Unconventional Multiple Ring Structure Formation from Evaporation

Jul 2, 2012 - single ring deposit observed in a coffee ring stain.1 In this case, a nonvolatile solute is transported to and trapped at the pinned con...
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Unconventional Multiple Ring Structure Formation from Evaporation-Induced Self-Assembly of Polymers Wuguo Bi, Xiangyang Wu, and Edwin K. L. Yeow* Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371 S Supporting Information *

ABSTRACT: The formation of multiring deposits of poly(2vinylpyridine) (P2VP) from the evaporation of a P2VP-(2,6lutidine + water) drop on a glass substrate does not conform to the conventional pinning−depinning mechanism. Instead, ringlike deposits are formed when the droplet undergoes several cycles of spreading and receding where, for each spreading event, a P2VP ridge is formed at the contact line when the polymer flows toward the outward advancing edge. The complex interplay between an outward solutal-Marangoni flow due to a higher concentration of the polymer at the contact line and an inward solventMarangoni flow arising from the differences in volatilities and surface tensions of the pure solvent components plays an important role in enhancing the droplet spreading rate. The newly discovered surface patterning mechanism has important implications in the development of novel techniques for inducing self-assembly of functional materials from evaporating drops.

1. INTRODUCTION The evaporation of a sessile drop is a simple, convenient, and cost-effective method of creating self-assembled patterns of functional materials (e.g., polymers, semiconductor particles, and biological systems such as DNA) on solid substrates. The self-organization of the solute, upon drying of the sessile drop, has been demonstrated to give rise to different pattern morphologies including the well-known single coffee ring stain,1,2 multiring structures3−6 and uniform film deposits.1,2 Achieving a homogeneous and well-defined distribution of functional materials is highly important in the biomedical,7 nanotechnology,8 and inkjet printing industries.6,9−11 Therefore, a clear understanding of the factors that control the solute patterning process is necessary. Deegan et al. have proposed a physical model to explain the single ring deposit observed in a coffee ring stain.1 In this case, a nonvolatile solute is transported to and trapped at the pinned contact line (CL) of a stationary drying droplet by an outward capillary flow arising from both CL pinning and enhanced solvent evaporation at the edge of the sessile drop. On the other hand, multiple rings are observed when the CL undergoes a series of pinning−depinning events with the solute flowing toward the edge and accumulating at the CL when it is pinned (i.e., “stick”).3−6 Depinning occurs when either a dominating inward capillary force or an evaporation-induced rupture of the solvent at the edge (or close to it) causes the CL to “slip” to a new position such that a new solute ring is subsequently formed. The repeated pinning and depinning cycles give rise to a continuous stick−slip motion that is responsible for the formation of multiple solute rings. In particular, Adachi et al. have examined the surface morphology of concentric stripe patterns created upon drying of an aqueous drop containing © 2012 American Chemical Society

polystyrene particles (PS) and have proposed that the stick− slip motion is periodic.3 Subsequently, Shuylovich et al. revisted the mechanism behind the formation of multiple PS rings and have shown that the sequence of pinning−depinning events is stochastic, rather than periodic, in nature.4 More recently, Maheshwari et al. investigated the stick−slip motion involved during the evaporation of aqueous drops of DNA and have found that after CL depinning, the receding edge is repinned at a point where precipitation of the DNA has occurred.5 The evaporation of solutions trapped within the confined space of a sphere-on-flat geometry to yield surface patterns has also been widely reported.12,13 In this case, the spontaneous formation of well-ordered and periodic concentric rings results from the controlled and repetitive pinning and depinning of the CL.12,13 Apart from experimental studies, theoretical modeling has also been employed to examine the formation of multiple ring patterns arising from the stick−slip motion.14−16 The study of polymer deposition from evaporating droplets has recently attracted a lot of attention.6,17−23 Several types of polymer pattern on surfaces have been reported including ringlike structures, craterlike structures, dotlike structures, and films with constant thickness. Poulard and Damman have shown that by using different solvent−polymer pairs, various morphologies of solute deposit can be achieved.18 In particular, when the polymer accumulates at the droplet edge, a solute concentration gradient is created between the peripheral and interior of the drop which subsequently drives a Marangoni flow capable of moving the polymer either away (outward) or Received: February 17, 2012 Revised: July 1, 2012 Published: July 2, 2012 11056

