow. The initial foam head volume of 11 mLdecreased to a nal volume of 1.5 mL aer 7 minutes of lightirradiation. In contrast, when UV irradiation was applied tofoams aer 24 hours of drainage (3 � 0.1), the entire foam headrapidly collapsed in �60 seconds. Thus, the time necessary forfoam destabilization decreases with an increase in the age ofthe foam, which corresponds to a reduced average water frac-tion in the foam at the time of illumination.

The profound link between foam destabilization rate andfoam water content can be interpreted by a combination ofseveral effects. One important factor of stability is the thicknessof the lms between the bubbles. The bubbles in freshly formedwet foams are separated by thick lms of liquid, making thesystem more resistant to coalescence. Aer liquid drains outunder the action of gravity and lm thickness decreases, thelms become more sensitive to structural disturbances andeasier to destabilize.2,15,35,40 As mentioned above, the transitionof tubes into micelles upon UV irradiation probably leads tointerfacial ows; such ows would have a greater impact onfoam stability in the case of drier foams, which contain thinnerlms. Another relevant parameter that changes during foamaging is the depth of UV light penetration into the foam. In the

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literature, it has been shown that the amount of light trans-mitted inside foam depends on the volume fraction of water inthe foam head.41–43 Lower foam water fractions result in a higherintensity of transmitted light at any given depth into the foam.In our case, the water fraction as a function of the foam agedecreases from 30% to 10% (v/v). This change probably plays arole in the depth of light penetration into the foam, wheredeeper penetration and faster foam collapse are facilitated bysample age. Finally, it is important to note that an increase infoam age leads to an increase in the ratio between CPB andwater content. Wet foam contains more water to absorb thethermal energy generated by CBP in comparison to dry foam.Therefore it would take more time to heat wet foam than dryfoam. The combination of all these possible effects can explainwhy dry foam is destroyed more easily in comparison with wetfoams. However, at this preliminary stage it is difficult to eval-uate and to deconvolute the intricate impact of all concreteparameters on the foam destabilization rate.

Effect of 12-HSA concentration and CBP concentration

All results reported in the previous sections were obtained withfoams made from solutions of 10 g L�1 12-HSA and 1 g L�1 CBP.We also determined how the foam destabilization rate under UVillumination can be tuned by varying the concentrations of 12-HSA and CBP. First, we studied the effect of fatty acid concen-tration on the rate of foam destabilization in UV 10 minutesaer foam production. All foams tested were made at a xedCBP concentration of 1 g L�1 (Fig. 5b). The rate of foam desta-bilization increased with fatty acid concentration up to 10 g L�1,aer which it decreased. To explain these results, we measuredthe average water fraction for all foams 10 minutes aer foamformation. We observed that the average fraction of water in thefoam increased linearly with fatty acid concentration: from 3 �0.12 for a fatty acid concentration of 5 g L�1 to 3� 0.45 for a fattyacid concentration of 40 g L�1 (Fig. SI3b†). An increase in fattyacid concentration leads to an increase in the viscosity of thecontinuous phase,44 which is known to reduce the rate of liquiddrainage from the foam.45 We found that the concentration offatty acid, which optimizes bulk viscosity and liquid content forrapid foam destabilization, ranges from 7.5–10 g L�1 of fattyacid in the bulk. We hypothesize that the viscosity of the fattyacid tube solution has to be high enough to trap a considerableamount of CBP inside the foam liquid channels, but at the samebe low enough to form foams which are not very wet. As shownin the previous section, themost important factor in the processof foam destabilization by UV is the average water volumefraction: the higher the water fraction, the lower the rate offoam destabilization. Since foam water fraction is highlydependent upon the concentration of 12-HSA, the rates of foamcollapse can be controlled by varying the fatty acidconcentration.

We also characterized the effect of CBP concentration onfoam stability and rate of foam destabilization. The concen-tration of CBP in the foamwas varied from 0 to 7.5 g L�1 with 12-HSA concentration xed at 10 g L�1. Even at a low CBPconcentration (0.5 g L�1), we were able to achieve foam

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destabilization by UV irradiation (Fig. 5c). We observed thatfrom concentrations between 0.5 and 7.5 g L�1 of CBP, anincrease in CBP concentration led to an increase in foamdestabilization rate. In order to understand these results, wemeasured the water fraction at ambient conditions for all foamsamples 6 minutes aer foam formation. The water fractionincreased slightly with an increase in the CBP concentration inthe range tested (Fig. SI3c†). This increase in water fraction withincreasing CBP concentration did not a play a large role in thefoam destabilization rate under UV illumination. In this case,the most important factor is the CBP concentration inside thefoam: more particles in the foam lamella and plateau borderslead to faster foam destabilization by local photothermaleffects. These results guide us to a few general conclusions: rst,the foam destabilization rate increases with a decrease in theaverage volume fraction of water in the foam, which is affectedby the fatty acid concentration and viscosity of the continuousphase of the foam; second, the increase of CBP concentration inthe foam leads to an increase in foam destabilization rate.

