Forced Spreading of Aqueous Solutions on Zwitterionic Sulfobetaine

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Forced Spreading of Aqueous Solutions on Zwitterionic Sulfobetaine Surfaces for Rapid Evaporation and Solute Separation Cyuan-Jhang Wu,† Vickramjeet Singh,† Yu-Jane Sheng,*,‡ and Heng-Kwong Tsao*,†,§ †

Department of Chemical and Materials Engineering, National Central University, Jhongli, Taiwan 320, R.O.C. Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C. § Department of Physics, National Central University, Jhongli, Taiwan 320, R.O.C. ‡

S Supporting Information *

ABSTRACT: Solute separation of aqueous mixtures is mainly dominated by water vaporization. The evaporation rate of an aqueous drop grows with increasing the liquid−gas interfacial area. The spontaneous spreading behavior of a water droplet on a total wetting surface provides huge liquid−gas interfacial area per unit volume; however, it is halted by the self-pinning phenomenon upon addition of nonvolatile solutes. In this work, it is shown that the solute-induced self-pinning can be overcome by gravity, leading to anisotropic spreading much faster than isotropic spreading. The evaporation rate of anisotropic spreading on a zwitterionic sulfobetaine surface is 25 times larger as that on a poly(methyl methacrylate) surface. Dramatic enhancement of evaporation is demonstrated by simultaneous formation of fog atop liquid film. During anisotropic spreading, the solutes are quickly precipitated out within 30 s, showing the rapid solute−water separation. After repeated spreading process for the dye-containing solution, the mean concentration of the collection is doubled, revealing the concentration efficiency as high as 100%. Gravity-enhanced spreading on total wetting surfaces at room temperature is easy to scale-up with less energy consumption, and thus it has great potentials for the applications of solute separation and concentration.

1. INTRODUCTION Solute separation of aqueous mixtures is important in industrial applications such as seawater desalination, food preservation, and wastewater treatment. A water−solute separation system usually involves multieffect distillation under high temperature by spraying aqueous droplets into hot airflow. The water is evaporated toward condensate trap while the precipitated solutes are collected at the bottom. During the process, the evaporation rate of water is closely related to heat convection and depends on the drop size. However, this system has high energy consumption and requires a sophisticated spray nozzle (atomizer) to achieve high energy efficiency and water extraction.1−4 The efficiency of the separation process is mainly dominated by water vaporization, and thus the enhancement of the evaporation rate plays a role of crucial importance. Evaporation is vaporization of a liquid from the liquid−gas interface to the unsaturated gas phase. The evaporation rate of water is affected by many factors including relative humidity, air flow rate, temperature, pressure, and interfacial area. The more the liquid−gas area per unit liquid volume is, the higher the evaporation rate is. Consequently, the evaporation rate of aqueous droplets deposited on substrates relies on the drop shape which is determined by the wettability of water on the substrate. It is generally difficult for an aqueous drop to spread on typical surfaces due to partial wetting and self-pinning. However, as the hydrophilicity of the substrate is increased, the liquid−gas surface area grows. Therefore, the evaporation rate © XXXX American Chemical Society

of the drop rises with decreasing the contact angle at a given volume. When the contact angle reaches zero or the spreading coefficient exceeds zero, the substrate is referred to as the total wetting surface and the liquid drop spreads spontaneously.5 Since the interfacial area grows continuously, the evaporation rate of water on a total wetting surface is expected to be much higher than that on a typical surface. The wettability of a drop (l) on a surface (s) in ambient gas (g) is the consequence of the competition among solid−gas (γsg), solid−liquid (γsl), and liquid−gas (γlg) interfacial tensions. The spreading coefficient defined as S = (γsg − γsl) − γlg is used to determine the occurrence of spontaneous spreading. For S > 0, the liquid drop spreads spontaneously on the surface, corresponding to the total wetting behavior. On the contrary, as S < 0, the droplet wets the surface partially, exhibiting a contact angle.6,7 In general, the total wetting behavior can be demonstrated by two approaches, liquid drops with ultralow surface tension (γlg) or solid surfaces with high surface free energy (γsg). An example of the former is polydimethylsiloxane drops (low γlg) spreading spontaneously on various surfaces.8,9 The spreading dynamics can be described by Tanner’s law. The latter is often involved with continuously spreading of pure solvents such as water and long chain alkane on zwitterionic Received: April 22, 2017 Revised: June 20, 2017 Published: July 14, 2017 A