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to react under stirring for 4 h. The labeled P2VP was precipitated out by adding the DMSO P2VP solution into DI water and recovered by centrifugation. The procedure for removing free dyes is as follows: the recovered P2VP was first dissolved in MeOH and then precipitated out by adding the MeOH P2VP solution into DI water before extraction by centrifugation. The free dye removal procedure was repeated at least three times. 2.3. Sample Preparation. Binary mixtures of DI water and 2,6lutidine in various compositions (i.e., wt % of 2,6-lutidine:wt % of water = 60%:40%, 70%:30%, and 90%:10%) were used in the widefield microscopy and confocal fluorescent experiments. Different initial concentrations of P2VP solutions (i.e., 0.0002, 0.001, 0.002, and 0.008 g of P2VP dissolved in 1 g of solvent) were prepared by dissolving the labeled and unlabeled P2VP in the binary solvent such that the final concentration of the labeled P2VP is ca. 10−8−10−9 M and ca. 10−6 M for the wide-field microscopy and confocal fluorescence microscopy experiments, respectively. The glass coverslips (Menzel-Glaser, thickness #1) were precleaned by sonication in acetone for 10 min followed by sonication in aqueous sodium hydroxide for 30 min and then sonication in DI water for 10 min. The last step was repeated three times. The coverslips were further heated in a “piranha” solution (70% H2SO4 and 30% H2O2 v/v) for 1 h and then sonicated in DI water for at least three times. The cleaned coverslips were subsequently rinsed with DI water and dried by blowing argon over the coverslips before being exposed to ozone in an UV−ozone reactor (PSD Series, NovaScan) for 30 min. The coverslips were used immediately after cleaning. 2.4. Contact Angle, Surface Tension, Refractive Index, Fluorescence Correlation Spectroscopy, and Confocal Fluorescent Imaging Measurements. An optical contact angle and surface tension meter (KSV CAM 101) is used to measure the contact angles of the sessile drops (0.5 ± 0.1 μL) and the surface tensions of the binary mixtures. Each measurement was repeated at least three times and an average value taken. The accuracy of the optical contact angle instrument is ±0.1°, and the results are reported to no more than 1 decimal place. Reliable contact angles lower than 10° are obtained when the baseline, connecting the two extreme ends of the drop, is clear and can be accurately set for fitting. The fitting is performed using KSV CAM800 software which converts the profile of the drop into a curve for which the contact angle is calculated. In general, images used in this work have unambiguous baselines, and good fitting curves are used to describe the drop (see Figure S1 in Supporting Information). All experiments were conducted in an environment with a temperature of 23 °C and relative humidity of 60 ± 5%, and the results were consistently reproducible. For a 0.5 μL sessile drop with a solvent composition of wt % of 2,6-lutidine:wt % of water = 60%:40%, the radius of the drop (∼1.5 mm) is shorter than the capillary length (κ−1 = (γ/ρg)1/2 ≈ 2 mm, where γ is the surface tension of the liquid (39.0 mN m−1), ρ = 989.5 kg m−3 26 is the liquid density, and g is earth’s gravity) of the binary mixture, and flattening of the drop by gravity can therefore be neglected.27 The refractive indices of water, 2,6-lutidine, and (2,6-lutidine + water) mixture were measured using an Abbe refractometer. The diffusion coefficient of dye-labeled P2VP (∼10−9 M) in a 2,6-lutidine solution contained within a well was measured using fluorescence correlation spectroscopy (FCS). The FCS measurements were performed using a timeresolved confocal microscope (MicroTime 200, PicoQuant) and details regarding the experiment are given in the Supporting Information. A confocal fluorescence microscope (Nikon, Eclipse TE2000-E) equipped with a 633 nm HeNe laser and a 2×, N.A. 0.06 (Nikon) objective lens is used to record the confocal fluorescent images. 2.5. Wide-Field Microscopy (WFM). The wide-field fluorescence microscopy, WFM, setup consists of a microscope (IX 71, Olympus) and a 633 nm HeNe laser source (35 mW, Melles Griot). The excitation light was tuned to be circular polarized using λ/4 and λ/2 waveplates and expanded via a beam expander (Linos) before being focused onto the back-focal plane of a water immersion objective lens (60×, N.A. 1.2, Olympus) or (20×, N.A. 0.4, Olympus). An excitation filter (Z633/10, Chroma) was used to filter the excitation light. The

