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Origins of the Complex Motion of Advancing Surfactant Solutions B. Frank and S. GaroP Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received July 8,1994. In Final Form: September 26, 1994@ We have examinedthe fundamental mechanismswhich drive the varied behaviors of advancingsurfactant solutions. We show that surfactantconcentrationgradients and solution mobility are required for dendritic spreading, which occurs when an ionic surfactant spreads on a surface of the same charge. When the surfactant has the opposite charge as the ionized substrate, it can strongly adsorb to the surface ahead ofthe advancing solution. Inhibition of the fluid advance by surfactant adsorption at the contact line alters the dendritic pattern and can change the spreading to stick-jump motion. By tuning the strength of the surfactant-surface interaction, we have observed a spectrum of spreading behaviors from dendritic to stick-jump.
Introduction Due to their surface activity, surfactants dramatically affect wetting and spreading. In fact, they are used to control wettingin many technological Clearly, the ability to control wetting arises from the self-assembly of surfactant molecules at the solid-liquid, solid-vapor, and liquid-vapor interfaces and the attendant changes in interfacial energies. However, the molecular structure of the surfactant assembly and the detailed effect of that structure on wetting remain the topic of extensive scientific intere~t.~-l' Aqueous solutions of different surfactantsexhibitwidely varying wetting behavior. Some surfactant solutions spread to zero contact angle. Other surfactant solutions, on wetting surfaces with a strong affinity for the surfactant, exhibit a high contact angle. This high angle is sometimesattained via autophobing, a process where the solution first spreads across the surface and then retracts. Autophobing occurs because the surfactant adsorbs to the clean surface, lowering the surface energy so that the solution no longer wets the s u r f a ~ e . ~ J ~ J ~ Examining the advancing contact line microscopically reveals even richer behavior. Solutions which autophobe progress across the surface via an unsteady, or "stickjump", motion. The contact line pins to the surface, and the liquid-vapor meniscus deforms. The line then depins and jumps forward. The details of this motion have been Abstract published in Advance ACS Abstracts, December 1, 1994. (1)Leger, L.; Joanny, J. F. Rep. Prog. Phys. 1992,55,431. (2)Neumann, A. W.In Wetting, Spreading, and Adhesion; Padday, J. F., Ed.; Academic Press: New York, 1978;pp 3-35. (3)Johnson, R. E., Jr.; Dettre, R. H. In Wettability; Berg, J. C., Ed.; Marcel Dekker, Inc.: New York, 1993;Chapter 1. (4)Birch, W.R.;Knewtson, M. A.; Garoff, S.;Suter,R. M.;Satija, S. Colloids Surf 1994,89, 145. ( 5 ) Birch, W. R.; Knewtson, M. A.; Garoff, S.;Suter, R. M.; Satija, S. Langmuir, in press. (6)Birch, W.R. The Microscopic and Molecular Structure of Precursing Thin Films of Surfactant Solutions on Silicon OxiddSilicon Surfaces, Thesis, Carnegie Mellon University, Pittsburgh, PA, 1993. (7)Blake, T. D. In Wettability;Berg, J. C., Ed.; Marcel Dekker, Inc.: New York, 1993;Chapter 5. (8)Blake, T. D. In Surfactants; Tadros, Th. F., Ed.;Academic Press, Inc.: New York, 1984. (9)Bose, A. In Wettability; Berg, J. C., Ed.; Marcel Dekker, Inc.: New York, 1993:ChaDter 3. (10)Swalen, J. D.;et al. Langmuir 1987,3,932. (11)Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, Y.; Eilers, J. E.; Chang, J. C. J. Am. Chem. SOC.1991,113,1499. (12)Zisman, W.A. Adu. Chem. 1964,43,1. (13)Novotny, V.J.; Marmur, A. J.Colloid Interface Sci. 1991,145, 355. @
studied as a function of the average speed of the contact line.14J5 The motion is ascribed to the adsorption of a surfactant barrier on the solid surface ahead of the advancing front. As in the picture of macroscopic autophobing, the solution is assumed to be unable to wet this adsorbed barrier and the contact line is pinned. Experiments directly probing this barrier and developing a complete molecular scale explanation of stick-jump behavior have not been performed. Advancing contact lines of some surfactant solutions exhibit dendritic pattern formation.l6-lg The dendrites occur at the contact line of the advancing surfactant solution. The character of the pattern depends on the thickness of pre-existing fluid layers as well as the composition of the solution. Some controversy remains over the necessity of pre-existent fluid films for dendritic spreading. Present models attribute the dendrite formation to Marangoni flow driven by transient surfactant concentration gradients along the liquid-vapor interface at the advancing contact line.18Jg In this paper we demonstrate that the formation of dendriticpatterns requires a repulsive or weakly attractive surfactant-surface interaction and that the same surfactant can exhibit dendritic or stick-jump behavior depending on the substrate. We demonstrate the requirement of a concentration gradient by experiments on surfaces where we quench these gradients. Dendritic spreading does occur on