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Figure 1. Variation of wetting area of (a) 1 wt % cellobiose and (b) 1 wt % Dextran aqueous drops on SBSi surfaces.

sulfobetaine surfaces (high γsg). Its spreading motion can be depicted by the simple power law as well but the exponent is smaller than that in Tanner’s law.5,8 Unfortunately, the total wetting behavior on a substrate with high surface energy is impeded when the pure liquid contains nonvolatile additives. It is referred to as self-pinning and generally attributed to the additive confinement at the triplephase contact line.10−13 As a colloidal droplet (e.g., colloidal particles in decalin) is placed on a clean glass surface, the liquid exhibits the total wetting behavior first. Note that decalin has low surface tension while the clean glass possesses high surface energy. In the meantime, the particles are carried toward the edge of the droplet due to the outward spreading flow.10 It is different from the typical coffee ring effect in which the outward flow is driven by differential evaporation.14−16 Eventually, the particles confined at the edge generate a hydrodynamic resistance (capillary force) to hinder continuous spreading, leading to the self-pinning phenomenon.17 In order to achieve the goals of rapid evaporation and solute separation, the self-pinning behavior on an intrinsically total wetting surface must be overcome. In this work, many kinds of solutes including small molecules and polymers are used to observe the self-pinning behavior on total wetting surfaces, as described in section 3.1. Then, as shown in section 3.2, this undesired self-pinning behavior can be circumvented by external forces such as gravity. The spreading of a drop containing solutes is not only continuous but also enhanced. Finally, as described in section 3.3, such gravity-driven spreading can enhance the evaporation rate dramatically and possesses the advantage of less energy consumption for solute separation. Therefore, gravity-driven spreading on a total wetting surface has the potential in the solute-water separation and concentration applications.

aqueous solution (0.4%) were obtained from Sigma-Aldrich Co. (USA). Anhydrous ethanol (99.5%) was acquired from Echo Chemical Co. (Taiwan). All chemicals in this work are analytical grade and used as received without further purification. The glass slides were bought from Yancheng Guanghui Medical Products Factory (China). The poly(methyl methacrylate) surface (PMMA) was purchased from Kow-Yi Co. (Taiwan). 2.2. Synthesis of Zwitterionic Sulfobetaine Silane (SBSi). The synthesis of SBSi had been reported in the previous works.18,19 A total of 5 g of DMASi and 3 g of 1,3-propane sultone were dissolved in 20 mL of acetone and 5 mL of acetone, respectively. The above two solutions were mixed and reacted for 6 h (hrs) at room temperature in nitrogen environment. Afterward, the white suspension was filtrated and rinsed by 200 mL of acetone. Eventually, the collected white powders, SBSi, were freeze-dried in vacuum for 6 h and stored in a refrigerator. 2.3. Fabrication of the Zwitterionic SBSi Surfaces. The glass slice was treated by O2-plasma for about 10 min in order to increase the functional hydroxyl groups. The pretreated glass slice was immersed into the mixture containing 100 mg of SBSi, 60 mL of ethanol, and 120 mL of deionized water. After hydrolysis and condensation reaction for 12 h, the modified glass slice was washed by ethanol to remove the unreacted monomers and finally annealed at 80 °C for about 1 h. 2.4. Preparation of the Burned Glass. The glass slice was simply exposed to the flame of Mastrad burner about 10 s for 5 times and cooled in ambient condition. After the burned glass was is returned to room temperature, it was used for the total wetting experiment immediately to avoid the decay of superhydrophilicity. 2.5. Characterization. The top-view shape evolution of aqueous droplets on total wetting surfaces including SBSi surface and burned glass slice were recorded by charge coupled device (CCD) camera and transformed into enlarged images. The wetting area of the drop on a horizontal surface and the edge length of the spontaneous flow on an inclined surface were determined by the software of ImageJ. The precipitated stain was observed by Olympus BX51 optical microscope (OM) and the photos were recorded by SAGE SGHD260 camera. The UV absorption of the dye TB was analyzed by UV−visible spectrometer (Jasco, V-660). In order to identify the variation of the TB concentration in the drop, the calibration line of UV absorption at the wavelength 600 nm versus TB concentration has been acquired.

