Solute Concentration-Dependent Contact Angle Hysteresis and

Because of such a feature associated with a real surface, a hysteresis loop is formed in a plot of CA versus drop volume by the process of inflation/d...
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Solute Concentration-Dependent Contact Angle Hysteresis and Evaporation Stains 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, Department of Physics, National Central University, Jhongli, Taiwan 320, R.O.C. S Supporting Information *

ABSTRACT: The presence of nonvolatile solutes in a liquid drop on a solid surface can affect the wetting properties. Depending on the surface-activity of the solutes, the extent of contact angle hysteresis (CAH) can vary with their concentration and the pattern of the evaporation stain is altered accordingly. In this work, four types of concentration-dependent CAH and evaporation stains are identified for a water drop containing polymeric additives on polycarbonate. For polymers without surface-activity such as dextran, advancing and receding contact angles (θa and θr) are independent of solute concentrations, and a concentrated stain is observed in the vicinity of the drop center after complete evaporation. For polymers with weak surface-activity such as poly(ethylene glycol) (PEG), both θa and θr are decreased by solute addition, and the stain pattern varies with increasing PEG concentration, including a concentrated stain and a mountain-like island. For polymers with intermediate surface-activity such as sodium polystyrenesulfonate (NaPSS), θa descends slightly, but θr decreases significantly after the addition of a substantial amount of NaPSS, and a ring-like stain pattern is observed. Moreover, the size of the ring stain can be controlled by NaPSS concentration. For polymers with strong surface-activity such as poly(vinylpyrrolidone) (PVP), θa remains essentially a constant, but θr is significantly lowered after the addition of a small amount of PVP, and the typical ring-like stain is seen. difference between the advancing and receding angle (Δθ = θa − θr). The existence of CAH is generally explained by two mechanisms, the defect model which is associated with hydrophilic blemishes or surface roughness5 and the adhesion hysteresis that refers to the restructuring of the solid surface after wetting.2 In the former mechanism, the amplitude of the hysteresis varies as the area density of defects and the square of the maximum pinning force, which depends on the defect’s wettability, size, and shape.5 In the latter mechanism, the solid− liquid interfacial tension γsl is reduced to γ′sl because of surface relaxation (rearrangement). As a result, CAH involves two solid−liquid interfacial tensions, and the Young’s equation gives θa associated with γsl and θr with γsl′ . CAH is also manifested through the pinning−depinning behavior of the contact line,

1. INTRODUCTION The wettability of a solid surface by a liquid drop can be expressed by the static contact angle (CA) between the gas− liquid and solid−liquid interfaces. The CA (θ) is related to the interfacial tensions of solid−gas (γsg), solid−liquid (γsl), and liquid−gas (γlg) by Young’s equation,1 cos θ = (γsg − γsl)/γlg. For an ideal flat surface, the CA is unique and depends on the chemical composition of the substrate. However, on a real (nonideal) surface, the CA can exist in a range of angles, θr ≤ θ ≤ θa. Here, θa is referred to as the advancing CA beyond which the drop begins to expand. In contrast, θr is depicted as the receding CA below which the drop starts to withdraw. Because of such a feature associated with a real surface, a hysteresis loop is formed in a plot of CA versus drop volume by the process of inflation/deflation. As a result, the CA depends not only on its current environment but also on its past state. This phenomenon is known as the contact angle hysteresis (CAH),2−4 which is generally illustrated in terms of the © 2014 American Chemical Society