Generalization of our approach – use of solar irradiation andmagnetic particles

In order to conrm the generality of the technique, we testedfoam samples made with different types of CBP. In all cases, thepresence of CBP inside 12-HSA foams led to rapid foam desta-bilization by UV (Fig. SI4†). As CBPs have high absorptionthroughout the whole UV-Vis-NIR spectrum, we irradiatedfoams containing 12-HSA tubes with and without particles withnatural sunlight. We also placed CBP control foams of identicalcomposition under a black cover at the same location. It isimportant to note that the power of solar illumination(�100 mW cm�2) is almost 4 times lower than the power of theUV light used in the laboratory experiments.46 Remarkably, aer5 hours of solar irradiation at an average outside temperature of16 �C, we observed that only the foams containing CBPexhibited some degree of foam destabilization. The illuminatedfoams not containing CBP remained unchanged (Fig. SI5†). Thenon-illuminated CBP control foam also remained stable aer 5hours of testing. We repeated the solar experiments in triplicateand obtained exactly the same foam destabilization patterneach time. Thus, these ultrastable 12-HSA/CBP foams can alsobe destroyed by natural sunlight.

The synergistic response of 12-HSA tubes and CBP to UVillumination enabled the making of photo-thermoresponsivefoams; furthermore, in order to obtain foams which respond tomultiple stimuli, we extended the above approach by replacingCBP with carbonyl iron (CI) particles similar to the ones usedearlier to obtain magneto-responsive foams.20 Metallic particlesare known to also be intense absorbers of light irradiation andthe use of CI particles in place of CBP still resulted in thedestabilization of the foam system by UV (Fig. 6). In addition, wewere also able to easily destabilize these foams at roomtemperature by the application of a magnetic eld – similar tothat shown in previous work.20 Therefore, by mixing magneticparticles with 12-HSA tubes, we easily obtained the rst photo-thermo-magneto responsive foams.

This journal is ª The Royal Society of Chemistry 2013

Fig. 6 Illustration of the ability of 12-HSA foam with CI particles to respond tothree different external stimuli. Photographs of 12-HSA foams with CI particles (a)before and after (b) an increase in temperature; (c) UV irradiation; and, (d)exposure to a magnetic field.

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The destabilization of fatty acid and CI stabilized foams bymagnetic eld also turned out to be strongly contingent on theage of the foams. The exposure of fresh foams to a magneticeld resulted in the movement of magnetic particles throughthe liquid channels towards the eld source without foamcollapse. On the other hand, aged foam samples rapidlycollapsed toward the source of the magnetic eld (Fig. 6). Thesedistinct behaviours resulted from the difference in watercontent between fresh and aged foams. In fresh foams with CIparticles, the average water content was around 0.2. The thickwater lms in the wet foams allow the particles to migratetoward the source of the eld without disturbance to the foamstructure. In foams aged for 48 h, the average water fraction wasaround 0.08. The reduction of water in the foam during agingleads to thinner lms as well as the jamming of magneticparticles in the plateau borders and lm lamellae. This resultsin lm stretching and rupture when the iron particles movetoward the magnet. Similar effects of water fraction on thecollapse of magnetically responsive foams are discussed indetail in our previous work.37

Conclusions

We report a novel simple approach to generate photo-thermo-responsive foams by combining the thermal reconguration offatty acid tubes with photothermal heating by particles. Thesefoams were sterically stabilized by 12-HSA tubes, but destabi-lized by the transition of tubes to micelles in the foam lm andplateau borders aer a localized change in temperature inducedby light irradiation. This photothermal response can be ach-ieved over a wide range of 12-HSA tube concentrations, CBPconcentrations and under UV or solar irradiation. The foamdestabilization rate can be tuned by changing the fatty acid andCBP concentrations in the foam, as well as the foam age (waterfraction).

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Moreover, the CBP could be easily replaced by magneticparticles, resulting in thermo-photo-magneto-responsivefoams. To our knowledge this is the rst demonstration offoams which can be manipulated by three different externalstimuli: temperature, light and magnetic eld. Thus, ourapproach based on fatty acid surfactants in combination withdifferent types of light absorbent particles, yields systemsexhibiting excellent foamability and stability controllable bymultiple types of external stimuli. These ndings are of interestfrom both fundamental and applied perspectives, since 12-hydroxystearic acid is a green surfactant available in abundantquantities. In addition, the particles used in this study areinexpensive and commercially available. The use of light trig-gering offers signicant advantages over other stimuli such astemperature and magnetic elds since it can be applied from adistance, avoiding direct contact; and, unlike conventionalstimuli like pH and ionic strength, it does not change thesystem composition. This simple system can nd applicationsin diverse industries where stabilization and controlled desta-bilization of foam is desired, such as textile, petrochemical,washing, environmental cleanup and material recoveryprocesses.

