Article pubs.acs.org/Langmuir
Evaporation Stains: Suppressing the Coffee-Ring Effect by Contact Angle Hysteresis Yueh-Feng Li Department of Chemical and Materials Engineering, National Central University, Jhongli, Taiwan 320, R.O.C.
Yu-Jane Sheng* Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C.
Heng-Kwong Tsao* Department of Chemical and Materials Engineering, and Department of Physics, National Central University, Jhongli, Taiwan 320, R.O.C. ABSTRACT: A ring-shaped stain is frequently left on a substrate by a drying drop containing colloids as a result of contact line pinning and outward flow. In this work, however, different patterns are observed for drying drops containing small solutes or polymers on various hydrophilic substrates. Depending on the surface activity of solutes and the contact angle hysteresis (CAH) of substrates, the pattern of the evaporation stain varies, including a concentrated stain, a ringlike deposit, and a combined structure. For small surface-inactive solutes, the concentrated stain is formed on substrates with weak CAH, for example, copper sulfate solution on silica glass. On the contrary, a ringlike deposit is developed on substrates with strong CAH, for example, a copper sulfate solution on graphite. For surface-active solutes, however, the wetting property can be significantly altered and the ringlike stain is always visible, for example, Brij-35 solution on polycarbonate. For a mixture of surface-active and surfaceinactive solutes, a combined pattern of a ringlike and concentrated stain can appear. For various polymer solutions on polycarbonate, similar results are observed. Concentrated stains are formed for weak CAH such as sodium polysulfonate, and ring-shaped patterns are developed for strong CAH such as poly(vinyl pyrrolidone). The stain pattern is actually determined by the competition between the time scales associated with contact line retreat and solute precipitation. The suppression of the coffee-ring effect can thus be acquired by the control of CAH. contact line, the radial outward flow is thus induced by the differential evaporation rates across the drop. Contact line pinning of the drying drop on the surface leads to liquid flow from the interior to replenish the liquid evaporating from the edge. The resulting edgeward flow can carry nearly all of the dispersed solutes toward the edge and deposit them in the vicinity of the contact line to form a ringlike stain. Evidently, the appearance of the coffee-ring effect involves three ingredients for a drying droplet: nonvolatile solute, outward flow, and contact line pinning. Because inhomogeneous distributions of residual deposits may compromise the overall performance of systems such as fluorescent microarrays in combinatorial analysis,15,16 the suppression of the coffee-ring effect becomes an important
1. INTRODUCTION The drying of a droplet containing nonvolatile solutes, initially dispersed over the entire drop, is an everyday phenomenon, and it commonly leaves a ring of solute deposited on a surface rather than a uniform spot. Although droplet drying is often observed, it gives rise to surprisingly rich morphologies that depend on the contact line geometry, solute size and chemistry, and substrate−solvent interaction.1−3 In addition to influencing inkjet printing, washing, and coating processes,4−9 droplet drying provides a potential means to write or deposit a fine pattern on a surface. Therefore, it can serve as an important small-scale fabrication process for surface patterning, selfassembly, and size-discriminative transportation and separation of nanoparticles without resorting to any external forces.10−12 The formation of the well-known coffee-ring pattern after the liquid drop evaporates has been successfully explained by Deegan et al.13,14 Because the evaporative flux grows with the radial position (r) and becomes divergent as r approaches the © XXXX American Chemical Society
Received: March 13, 2013 Revised: May 6, 2013
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than that of a typical surfactant, some of them can interact strongly with the substrate, thus leading to strong CAH. For an aqueous drop containing surface-inactive solutes on a substrate with weak CAH, surfactant molecules can be added to alter the wetting property. In the current study, dilute solutions are used to eliminate the early precipitation effect. The stain patterns of drying drops containing various solutes are observed on various substrates. A ringlike stain does not always appear. The variation of the contact angle and base diameter with the drop volume during evaporation are analyzed to explain the stain pattern. It is found that the wetting property, particularly the receding contact angle, is the key role in the formation of the ringlike structure.