toward (inward) the center. Kajiya et al. have also noted that a dotlike deposit is formed when the surface tension gradient, generated from an inhomogeneous polymer concentration, induces an inward solutal-Marangoni force that pushes poly(N,N-dimethylacrylamide) into the center of an aqueous drop.20 An effective method of creating a uniform dotlike deposit is to utilize a binary mixture droplet consisting of two solvents with different boiling points.6,9−11 Due to the enhanced evaporation at the edge of the drop, the composition at the CL will shift toward a larger fraction of the solvent component with the higher boiling point. The surface tension at the CL will therefore be different from that of the bulk liquid, hence establishing a surface tension gradient that drives a solventMarangoni force. Pesach and Marmur have discussed the effects of the differences in volatilities and surface tensions between the two solvent components on the overall direction of the solvent-induced Marangoni flow (i.e., inward vs outward solvent-Marangoni flow).24 Subsequently, de Gans and Shubert observed that by adding acetophenone to ethyl acetate, the ringlike structure of polystyrene in neat ethyl acetate solvent is converted to a dotlike deposit in the binary mixture when a solvent-Marangoni flow effectively transports the solute away from the edge and into the drop interior.6 It has been shown that polymer-based arrays of well-defined dots and highly ordered crystalline structures obtained from evaporating binary droplets are capable of enhancing the working efficiency of devices prepared using inkjet printing technology.6,9−11 However, the exact nature of the surface patterning process and the complex interplay between evaporation, Marangoni flow, and CL movement of a binary drop are still not well understood, and further research is warranted. The evaporation of solute−solvent drops has previously been characterized using several experimental techniques such as optical microscopy and particle image velocimetry.12,22 These techniques, however, do not provide detailed information on the dynamics of the motion of both the CL and the polymer moving in close vicinity to the leading edge. Such information is important as it provides a more complete picture of the overall surface patterning mechanism. In this study, we will utilize wide-field microscopy (WFM) to visualize, in real-time and with minimal intermittent breaks,25 the behavior of the CL and the solute, poly(2-vinylpyridine) P2VP, close to the CL in order to elucidate the driving force behind the self-assembled multiring pattern of an evaporating binary solvent (i.e., 2,6lutidine + water) drop. In this study, it will be shown that the formation of multiple ring structures does not conform to the conventional CL pinning−depinning mechanism but is instead driven by repeated CL spreading and retraction.

2. EXPERIMENTAL SECTION 2.1. Materials. Amino-terminated poly(2-vinylpyridine) P2VP (Mw = 140,600 and Mw/Mn = 1.04), and Atto647N NHS-ester dye were obtained from Polymer Source Inc. and Atto-Tec GmbH, respectively. Anhydrous dimethyl sulfoxide (DMSO, 99.9%, Aldrich), 2,6-lutidine (98+%, Alfa Aesar), sulfuric acid (H2SO4, 96.7%, Schedelco), hydrogen peroxide (H2O2, 30%, Scharlau), and methanol (MeOH, HPLC/spectroscopy grade, Tedia) were used as received. 2.2. Labeling of P2VP with Atto647N. The amino-terminated poly(2-vinylpyridine) was labeled by the reaction of its terminal amino group, NH2, with the NHS-ester moiety of Atto647N. A 100 mg amount of amino-terminated poly(2-vinylpyridine) was dissolved in 1 mL of anhydrous DMSO at 40 °C, followed by the addition of 1 mg of Atto647N NHS-ester into the solution. The mixture was then allowed 11057

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fluorescence was passed through a dichroic mirror (Z633rdc, Chroma) and an emission filter (HQ645lp, Chroma) and subsequently detected by a highly sensitive CCD camera (CascadeII 512B, Photometrics) after magnification with a 3.3× camera lens. The dimension of a widefield fluorescence frame was measured to be 41 × 41 μm2 for a 60×, N.A. 1.2, water objective lens (Olympus), and 123 × 123 μm2 for a 20×, N.A. 0.4, air objective lens (Olympus), using a stage micrometer, and the power density was adjusted to be about 0.512 kW cm−2. The integration time of the CCD camera is 34.6 ms per frame.