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium chloride (99.5%) was bought from Showa Chemical Ind. Co. (Japan). Dextran (MW = 500k) and D(+)cellobiose (98%) are the products of Alfa Aesar Co. (USA). The polyacrylamide (MW = 1.5k) aqueous solution (50%) was obtained from Polysciences Inc. (USA). (N,N-Dimethylaminopropyl)trimethoxysilane (DMASi) was purchased from Gelest Inc. (USA). 1,3-Propane sultone (99%), acetone (99.9%), and trypan blue (TB)

3. RESULTS AND DISCUSSION 3.1. Solute-Induced Self-Pinning Behavior on Total Wetting Surfaces. Zwitterionic sulfobetaine silane (SBSi) surface is known as a superhydrophilic surface. As a liquid drop B

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out at the elapsed time 10 s. This experimental result supports our proposed mechanism of self-pinning. 3.2. Overcoming Self-Pinning by Gravity-Driven Spontaneous Spreading. While a drop stays statically on a partially wetting surface, it spreads spontaneously on a totally wetting surface. The evaporation rate of the latter is dramatically greater than that of the former because of the difference of the liquid−gas interfacial area. However, as mentioned earlier, when the drop contains nonvolatile solutes, the self-pinning behavior appears and spontaneous spreading is halted. If the contact line pinning on the totally wetting surface induced by solutes can be relinquished, spontaneous spreading may reoccur, leading to the increment of the interfacial area. In general, the fluid on a partially wetting surface can be set in motion by inclination. As the critical tilted angle is exceeded, the gravitational force dominates over the capillary force associated with contact angle hysteresis and thus the drop starts to slide downward. In this work, the approach based on the gravity-driven motion is adopted to overcome self-pinning and the corresponding spreading behavior is observed. On a partial wetting (or self-pinning) surface, evaporation leads to the withdrawal of the contact line and the deposition of colloidal particles or nonvolatile solutes near the edge.13−15,20,21 The balance of the forces acting on the particles near the contact line is often performed to estimate the extent to which particles enhances the contact line pinning.13 In principle, one can apply the same concept to determine the inclination required to induce anisotropic spreading for a specific system. In our system, however, the particle size is time-varying and depends on the crystallization process of solutes near the edge, causing self-pinning. That is, this self-pinning behavior induced by nonvolatile solutes is much more complicated than the typical coffee ring effects. In fact, it involves advancing of the contact line (flow), solute crystallization near the edge (evaporation rate), the resistant force associated with surface roughness developed by solute crystallization, and the magnitude of the driving force (e.g., gravity). As a result, a quantitative model is not straightforward to establish. In order to observe the motion of the aqueous drop containing nonvolatile solutes clearly, the diazo dye, trypan blue (TB), is used. The initial volume of the drop is 5 μL and the concentration of TB is 1 mM. As the SBSi surface is tilted, it is found that the self-pinning behavior disappears and the drop spreads anisotropically. Figure 2a shows the time variation of

