“Micro-Pottery”sMarangoni Effect Driven Assembly ofAmphiphilic Fibers

Janhavi S. Raut,* Pradeep Bhattad, Aditi C. Kulkarni, and Vijay M. Naik

Unilever Research India & Hindustan Lever Research Centre, 64 Main Road,Whitefield, Bangalore-560066, India

Received May 17, 2004. In Final Form: August 6, 2004

We report spontaneous supra-assembly of fibrous surfactant crystallites at the air-solution interfaceresulting in spectacular arrays of two-dimensional spiral and three-dimensional “micro-pottery”-likesuperstructures. Surface pressure differential driven bending of the embryonic fiber nuclei and Marangoniconvection driven fiber migration/alignment appear to be the causal factors behind this phenomenon. Theassemblies form at specific crystal-growth velocities dictated by the relative time scales for fiber bending/alignment and their rigidification/immobilization as they grow. Although our studies are restricted to aspecific class of amphiphiles, namely, alkaline metal salts of linear fatty acids, the phenomenon shouldbe generic to amphiphilic molecules that crystallize into flexible fibers.

Nature is replete with examples of spontaneouslyformed, spectacular shapes of crystalline supramolecularassemblies, arising solely out of either the intrinsic packingcharacteristics of molecules or their combination withpatterns of external energy and mass transport. Am-phiphilic molecules in particular organize themselves intoa range of beautiful liquid crystalline or crystallineforms.1-6 Crystallization of one class of amphiphiles,namely, alkaline metal salts of linear fatty acids, havebeen the subject of a large number of scientific studies,for nearly a century, because of their ready availabilityand commercial importance.7-9 It is well documented thatthey crystallize from aqueous solutions in the form of flatplatelets, ribbons, or twisted fibers.9-11 Despite being asubject of such exhaustive study, we report for the firsttime observations of harmonious intricate designs ofpottery-like tertiary structures formed spontaneously bycrystalline fibers of one typical member of the salt family,namely, sodium myristate (NaMy). In addition to itsaesthetic charm, we believe that once the underpinningfactors and mechanisms are fully understood, the phe-nomenon can be harnessed for potential applications infabrication of encapsulates, microporous membranes,templates for microelectronics, and so forth.

When an isotropic aqueous solution of NaMy is cooledfrom a temperature well above its Kraft boundary (59°C),9 ribbonlike flat fibers are seen to crystallize out ofsolution.10,11 The fiber structure that has been well

characterized in the past is lamellar in nature with a dspacing of 36.2 Å as obtained from X-ray diffractionstudies.11 Time-resolved microscopy experiments haveshown that the fibers first grow in length and then inthickness.9 When the concentration of NaMy is in excessof 5% (w/w), the crystallized fibers are seen to form asolidified random network (“gel”) having a pore space filledwith the equilibrated solution phase.11 The tertiary fiberassemblies were observed while carrying out detailedstudies on the NaMy microstructures, crystallized underdifferent conditions. Figure 1a-c shows surface micro-graphs of 10% NaMy solutions deposited on a glass slidecooled to ambient temperature at varying rates. At acooling rate of 4 °C/min a random network of fibers wasseen (Figure 1a). However, when the cooling rate waslowered to 2 °C/min, we started to see some unusualphenomena (Figure 1b). The fibers now tended to alignthemselves into bundles, as well as showed bending andcurling. Some fibers formed circular ring structures. Whenthe cooling rate was further reduced to 1 °C/min, a neatarray of close-packed spiral patterns of fibers was observedin the micrographs (Figure 1c). Thus, changing the cooling

* To whom the correspondence should be addressed. E-mail:janhavi.raut@unilever.com.

(1) Yan, D.; Zhou, Y.; Hou, J. Science 2004, 303 (5654), 65-67.(2) Dubois, M.; Deme, B.; Krzywicki, T. G.; Dedieu, J. C.; Vautrin,

C.; Desert, S.; Perez, E.; Zemb, T. Nature 2001, 411, 672-675.(3) Zhang, S. Nat. Biotechnol. 2003, 21, 1171-1178.(4) Zastavker, Y. V.; Asherie, N.; Lomakin, A.; Pande, J.; Donovan,

J. M.; Schnur, J. M.; Benedek, G. B. Proc. Natl. Acad. Sci. U.S.A. 1999,96, 7883-7887.

(5) Fuhrhop, J.-H.; Helfrich, W. Chem. Rev. 1993, 93, 1565-1582.(6) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A.