issue for the control of the deposit pattern. The general approach to suppressing the formation of the ring stain is to alter the flow induced by the liquid evaporating from the drop edge. The evaporation can result in a nonuniform temperature profile along the droplet surface.17,18 The thermal Marangoni flow inside a droplet is therefore induced by a surface tension gradient. When the Marangoni effect is weak, all of the surface liquid moves outward and a dense ring stain is formed. However, when the Marangoni effect is strong, a stagnation point where the surface flow changes its direction appears on the droplet surface. As a result, particles within the stagnation point are redistributed back to the central region of the droplet. The coffee-ring effect of evaporating colloid drops can also be altered by the surfactant-induced Marangoni effect that is induced by the variation in surfactant concentration along the air−water interface.19 Electrowetting is another approach to controlling colloidal self-assembly in evaporating drops.20 Alternating-current frequencies ranging from a few hertz to a few tens of kilohertz prevent contact line pinning and promote strong internal flow. The latter counteracts the evaporation-driven outward flux and leads to the suppression of coffee-ring stains formed by colloidal particles of various sizes and DNA solutions. Note that electrowetting has the disadvantage that it requires conductive or highly polarizable liquids. In addition to the flow pattern, the shape of the suspended particles is also shown to eliminate the coffee-ring effect based on shape-dependent capillary interactions.21 Uniformly deposited stains can be obtained by ellipsoid particles that form loosely packed or arrested structures on an air−water interface during evaporation. These structures prevent the suspended particles from reaching the drop edge, and a uniform coating is thus ensured. In previous studies of the coffee-ring effect, micrometer-sized particles such as polystyrene are frequently used as nonvolatile solutes in the evaporating drop. Sometimes, polyelectrolytes such as DNA17,22 and sodium polystyrene sulfonate (NaPSS) are employed.23 In that works, silica glass, which is quite hydrophilic, is often adopted as the substrate on which the solute-containing water drop is placed. However, one would wonder whether the properties of nonvolatile solutes and substrates affect the stain pattern of a drying drop. Therefore, several questions about the coffee-ring effect arise naturally. (i) Does the coffee-ring effect always occur on a hydrophilic surface? (ii) Does the coffee-ring effect rely on the properties of nonvolatile solutes, such as the shape and size? (iii) How does the interaction between the nonvolatile solute and substrate influence the formation of the ringlike stain? (iv) How does the surfactant (surface-active solute) influence the appearance of the coffee-ring effect? Contact line pinning is the necessary condition for coffeering stain formation. Because it is closely related to contact angle hysteresis (CAH), wetting properties such as the advancing and receding contact angles play an important role in the final pattern of the solute deposit. It is known that the wetting property depends on the substrate and surface activity of the solute dissolved in the water drop. In this work, various hydrophilic substrates such as silica glass, acrylic glass, polycarbonate, and graphite are employed. Their wetting properties are significantly different. Moreover, various small nonvolatile solutes such as metal salts, saccharides, and polymers are used. Their surface activities in terms of the surface tension change of water differ from each other. Although their influence on surface tension is much weaker
II. MATERIALS AND EXPERIMENTAL METHODS A. Materials. Nonvolatile small solutes used for stain formation include metal salts, carbohydrates, polymers, and surfactants. For metal salts, copper(II) sulfate (Merck, Germany) and cobalt(II) sulfate (Wako, Japan) are employed, and their anhydrous appearances are gray-white and reddish, respectively. For carbohydrates, glucose (monosaccharide) (Sigma-Aldrich, Riedel-deHaen, USA) and cellobiose (disaccharide) (Alfa Aesar, USA) are used. The saturation concentrations of those small solutes are given as follows: 316 g/L (CuSO4 at 0 °C), 604 g/L (CoSO4 at 3 °C), 910 g/L (glucose), and 120 g/L (cellobiose at 25 °C). There are two types of polymeric additives: nonionic polymer and polyelectrolyte. The former has poly(ethylene glycol) 20K (PEG) (Sigma-Aldrich, Fluka, USA), Dextran 500K (polysaccharide) (Sigma-Aldrich, Fluka, USA), poly(vinyl alcohol) 130K (PVA) (Sigma-Aldrich, Fluka, USA), and poly(vinyl pyrrolidone) 360K (PVP) (Acros, USA). The latter includes sodium polystyrene sulfonate 70K (NaPSS) (Aldrich, USA) and poly(diallyldimethylammonium chloride) (PDDC) 100−200K (20 wt % Aldrich, USA). Nonionic surfactant Brij-35 ((C2H4O)23C12H25OH) (Acros, USA) is also used, and it is a white waxy solid at room temperature. The aqueous solution for an evaporative drop is prepared by dissolving a nonvolatile solute in deionized water. Because dilute solutions are considered, the concentration of various solutes is generally about 0.05 wt %. B. Stain Formation. Substrates with different wetting properties are utilized for stain pattern formation, including silica glass, acrylic glass, polycarbonate, and graphite (SGL group, Germany). The slices of silica glass, acrylic glass, and polycarbonate are transparent and were purchased from Kwo-Yi Co. (Taiwan). The evaporation of aqueous drops is performed on substrates that are precleaned with deionized water and ethanol. Water drops with a volume of about 10 μL are dripped onto substrates and then placed inside a box with container desiccants. Typically, it takes about 2 h to complete the evaporation of the drop with a volume of about 10 μL. After complete evaporation of the water drops, various patterns are observed by microscopy (Optem Zoom 125C). Because dilute solutions are used, the amount of deposition is relatively small. To obtain clear images of evaporative deposition patterns on transparent substrates, the intensity and angle of incident light are varied to achieve maximum contrast between stain patterns and the substrate. C. Wetting Property Measurement. Contact angles (CAs) of those substrates and surface tension of the aqueous solutions (γlg) are analyzed with a CA goniometer, droplet shape analysis system DSA10MK2 (Krüss, Germany) at room temperature under the open-air condition with a relative humidity of 45−50%. When side-view images of the sessile drops on various substrates are taken, the values of CAs are determined by the tangent method (curve fitting by the general conic section equation) offered by the droplet shape analysis system software. The CA measurements are made on both the left and right sides of the droplet, and the averaged CA is reported. The CAH of the substrate is realized by measuring the advancing and receding CAs. The addition of liquid to the droplet with the needle syringe makes the contact line move outward with the advancing angle (θa), and the removal of liquid from the droplet with the needle syringe leads to B
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contact line pinning (receding pinning) until the receding angle is achieved. Afterward, the contact line moves inward with a receding angle of θr. Note that the rate of inflation and deflation should be kept as slow as possible to avoid disturbing the contact line with fluid flow. It is generally believed that the static contact angle is different from the advancing contact angle. The measurement of the static contact angle depends on the process of drop deposition. In our study, a slight inflation is always imposed right after drop deposition. As a result, the static angle is equal to the advancing angle. The surface tension can be estimated by analyzing the profile of a pendant drop, which can be described by the Young−Laplace equation for the balance between surface tension and hydrostatic pressure.