Received: April 14, 2014 Revised: June 11, 2014 Published: June 16, 2014 7716

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2.2. Stain Formation. The slices of transparent polycarbonate are used as the substrate in this work (Kwo-Yi Co., Taiwan). The evaporation of the drop with an initial volume of about 10 μL is performed on a polycarbonate substrate, which is placed inside a box with container desiccants. Note that all of the substrates are precleaned with deionized water and ethanol. Generally, it takes about 2 h to complete the evaporation process for a 10-μL drop. After complete evaporation of the aqueous drops, various patterns are observed by microscopy (Optem Zoom 125C), and the detailed features of the patterns are further acquired by optical microscopy (Olympus BX-51). To obtain clear images of the evaporation stain patterns on transparent substrates, the intensity of the incident light and the width of the aperture are varied to achieve a maximum contrast between the stain pattern and the substrate. 2.3. Wetting Property Measurement. The CA and the liquid− gas interfacial tension (γlg) of the polymeric solution on the polycarbonate are analyzed by the CA goniometer with the droplet shape analysis system DSA10-MK2 (Krüss, Germany) at room temperature under open-air conditions with a relative humidity of 45−50%. When side-view images of the sessile drops on the polycarbonate substrate are taken, the values of CAs are determined by the tangent mode (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 mean CA is reported. Note that each reported data is given based on more than three independent measurements. The CAH of the substrate is realized by measuring the advancing and receding CAs. The addition of liquid to the sessile droplet by a syringe makes the contact line move outward with an advancing angle (θa). The removal of liquid from the sessile droplet by a syringe is accompanied by contact line pinning until the receding angle is achieved. In general, the contact line moves inward with the receding angle (θr). The rate of inflation and deflation should be kept as slow as possible to avoid disturbing the contact line with fluid flow. Typically, the deflation is completed within 1 min to avoid the evaporation of the drop. Note that the volume of a static sessile drop remains essentially the same within 1 min.

and the transition corresponds to the advancing or receding CA. Therefore, θa and θr can be obtained by several approaches involving contact line movement.6−14 In the tilted plate method, a drop is placed on an inclined plate, and θa and θr are determined when the drop starts to slide down. In the Wilhelmy method, a thin plate is immersed into a liquid bath, and the relevant CAs are decided when the plate is pushed down or pulled up. In the dynamic sessile drop method, CAH can be observed by increasing or decreasing the volume of the sessile drop dynamically without changing its solid−liquid interfacial area. The evaporation method is a modification of the sessile drop method, and the receding angle is measured as the evaporating drop begins to dewet. The wetting behavior of a drying drop may be altered because of the presence of solutes. Moreover, after the evaporation of a droplet, a ring-like stain often appears. For example, the so-called “coffee ring” is commonly seen after spilling coffee.15,16 The appearance of the ring-like stain involves three ingredients for a drying droplet: nonvolatile solute, outward flow, and contact line pinning. As first explained by Deegan et al.,17,18 the dispersed solutes in the drying drop are carried toward the edge due to the edgeward capillary flows. Because of solute deposition at the vicinity of the contact line, a ring-like stain forms, rather than a uniform stain. Since the evaporation deposition provides a potential means to write or leave a pattern onto a surface exquisitely, it plays an important role in coating, inkjet printing, self-assembly, small scale fabrication processes for surface patterning, and sizediscriminative transportation and separation of nanomaterials without resorting to any external forces.19−26 In the previous study, drying patterns of a water drop containing various solutes on different hydrophilic substrates were observed, including a concentrated stain, a ring-like deposit, and a combined structure.27 The pattern of the evaporation stain depends on the surface activity of solutes and the CAH of substrates. For substrates with strong CAH such as graphite, the contact line of the drop tends to pin on the surface and forms a ring-like stain. On the contrary, the contact line of the drop dewets on the substrate with weak CAH like polycarbonate during evaporation, and a concentrated drying pattern is observed. Nonetheless, the addition of the solute with surface-activity is also able to alter the CAH of substrates and therefore change the stain pattern. Since the surface-activity of the solute often varies with its concentration, the wetting properties and evaporation stain may depend on the solute concentration as well. In this work, the wetting properties and stain patterns of drops containing various polymeric solutes on substrates with weak CAH (polycarbonate) are investigated. Four types of the wetting properties and stain patterns are identified: concentration independent, weak concentration dependent, intermediate concentration dependent, and strong concentration dependent.