3. RESULTS AND DISCUSSION 3.1. General Characterization. The (2,6-lutidine + water) mixture is a nonideal mixture and displays a positive deviation from ideality.28,29 Because the lower critical solution temperature of the binary (2,6-lutidine + water) solvent is ∼34 °C, water (W) and 2,6-lutidine (L) are completely miscible in the mixture compositions used here at 23 °C (i.e., single-phase mixture).26 According to the contact angle measurements, 2,6-lutidine completely wets the glass surface used in this study (contact angle ≈ 0°), whereas water forms a thin sessile drop with an initial contact angle of 3.3°(±0.3°), measured immediately after deposition of water on the glass surface (see Figure S3 in Supporting Information). Therefore, 2,6-lutidine has a greater affinity to spread on the glass surface. In addition, the binary solvent exhibits greater dewetting on the glass substrate as compared to the pure solvent components and forms a sessile drop with an initial contact angle of 14.3°(±0.6°) for a solvent composition of wt % of L:wt % of W = 60%:40% (Figure S3). The exact nature of the dewetting properties of (2,6-lutidine + water) mixture is not known. However, a simple model used to explain why a binary mixture, such as (2,6-lutidine + water), dewets a surface when the pure components will wet the same surface is given by Tronel-Peyroz and co-workers.30 The authors used thermodynamic calculations to derive an expression for the contact angle that is dependent on the local composition and surface tension fluctuations. In particular, the contact angle for (2,6-lutidine + water) mixture is nonzero because the surface tension of the binary mixture varies greatly with mixture composition.30 It is observed that while P2VP is soluble in neat 2,6-lutidine and (2,6-lutidine + water) mixture, the polymer is only sparingly soluble in neat water at room temperature. This is in line with the close to similar solubility parameter, δ, of P2VP (δ = 20.4 (J cm−3)1/2)31 with 2,6-lutidine (δ = 19.3 (J cm−3)1/2)32 as compared with water (δ = 47.9 (J cm−3)1/2)33 (i.e., “like” dissolves “like”).33 Given that the P2VP-2,6-lutidine solution undergoes complete wetting on a glass surface and the solubility of the polymer is reduced in water, the evaporation behavior of the pure solvents (i.e., 2,6-lutidine and water) containing P2VP is not considered further in this study. The relationship between the initial contact angle (θi) and the solvent composition of a binary drop with a P2VP concentration of 0.002 is illustrated in Figure 1. It is observed that θi of a binary drop with wt % of L:wt % of W = 60%:40% dips slightly from 14.3° in the absence of P2VP to 13.5°(±0.3°) when the polymer is dissolved in the binary mixture. When the proportion of L is increased to wt % of L = 65%, θi becomes 12.2°(±0.4°). The initial contact angle remains relatively unchanged for binary drops with solvent compositions ranging from wt % of L:wt % of W = 65%:35% to 75%:25%. When the wt % of L is larger than 75%, an increase in the concentration of 2,6-lutidine leads to a drastic drop in θi. A similar trend is observed for the relationship between θi and the composition of

Figure 1. The plots of the initial contact angle (θi, □) and surface tension (γ, Δ) vs wt % of L for a binary solvent with a P2VP concentration of 0.002. Error bars are given.

the binary mixture drop in the absence of P2VP (Figure S4 in Supporting Information). The relationship between the surface tension of the binary solvent (wt % of 2,6-lutidine:wt % of water = 60%:40%) and 2,6-lutidine against different concentrations of P2VP is presented in Figure 2. Clearly, the surface tension increases as the concentration of P2VP increases.

Figure 2. Plot of the surface tension (γ) of the binary solvent (wt % of 2,6-lutidine:wt % of water =60%:40%) (□) and 2,6-lutidine (○) against different concentrations of P2VP. Error bars are given.

3.2. Wide-Field Microscopy. Figure 3a shows the confocal microscope image of a typical multiring deposit left by a 0.5 μL P2VP-(L + W) drop on the surface of a glass coverslip. The initial concentration of P2VP is 0.002,34 and the solvent composition is wt % of L: wt % of W = 60%:40%. The

Figure 3. Confocal fluorescent images of a typical multiring deposit left by a 0.5 μL P2VP-(L(60%) + W(40%)) drop on a glass substrate at (a) high contrast and (b) low contrast (movie 1). Image a shows fine ringlike solute deposits (X) whereas image b shows the thick solute deposit (Y) formed at a later stage of the evaporation. The initial concentration of P2VP is 0.002. 11058

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Figure 4. WFM images of a contact line CL (from Movie 2, Supporting Information) at (a) the beginning of a spreading process (1.9 s after the start of measurement), (b) 6.1 s after the start of measurement, and (c) the end of the spreading process (14.0 s). A P2VP deposit is observed. I is inside of the droplet, and II is outside of the droplet. The arrow in image b indicates the direction of the outward spreading of the CL.