process, the pinning of the drop’s contact line is clearly seen in side views 1 and 2 in Figure 1a and in the variation of the base
III. RESULTS AND DISCUSSION The coffee-ring effect is generally observed on a hydrophilic substrate after drop evaporation. The necessary condition for the formation of the ringlike stain is outward flow, contact line pinning, and nonvolatile solutes (particles). However, contact line pinning on hydrophilic substrates does not always persist through the whole drying process. The withdrawal of the contact line may affect the final stain pattern and is closely related to the wetting behavior of the particle-containing drop on the substrate. Because stain formation due to solute deposition also involves the adsorption (deposition) of solutes (particles), the competition between contact line receding and particle deposition becomes the dominant factor in the stain pattern. Solute adsorption is related to the solute−substrate attraction, and the adsorbed layer may be able to modify the effective solid−liquid interfacial tension, which in turn alters the wetting behavior. In this study, the relationship between the stain pattern and the wetting behavior is explored. We focus on aqueous drops containing small solutes on hydrophilic substrates with various wetting properties. The wettability of an ideal flat solid in terms of the contact angle (θ) between the gas−liquid and solid−liquid interfaces can be depicted by Young’s equation,24 cos θ = (γsg − γsl)/γlg, where γsg, γsl, and γlg represent the interfacial tensions of solid− gas, solid−liquid, and liquid−gas, respectively. On a real (nonideal) surface, the CA is not unique and depends on whether the liquid is advancing over the surface or receding. This phenomenon is known as CA hysteresis, which is generally expressed in terms of the difference between the advancing and receding angle (Δθ = θa − θr). Note that on the basis of the liquid-induced defects model for CAH25−27the CA hysteresis involves two solid−liquid interfacial tensions, γsl and γsl′ . Young’s equation with γsl gives the advancing angle θa whereas that with γsl′ due to surface rearrangement yields the receding angle θr. The hysteresis in adhesion energy is related to the decrease in the solid−liquid interfacial energy, γ′sl < γsl. CAH is accompanied by the pinning of the contact line as the drop volume is withdrawn. Because the time period associated with contact line pinning is related to when the contact angle reaches θr, the final stain pattern is expected to be influenced by the extent of CAH of the substrate. A. Formation of Concentrated Stain on Hydrophilic Substrates with Weak CAH. A pinned contact line is the necessary condition for the formation of the ringlike stain. Does the coffee-ring effect always occur on hydrophilic surfaces where contact line pinning evidently appears? First, consider the evaporation of an aqueous drop containing copper sulfate deposited on silica glass. The drop volume is 9.25 μL, and the initial concentration of the nonvolatile solute is 25 mM. The advancing and receding angles of water on silica glass substrates are θa ≈ 45° and θr ≈ 20°, respectively. During the evaporation
Figure 1. (a) Evaporation of a drop containing copper sulfate (CuSO4) on silica glass. (b) Variation of the contact angle with the volume of a water drop containing 25 mM CuSO4 on silica glass during evaporation.