3. RESULTS AND DISCUSSION In this work, the stain patterns of water drops containing various polymeric solutes on the substrates with weak CAH (polycarbonate) are investigated. The CAH of a pure water drop on polycarbonate is Δθ = θa − θr ≈ 18°. The addition of polymeric solutes possessing surface-activity, which may vary with their concentration, can lead to the change of the degree of the CAH, which is closely related to contact line pinning. That is, both the advancing and receding CAs may be altered as the concentration of polymeric solute is increased. Note that added polymers cannot be characterized as typical surfactant because they affect the surface tension very little. Four types of the wetting properties and stain patterns can be identified. The suggestive causation between wetting properties and deposition morphology will be established. Basically, added polymers can be categorized into concentration independent, weak concentration dependent, intermediate concentration dependent, and strong concentration dependent. 3.1. Surface-Inactive Polymer Solute (Concentration Independent). Consider a drop containing polysaccharide (dextran) as the nonvolatile solute and that the concentration of the solute is 0.05 wt %. The drop with the volume about 11 μL is deposited on a polycarbonate surface. The effect of the solute on the wetting property is shown in Figure 1a. The variation of the CA and base diameter (BD) with the droplet volume is acquired by the deflation method. As the droplet volume is decreased, the CA declines from θa ≈ 86.5° due to contact line pinning, which corresponds to constant BD. When

2. MATERIALS AND METHODS 2.1. Materials. There are two types of polymeric solutes used as the nonvolatile solutes in the drops: nonionic polymers and polyelectrolytes. The former contains dextran 500 K (polysaccharide) (Sigma-Aldrich, Fluka, USA), poly(ethylene glycol) 20 K (PEG) (Sigma-Aldrich, Fluka, USA), and poly(vinylpyrrolidone) 360 K (PVP) (Acros, USA). The latter includes sodium polystyrenesulfonate 70 K (NaPSS) (Aldrich, USA). The aqueous solution for an evaporative drop is prepared by dissolving a nonvolatile solute in the deionized water. 7717

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drop indicate that the contact line dewets significantly during evaporation. After complete evaporation, a concentrated stain of polysaccharide is clearly seen in the vicinity of the center of the drop. This result is caused by weak CAH associated with the substrate polycarbonate Δθ ≈ 18°, which is not altered by the addition of polysaccharide. However, a ring-like pattern with the width about 100 μm can be observed under a microscope, as demonstrated in the snapshot (7) of Figure 1b. This consequence indicates that at the final stage of drying, the polysaccharide concentration becomes so high along the rim of the tiny drop that deposition begins. 3.2. Surface-Active Polymer Solute (Weak Concentration Dependent). When an aqueous drop contains polymers with surface-activity, it is anticipated that both wetting behavior and stain pattern are different from those of dextran. Consider aqueous drops which have different concentrations of poly(ethylene glycol) (PEG). The drop with the volume about 10 μL is deposited on a polycarbonate substrate, and the deflation method is also used to investigate the variation of the CA and BD with the droplet volume. The influence of PEG on the wetting property is illustrated in Figure 2. In the deflation process of the drop containing PEG with low

Figure 1. (a) Variation of the contact angle with volume of a water drop containing dextran on polycarbonate during the deflation process. (b) Evaporation of a drop containing dextran on polycarbonate.

the CA reaches θr ≈ 68.3°, the CA remains unchanged, but the contact line starts to withdraw upon deflation. Since the advancing and receding CAs are essentially the same as those of the pure water drop, the influence of polysaccharide at low concentration is insignificant. In order to examine the influence of solute concentration, the wetting property of a drop containing polysaccharide of 5 wt % is investigated. As illustrated in Figure 1a, the variation of the CA and BD at high concentration with the volume is similar to those at low concentration. This result indicates that polysaccharide is surface-inactive and that the addition of such solutes cannot alter the wetting properties of the substrate. This concentration-independent feature can also be demonstrated through the surface tension measurement. γlg is about 71.5 mJ/m2 at low concentration (0.05 wt %) and is slightly lowered to 70.1 mJ/ m2 at high concentration (5 wt %). As a result, γsl is also insensitive to polysaccharide concentration according to Young’s equation. When an aqueous drop containing surface-inactive solutes such as inorganic salts or monosaccharides27 is deposited on a surface with weak CAH, the drying pattern tends to be a concentrated stain in the center of the drop because the contact line withdraws before solute precipitation. Figure 1b shows the dynamic sequence of the evaporation of a drop containing polysaccharide. The photos of top view and side view of the