for a typical 0.5 μL binary drop (wt % of L: wt % of W = 60%:40%) with an initial P2VP concentration of 0.002 is shown in Figure 5a. In general, the time variation in θ occurs in two

multiring structure of P2VP shows physical features similar to the self-assembled multiring patterns of other functional materials (e.g., polystyrene particles, DNA, and polymers) where pinning of the triple line is a necessary condition for the formation of ring deposits (i.e., by the advection and accumulation of solute at the pinned edge before depinning occurs).3−6 Two distinct pattern morphologies are observed in this study depending on the stage of evaporation. Fine ringlike solute deposits (e.g., rings found in region X for a typical droplet as shown in Figure 3a) are initially formed during evaporation. As drying proceeds, the droplet shrinks and significantly thicker CL rings (e.g., the thick ring labeled Y in Figure 3b for the same droplet formed at ∼270 s after the start of the experiment) are deposited toward the end of the evaporation and prior to complete dry up of the solvent (see the corresponding Movie 1; movie captions are provided in Supporting Information). We will first focus on the fine multiple rings. For the P2VP system studied here, the evolution of the steps leading to the formation of a ringlike solute deposit is visualized using WFM, and the time-lapsed WFM fluorescent images of a typical CL in motion during droplet evaporation are presented in Figure 4a− c (see the corresponding WFM Movie 2, Supporting Information). Movie 2 initially shows P2VP flowing outward to the edge of the drop (not captured by the camera) before the CL retracts rapidly to a new position (Figure 4a). From there, the CL immediately spreads outward (Figure 4b) with P2VP continuously flowing toward the advancing CL (i.e., CL is not pinned) and forming a polymer ridge at the edge. When the drop stops spreading and begins to recede, the P2VP ridge is left behind as a ringlike deposit on the substrate (Figure 4c). Multiple ring stains are formed by the repeated cycles of spreading and receding. Movie 3 (Supporting Information) shows the formation of several polymer rings formed by the repeated spreading−receding events as recorded using WFM. From the WFM Movies 2 and 3 (Supporting Information), we note that at the point when the CL recedes, there is already a significant amount of solute at the edge accumulated during the spreading process. This means that at the point of retraction, the solute did not suddenly accumulate at the contact line. Clearly, the ubiquitous stick−slip motion observed in previous works3−6,35 does not account for the surface patterning process observed here. It is noted that detailed information regarding the motion of the CL is not available from the confocal microscope (e.g., Movie 1, Supporting Information) due to its poorer temporal and spatial resolution as compared to WFM (e.g., Movie 2, Supporting Information). 3.3. Contact Angle Variation with Evaporation Time. The evolution of the contact angle (θ) with evaporation time

Figure 5. (a) Plots of the contact angle θ vs evaporation time t for sessile drops with compositions of wt % of L:wt % of W = 60%:40% (“curve a”, refer to the axis on the left-hand side of the graph) and 90%:10% (“curve b”, refer to the axis on the right-hand side of the graph) on a glass surface. The concentration of P2VP is 0.002. For “curve a”, the graphs of contact angle (θ) vs drying time (t) (panel i) and radius of drop (r) vs t (panel ii) for the first 50 s of evaporation are given in the inset. r is obtained from the KSV CAM800 software. (b) The histogram of the time taken for a series of CL (95) to complete a spreading event (i) and the histogram of the time taken for θ to decrease to a minimum value in 65 independent oscillation cycles (ii). The histograms were fitted to a Gaussian distribution that yielded a mean time of 8.0 and 6.9 s for i and ii, respectively.

different time scales, where for the short time scale, a periodic rise and fall in θ is observed (“curve a” of Figure 5a and inset). The graphs of θ vs drying time (t) and radius of drop (r) vs t for the first 50 s of evaporation are given in panels i and ii of the inset of Figure 5a, respectively. In this case, the decrease in θ for each cycle corresponds to the continuous reduction in the contact angle when the CL undergoes a spreading event to form a solute ring. In particular, for each oscillating cycle, it is 11059