diameter with droplet volume of Figure 1b. Because of contact line pinning, the contact angle decreases with decreasing droplet volume. Eventually, the contact angle reaches θr, and the contact line begins to withdraw as illustrated in side views 3−7 of Figure 1a. In the meantime, the contact angle remains at θ = θr as demonstrated in Figure 1b. Although pinning of the contact line of the drying drop is present, the ringlike deposit of CuSO4 along the perimeter is absent after the withdrawal of the contact line, as shown in the top view of Figure 1a. On the contrary, after the complete evaporation of water, all nonvolatile solutes are concentrated in the center of the drop, referred to as a concentrated stain. The above experiment involves a drop of an electrolyte solution (CuSO4) and a rather hydrophilic substrate (SiO2). To prove that a concentrated stain can be generally observed for small nonvolatile solutes, another experiment involving a drop of a neutral solution and a less hydrophilic substrate is conducted. Consider the evaporation of an aqueous drop containing cellobiose deposited on polycarbonate. The initial drop volume is 12 μL, and the initial concentration of the C
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solutes are transported outward. In stages II and III (θ ≤ θr), the withdrawal of the contact line further hinders solute deposition along the perimeter. Eventually, the solute concentration reaches its solubility, and precipitation or crystallization of the solute takes place. After the demonstration of the presence of concentrated stains on hydrophilic substrates, one may suggest that the distinct results between the concentrated stain and evaporation ring arise from the wetting characteristics of solutes or substrates. Because the CAH effect is weak on silica glass and polycarbonate, one would wonder whether the ringlike pattern can be formed by copper sulfate or cellobiose on a substrate with strong CAH. B. Formation of Ring Stains on Substrates with Strong CAH. One common reason for the formation of concentrated stains is attributed to hydrophobic substrates on which the pinning of the contact line is absent. However, our experimental results on both silica glass and polycarbonate show evident contact line pinning on hydrophilic substrates, but a ringlike deposit does not appear during the pinning period. Another reason is attributed to small solutes. It has been proposed that if the solutes are very small they are agitated by the thermal motion and convective motion and thus do not deposit in the corner near the triple line. Because the central part of the drop is the last region to dry, it has more solute deposition than that of the border. To examine the above explanation, aqueous droplets containing copper sulfate or cellobiose are deposited on smooth graphite surfaces for which θa ≈ 85° and θr < 10°. Because the advancing angle of the graphite surface is close to 90°, the wetting behavior seems like that of a hydrophobic surface. Nonetheless, the pinning of the contact line on graphite is very serious because of the very small value of θr. Figure 3a shows the variation of the side and top views of an evaporating drop containing copper sulfate on graphite. The initial concentration is also 25 mM. Obviously, the outcome on the graphite surface is distinctly different from that on silica glass. The ringlike stain due to the crystallization of copper sulfate along the perimeter of the drop is displayed. Figure 3b also illustrates that the base diameter of the drop remains the same but the contact angle continues to decay during evaporation. The pinned contact line lasts almost to the end of the drying process. The deposition of surface-inactive solute takes place only when the local concentration reaches the saturation point. The difference between the substrates with weak and strong CAH is that the precipitation event occurs after the contact line recedes in the former but before the contact line recedes in the latter. To examine further when the ring pattern starts to form on the graphite surface, we deflate the liquid drop containing copper sulfate quickly after its volume decreases from 10 to 2 μL as a result of evaporation. As demonstrated in Figure 4, there is no residue on the graphite surface after the suctioning of a partially drying drop. That is, the ringlike stain that is supposed to appear after the complete evaporation of a drop does not occur at all on the graphite surface because all solutes are removed by suction. Evidently, solute deposition does not happen before the deflation of the drop. Note that already deposited solutes cannot be easily removed by our deflation procedure, as will be discussed later for surface-active solutes. This consequence indicates that although small solutes without surface activity are carried toward the pinned contact line by outward flow, they are unable to deposit on the edge during the process of evaporation. The precipitation of surface-inactive solute occurs as the instantaneous local concentration reaches
nonvolatile solute is 0.05 wt %. The advancing and receding angles of water on polycarbonate substrates are θa ≈ 85° and θr ≈ 65°, respectively. Again, contact line pinning of a drying drop is clearly seen in side views 1−3 in Figure 2a and in the
Figure 2. (a) Evaporation of a drop containing cellobiose on polycarbonate. (b) Variation of the contact angle with the volume of a drying drop containing 0.05 wt % cellobiose on a polycarbonate substrate.
variation of the base diameter with droplet volume in Figure 2b. The retreat of the contact line at θ = θr does not leave a deposit of cellobiose along the perimeter, as demonstrated in the top view of Figure 2a. The aforementioned two examples show that it is not necessary that the ringlike stains always take place on hydrophilic substrates with contact line pinning. A concentrated stain appears instead. Three stages associated with a drying drop can be identified, as illustrated in Figures 1b and 2b. Owing to contact angle hysteresis, the contact line on a substrate is pinned as θa ≥ θ > θr and starts to shrink as θ = θr. The former corresponds to stage I, and the latter is stage II. When the droplet volume is too small (e.g., less than 1 μL), the evaporation rate is too fast for the drop to reach its equilibrium shape. Consequently, both the base diameter and contact angle decline with decreasing volume. This period can be identified as stage III. The outcome of concentrated stains indicates that the deposition of nonvolatile solutes near the contact line does not occur at all in stage I (θ > θr), even though the contact line is fixed and the D