N. Science 1989, 245, 1371-1374.(7) Madelmont, C.; Perron, R. Colloid Polym. Sci. 1976, 254, 581-

595.(8) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants;

Academic Press: San Diego, CA, 1994.(9) Darke, W. F.; McBain, J. W.; Salmon, C. S. Proc. R. Soc. London,

Ser. A 1920, 395-409.(10) Trager, O.; Sowade, S.; Bottcher, C.; Fuhrhop, J. H. J. Am. Chem.

Soc. 1997, 119, 9120-9124.(11) Liang, J.; Ma, Y.; Zheng, Y.; Davis, T. H.; Chang, H. T.; Binder,

D.; Abbas, S.; Hsu, F. L. Langmuir 2001, 17, 6447-6454.

Figure 1. Parts a-c show SEM surface micrographs of a 10%NaMy solution deposited on a glass slide and cooled at (a) 4,(b) 2, and (c) 1 °C/min. Scale bar ) 50 µm. (d) Cross-sectionalview of the surface pattern, scale bar ) 20 µm.

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rate resulted in transition from a random network to awell-organized spiral self-assembly of fibers at the freesurface. A cross section of the surface showed that eachof the surface spirals extended about 10-15-µm deep intothe bulk (Figure 1d), forming a bobbin-like structure withfibers tightly wound around it. The bulk region underneathshowed the presence of a random fiber assembly. Thus,the fiber assembly appeared to be a pure surface phe-nomenon. Image analysis of the surface spiral structureshowed that the spirals had an average hole of 1.2-µmdiameter (with a standard deviation of 0.5). The spiralassemblies could be obtained for concentrations rangingfrom 5-30%. Higher concentrations close to the liquidcrystalline region were not studied. At lower concentra-tions of NaMy (<2%), although fiber bending and ringformation was observed at low cooling rates, the well-packed array of spirals could not be obtained.

In addition to the two-dimensional assembly, it wasobserved that some of the spirals extended out into theair space, creating an array of three-dimensional micro-scopic pottery-like tertiary structures (Figure 2a). Thedensity of these three-dimensional structures was seen toincrease when a vacuum was applied in the air space(Figure 2b). This was done by sealing the vial containing50 vol % of the hot solution. On cooling as the air contracts,a suction force is created at the air-solution interface.Figure 2c shows a close-up of one such micro-potterystructure. The individual winding of the fibers is clearlyvisible in the “cup-and-saucer”-like arrangement. To studythe effect of surface curvature, air bubbles were introducedinto the hot NaMy solution. Figure 2d shows the insidesurface of an ∼100-µm diameter air bubble trapped in thesolidified “gel” on cooling. The two-dimensional and three-dimensional fiber assemblies are visible at the innersurface of the air bubbles. The assemblies were unaffectedby the curvature of the air-solution interface or itsorientation with respect to gravity, indicating that gravi-tational forces did not play a role in the self-assembly. InFigure 3, we present a collage of a variety of attractiveshapes of the spontaneously formed micro-pottery struc-tures that were observed at the air-gel interface. Thesestructures, typically 10-20 µm in size, displayed a varietyof shapes and surface-finish characteristics. Similar fiberassemblies were observed in homologues of NaMy. How-ever, at comparable solidification rates sodium stearateand palmitate showed a lesser propensity to form the spiralstructures as compared to myristate and laurate. Thestructures have been observed in both dehydrated (scan-ning electron microscopy, SEM) and hydrated samples(cryo-SEM). The structures in the hydrated samples werealso observed using optical microscopy (400× in reflectancemode).

With the objective to understand the genesis of thesupra-assembly, time-resolved microscopy studies of theNaMy crystallization were carried out. At solution con-centrations of 10% (w/w) and cooling rates of 4 °C/min,only straight fibers were seen to appear suddenly in thefield of view. However, when the cooling rate was loweredbelow 2 °C/min, fibers in preformed ring shapes appearedduring the initial stages of crystallization (Figure 4a).

Figure 2. (a) Emergence of three-dimensional micro-potterystructures at the free surface and (b) enhanced density of thestructures on application of a vacuum. The surface folds arebecause of drying; scale bar ) 200 µm. (c) Zoomed view of athree-dimensional micro-pottery structure; scale bar ) 10 µm.(d) Spiral patterns and “micro-pottery” structure on the innersurface of an air bubble in NaMy; scale bar ) 50 µm.