Figure 2. Variation of the contact angle with the volume of a water drop containing poly(ethylene glycol) on polycarbonate by the deflation method.

concentration (0.05 wt %), the CA starts to decrease from θa ≈ 77.6° due to contact line pinning, which corresponds to constant BD. When the CA reaches θr ≈ 57.2°, the CA remains unchanged, but the contact line begins to withdraw upon deflation. Note that the contact angles associated with pure water are θa ≈ 86° and θr ≈ 68°. This result indicates that due to the addition of PEG, the wetting property is evidently changed and that both the advancing and receding contact angles descend. The effect of solute concentration on the wetting property is further studied by examining a drop containing PEG with high concentration (5 wt %). As demonstrated in Figure 2, the variation of the CA and BD at high concentration with the volume is essentially the same as those at low concentration. This consequence reveals the weak surface-activity associated with the PEG. The characteristic of such concentration dependence is also manifested through the surface tension measurement. γlg is lowered to about 63.8 mJ/ m2 at low concentration (0.05 wt %) and is further lowered to 7718

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62.8 mJ/m2 at high concentration (5 wt %). As a result, the wetting property can be altered by adding PEG owing to its weak surface-activity but becomes saturated at high solute concentrations. Although the wetting property of an aqueous drop containing PEG is insensitive to the solute concentration, the evaporation pattern varies significantly with PEG concentrations, as shown in Figure 3a. Evidently, when the solute concentration is increased, the size of the drying pattern observed by top view grows larger. In addition, the side view at high concentration shows that the stain pattern exhibits a mountain-like island. The feature of this drying pattern is

distinct from that of the typical coffee ring stain and indicates that the solute deposition mechanism of the former is different from that of the latter. However, the variation of the CA with droplet volume during evaporation is essentially the same as that determined by the deflation method, as illustrated in Figure 3b. During the initial period of the evaporation process, the contact line is pinned, but no precipitation is observed. When the contact line begins to withdraw, the CA is maintained at θr, but precipitates are still not seen. However, near the end of the receding period, precipitates start to appear on the surface covered by the shrinking drop, as pointed out by the arrow in Figure 3b for high solute concentration (5 wt %). Note that at this moment, the bulk solute concentration does not reach the saturation limit. In fact, the precipitation may be attributed to segregation of the excess solute in the confined three-phase contact region at the edge of the spreading fluid.28−32 That is, solute deposition near the contact line may be initiated by such a wedge confinement effect. However, the dewetting of the contact line continues upon evaporation and is not affected by the presence of the precipitate. The changes of the contact line on the precipitate layer can be further elucidated by a sequence of the top views, as shown in Figure 3c. One can notice that the droplet withdraws toward the center of the precipitate layer. Because of continuous contact line withdrawal and persistent solute precipitation from the shrinking drop, a mountain-like shape of the precipitate is seen, as demonstrated in Figure 3c. From the image of the optical microscope, it is also clear that the thickness of the precipitate layer grows from the edge toward the center. Such an intriguing process of mountain-like stain formation is different from those associated with coffee ring stain and concentrated stain patterns.17,27 3.3. Surface-Active Polymer Solute (Intermediate Concentration Dependent). Obviously, the wetting behavior of a water drop on a substrate can be changed by adding surface-active solutes. Consider aqueous drops which have different concentrations of sodium polystyrenesulfonate (NaPSS). The drop with the volume about 10.5 μL is deposited on polycarbonate. Again, the influence of NaPSS addition on the wetting property is investigated through measuring the variation of the CA and BD with the droplet volume by the deflation method. As illustrated in the inset of Figure 4a, the advancing CA of the drop containing NaPSS with low concentration (0.05 wt %) is about θa ≈ 85.5°, and the BD starts to withdraw when the drop reaches its receding CA about θr ≈ 66.3°. Such wetting behavior is almost the same as those of pure water on polycarbonate. However, the effect of solute concentration on the wetting property can be clearly seen when a drop contains NaPSS at high concentration. Figure 4a shows the plot of the CA and BD against the drop volume at a concentration of 5 wt %. Because of the surface-activity of NaPSS, the advancing CA is decreased from 85.5° to 79.3°. Moreover, the BD remains unchanged, and the CA continues to descend until the drop volume becomes quite small, i.e., about 2.8 μL. Further deflation results in depinning of the contact line. Accompanying the withdrawal of the contact line, however, the CA is not kept at a constant value corresponding to the receding CA. Such a continuous decline of the CA with decreasing drop volume is generally observed for deflating small enough drops, e.g., Figure 1a. As a result, the receding CA is defined at the beginning of depinning, θr ≈ 32.1°. It is significantly lower than θr at low concentration. Note that such