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drop volume is reduced to 2 μL, the bulk concentration of copper sulfate becomes 0.125 M, which is still much less than the saturation concentration of about 2.2 M. Because the contact line remains pinned just before the drop becomes completely dry, the solute concentration in the vicinity of the pinned contact line eventually reaches its saturation solubility and therefore precipitates out to form a ringlike stain in the final period of evaporation. The aforementioned two experimental results on graphite surfaces clearly show that a hydrophilic surface possessing strong CAH (a small receding contact angle) is necessary to form a ringlike stain by surfaceinactive solutes because the contact line can be pinned persistently. The growth of the coffee stain has been observed microscopically under real-time conditions.28 Inside the coffee stain, several distinct phases of growth, from the first growth inside a wedge to the delamination process, are clearly seen. During the formation of ring-shaped colloidal stains, the transition from ordered crystals to disordered packing is also observed.29 When the deposition rate is low, particles have time to arrange by Brownian motion and form an ordered phase. At the end of the droplet’s life, there is no time for such rearrangement and the particles form a disordered phase. In our study, we focus on macroscopic patterns, and the foregoing results are simply considered to be a ringlike stain. Moreover, the deposition rate of small solutes on substrates with weak CAH is so slow that the contact line recedes before the occurrence of deposition. It has been shown that the drop edge can be prevented from receding by micrometer-sized particles near the contact line.30 If the contact angle reduction due to evaporation can be hindered by protruding particles at the drop edge, then the contact line remains pinned. On the basis of this model, the criterion for contact line pinning is given theoretically. In this work, nonvolatile solutes are small ions, molecules, and polymers instead of micrometer-sized particle. As a result, the foregoing mechanism cannot apply to our system. C. Ringlike Stain Induced by Surface-Active Solutes. The stain pattern is first realized by the wetting property of a pure water droplet on the substrate, particularly the receding contact angle. On substrates with weak CAH such as acrylic glass and polycarbonate, the contact line cannot be pinned for a long enough time so that solute deposition can take place. That is, the time scale of contact line retreat (τret) is shorter than that of solute deposition (τspre), τret < τspre. For example, the advancing and receding contact angles of a pure water drop on acrylic glass are θa ≈ 75° and θr ≈ 60°, respectively. The contact line retreat happens as θ ≤ θr. To have a ringlike stain on a hydrophilic substrate with weak CAH, the liquid droplet must contain surface-active solutes such as surfactants that can alter the wetting property of a water droplet on the substrate. Consider the evaporation of an aqueous drop containing detergent Brij-35 deposited on acrylic glass. The drop volume is 11 μL, and the initial concentration of the nonvolatile solute is 0.05 wt %. The advancing and receding contact angles of this drop on acrylic glass substrates have been reduced to θa ≈ 45° and θr ≈ 10°, respectively. As depicted in Figure 5a for the time variation of the side and top views of a drying drop containing small surface-active solutes, the contact line is always pinned. Moreover, at the end of the evaporation process, the ringlike stain is evidently demonstrated. Figure 5b shows the variation of the contact angle and base diameter with the droplet volume. The continuous decay of the contact angle and the constant
Figure 3. (a) Evaporation of a drop containing CuSO4 on graphite. (b) Variation of the contact angle with the volume of a drying drop containing 25 mM CuSO4 on a graphite substrate that possesses strong contact angle hysteresis.
Figure 4. Residues on graphite after the suctioning of a partially drying drop.
its saturation point. For the suction experiment in Figure 4, the fact that no residue was found after removing the 2 μL is simply because the evaporation droplet is still not saturated. As the E
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the surfactant layer adsorbed onto the relaxed surface is modified accordingly. The receding contact angle of this system is then given by cos θr* = (γsg − γsl*′)/γlg*. Again, the result, cos θ*r < cos θr, is obtained because of γ*sl ′ < γ′sl. Thus, one has θ*r < θr. Also, because γ*sl ′ < γ*sl , θ*r < θ*a . Effectively, the addition of surfactant leads to a significant reduction of the receding contact angle, and thus the contact line can be lastingly pinned during the evaporation process. The formation of the ringlike stain on a substrate with weak CAH can thereby be achieved by the addition of surface-active solutes. Again, one would wonder when the ringlike pattern starts to form during the evaporation process. Note that for graphite surfaces, ring-shaped residues of surface-inactive solutes form during the last stage of evaporation. The deflation experiment is able to reveal the status of solute deposition. Consider the liquid drop containing Brij-35, which is deflated quickly after its volume decreases from 10 to 2 μL as a result of evaporation. As demonstrated in Figure 6, residues of surfactant appear clearly
Figure 5. (a) Evaporation of a drop containing surfactant Brij-35 on acrylic glass. (b) Variation of the contact angle with the volume of a water drop containing 0.05 wt % Brij-35 on acrylic glass during evaporation.
Figure 6. Residues on acrylic glass after the suctioning of a partially drying drop.