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observed that r increases gradually when θ decreases (e.g., from 0 to 8 s), corresponding to a spreading process of the contact line. When θ increases rapidly (e.g., from 8 to 10 s), the radius drops significantly, indicating a receding process. Furthermore, the histogram of the time taken for a series of CL (95) to complete a spreading event (from WFM) and the histogram of the time taken for θ to decrease to a minimum value in 65 independent oscillation cycles are in good agreement with each other (Figure 5b). The histograms were fitted to a Gaussian distribution that yielded a mean CL spreading time of 8.0 s and a mean θ decay time of 6.9 s. When the contact angle becomes equals to the “receding” angle, the CL retracts to a new position and a rapid rise in θ is observed for each cycle (inset of Figure 5a). The actual reason why the receding edge eventually stops before the next spreading event takes place is not known. A plausible explanation could be because when the CL recedes and θ increases, the CL reaches a position where the force of traction pulling the triple line toward the center of the drop (i.e., γlv cos θ + γsl, where γlv is the liquid/vapor interfacial tension and γsl is the solid/liquid interfacial tension) is balanced by the solid/vapor interfacial tension γsv and Marangoni force that pull the CL away from the center.36 We will next briefly consider the time-dependent change of the contact angle over the long time scale. In this case, θ varies according to the relatively slow evaporation rate and the timedependent change in the composition of the binary solvent.37 In particular, during the time period when fine solute rings are formed from spreading−receding events, the contact angle of the mixture changes according to the change in the composition of the mixture (Figures 1 and 5a). Apart from the relatively small oscillation of the contact angle taking place in the short time scale ( 75%) remaining in the solvent during evaporation (Figure S5 in Supporting Information). This is possible since water (vapor pressure, P = 23.8 mmHg at 25 °C) is known to evaporate faster than 2,6-lutidine (P = 5.65 mmHg at 25 °C). However, we do not elaborate on the latter behavior in this work because the disentanglement of the roles played by pinning and composition change on the contact angle is nontrivial and is beyond the scope of this study. 3.4. Spreading and a Tentative Model. The speed of an advancing CL when it spreads to form a ring deposit is investigated by plotting the change in drop radius ΔR(t) (= R(t) − R(0)) as a function of time t for a spreading event obtained from the WFM experiment, and a typical example is given in Figure 6 (corresponding to the CL in Figure 4 and Movie 2 (Supporting Information)). For each spreading event, we consider the initial time (i.e., t = 0 s) to be the time when the drop begins to spread, and R(t) and R(0) are the radii of the drop at time t and at t = 0, respectively. The ΔR(t) vs t plot in Figure 6 is best described using a power-law expression ΔR(t) ∼ tα, where the power-law exponent α = 0.9. The histogram for the distribution of α for a collection of 30

Figure 6. A typical plot of ΔR(t) vs t for a CL undergoing spreading is fitted to a power-law expression, ΔR(t) ∼ tα with α = 0.9. The inset shows the histogram of α for 30 independent spreading CLs.

independent spreading events (see inset of Figure 6) indicates that α lies between 0.4 and 1. The dynamics of spreading is highly complicated27,36,38 and is usually accounted for by invoking the Tanner’s law,39 where the relationship R(t) ∼ tα with α = 0.1. Because R(0) is timeindependent for each spreading event, ΔR(t) ∼ t0.1 if Tanner’s law is obeyed. The superspreading phenomenon occurs when a drop spreads rapidly (i.e., α > 0.1), and the presence of a precursor film40 has been proposed to aid in the superspreading process by promoting an outward Marangoni force.41−43 On the other hand, the receding dynamics of an evaporating drop containing a volatile liquid is best described by R(t) ∼ (t0 − t)1/2 where t0 is the time when the drop vanishes.27,44 Because the Tanner’s law is unable to explain the large α values observed in Figure 6, the tentative model proposed here, based on the preexistence of a precursor film40 (see Scheme 1) and Marangoni forces, provides a reasonable insight into the surface patterning process seen in Figures 3a and 4. Scheme 1. Schematic Drawing of (Half) a Sessile Drop

Because 2,6-lutidine, as compared to water, has a greater propensity to spread on glass (see Figure S3), the transition region between the precursor film and the edge of the drop is likely to be 2,6-lutidine-rich (Scheme 1). Given that P2VP prefers to be solubilized in an environment rich in 2,6-lutidine at room temperature, the polymer will therefore aggregate at the transition region of the advancing edge Therefore, a sharp increase in both the P2VP concentration and surface tension of the transition region, as compared to the adjacent solvent (Scheme 1), leads to a surface tension gradient (Δγ) that drives an outward solutal-Marangoni flow that aids in the CL spreading.24,41−43 Furthermore, the elevated spreading rate can also be attributed to the relaxation of the no-slip boundary condition because of the presence of the precursor film.41−43 As the binary solvent drop undergoes spreading and the contact angle decreases (inset of Figure 5a), P2VP molecules are rapidly transported to the advancing CL, via the solutal11060