dry surfaces that are sufficiently clean; but it is preceded by fast transient films of fluid spontaneously drawn across the surface. We show that dendritic spreading is halted by surfactant barriers which inhibit formation of this transient film. Finally, we unify stick-jump and dendritic behavior by showingthat tuning the surfactant-surface interaction strength causes a spectrum of behaviors between these two extremes. In the next section we address the experimental procedures and techniques used. We detail the critical cleaning procedures of our surfaces and the accuracy of the optical techniques. We then present our results and discuss the implications of those results for the proposed (14)Cohen Stuart, M. A.; Cazabat, A. M. Prog. Colloid Polym. Sci. 1987,74,64. (15) Princen,H. M.; Cazabat,A. M.;CohenStuart, M. A.; Heslot, F.; Nicolet, S.J. Colloid Interface Sci. 1988,126,84. (16)Marmur, A.; Lelah, M. D. Chem. Eng. Commun. 1981,13,133. (17)Troian, S.M.;Wu, X. L.; Safran, S.A. Phys. Rev. Lett. 1989,62, 1496. (18)Troian, S.M.; Herbolzheimer, E.; Safran, S.A. Phys. Rev. Lett. ISSO,65,333. (19)Elender, G.; Sackmann, E. J.Phys. II 1994,4,455.
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models of dendritic pattern formation. Finally we discuss our observations showing that dendritic and stick-jump spreading are extreme behaviors which can be bridged by the proper choice of surfactant-surface interaction.
Experimental Section We used microscopy, rotating analyzer ellipsometry (RAE), and optical reflectance to study the behavior of advancing surfactant solutions. In this section, we begin by discussing the materials, sample preparation, and the geometry and conditions of our experiments. We then describe our microscopy methods. Finally, we discuss our ellipsometry and reflectance techniques with a detailed discussion of the accuracy of these methods. Sample, Materials Preparation, Experimental Geometry. We explored the spreading behavior of five surfactants: a cationic surfactant, cetyltrimethylammonium bromide (CH3[CH2115N[CH313Br7CTAB);an anionic surfactant, sodium dodecyl sulfate (CH3[CH2l11OS03Na7SDS); three ethylene oxide surfactants, ethylene glycol monododecyl ether (CH3(CH2)110CH2CH20H, El), triethylene glycol monododecyl ether (CH3(CH2)11(OCH2CH2)30H7E3), and hexaethylene glycol monododecyl ether (CH3(CH2)11(OCH2CH2)6OH7E& All surfactants were used as received from Fluka Chemical Corp. (Ronkonkoma, NY).The CTAB was of purum grade, the SDS was puriss grade, and the E, surfactants were assayed at >98%purity. We made aqueous solutions using RO pure water (from a Barnstead Nanopure I1 water system with post-treatment to remove organic impurities) with resistivity >17.5 MQ cm. Unless noted otherwise, all solution concentrations were 0.1 critical micellar concentration (cmc). cmc's for the surfactants are SDS 8 x M,20CTAB low3M,20 9x M,20C12E3 5.5 x M,21and C12E1 9 x M.22 Our substrates were the native oxide on silicon wafers and polished sapphire disks. The silicon wafers were received from Unisil Corp. (Mountain View, CA) and the sapphire disks were received from Hereus Amercil (Sayreville, NJ). Ellipsometry and neutron and X-ray reflectivity show the oxide on the silicon wafers to be about 15-20 A thick with a root-mean-squared roughness less than 5 Our cleaning technique achieved zero contact angle against water and maintained the charged nature of the silicon oxide and sapphire surfaces. The silicon substrates were soaked for 20 min in a saturated solution of potassium dichromate in 36 N sulfuric acid (Fisher ACS grade) and then rinsed in RO pure water. They were then soaked in 6 N hydrochloric acid (Fisher ACS grade)for 20 min and finally rinsed thoroughly with purified water. While still macroscopically wet from the final rinse, a few drops of 1 cmc CTAB solution were placed on the silicon samples and autophobing12to drops with 30-50" contact angles was observed. We then spun the samples dry to remove these drops. The resulting protective autophobed monolayer of CTAB was removed by exposing the samples to light from a W lamp, supplied by BHK, Inc. (Pomona, CAI, for 15-20 min. The resulting samples were dry and were readily wet by water to zero contact angle. We treated the sapphire samples similarly, with the followingexceptions: They were not soaked in the acid solutions but rather exposed for about 10-15 s to prevent any possible etching or roughening of the surface. Further, 1 cmc SDS was utilized as the autophobic solution for the sapphire samples. Substrates were used immediately following these cleaning procedures or were coated by monolayers as described below. The differing behaviors of cationic (CTAB) and anionic (SDS) solutions on our cleaned substrates clearly indicated their ionizable nature (near pH 7,negatively charged for the silicon oxide and positively charged for the ~apphire).2~ First, when a thick water layer was depositedon the cleaned sapphire substrate and a drop of SDS solution was added, the fluid autophobed into droplets. In contrast, when a drop of CTAB solution was added,
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A.