on the surface of acrylic glass after the suctioning of a partially drying drop. Long before the complete evaporation of a drop, the ringlike pattern is already formed. This consequence indicates that the surface activity of Brij-35 enhances the deposition of small solutes along the pinned contact line. Unlike the precipitation of surface-inactive solutes, the deposition of surface-active solutes is caused by adsorption, which can take place at low solute concentrations. Another distinct feature associated with the ringlike stain formation induced by surfactant addition is solute deposition at locations besides the edge. Although surface-active solutes are carried toward the pinned contact line to deposit, a significant amount of solutes has already adsorbed onto the surface before they reach the edge. Figure 6 demonstrates that even the forced convection driven by suction during the deflation experiment is unable to eliminate the adsorbed residues. Obviously, the “clean ring” formed by surface-inactive solutes on the substrate with a low, intrinsic receding angle is quite different from the “dirty ring” formed by surface-active solutes that lower the receding angle effectively. The former is driven by solute precipitation, and the latter is caused by solute adsorption. Consequently, in the former case, solutes deposit in the vicinity of the edge, and in the latter case, solute deposition is observed everywhere in the liquid−solid contact region. Nonetheless, the deposition is more pronounced near the border because of outward flow. D. Combined Pattern of a Drying Drop Containing Surfactant and Surface-Inactive Solute. If the droplet
value of the base diameter signify the phenomenon of contact line pinning due to surfactant addition. Note that a drying drop containing surface-inactive solutes such as glucose on acrylic glass would leave a concentrated stain instead of a ringlike stain. However, the presence of the surfaceactive solute modifies the behavior of the contact line. The function of a surface-active solute that is amphiphilic is not only to reduce the surface tension (γlg) but also to change the solid− liquid interfacial tension. The former has been realized as the accumulation of amphiphilic molecules at the liquid−gas interface, and thus γlg is reduced to γlg*. Similarly, the latter is attributed to the adsorption of surface-active solutes on the substrate, and thereby the solid−liquid interfacial tension is lowered from γsl to γ*sl . Following Young’s equation, the advancing contact angle for this system is determined by cos θ*a = (γsg − γsl*)/γlg*. The reduction in both γlg and γsl leads to the result, cos θ*a < cos θa. Therefore, the advancing contact angle declines because of the presence of surfactant (θ*a < θa). Upon surfactant addition, the change in the receding contact angle θ*r can also be understood from Young’s equation. On the basis of the liquid-induced defects model for CAH,25−27 the microstructure on the substrate surface will be rearranged after the contact of the substrate with liquid and surfactant. As microstructural relaxation is achieved, the solid−liquid interfacial tension is further lowered from γ*sl to γ*sl ′ because F
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about 10 μL initially, and it declines with time during evaporation. Nonetheless, the contact line remains pinned because the receding contact angle is significantly reduced by surfactant. At the beginning of the final stage of evaporation, the drying drop has a volume of less than 1 μL and the contact angle is less than 10°, as shown in the first photograph in Figure 7b. The ringlike stain formed by surfactant deposition has already developed but cannot be seen in this top view. If this drop is quickly dried by suctioning, then a ringlike pattern will appear. As time proceeds, the contact line starts to recede. Nonetheless, the withdrawal of the contact line is not uniform along the perimeter but only from a corner, as demonstrated in the second photograph. As the drop is gradually shrinking, most of the contact line retreats but a small part remains pinned, as illustrated from the third to eighth photographs. Because the CoSO4 concentration grows with decreasing drop volume, the pink color is evident as depicted in the ninth photograph. Eventually, the precipitation of CoSO4 occurs near the pinned contact line. This result illustrates that the separation of a mixture can be achieved on an evaporation stain based on the difference in surface activity. Obviously, there are three time scales involved for such a combined pattern. The ringlike pattern forms first, and thus the time period at which detergent adsorb substantially (τdads) is the shortest. Latter on, the contact line begins to recede; therefore, the time period at which the contact line begins to retreat (τret) is intermediate. Finally, the surface-inactive solutes precipitate, and the time period for observing significant precipitation (τspre) is the longest. It is worth comparing the process of this combined pattern with that of the ringlike pattern on graphite (strong CAH). Both drying processes involve a small receding contact angle; therefore, the contact line of a drop has to be pinned for a while during evaporation. However, in the latter process, contact line pinning lasts until the precipitation of the surface-active solute. That is, τret is greater than τspre, and the simple ringlike pattern is always seen on a graphite surface. This consequence reveals that contact line pinning caused by graphite itself is stronger than that induced by surfactant adsorption. E. Stain Patterns of a Drying Drop Containing Polymers. In the aforementioned experiments, we have shown that the surface activity of small solutes plays a key role in the determination of the pattern of the evaporation stain. For the solute with no surface activity, the concentrated stain is always formed for substrates with weak CAH; therefore, one might suspect that its diffusivity is too large for it to be deposited along the perimeter. To examine the effect of the diffusivity, we investigate the polymeric solute whose molecular size is much larger than that of the typical solute. According to the Stokes−Einstein relation, the diffusivity is about 10−9 m2/s for a molecular size of a few angstroms and it becomes 10−11 m2/s for 10 nm. Two types of polymers are considered, including nonionic polymers and polyelectrolytes. The drops containing various dilute polymer solutions (0.02 wt %) are drying on the surface of polycarbonate, which possesses weak CAH for drops of pure water. As shown in Figure 8, two types of stain patterns are observed. A ringlike stain is observed for PVA and PVP. However, a concentrated stain is seen for dextran, PEG, PDDC, and NaPSS. It is evident that the appearance of the concentrated stain for large solutes cannot be attributed to the diffusivity of the solute. On the basis of the proposed mechanism associated with small solutes, it is anticipated that
contains a mixture of surface-inactive and surface-active solutes, what kind of stain shape would occur after evaporation? Consider an aqueous drop containing 0.05 wt % Brij-35 and 1 wt % surface-inactive solutes on the surface of polycarbonate. Note that the surface-inactive solute concentration is high compared to that in previous experiments so that its stain can be clearly distinguished from that of surfactant. Figure 7a shows
Figure 7. (a) Stain patterns of drying drops containing both surfaceinactive solutes (1 wt %) and surfactant (0.05 wt %). (b) Time variation of the drop shape (top view) and precipitation of the surfaceinactive solute (CoSO4). The ringlike pattern formed by surfactant is indicated by arrows.