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3.5. Effects of Solvent and Solute Compositions. We examine the effects of solvent composition, while maintaining the initial P2VP concentration fix at 0.002, on the morphology of the self-assembled surface pattern. When the fraction of 2,6lutidine is increased such that wt % of L:wt % of W = 70%:30%, the number of fine solute rings significantly decreases and the time taken to reach the stage where thick P2VP rings are deposited is shortened. An example is illustrated in the confocal microscope image Figure 7a (see corresponding Movie 5,

Marangoni effect and a curvature-induced Laplace pressure, to form a P2VP ridge at the leading edge. Apart from the outward solutal-Marangoni force, an inward Marangoni flow that opposes spreading and which arises from the different volatilities and surface tensions of the pure solvent components must be taken into account.23,24 Because water has a boiling point lower than that of 2,6-lutidine and the evaporation rate is enhanced at the edge, the major component of the remaining solvent in the vicinity of the edge during evaporation is 2,6-lutidine which has a surface tension (31 mN m−1) lower than that of the bulk binary solvent (>31 mN m−1 for the time period when fine solute rings are formed) (Figure 1). From the WFM experiment, it is observed that while most of the P2VP chains close to the edge flow toward and accumulate at the CL, P2VP chains located away (e.g., 10−20 μm)45 from the CL and near the free surface experience a larger solvent-Marangoni force and are carried back from the outer to the inner regions of the drop via the solvent-Marangoni flow (Movie 4, Supporting Information). The solutal-Marangoni flow is thus effective over a short distance d1 spanning from the P2VP-rich end of the edge (i.e., transition region) to the adjacent solvent region A with a lower P2VP concentration (Scheme 1), and the solvent-Marangoni force operates between region A, which is relatively richer in 2,6-lutidine due to the preferential evaporation of water, and the bulk binary mixture over a distance d2. A large solutal-Marangoni number (Ma > 106, see Supporting Information) indicates that the solutalMarangoni flow is important in describing spreading dynamics. Thermal-induced Marangoni flow46 is not considered because it has previously been shown that for a polymer-based droplet system, the solutal-Marangoni force is significantly stronger than the thermal one.22,23 The distribution of α values observed in Figure 6 indicates that the rate of spreading is not constant throughout the CL and may vary with different spreading cycles. Figure 3a clearly shows that the top left edge of the drop is stationary during evaporation because the CL is strongly pinned to the substrate whereas the mobile parts of the CL are not dominated by efficient pinning and can therefore undergo repeated spreading and retraction to form the overall skewed multiring structure. For instance, inhomogeneous distributions in surface roughness, chemical heterogeneities, different amounts of solute trapped at the edge, and varying solute− (or solvent−) substrate interaction are capable of reducing the speed of the CL to different extents depending on the location of the CL on the substrate. An interesting feature, observed from the WFM experiment, is the instability of the polymer ridge to small perturbation which leads to the formation and growth of P2VP fingers. From Movies 2 and 3 (Supporting Information), we note that the P2VP fingers grow by constantly moving along the CL in either direction and coalescing with neighboring fingers to form larger ones. In addition, a portion of the newly arrived P2VP chains at the thin regions (i.e., valleys with relatively low P2VP concentrations) are attracted to thicker regions (i.e., fingers with relatively high P2VP concentrations) possibly by the solutal-Marangoni effect and osmotic pressure imbalance that push the chains to move from low to high concentrations.17 Gonuguntla and Sharma have also ascribed the periodic polymer patterns observed along an evaporating droplet edge to thermocapillary effect arising from a temperature difference between the valleys and fingers and the presence of Benard convection cells along the CL.17

Figure 7. Typical confocal fluorescent images of a stain deposited on a glass substrate by a drop of binary solvent with (a) wt % of 2,6lutidine:wt % of water = 70%:30% (high contrast image and see corresponding Movie 5, Supporting Information) and (b) 90%:10% (low contrast image and see corresponding Movie 6, Supporting Information). The initial concentration of P2VP is 0.002.