Figure 3. (a) Photos of evaporation stains of a drop containing poly(ethylene glycol) at different concentrations. (b) The variation of the contact angle and base diameter with the volume of a drop containing 5 wt % poly(ethylene glycol) on polycarbonate during the evaporation process. (c) The dynamic process of evaporation of a drop containing 5 wt % poly(ethylene glycol). 7719

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Figure 4. (a) Variation of the contact angle with the volume of a water drop containing poly(sodium 4-styrenesulfonate) on polycarbonate during the deflation process. (b) The change of advancing and receding contact angles of a water drop containing poly(sodium 4-styrenesulfonate) with different concentrations. (c) The variation of the contact angle and base diameter with the volume of a drop containing 5 wt % NaPSS on polycarbonate during the evaporation process.

a definition differs from the typical one of θr corresponding to a limiting value of CAs. The influences of the polymer concentration (NaPSS) on both θa and θr are further demonstrated in Figure 4b. Evidently, as NaPSS concentration is increased, the advancing CAs are slightly decreased, but the receding CAs are substantially descended. The weak effect of NaPSS concentration on θa indicates that NaPSS is not a typical surface-active agent which lowers γlg significantly by a small amount of addition. In contrast, the strong influence of polymer addition on θr reveals that γ′sl is sensitive to NaPSS concentration. According to the surface relaxation model (from γsl to γsl′ ), γsl is determined by the initial deposition of NaPSS, and γsl′ resulted from the subsequent relaxation of the adsorbed polymers due to their negative charges and chain adjustments. Depending on the bulk

polymer concentration, the adsorbed amount and packing arrangement of NaPSS vary. This consequence alters the extent of surface relaxation, which in turn changes the depinning− pinning behavior of the contact line. However, when the NaPSS concentration is high enough, both θa and θr become insensitive to polymer concentration as demonstrated in Figure 4b. Figure 4c shows the change of the CA and BD with the volume of a drop containing 5 wt % NaPSS initially on polycarbonate during evaporation. The results are essentially the same as that in Figure 4a obtained by deflation. Note that the polymer concentration remains the same in Figure 4a but grows in Figure 4c. This consequence is consistent with the result of Figure 4b. The evaporation stain is also dependent on NaPSS concentration. At lower concentration, the extent of CAH 7720

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PVP gives quite low θr at low solute concentrations, and thus, ring-like stains are shown. Evidently, they possess strong surface-activity, which can be manifested by the influence of their polymeric solute concentration. Figure 6 shows the