is deposited on such a surface, both water and oil show the total wetting behavior (contact line expanding continuously).5 The drop spreads initially to a maximum radius and then shrinks due to evaporation.10,11 However, spontaneous spreading vanishes as some nonvolatile solutes such as small molecules or polymer are added into the pure liquid. Note that the contact line pinning on a partial wetting surface is different from self-pinning on a total wetting surface. For the former, the motion of the contact line stops immediately after the drop deposition. For the latter, nonetheless, the contact line moves outward for a while and then stops. When a water drop of 5 μL containing 1 wt % cellobiose is placed on the SBSi surface, the wetting area of the liquid drop will grow with time at first. Afterward, the wetting area reaches its maximum at about 65 mm2 in the time period of 60 s, and then it remains essentially the same for at least 130 s, as shown in Figure 1a. The contact line pinning is clearly observed on such a total wetting surface. Eventually, the water droplet is evaporated completely, and a precipitated pattern as illustrated in the inset appears. When the solute size is increased, the self-pinning phenomenon becomes more prominent. Figure 1b demonstrates the variation of the wetting area with time for larger solute Dextran. The wetting area of the droplet of 5 μL containing 1 wt % Dextran grows to its maximum (32.8 mm2) within 10 s and then maintains unchanged for at least 300 s. Compared to small molecules, the occurrence of self-pinning is more rapid for polymers and thereby the maximum wetting area of the latter is smaller. During the spreading process, the solute is carried by the outward flow to the contact line. The self-pinning behavior occurs probably due to the emergence of “surface roughness” developed by the accumulation of solutes at the edge. It is expected that solutes of large sizes (e.g., polymer) yield larger scale of roughness, and thus self-pinning takes place faster. Note that the contact line pinning associated with partial wetting surfaces takes place almost instantaneously. Our experiments reveal that the spontaneous spreading behavior of a water droplet on a total wetting surface can be effectively halted by the solute addition, and the influence of the solute seems to be more significant as its size is increased. During outward spreading of the pure water drop on a horizontal SBSi surface, evaporation also occurs simultaneously.5 The evaporation rate is proportional to the liquid−gas area which is generally proportional to the wetting area. Typically, the flux of evaporation is the greatest near the edge of the drop based on the solution of the steady-state diffusion equation.16 As a result, the evaporation rate on a total wetting surface grows with increasing the wetting area. The withdrawal of the contact line can be seen within about 90 s for a pure water drop.5 However, solute-induced self-pinning stops the expansion of the wetting area on a total wetting surface and thus reduces the evaporation rate. Because it takes a finite time for the contact line pinning to occur, the mechanism of selfpinning is probably related to evaporation near the contact line. Following the concept of the formation of the coffee ring, the solutes are carried by the outward flow and accumulated at the edge.14,15 Once the saturation point is reached, the solutes precipitate and protrusions of deposit are formed near the contact line. Spontaneous spreading is then inhibited by the capillary force, similar to the pinning mechanism responsible for the colloidal drop.10,17 For an aqueous solution of Dextran deposited on the SBSi surface, a ring-like pattern of precipitated polymer is observed at the contact line after the drop is sucked

Figure 2. (a) Time evolution of TB spreading flow on the SBSi surface and (b) variation of length of TB spreading flow on SBSi surfaces with time. The inset shows the log−log plot of the length versus time. C

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Langmuir the top view of the spreading drop at the tilted angle 10°. During the process of anisotropic spreading, the front side of the drop moves down continuously. Nonetheless, its rear side remains essentially pinned at the original position. This phenomenon is distinctly different from that observed on a partially wetting surface. The former looks like fluid flow driven by the gravitational pressure while the latter exhibits either a motionless drop or a sliding one. As time passes, the length of the spreading drop grows due to gravity-driven thin film flows, but the thickness at the rear side tends to decrease rapidly. Accordingly, the wetting area rises dramatically with time, in contrast to that associated with spontaneous spreading on a horizontal surface with negligible evaporation and selfpinning.5,8 The variation of the length (L) with time can be realized from the following simple scaling based on the thin film flow on an inclined plane with tilted angle (α). The fluid velocity (u) driven by gravity (ρg) against the viscous resistance (μ) is proportional to the square of the film thickness (δ)22 pgδ 2 sin a u≈ μ

Figure 3. Variation of the length of a drop with 1 wt % NaCl(aq) spreading flow on SBSi surfaces with time. The inset shows the log− log plot of the length versus time.

about six times larger than that of the former. This significant increment of the wetting area leads to rapid evaporation of the drop. As a result, gravity-driven spreading has the potential for the applications of concentration and separation of solutes. 3.3. Applications of Concentration and Evaporation Enhancement. During the process of anisotropic spreading, the rapid growth of the wetting area results in the increase of the liquid−gas interfacial area which accelerates the evaporation rate. Moreover, the thickness of the liquid film decreases substantially with time because of the increment of the area and evaporation. As a result, the drying rate of the liquid film is significantly enhanced and the rate of the formation of the solute deposit is very fast. In this work, the spreading and drying processes are recorded for various aqueous solutions of salts, molecules, and polymers. As shown in Figure 4a,b for drops of 5 μL containing 1 wt % NaCl and polyacrylamine (PAM), respectively, the solutes are quickly precipitated out at the rear side of the aqueous drop within 30 s. The complete processes are given in Figure 4 and the movies are provided in the Supporting Information, videos S1 and S2. The fact that the drying region propagates from the rear side to the front side