(20) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; National Bureau of Standards, US. Department of Commerce: Washington, DC, 1970. (21) Lu, J. R.; Lee, E. M.; Thomas, R. K.; Penfold, J.; Flitsch, S. L. Langmuir 1993,9,1352. (22) Estimate based upon the values of other ethylene oxides. (23) Mullins, W. M.; Averbach, B. L. Surf. Sci. 1988,206,41.
Figure 1. Sample seen edge-on suspended above a dish of fluid. The holder can translate the sample into the fluid reservoir at controlled speeds. the fluid layer thinned in the center exhibiting a slowly draining film and a thick ridge of fluid around the periphery of the substrate. On the silicon oxide, the CTAB and SDS behaviors were reversed. For some experiments, we coated the silicon substrate with a monolayer of surfactant. This was accomplished by dipping the substrate into a 0.5 cmc surfactant solution and then slowly withdrawing it. For CTAB solutions on silicon oxide substrates, the substrate emerged dry, showing that an autophobed layer had formed. Steady retreat of the solution as the sample was withdrawn and uniform condensation of microdroplets when the coated sample was exposed to saturated vapor indicated uniform depositionof the surfactant layer. This uniformity was confirmed using ellipsometry. Neutron and X-ray reflectivity studies showed that these monolayers were not densely packed, with their hydrocarbon chains containing many gauche ~onformers.~ The contact angle of about 40" against water was also less than that observed for surfaces with densely packed methyl groups.12 For SDS coating of the silicon oxide substrates, the receding surfactant solution exhibited a near zero contact angle and left a slowly thinning film. Careful control of the withdrawal speed gave a uniform horizontal pattern of optical fringes which signify deposition of a uniform surfactant monolayer. The SDS molecules in the deposited monolayer were also not densely packed (-28 A2/molecule),with many gauche conformersin their hydrocarbon chain^.^ Ellipsometric measurements and vapor-condensation tests on these surfaces also indicated a uniform monolayer had been deposited. In our experiments, the substrate was held vertically, either fixed above a dish of surfactant solution, fixed with the bottom edge touching the solution, or advancing into the solution a t a fEed speed (see Figure 1). We performed all our measurements open to the ambient lab environment where the temperature varied from 19 to 23 "C. The relative humidity varied from 15 to 60%;however ellipsometric measurements show that there is little change in water adsorption on our substrates over this humidity range. These variations had no detectable effect on the results reported here. Optical Microscopy. To measure the microscopic structure of the advancing surfactant solution, we imaged the advancing front using reflectance video microscopy.% In these experiments, we drove the substrate into surfactant solution at about 350 p d s . The sample was illuminated with light coaxial with the optic axis of the microscope (see Figure 2). The flat substrate abovethe advancing front reflected light back into the microscope, appearing bright in the resulting images. The curved liquidvapor interface reflected light away from the microscope, appearing dark in the images. We utilized magnifications from 2 to 50 p m l p i ~ eto l ~examine ~ the spreading behaviors at several length scales. Careful lighting conditionswere critical to produce images with sufficient contrast to analyze the structure of the advancing contact line. The images were recorded on videotape and were analyzed frame by frame, providing a time resolution of V30th of a second. (24) Nadkarni, G. D.; Garoff, S. Langmuir 1994,10,1618. (25) A pixel is the imaging element of our CCD camera, with 640 by 480 pixels composing the entire array.