the stain patterns with and without surfactant for the sake of comparison. In the absence of surfactant, concentrated stains are distinctly observed for CoSO4, glucose, and PDDC. Neutral polymer PDDC will be shown to be surface-inactive later. The precipitation of solutes occurs after the withdrawal of the contact line to the neighborhood of the center of the drop. However, the addition of surfactant leads to the persistence of contact line pinning. Consequently, the ringlike pattern associated with surfactant adsorption is seen in the case with Brij-35. As discussed earlier, the deposition of surfactant in an area other than the perimeter of the drop is also observed. However, it is interesting to find that the surface-inactive solute does not deposit along the perimeter to form a ringlike pattern. Instead, a concentrated stain of surface-inactive solute appears. Moreover, it is located in the vicinity of the perimeter instead of the center of the drop. This consequence reveals the competition between the deposition of the surface-inactive solute and the withdrawal of the contact line. Our experimental result indicates that the latter event takes places earlier than the former one. To examine our explanation for the combined pattern of ringlike and concentrated stains, the time variation of the contact line and the precipitation of the surface-inactive solute in the final stage is observed for a drop containing Brij-35 and CoSO4 on a polycarbonate surface. The volume of the drop is G
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angle (i.e., θr ≈ 30°). This consequence indicates that PVA and PVP are surface-active and thereby the ringlike stain pattern is developed. The fact that the receding contact angle is well correlated with the stain pattern reveals that CAH is relevant in determining the pattern of the evaporation stain and is mainly driven by the reduction of the solid−liquid interfacial tension induced by the adsorption of added polymers.25−27 Obviously, there are three characteristic time scales involved after droplet deposition. The time period at which a significant adsorption of polymers is observed is defined as τpads. Similarly, the time period at which a significant precipitation of polymers is seen is represented by τppre. The third time scale τret denotes the time period at which the contact line starts to retreat. τpads and τppre depend on the surface activity of the polymeric solute. For drops containing surface-inactive polymers on polycarbonate, one has τpads ≫ τppre ≫ τret. That is, polymer adsorption is negligible and the precipitation of solutes takes place after the contact line is withdrawn substantially. However, for surfaceactive polymers, one has τpads ≪ τppre ≪ τret. That is, the receding angle is lowered by polymer adsorption very soon. Later on, the precipitation of the solutes occurs far before the retreat of the contact line. It is worth noting that for drops containing surface-inactive solutes on graphite (strong CAH) one has τsads ≫ τret≫ τspre. Because the intrinsic receding angle associated with the water−graphite contact is very low, the contact line pinning persists even after the process of solute precipitation is accomplished.
Figure 8. Stain patterns of small and polymeric solutes with a concentration of 0.02 wt %.
PVA and PVP are surface-active but dextran, PEG, PDDC, and NaPSS are not. The results of the surface tension measurement are given in the second column of Table 1. Because of the Table 1. Influence of Solute Addition on the Wetting Properties and Stain Pattern for the Polycarbonate Substratea solute
γlg (mN/m)
θa (deg)
θr (deg)
Δθ (deg)
stain pattern
water copper sulfate cellobiose Brij-35 NaPSS (70K) PDDC (100−200K) dextran (500K) PEG (20K) PVA (130K) PVP (360K)
70.1 70.1 69.8 41.6 70.1 69.5 70.0 60.1 55.6 66.2
84.7 81.9 82.0 65.0 73.8 77.5 83.8 74.5 84.9 81.7
65.6 64.9 64.3 24.5 58.4 57.3 65.9 50.7 30.3 31.1
19.1 17.0 17.7 40.5 15.4 20.2 17.9 23.8 54.6 50.6
concentrated concentrated ringlike concentrated concentrated concentrated concentrated ringlike ringlike
a
IV. CONCLUSIONS A ring-shaped stain is often seen on a hydrophilic substrate after the evaporation of the drop containing small nonvolatile solutes owing to edgeward flow and pinned contact line. The retreat of the contact line, which may influence the final stain pattern, is related to the wetting behavior of the solutecontaining drop on the substrate, such as contact angle hysteresis. Because the wetting-relevant characteristics of solutes and substrates are able to affect the wetting behavior of a liquid drop on a surface, the stain pattern after evaporation can be altered by them as well. In this work, the wetting characteristics that control the deposition pattern are explored. By employing various small solutes and substrates with different wetting characteristics, different types of stain patterns associated with a drying drop are observed. Dilute solutions are used to prevent the early precipitation effect. The pattern of evaporation stains is not sensitive to the initial solute concentration in the dilute regime. For surface-inactive solutes such as metal salt and glucose, the final stain pattern depends on the extent of CAH associated with the substrate. For substrates with weak CAH such as silica glass and polycarbonate, a concentrated stain is formed instead of a ringlike stain. On such surfaces, the receding contact angle is easily reached and the gradual withdrawal of the contact line hinders solute deposition along the perimeter of the drop. On the contrary, for substrates with strong CAH such as graphite, the coffee-ring effect always appears. On those surfaces, the contact line can be pinned persistently because θr is difficult to reach. In fact, the solute concentration arrives at its saturation solubility and precipitates out to form a ringlike stain before the contact line starts to retreat. However, the deflation experiment associated with the suction of a partially drying drop shows the absence of the ringlike residues on the graphite surface. This consequence confirms that the deposition of surface-inactive solutes takes place during the final period of evaporation.