(Δθ) is smaller, and thus, it is easier for the contact line to withdraw initially during evaporation. However, because the polymer concentration becomes high enough eventually, the contact line repins, and a smaller size of the ring stain will be developed. As illustrated in Figure 5, the size of the drying

Figure 5. Photos of evaporation stains of a drop containing NaPSS with different concentrations. The typical coffee ring can be observed at sufficiently high concentrations.

pattern grows with increasing polymer concentration. Nonetheless, when the concentration is sufficiently high, e.g., 5 wt %, initial depinning of the contact line vanishes, and the typical coffee ring stain is observed. This consequence reveals that the size of the ring stain can be controlled by the concentration of the surface-active polymer solute. It is obvious that the pinning strength of the contact line grows with increasing polymer concentration and influences the size of the stain pattern. In order to enhance the pinning strength, NaPSS has to lower the solid−liquid interfacial tension in the vicinity of the contact line. A common way to do so is the deposition of surface-active solutes on the substrate, which can be examined by deflating the drop completely at a particular point in time. Photo i in Figure 4c demonstrates the presence of a ring-like pattern obtained by a quick deflation at 30 seconds after the deposit of the drop. Since the time period for evaporation is very short, such a ring-like stain is unlikely formed by the outward flow associated with the typical coffee ring. Similar to the precipitation of PEG, the wedge confinement effect associated with the excess solute near the contact line initiates solute deposition at the edge of the fluid,28−32 which in turn lowers γ′sl and thereby enhances the pinning strength. As time proceeds, inhomogeneous evaporation on the drop surface induces outward flow and leads to serious solute accumulation in the neighborhood of the contact line. Consequently, more solutes are deposited along the pinned contact line. As illustrated in photo ii in Figure 4c, the thickness of the ring-like pattern grows with time. Since the pinning strength reaches its maximum for a sufficiently thick deposited layer, depinning of the contact line occurs eventually at a small drop volume (V ≈ 2 μL). This outcome is different from that in the system containing surfactant additive or graphite substrate.27 In the latter cases, contact line pinning persists until complete evaporation. 3.4. Surface-Active Polymer Solute (Strong Concentration Dependent). At low polymeric solute concentration (e.g., 0.02 wt %), dextran, PEG, and NaPSS behave like surfaceinactive solutes.27 Thereby, concentrated stains are seen on the polycarbonate substrate. However, as the solute concentration is increased, PEG and NaPSS exhibit, respectively, weak and intermediate surface-activity, while dextran still shows no surface-activity according to the receding CA. As a result, various patterns of evaporation stain are observed. In contrast,

Figure 6. Change of surface tension and contact angles of a water drop containing PVP with different concentrations.

variation of the advancing and receding CAs with the PVP concentration. As the solute concentration is increased, the advancing CA remains roughly a constant, about 85.5°. This result is consistent with that of the surface tension, whose value is about 68 mJ/m2 and seems to be independent of the polymer concentration. Obviously, PVP is not a typical surface-active agent like detergent, which reduces θa through lowering γlg. However, the receding CA descends significantly with increasing the polymer concentration. At extremely dilute concentration (10−4 wt %), both the advancing and receding CAs are essentially the same as those of pure water. The value of θr drops quickly from 68.5° to 34.2° at 0.1 wt % and becomes saturated for sufficiently high concentrations. This consequence reveals that unlike γlg, the solid−liquid interfacial tension (γsl′ ) is very sensitive to the PVP concentration. In fact, the addition of a small amount of PVP leads to serious CAH owing to its strong surface-activity. Owing to the strong CAH, contact line pinning of a drop containing PVP persists during evaporation as demonstrated in Figure 7a. As a result, a typical ring-like stain of PVP is left after complete drying, as shown in Figure 7b.27 The width of the ring is about 250 μm. In contrast, for a drop containing surfaceinactive solutes such as CuSO4, the depinning behavior of the contact line occurs due to the weak CAH. Thereby, a concentrated stain is formed eventually as shown in Figure 7b.27 If the droplet consists of a mixture of surface-active and surface-inactive solutes, it is anticipated that surface-inactive solutes are able to deposit in a ring-like pattern because of the contact line pinning associated with the surface-active solute. Consider a water drop containing both PVP and CuSO4 on the surface of the polycarbonate. As illustrated in Figure 7b, a ringlike stain is developed, and its width is significantly greater than 7721