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E
Figure 2. Schematic diagram of optics for reflectance microscopy: S, sample; F, fluid; L, light source; H, half-silvered mirror; M, microscope; C, camera. Dashed arrows are incident light rays. Solid arrows are reflected light rays. Rotating Analyzer Ellipsometry and Optical Reflectance. We used RAE and optical reflectance to probe changes in solution film thickness. For both techniques, our instrument was a modified Gaertner Auto-Gain ellipsometer, L104B (Gaertner Scientific Corp., Chicago, IL), utilizing a He-Ne laser (wavelength6328A). Our beam size limited the spatial resolution to about 1mm (vertical) x 1.5 mm (horizontal) and the rotating analyzer allowed measurements up to 0.5 Hz. The ellipsometer was set for an angle of incidence of 70.00 f 0.05", which is near the 76" principal angle of incidence for a silicon crystal with its native oxide.26 For RAE, the substrate was fxed with its bottom edge 250pm above the surface of the bulk solution. We probed changes in film thickness near the bottom edge of the sample. For the films studied, only the change in the ellipsometric parameter A is sensitive to film thickness changes.27 The error of f0.05" in A, dominated by electronic noise, was determined by repeated measurements of a single static sample. This corresponds to an error in thickness of an equivalent water film of 0.3 A. As discussedbelow, no reasonable variation in parameters describing the substrate and film affected the measurements of changes in film thickness. For optical reflectance measurements we brought the sample into contact with the solution and probed the film about 1mm above the macroscopiccontact line of the resulting meniscus. To perform these measurements the rotation of the analyzer of the RAE was stopped and the analyzer was set to null the signal from the bare clean substrate. Intensity measurements were triggered at up to 100 Hz, allowing us to study faster film growth phenomena. The minimum intensity signal occurred at an analyzer setting within 0.1" of the value predicted for our substrates. As the fluid films formed on the surface, changes in the optical reflectance were measured. We utilized a standard model for our substrate to convert the change in reflected intensity measured to a film thickness. In our modelingwe fixed the index of the silicon(n= 3.858 0.018i)28 and the oxide (n = 1.49).29 To account for the values of both ellipsometric parameters Y and A, we required an index grade between the silicon crystal and the native oxide. For simplicity we approximated a linear grade by six equally thick layers. We allowed the total thickness of the grade and of the outer oxide layer to be fit as adjustable parameters. Any adsorbed water or other foreign material on the bare surfaces measured could be included in the outer oxide layer without appreciable error in the measurement of changes in film thickness. By measuring several samples, we found that the grade was about 6 f2 A with the outer oxide being 14 f2 A. Given this model, we then treated any changes in intensity as growth of a water layer (n = 1.33) on this substrate. Possible variations in this model, as well as fluctuations in the signal, lead to uncertainties of about 20% in our film thickness measurements with this technique.
+
(26)Archer, R. J. Manual on Ellipsometry; Gaertner Scientific Corporation: Chicago, IL, 1968. (27) See, for example: Thompkins, H. G. A User's Guide to Ellipsometry; Academic Press, Inc.: New York, 1993. (28)Tompkins, H. G.A User's Guide to Ellipsometry;AcademicPress, Inc.: New York, 1993; p 213. (29)Within the range of literature values. See, for example: Tompkins, H. G. A User's Guide to Ellipsometry; Academic Press, Inc.: New York, 1993. Palik, E. D. Handbook of Optical Constants of Solids; Academic Press, Inc.: New York, 1985. Chandler-Horowitz, D.; Candella, G. A. Appl. Opt. 1982,21, 2972.
Figure 3. Stick-jump motion of CTAB solution on silicon oxide: (a) contact line immediately before a jump; (b) wave propagating across the contact line from right to left during the jump.