(1)

The drop volume is assumed to be constant WLδ ≈ constant

(2)

where W is the width of the droplet and remains unchanged. Thus, one has δ ≈ L−1. Since u = dL/dt, one has

dL ≈ δ 2 ≈ L−2 dt

(3)

Therefore, the scaling law is obtained

L ≈ t 1/3

(4)

Figure 2b illustrates the plot of the length (L) against time (t) obtained in our experiments. The log−log plot is shown in the inset and a linear line is acquired. The slope is about 1/3, which agrees with our scaling law. Note that the same result is acquired if anisotropic spreading is performed for a pure water drop. This consequence indicates that self-pinning behavior of a liquid drop containing solutes on the SBSi surface can be eliminated by tilting the plane corresponding to gravity-driven spreading. Such gravity-driven spreading occurs not only on the SBSi surface but also on all kinds of total wetting surfaces. Figure 3 shows that a drop of 5 μL with 1 wt % NaCl(aq) is placed on an inclined burned glass (α = 10°) which is commonly known as a total wetting surface due to high surface energy. The water droplet also overcomes self-pinning and spreads downward. The exponent remains at about 1/3. Note that the long-term spreading kinetics on inclined surfaces can differ from the 1/3 law due to evaporation. The exponent of gravity-driven spontaneous spreading (∼1/ 3) is significantly greater than that of isotropic spontaneous spreading on a horizontal surface.8,23 Typically, the exponent in Tanner’s law (L ≈ tβ) is about 1/1024 and often observed in spontaneous spreading of polydimethylsiloxane drops (low surface tension liquid). Nonetheless, the exponent becomes even smaller (∼1/20) for a water or hexadecane drop spreading on the SBSi surface (high surface energy substrate). This result indicates that the spreading rate of gravity-driven spreading is much greater than that of isotropic spreading. For comparison, 5 μL water drops are placed on the horizontal and inclined (α = 10°) SBSi surfaces. After 60 s, the wetting area of the latter is

Figure 4. Rapid precipitation at the rear-part of the aqueous (a) NaCl and (b) polyacrylamide spreading flow on inclined SBSi surfaces. D

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Langmuir reveals that the thickness of the spreading film is increased from the rear side to the front side. Moreover, the solute’s deposit appears at the edge first, indicating the existence of the edgeward flow frequently observed in the coffee ring effect.14−16 That is, the evaporation rate along the edge is higher than that in the drop center. The aforementioned results indicate that gravity-driven spreading on a total wetting surface can be utilized for rapid solute−water separation. The enhancement of water evaporation by the expansion of the interfacial area can be demonstrated by the simultaneous formation of fog atop the liquid film. As illustrated in Figure 5,

different surfaces. The mean evaporation rate is about E = 5.51 × 10−3 g/(cm3·s) for a spreading drop driven by gravity with the wetting area 340 mm2 (Figure 6d) at the relative humidity 40%. Here E is defined as the ratio of the weight loss to the product of the drop volume and the time of drying. The time of drying is the time period required for a droplet to vanish completely. Since the weight and volume of the drop are known in the beginning, the mean evaporation rate can be simply estimated. Note that E = 3.30 × 10−4 and 4.96 × 10−4 g/ (cm3·s) for aqueous drops on PMMA (Figure 6a) and glass (Figure 6b) surfaces, respectively. The contact angle is 72° for the former and 40° for the latter. Obviously, the evaporation rate is enhanced at least 10 times for gravity-driven spreading. The evaporation rate can be increased more if the wetting area is elevated furthermore. For example, gravity-driven spreading is performed by tilting the total wetting surface leftward and rightward alternatively (Figure 6e). The wetting area of a 10 μL drop can be as large as about 600 mm2 within 2 min. Thus, the evaporation rate is raised to 8.26 × 10−3 g/(cm3·s), which is 25 times of that on a PMMA surface. As a drop spreads for a distance, a significant amount of solute left behind appears on the surface. The solute concentration of the front side of the liquid film may change due to two factors: water evaporation and solute deposition. The former raises the concentration while the latter reduces it. In our experiment for a 10 μL drop with 10 mM TB spreading for about 6 cm, the front side of the drop acquired by a pipet shows an increase of 20% in concentration. Since there exists a peak at the wavelength of 600 nm for TB, UV−visible spectrometer is used to analyze the TB concentration. This consequence indicates the solute enrichment can be achieved because the evaporation rate exceeds the deposition rate. If gravity-driven spreading is repeated on the same surface, the precipitated solutes on the SBSi surface will be redissolved by the subsequent spreading droplet. It is found that the concentration efficiency is significantly increased after repeating the process a few times because of the joint effort of redissolution of solutes and water evaporation. The averaged TB concentration of the six drops collected at the end of the spreading film is increased from 10 to 20 mM after the process is repeated 6 times. That is, the concentration efficiency of the whole process is as high as 100%. This concentration method based on the total wetting surface shows the advantages of high efficiency, easy to scale-up, and less energy consumption. Note that the gravity-driven spreading technique may be limited by the droplet size. However, the approach of forced spreading on a total wetting surface can be generalized to any drop size as long as the mechanical force can be applied to spread the drop.