At this stage, we can only speculate on the balance of thecontributions from each of the proposed mechanisms to foamdestabilization and how the transition from tubes into micellesaffects foam stability. We demonstrate that the combination ofmany effects at different length scales leads to rapid lamellaerupture and foam collapse. The rapid transition of tubes intomicelles results in drastic changes to the shape of the lamellaeandmenisci in the foam.8 The fragile foam lamellae can rupturedue to bulk or surface disturbances. For example, the process oftransitioning from tubes to micelles in the foam lamellae andPlateau borders can cause local gradients in surface tension,resulting in foam destabilization by Marangoni effects.47 Themechanisms related to the destabilization of these photo-thermo-responsive foams can nd application in a broad rangeof other disperse systems and would be an interesting topic forfuture work.

Experimental sectionMaterials

Experiments were performed using 12-hydroxystearic acid (12-HSA) (Sigma-Aldrich, 75% purity). Carbon black particles (CBP)were a gi from Cabot, Inc. (Billerica, MA, USA). Two types ofcarbon black particles were used: Black Pearl� 880 and Mon-arch� 800. Surface treated iron particles were preparedaccording to Lam et al.,20 by mixing �21 g carbonyl iron (CI)powder (avg. dia. 4.5–5.2 mm, Sigma) with 35 mL solution of0.2 M oleic acid in methanol. The methanol and excess oleicacid were washed away prior to use.

The absorbance spectra of CBP and 12-HSA mixtures weremeasured using a Jasco 550 UV-Vis spectrophotometer (JASCO,Inc., USA) at room temperature between the wavelengths of 200to 900 nm. Samples were placed into quartz cuvettes with a pathlength of 1 cm (Fig. SI6†). The sizes of the carbon black particleswere characterized using transmission electron microscopy

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(TEM, JEOL 2010F, JEOL Ltd., Tokyo, Japan). For both BlackPearl� 880 and Monarch� 800, we observed that nanoparticles20 nm in size were aggregated into larger clusters. These clus-ters most likely represent the morphology of the carbon blackparticles in the foam. The average size of the Black Pearl� 880aggregates was found to be �330 nm, and the average size ofMonarch� 800 aggregates was found to be �320 nm aersample sonication. The TEM samples were viewed at 200 kVacceleration voltage (Fig. SI7†).

Sample preparation

12-Hydroxystearic acid was weighed in a sample tube into whichMilli-Q water (Millipore RiOs 16 RO system, EMD MilliporeCorporation, Billerica, MA, USA) was added to obtain thedesired concentration. Next, we mixed in the desired volume ofa 1 M stock solution of the counter-ion (ethanolamine (Sigma-Aldrich, 99% purity)) to obtain a molar ratio of 2 between thecounter-ion and 12-HSA. The mixture was heated at 80 �C for 15min until all fatty acid solids were dispersed. The samples werethen vigorously vortexed and cooled to room temperature. Thusformed 12-HSA tubules are thermoresponsive and molecularlyassemble into micelles at around 45 �C as shown by Small AngleNeutron Scattering (SANS) measurements (Fig. SI1a†). 12-HSAmolecules assemble into micron-size tubules starting at aconcentration of 1 g L�1, and have the same structure for allconcentrations studied here (Fig. SI1b†).32 In order to preparethe mixture of 12-HSA tubes and carbon black particles, wesimply added the desired amount of particles to the solutioncontaining fatty acid tubules. The mixture was vortexed andsonicated for two minutes.

Foam preparation and characterization

Foam samples were prepared by vigorous handshaking of glasscontainers (16 mm internal diameter, 125 mm height) con-taining 8 mL of the 12-HSA–CBP mixture. The mixture wasagitated for 40 s and all foam samples were produced by thesame operator. The cylinders were sealed except for when thefoam was exposed to UV irradiation.

Testing under UV illumination

All the samples were irradiated from the top with a 100 W UVlamp (OmniCure S1000, Efsen Engineering A/S, Denmark)equipped with a 320–500 nm band pass lter. The foam volumeevolution upon UV irradiation was recorded in digital video usinga Canon EOS 5D Camera with a 100 mm macro lens. Then, byimage analysis using ImageJ soware, the evolution of the foamvolume as a function of UV illumination time was measured.

The change in sample temperature throughout the UV irra-diation process was monitored using a FLIR SC300 infraredcamera, which can measure sample temperatures in thefollowing temperature ranges: �120 �C to 120 �C and 0 �C to350 �C (FLIR Systems, Inc.). The camera has an uncooledmicrobolometer detector and a temperature reading accuracy of�2 �C; and, was used in conjunction with the ExaminIR�soware to produce a thermal map of the samples during foamcollapse.

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Acknowledgements

The authors gratefully acknowledge the U.S. Army ResearchOffice (grant 56041CH) and partially the NSF Research TriangleMRSEC on Programmable So Matter (DMR-1121107) fornancial support of this study. We acknowledge Cabot, Inc.(Billerica, MA, USA) for the carbon black particle samples utilizedin this study. We thank the Laboratoire Leon Brillouin (LLB) forthe allocation of neutron beam time on the spectrometer PAXY.We gratefully acknowledge the assistance of our local contact,Fabrice Cousin, during the neutron scattering run.We also thankDr Arnaud St. Jalmes for the useful discussions.

Notes and references

1 R. K. Prud'homme and S. A. Khan, Foams – Theory,measurements and applications, Marcel Dekker, New York,1996.

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