Results and Discussion Our experiments systematically probe the models of dendritic spreading of surfactant solutions on solid surfaces. Further, they show a continuousbridge between the dendritic behavior exhibited by some surfactant solutions and the stick-jump behavior of others. Within this section,we first demonstrate that the same surfactant solution exhibits either dendritic or stick-jump spreading dependingon the surfactant-surface interaction. We then present results showingthat dendritic spreading requires surfactant concentration gradients along the advancing liquid-vapor interface. Next, we show that precursing films of fluid are present for dendritic spreading and that such films spontaneouslyform on nominally dry substrates if the substrates are sufficientlyclean. Then, we describe experiments demonstrating that this precursing film is stopped by surfactant barriers spontaneously formed in front of advancing solutions of surfactants which exhibit stick-jump rather than dendritic spreading. Finally, we discuss our observationsof the spreading of ethylene oxide surfactants, which show a clear transition from stickjump to dendritic spreading. Surfactant-Surface Interactions. Stick-jump spreading occurs when the interaction between the surface and the surfactant head group is strongly attractive; dendritic spreading occurs when the interaction is repulsive. Drops of 0.1 cmc CTAB solution exhibit autophobic behavior when placed on silicon oxide. Carefully placed drops show an increase in contact angle of 5-10' over the course of about 30 s. The final angle is about 25". When a vertical substrate is forced into such a solution, we observe stick-jump behavior similar to that seen by others.14J5 The contact line moves by pinning to the surface, moving down with the surface, depinning from the surface, and jumping upward (see Figure 3). At 0.1 cmc, SDS exhibits a 0" contact angle on the silicon oxide,
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time (sec) Figure 5. Reflected intensity over time for a wafer touched to 0.5 cmc SDS solution (a), pure water (b), and 0.5 cmc CTAB solution (c). An intensity change of 0.01 is about 10A. Signals have been offset for clarity.
Figure 4. Dendritic spreading of SDS solution on silicon oxide: (a) beginning of the dendritic spreading; (b) after 4 s near the conclusion of dendritic formation. Note the interconnected branches.
and dendriticspreading,such as shownin Figure 4 (similar to that described by others17J9),is observed for a dry, vertical substrate brought into contact with a reservoir of SDS solution. We observe qualitatively similar patterns for SDS solutions with concentrations up to several times the cmc. The SDS and CTAB solutions reverse their behavior when the charge ofthe substrate is reversed. On sapphire, CTAB solutions exhibit low contact angles and dendritic spreading while similar concentrations of SDS solutions exhibit autophobicbehavior and stick-jump motion. The dendritic patterns formed by CTAB on sapphire are qualitatively similar to that of SDS on silica, and the autophobingof SDS on sapphire is similar to that of CTAB on silicon oxide. Thus, the behavior of an advancing solution is not a property of the surfactant but rather a property of the head group-surface interaction. Autophobic behavior, nonzero final contact angles, and stick-jump spreading occur when the head-surface interaction is attractive, i.e., for the cationic CTAB on silicon oxide which is negatively charged near pH 7 and for the anionic SDS on sapphire which is positively charged near pH 7. Final contact angles of 0" and dendritic spreading occur when the head-surface interaction is repulsive, i.e., for CTAB on sapphire and SDS on silicon oxide. Surface ConcentrationGradient. Our experiments provide evidence that dendritic spreading requires a gradient of surfactant concentration along the liquidvapor interface of the advancing solution front. In two experiments we observe that SDS precoated on the solid surface suppresses dendritic spreading. (1)On a clean, bare substrate, dendritic spreading occurs on first contact with the solution. The dendrites propagate up the surface and then retreat down the surface. Ellipsometric measurements show that the receding dendrites leave an
uneven surfactant layer behind. If the substrate is immersed further into the solution, dendrites do not form until the contact line reaches a position not previously contacted by the dendrites and coated by surfactant. (2) SDS solutions spread smoothly to a 0" contact angle over a silicon oxide surface precoated with a uniform monolayer of SDS. No dendrite formation is observed. Neutron and X-ray reflectivity studies show that our precoated surface has a higher SDS concentration than the bulk liquid-vapor interfa~e.~*~ These studies also show that preadsorbed SDS is rapidly desorbed from the silicon oxide surface to the liquid-vapor interface of the film ahead of the bulk meniscus. Thus, SDS molecules deposited on the solid surface before the advance of the solution (either by previous dendrites or designed preadsorption)desorb,removing concentrationgradients on the liquid-vapor interface and thereby inhibiting dendritic spreading. Fluid Mobility. We observe that, while the substrate need not be prewet by a fluid film, mobility of water ahead of the advancingfront is necessary for dendritic spreading. Although our experiments are carried out in ambient humidity where the amount of water adsorbed to the silicon oxide is on the order of a m ~ n o l a y e r ,our ~ ~optical *~~ reflectivity studies show a rapid, precursing film of water spreading across the surface ahead of the dendrites of solution. Figure 5 shows the temporal changes in optical reflectivity of surfaces ahead of an advancing meniscus on a vertical substrate. For a clean, dry silicon oxide surface, a film appears in front of the laser beam shortly after touching the bottom of the surface to a bulk water bath. At the position of the monitoring laser beam (about 1mm above the final meniscus location for the data in Figure 5), the film achieves a maximum thickness on the order of 150 A (assuming the film has the refractive index ofwater) and lasts for several seconds. Af'ter the maximum film thickness is reached, the film at the position of the laser beam thins leaving residual material -2 A thick. The maximum thickness of the transient film decreases (30)Gee, M.SurfaceForcesin Thin Liquid Films on Quartz. Thesis, University of Melbourne, 1987. (31)Pashley, R. M.; Kitchener, J. A. J. Colloid Interface Sci. 1979, 71,491.