Figure 5. (a) Equipment scheme of evaporation observation and (b) rapid evaporation and condensation of TB aqueous spreading flow on the inclined SBSi surface.

when an aqueous drop containing TB spreads on the inclined SBSi surface (α = 10°), the water vapor evaporated from the drop is immediately condensed on the upper glass slice which is 2 mm away and cooled to 20 °C. The developing shape of the fog is similar to that of the spreading film, revealing that the enhancement of evaporation is achieved by gravity-driven spreading. The water vapor evaporated from the liquid film will diffuse in every direction. If it encounters the upper glass, the fog is formed. There is no reason to expect the shape of the fog to be exactly the same as that of the spreading film. Figure 6 shows the wetting area of TB aqueous droplets of 10 μL on

4. CONCLUSIONS While the wetting area of a drop on total wetting surfaces (SBSi surface and burned glass) grows with time due to spontaneous spreading, the self-pinning behavior has been observed upon addition of nonvolatile solutes (cellobiose and Dextran). Both the spreading time and wetting area of aqueous drops (5 μL) are significantly reduced from 130 s and 65 mm2 for cellobiose solution to 10 s and 33 mm2 for Dextran solution. Self-pinning phenomenon becomes more prominent with increasing solute size. It can be attributed to the accumulations of solutes near the contact line driven by both spreading flow and differential evaporation, generating the capillary resistance to impede continuous spreading.

Figure 6. Top-view photos of the wetting area of TB aqueous droplet on various surfaces. E