Figure 3. Collage of various “micro-pottery” structures observed at air-gel interface; scale bar ) 10 µm.

Letters Langmuir, Vol. 21, No. 2, 2005 517

The process of ring formation by the fiber, thus, could notbe resolved because the rings emerged abruptly in thefield of view as the fiber thickness grew and became largeenough to be visible under the optical microscope.

Wepropose that theobservedphenomenon is a combinedresult of a number of factors coming together under theexperimental conditions: (i) initiation of crystallizationat the interface, driven by excess concentration of surface-active molecules at the interface; (ii) flexing of theembryonic filaments under the influence of Brownianprocesses and on interactions with neighboring fibers; (iii)closure of bent filaments into rings at the air solutioninterface under the influence of surface pressure dif-ferentials; (iv) surface pressure gradient and Marangoniflow driven migration/alignment of filaments leading toformation of parallel assemblies and winding of thefilaments around closed rings residing at the interfaceleading to the emergence of spiral/bobbin-like superstruc-tures; and (v) squeezing out of some spirals into the vaporspace under a pressure differential across the interfaceresulting in the generation of the micro-pottery structures.Phenomena i and ii have been studied before.12,13 Theabsence of well-packed spirals at low surfactant concen-trations probably results from a relatively lower densityof the embryonic fibers that do not see each other and,hence, continue to grow straight over longer distances.

The closing-in of bent flexible filaments to form ringstructures under the influence of surface pressure gra-dients is an unreported phenomenon. It can be visualizedto happen when a partially bent filament at the air-solution interface is acted upon by differential surfacepressure arising out of asymmetric surface concentrationprofiles of the crystallizing surfactant across the filament14

(Figure 4b). Ring closure can take place if the bendingmoment due to differential surface pressure exceeds theresistance offered by the filament to bend, that is,

where ∆P is the difference in surface pressure (N/m) acrossthe filament, E is the bending modulus (N/m2), w is thefiber width (m), and R is the radius of the nascent ring(m). κ is an appropriate shape factor. Because R ) L/2π,where L is the fiber length, the relationship qualitativelyshows that crystals with only small “w/L” ratios can formrings. For a surface pressure differential of 30 mN/m,assuming κ ) 1 and R ) 10 µm, the ring formation wouldcease when the fiber thickness grows to ∼0.3 µm, for aconservative bending modulus E of 100 MPa. It is, thus,apparent that the evolution/development of the spiralstructures is frozen into a latent pattern long before thefilament thickness grows large enough for visibility. Thewell-developed assemblies at slower cooling rates can,thus, be attributed to the longer time available for ringformation before w increases causing the fibers to becomeinflexible. Similarly, the lower propensity of the stearateand palmitate molecules to form the spirals can also beascribed to the higher intrinsic w/L ratios of the fibers,causing higher stiffness. The rings once formed providethe template for further development of the spiralsuperstructure. The surface concentration gradients con-tinue to create stress imbalances of the diffuso-capillarytype resulting in migration of the individual crystallites.Numerous examples of surface/bulk movements causedby local differences of surface tension (Marangoni effects)during crystal growth/solubilization or evaporation arecited in the literature,15-18 the earliest reported phenom-enon probably being the camphor dance, dating back to1686.19 The surfactant depletion during crystallization inthe region between preformed rings or fibers causes themto move laterally toward each other in a self-organizingmanner (Figure 4c,d). The stresses can also generateconvective flows at the surface.19 These flows could leadto a swirling motion of the solution around isolated ringsoriginating from the centripetally directed flows resolvinginto downward and tangential components on encounter-ing the rings. The cross-sectional view of the surfacepatterns (Figure 1d) indicates that the surface swirls canpenetrate about 10-15 µm into the bulk. The flows causealignment of the filaments along the streamlines whereinthey grow further from the aligned state. The creation ofthree-dimensional “micro-pottery” structures that extendout from the spiral surface patterns could be attributedto two factors, namely, the normal capillary stress on thering structures resulting in a compressive action on the

(12) Ries, H. E. Nature 1979, 281 (5729), 287-289.(13) Isambert, H.; Maggs, A. C. Macromolecules 1996, 29, 1036-

1040.

(14) Mikhailov, A.; Meinkohn, D. In Stochastic Dynamics;Schimansky-Geier, L., Poschel, T., Eds.; Springer: Berlin-Heidelberg,1997; pp 334-345.