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as regions farther fromthe final meniscus are probed. For of ellipsometric parameter A as changes in film thickness a clean, dry surface brought into contact with SDSsolution, due to the swelling of the films by water. A CTAB layer a similar f l m appears but is followed by the dendrites on a silicon oxide substrate swells about 0.9 f 0.3 A. In passing in front of the laser beam. contrast, an SDS layer swells 3.1 f0.6 A, 3-4 times more Even when a concentration gradient of surfactant is than Cl'AB, adsorbing an amount of water comparableto present along the liquid-vapor interface, dendritic spreadthat adsorbed onto a bare, uncoated substrate under the ing cannot occur without the presence of water on the same conditions. Even under saturated water vapor surface ahead of the advancingfront. Using ellipsometric conditions,previous ~ t u d i e shave ~ - ~ shown that the strong measurements, we have seen that the uneven monolayer attractive interaction between CTAB head groups and of SDS adsorbed to the solid surface by the propagation the silicon oxide surfaces only allow 3-4 A of water to and retreat of dendrites does not diffuse across the ambient enter the head group region of a preadsorbed CTAB surface on time scales relevant to spreading processes monolayer. Under similar conditions, the repulsive addressed here. This is true even when the surface is interactions between SDS and silicon oxide cause a still attached to a bulk meniscus. However, we can cause preadsorbed SDS monolayer to desorb and a water film, the dendritic spreading to resume into a clean area of the over 100 A thick, to form between the surface and substrate by blowing water-saturated air onto the surface surfactant mon~layer.~ Thus, our experiments show that at and ahead of the position of previous dendrites. As a water is strongly inhibited from entering barriers of microscopic f l m of water condenses, the dendritic spreadadsorbed CTAB molecules. This restriction of water ing resumes. No dendritic spreading was observed when mobility in barriers formingahead of an advancing CTAB air saturated with water vapor condensed on a surface solution prevents the formation of the dendrites which uniformly precoated with an SDS monolayer. would otherwise form due to the existing surfactant Thus, dendritic spreading requires both a surfactant concentration gradient during the unsteady motion of the concentration gradient and water to mobilize the surfacCTAB contact line. tant. However, the water does not have to be present We confirm the inhibition of dendritic spreading by a before the surfactant solution attempts to spread. The CTAB barrier by observing that a CTAB coating will stop clean, high-energy oxide surface drives the spreading of the dendritic spreading of an advancing SDS front. We ultrathin precursing films which provide the mobility of coat half of a silicon oxide surface with a CTAB monolayer fluid needed for dendritic spreading. and allow SDS to advance across the other half. The Suppression of Water Mobility and Dendritic dendritic spreading of the SDS solution proceeds normally Spreading. CTAB spreading on dry silicon oxide shows until it reaches the area coated with CTAB. At this point, stick-jump behavior rather than dendritic spreading (see it stops along the sharp line correspondingto the boundary Figure 3). Further, we do not observe the transient fluid of the CTAB layer. films seen for pure water or SDS solutions on silicon oxide Transition Behaviors. We demonstrated above that (seeFigure 5). Barriers of surfactant molecules,previously the strong adsorption of CTAB ahead of the advancing postulated to adsorb ahead of the meniscus of a CTAB meniscus stops dendrite formation. The contact angle is solution,16 could suppress these transient films. To also affected by the strength of adsorption to the solid investigate this possibility, we first explored the barriers We can tune the surfactant-surface interacof CTAB molecules produced upon autophobingof a CTAB tion, and thus the adsorption strength, by using dipolar solution. The barriers cause a nonzero contact angle for surfactants instead of ionic surfactants. We used El, EO, both water and the solution itself. The effect of this barrier and E6 to span the strength of the surface-surfactant on macroscopic menisci of water is explicitly seen by interaction between the repulsion of SDS and the strong observing a drop of pure water spreading toward an attraction of CTAB. The contact angles of 5-pL drops of autophobed, stationary drop of CTAB solution on a 0.1 cmc solutions on silicon oxide increase from SDS (0") horizontal, silicon oxide surface. The water contact line through E l (