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(5) Wu, C.-J.; Huang, C.-J.; Jiang, S.; Sheng, Y.-J.; Tsao, H.-K. Superhydrophilicity and Spontaneous Spreading on Zwitterionic Surfaces: Carboxybetaine and Sulfobetaine. RSC Adv. 2016, 6, 24827. (6) Harkins, W. D. Feldman, Films. The Spreading of Liquids and the Spreading Coefficient. J. Am. Chem. Soc. 1922, 44, 2665−2685. (7) Wu, C.-J.; Li, Y.-F.; Woon, W.-Y.; Sheng, Y.-J.; Tsao, H.-K. Contact Angle Hysteresis on Graphene Surfaces and Hysteresis-free Behavior on Oil-infused Graphite Surfaces. Appl. Surf. Sci. 2016, 385, 153−161. (8) Tanner, L. H. The Spreading of Silicone Oil Drops on Horizontal Surfaces. J. Phys. D: Appl. Phys. 1979, 12, 1473−1484. (9) deGennes, P. G.; Brochard-Wyart, F.; Quéré, D. Capillary and Wetting Phenomena: Drops, Bubbles, Pearls Waves; Springer Science + Business Media: NewYork, 2004. (10) Weon, B. M.; Je, J. H. Self-Pinning by Colloids Confined at a Contact Line. Phys. Rev. Lett. 2013, 110, 028303. (11) Deegan, R. D. Pattern Formation in Drying Drops. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2000, 61, 475−485. (12) Weon, B. M.; Je, J. H. Capillary Force Repels Coffee-Ring Effect. Phys. Rev. E 2010, 82, 015305. (13) Chhasatia, V. H.; Sun, Y. Interaction of Bi-dispersed Particles with Contact Line in an Evaporating Colloidal Drop. Soft Matter 2011, 7, 10135−10143. (14) Li, Y.-F.; Sheng, Y.-J.; Tsao, H.-K. Evaporation Stains: Suppressing the Coffee-Ring Effect by Contact Angle Hysteresis. Langmuir 2013, 29, 7802−7811. (15) Li, Y.-F.; Sheng, Y.-J.; Tsao, H.-K. Solute ConcentrationDependent Contact Angle Hysteresis and Evaporation Stains. Langmuir 2014, 30, 7716−7723. (16) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Capillary Flow as the Cause of Ring Stains from Dried Liquid Drops. Nature 1997, 389, 827−829. (17) Sangani, A. S.; Lu, C.; Su, K.; Schwarz, J. A. Phys. Rev. E 2009, 80, 011603. (18) Litt, M.; Matsuda, T. Siloxane Zwitterions: Synthesis and Surface Properties of Crosslinked Polymers. J. Appl. Polym. Sci. 1975, 19, 1221−1225. (19) Yeh, S. B.; Chen, C. S.; Chen, W. Y.; Huang, C. J. Modification of Silicone Elastomer with Zwitterionic Silane for Durable Antifouling Properties. Langmuir 2014, 30, 11386−11393. (20) Yang, X.; Li, C. Y.; Sun, Y. From Multi-Ring to Spider Web and Radial Spoke: Competition between the Receding Contact Line and Particle Deposition in a Drying Colloidal Drop. Soft Matter 2014, 10, 4458−4463. (21) Hurth, C.; Bhardwaj, R.; Andalib, S.; Frankiewicz, C.; Dobos, A.; Attinger, D.; Zenhausern, F. Biomolecular interactions Control the Shape of Stains from Drying Droplets of Complex Fluids. Chem. Eng. Sci. 2015, 137, 398−403. (22) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. Transport Phenomena, 2nd ed.; John Wiley and Sons: New York, 2002. (23) Bonn, D.; Eggers, J.; Indekeu, J.; Meunier, J.; Rolley, E. Wetting and Spreading. Rev. Mod. Phys. 2009, 81, 739−805. (24) Liao, Y.-C.; Li, Y.-C.; Wei, H.-H. Drastic Changes in Interfacial Hydrodynamics due to Wall Slippage: Slip-intensified Film Thinning, Drop Spreading, and Capillary Instability. Phys. Rev. Lett. 2013, 111, 136001.

In this work, the solute-induced self-pinning is relinquished by gravity-driven flow and anisotropic spreading occurs. During the process, the rear-side pinning and front-side flowing of the drop lead to the dramatic growth of the wetting area. After 1 min, the wetting area of the water drop (5 μL) on the inclined SBSi surface (10°) grows about 6 times greater than that on the horizontal surface. This gravity-driven spreading can be depicted by a simple power law, and its exponent of 1/3 is significantly larger than that of typical isotropic spreading (1/ 10). The wetting area can be further increased by tilting the SBSi surface leftward and rightward alternatively. For a 10 μL drop, the wetting area can become as large as about 600 mm2 within 2 min. As a result, the evaporation rate is raised to 8.26 × 10−3 g/(cm3·s), which is 25 times of that on a PMMA surface. During anisotropic spreading, the solutes are quickly precipitated out within 30 s, indicating the application for rapid solute−water separation. After repeating gravity-driven spreading, the mean TB concentration of the collected drops is increased from 10 to 20 mM, revealing high concentration efficiency (100%). This dramatic enhancement of evaporation is easy to scale-up with less energy consumption. Therefore, it possesses good potentials for the development of solute separation and concentration at room temperature.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01384. Video of the spreading and drying processes. (AVI) Video of the spreading and drying processes. (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yu-Jane Sheng: 0000-0002-3031-8920 Heng-Kwong Tsao: 0000-0001-6415-8657 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work is supported by Ministry of Science and Technology of Taiwan and Industrial Technology Research Institute of Taiwan.

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REFERENCES

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DOI: 10.1021/acs.langmuir.7b01384 Langmuir XXXX, XXX, XXX−XXX