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Table 1. Summary Table of Wetting Properties of Polymeric Solutes on Polycarbonate solute

concn (wt %)

γlg (mJ/m2)

θa (deg)

θr (deg)

Δθ (deg)

dextran

0.05 5 0.05 5 0.05 5 10−4 0.1

71.5 70.1 63.8 62.8 71.2 58.6 67.5 66.2

86.5 87.6 77.6 76.5 85.5 79.3 85.5 84.6

68.3 70.2 57.2 55.5 66.3 32.1a 68.5 34.2a

18.2 17.4 20.4 21.0 19.2 47.2 17.0 50.4

PEG NaPSS PVP a

The receding contact angle defined at the beginning of contact line depinning. Figure 7. (a) Evaporation of a drop containing both PVP (0.05 wt %) and CuSO4 (0.05 wt %). (b) The stain patterns of PVP only, CuSO4 only, and the mixture.

Table 2. Summary Table of the Evaporation Behaviors and Deposition Morphology

that associated only with PVP. Evidently, this thick ring contains two distinct layers. The outer layer is formed by PVP deposition during the early period of evaporation, while the inner layer corresponds to the crystallization of CuSO4 in the final stage. It should be noted that in the final stage, the drop becomes a thin film with its height much less than its BD due to contact line pinning. Eventually, film thickness reaches the critical thickness and becomes unstable. That is, the water film dewets and finally ruptures. In Figure S1a of the Supporting Information, the dewetting region is indicated by the arrows and the PVP ring is clearly seen. As time proceeds, the dewetting region expands toward the PVP outer ring. Ultimately, the ring-like film dries, and thus, the inner ring stain of CuSO4 is produced. Sometimes, the water film ruptures at the perimeter because of the inhomogeneity of PVP deposition. The location along the perimeter where PVP deposition is scarce is unable to pin the contact line, and therefore the rupture takes place easily at this spot. Figure S1b (Supporting Information) demonstrates that the final stain is a ring-like pattern with an opening.

tration-dependent size, while the latter yields a shrunken ringlike stain with concentration-dependent size.

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ASSOCIATED CONTENT

S Supporting Information *

Evaporation process of a drop containing the mixture of PVP and CuSO4 at the final stage. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSIONS The presence of nonvolatile solutes in a liquid drop on a solid surface can affect contact angle hysteresis. In this work, watersoluble polymers are used as additives. Depending on the surface-activity of the solutes, the extent of CAH can vary with the concentration of the solute. Four types of concentrationdependent CAH can be identified: (i) concentration independent (dextran), (ii) weak concentration dependent (PEG), (iii) intermediate concentration dependent (NaPSS), and (iv) strong concentration dependent (PVP). The wetting properties of polymeric solutes on polycarbonate are summarized in Table 1. Those polymeric solutes are not typical surfactants because they alter the receding CA rather than surface tension. The formation of the evaporation stain of a drop containing solute is closely related to the pinning− depinning behavior of the contact line, which corresponds to CAH. As a consequence, four types of evaporation stains can be observed for polymeric solutes. The evaporation behaviors and deposition morphology is summarized in Table 2. Note that the stain patterns formed by PEG and NaPSS are distinctly different from the typical patterns, coffee ring and concentrated stain. The former gives a mountain-like stain with concen-

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AUTHOR INFORMATION

Corresponding Authors

*(Y.-J.S.) E-mail: [email protected]. *(H.-K.T.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was supported by Ministry of Science and Technology of Taiwan. REFERENCES

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dx.doi.org/10.1021/la501438k | Langmuir 2014, 30, 7716−7723