The solution concentration is fixed at 0.02 wt %.
dilute solutions, the surface tensions of polymer solutions for dextran, NaPSS, and PDDC are essentially the same as that of pure water. On the contrary, the surface tensions of polymer solutions for PEG, PVA, and PVP are significantly lower than that of pure water. This consequence qualitatively reveals the surface activity associated with those polymers. Although the liquid−gas interfacial tension (surface tension) is generally used to describe the surface activity, the change in solid−liquid interfacial tension due to solute adsorption is more important in understanding CAH25−27 because the receding angle is a more relevant wetting property for contact line withdrawal. The results of contact angle measurements for various solutes on polycarbonate are listed in the third and fourth columns of Table 1. The advancing contact angles of all polymers solutions are similar and vary within 10° (i.e., θa ≈ 74−85°). However, the surface activity in terms of the receding contact angle can be classified into two groups. Polymer solutions including NaPSS, PDDC, dextran, and PEG behave just like pure water does on the surface of polycarbonate (i.e., θr ≈ 51−66°). That is, they are surface-inactive; therefore, the concentrated stain pattern is formed as illustrated in Figure 8. However, the polymer solution of PVA or PVP leads to a very small receding contact H
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ACKNOWLEDGMENTS This research work is supported by the National Science Council of Taiwan.
Because the withdrawal rate of the contact line is less than the solute deposition rate, ring-shaped colloidal stains are formed. Note that ordered crystals are observed initially but disordered packing is seen at the end of the droplet’s life.29 To acquire a ringlike stain on a hydrophilic substrate with weak CAH such as acrylic glass, the wetting property of the liquid drop on the substrate must be modified and can be achieved by surface-active solutes. The addition of surfactant leads to the reduction of the advancing contact angle. Moreover, the receding contact angles are significantly lowered on the basis of the liquid-induced defect model, and the contact line can be pinned persistently. Thus, the ringlike pattern is easily observed for a drying drop containing surfactant. The deflation experiment also shows the presence of the ringlike residues. This result illustrates that the coffee-ring effect occurs because of surfactant adsorption long before complete evaporation. In addition, besides the edge, solute deposition is seen everywhere in the liquid−solid contact region. The dirty ring formed by surface-active solutes on substrates with weak CAH is quite different from the clean ring formed by surfaceinactive solutes on substrates with strong CAH. When the drop contains a mixture of surface-active and surface-inactive solutes such as Brij-35 and CoSO4, the combined pattern of ringlike and concentrated stains can appear on a substrate possessing weak CAH with respect to pure water, such as polycarbonate. After the analysis of the shape evolution of the drying drop during the final stage, it is found that a pinned contact line is induced by surfactant adsorption and the occurrence of surfactant precipitation leads to the formation of the ringlike stain. The withdrawal of the contact line starts from a corner, and eventually most of the contact line recedes toward the everlasting pinned contact line. Eventually, the precipitation of surface-inactive solutes takes place in the vicinity of the pinned contact line, and the concentrated stain is developed. This result demonstrates that the separation of a mixture can be achieved on an evaporating stain on the basis of their differences in the surface activities of the components. Evidently, the pattern of an evaporating stain can be determined by the competition between the receding contact line and the small solute precipitation (adsorption). The former relates to CAH, and the latter depends on the surface activity. For the drying drop containing surface-inactive solute on the substrate with weak CAH, one has τspre ≫ τret and the concentrated stain forms. In contrast, on the substrate with strong CAH, one has τret ≫ τspre, and the ringlike stain develops. To examine our model for the pattern of evaporating stain, drying drops containing large polymeric solutes on the substrate with weak CAH are considered. The results are consistent with our explanation for small solutes. The concentrated stain is observed for polymers leading to weak CAH (Δθ ≈ 20°), and the ringlike pattern is seen for polymers resulting in strong CAH (Δθ ≈ 50°). In summary, the suppression of the coffee-ring effect can be acquired by the control of contact angle hysteresis.
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