Supporting Information) where a typical drop with wt % of L:wt % of W = 70%:30% initially forms fine multiple rings via spreading−receding cycles followed by thicker solute deposits at ∼150 s after the start of the experiment. The latter is formed by the pinning−depinning mechanism (WFM movie not shown). Upon increasing the wt % of L in the binary mixture to 90%, the sessile drop forms only thick ring deposits (i.e., fine multiple ring structures are absent) (see confocal microscope image Figure 7b and corresponding Movie 6 (Supporting Information)). Furthermore, the plot of θ vs evaporating time does not display any periodic oscillation in θ (see “curve b” in Figure 5a). This suggests that for solvent compositions with a high 2,6-lutidine fraction, the sessile drop does not undergo repeated spreading and receding, and the CL is efficiently pinned to the substrate such that the solute is transported to the stationary edge by advection. This is in accordance with the pinning mechanism proposed by Deegan et al.1 to explain the formation of coffee ring stains and is clearly seen in the WFM Movie 7 (Supporting Information) for a droplet with wt % of L = 90% where the contact line is pinned to the substrate and the solute is accumulated along the stationary edge of the drop. A possible explanation is the greater affinity of the 2,6-lutidine component in the binary mixture at the leading edge to wet the glass surface (Scheme 1). The transition region is therefore richer in 2,6-lutidine and hence P2VP, as the relative concentration of 2,6-lutidine in the binary mixture increases which will cause the CL to effectively pin to the glass surface either by a strong solvent−substrate or solute−substrate interaction. For the binary mixture with wt % of L:wt % of W = 60%:40%, the droplet needs to evaporate for a longer period of time before reaching the appropriate solvent composition to effect complete pinning of the CL and formation of thick solute deposits. Therefore, the mechanism for the formation of thick solute rings found at the later stage of evaporation (i.e., pinning and depinning of the CL) is different 11061

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Figure 8. Typical confocal fluorescent images (high contrast) of a stain deposited on a glass substrate by a binary mixture drop (wt % of 2,6lutidine:wt % of water =60%:40%) with an initial P2VP concentration of (a) 0.0002 (see corresponding Movie 8, Supporting Information) and (b) 0.001. The drops are displaced from their initial positions. (c) Typical confocal fluorescent image (high contrast) of a stain deposited on a glass substrate by a binary mixture drop (composition wt % of 2,6-lutidine:wt % of water = 60%:40%) with a P2VP concentration of 0.008 (see corresponding Movie 9, Supporting Information).

from the case for fine solute rings which involves repeated spreading and receding of the CL. To examine the effects of solute concentration, the initial amount of P2VP is varied while maintaining the same solvent composition (wt % of L:wt % of W = 60%:40%). We observed that when the initial concentration of P2VP is reduced from 0.002 to 0.001 and 0.0002, the droplet moves on the glass substrate with fine ringlike deposits trailing behind the tracks as evaporation progresses (i.e., the drop is displaced from its initial position) (see confocal microscope image Figure 8a and the corresponding Movie 8 (Supporting Information), and Figure 8b). The reason behind the lateral movement of the drop is unclear at the present moment. A plausible explanation may be due to an imbalance of Marangoni force arising from a heterogeneous vapor pressure distribution of the solvent components. 23 On the other hand, when the P2VP concentration is high (e.g., 0.008), multiple ring structures are initially formed by the spreading and receding mechanism followed by thicker P2VP deposits (see confocal microscope image Figure 8c and corresponding Movie 9 (Supporting Information)). However, the number of fine ringlike solute deposits decreases as the concentration of P2VP increases. When the concentration of P2VP exceeds 0.015, only thick solute rings are observed when the CL is pinned to the substrate (data not shown). Clearly, at an initial high polymer concentration (e.g., 0.008), sufficient amounts of P2VP can aggregate at the 2,6-lutidine-rich transition region at the leading edge to induce complete pinning of the CL at short times whereas at an initial intermediate polymer concentration (e.g., 0.002), the amount of P2VP is insufficient to drive an efficient pinning process and the solutal Marangoni-induced spreading of the CL becomes important. At an initial low P2VP concentration (e.g., 0.0002), too few P2VP chains are available for trapping at the transition region to immobilize even a small portion of the CL, and the drop moves about on the glass surface.

gradient created as a result of the solutal-Marangoni effect aids in enhancing the droplet spreading rate. This study clearly shows that the formation of multiple ring structures is not restricted solely to a stick−slip motion and that other types of mechanisms may be in operation depending on the judicious selection of the solvent and/or solute.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. Eight movies and their captions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Financial support from the Singapore Ministry of Education MOE Tier 1 grant (RG60/11) is acknowledged.

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4. CONCLUSIONS In conclusion, we have shown that multiple ring deposits of P2VP from the evaporation of a binary (2,6-lutidine + water) drop with an appropriate solvent composition on a glass substrate are not formed via the conventional pinning− depinning mechanism.3−6 Instead, ringlike solute deposits are formed when the droplet undergoes several cycles of spreading and retraction where for each spreading event, the P2VP flows toward the outward advancing edge, forming a polymer ridge that is deposited when the CL recedes. A surface tension 11062

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