(15) Ostrach, S. J. Fluids Eng. 1983, 105, 5-20.(16) Berg, J. C.; Boudart, M.; Acrivos, A. J. Fluid Mech. 1966, 24 (4),

721-725.(17) Wang, C. P.; Liu, X. J.; Ohnuma, I.; Kainuma, R.; Ishida, K.

Science 2002, 297 (5583), 990-993.(18) Daniel, S.; Chaudhury, M. K.; Chen, J. C. Science 2001, 291

(5504), 633-636.(19) Scriven, L. E.; Sterling, C. V. Nature 1960, 187, 186-188.

Figure 4. Plate (a) presents a series of time-resolved trans-mission optical micrographs captured during crystallization ofNaMy (10% w/w) showing emergence of preformed rings in thefield of view, at a cooling rate of 1 °C/min; scale bar ) 50 µm.Plates (b)-(e) show our conjectures regarding the process ofthe formation of rings and multifiber assemblies as a result ofsurface pressure gradients. The colored frames are schematicconcentration maps wherein red indicates high concentrationand blue indicates low concentration. The black arrows indicatethe direction of the net surface pressure driven forces. Plate (b)shows the process of closure of a fiber bent to a critical curvaturedue to centripetal forces acting on it. Plates (c) and (d) illustratethe self-organization of crystallites (rings or fibers) due to thesurfactant depletion between them as they come sufficientlyclose to each other during crystallization. Plate (e) illustratesthe squeezing out of assembled concentric rings into vapor spaceresulting in three-dimensional structures.

(∆P/wE)κ g (w/R)2

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inner rings (Figure 4e) and pressure differentials createdacross two sides of the interface during cooling, due todifferences in the thermal expansion coefficients (water) 1.53 × 10-4 K-1 and air ) 3.5 × 10-3 K-1). Parts of thespiral structures that are weak get squeezed/sucked outinto the vapor space resulting in the three-dimensionalstructures. The variety in the shapes of the three-dimensional structures could be a consequence of thevarying location of the weak ring junctions that getdisplaced first, subsequently drawing out the rest of thestructure.

We believe that this is the first sighting of suchspontaneous emergence of tertiary assemblies of surfac-tantmolecules.Our hypothesis for the fiber assemblybeingright, the phenomena should be generic to amphiphilescrystallizing into fibers (with a high dependence of thesurface tension, γ, on the surfactant concentration, C; i.e.,high |δγ/δC| in the relevant concentration range) underappropriate conditions of crystal growth rate and fiberflexibility. Clearly the influence of the Marangoni effecton an ensemble of crystallizing fibers is far more dramaticthentherandomcamphordance19 or tinwaltz20,21 exhibitedby dissolving isolated particles as reported earlier. Po-tential applicability of this phenomenon in the fabricationof the microporous membrane and micro-capsules forcontrolled release can be easily visualized. Like the tubule

structures formed by chiral amphiphiles,22 the micro-pottery structures too should be amenable to metal plating,opening up new opportunities in microelectronics. Thephenomenon may also provide new leads for elucidatingmechanisms of pattern formations encountered at phaseboundaries in biological systems (e.g., spiral ridge devel-opment on fingertips).23,24 Although we attempt to proposea plausible hypothesis to explain the phenomenon, ques-tions such as (i) what are the necessary and sufficientconditions for the tertiary fiber assemblies to develop, (ii)how can the shapes and ordering of the patterns becontrolled, and so forth remain unanswered. Furtherexperimental and theoretical probing is warranted on thisfront.

Acknowledgment. The authors would like to ac-knowledge Dr. Hari Koduvely and Smita Gadre of UnileverResearch, India, for the useful discussions on crystalgrowth and morphology of surfactant systems.

Supporting Information Available: Materials andmethods, SEM surface micrographs of NaMy microstructures,and spiral patterns and three-dimensional “micro-pottery”structure on the inner surface of an air bubble in sodium myristateand sodium laurate. This material is available free of charge viathe Internet at http://pubs.acs.org.

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(20) Wilson, E. Chem. Eng. News 2000, 78 (48), 4.(21) Schmid, A. K.; Bartelt, N. C.; Hwang, R. Q. Science 2000, 290

(5496), 1561-1564.

(22) Schnur, J. M. Science 1993, 262, 1669-1676.(23) Penrose, L. S. Br. Med. J. 1968, 2, 321-325.(24) Jain, A. K.; Prabhakar, S.; Pankanti, S. Pattern Recognition

2002, 35 (11), 2653-2663.

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