Self-Propulsion and Shape Restoration of Aqueous Drops on

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Self-Propulsion and Shape Restoration of Aqueous Drops on Sulfobetaine Silane Surfaces Vickramjeet Singh, Cyuan-Jhang Wu, Yu-Jane Sheng, and Heng-Kwong Tsao Langmuir, Just Accepted Manuscript • Publication Date (Web): 28 May 2017 Downloaded from http://pubs.acs.org on May 28, 2017

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Self-Propulsion and Shape Restoration of Aqueous Drops on Sulfobetaine Silane Surfaces Vickramjeet Singha, Cyuan-Jhang Wua, Yu-Jane Shengb*, Heng-Kwong Tsaoa*

a

Department of Chemical and Materials Engineering, National Central University, Jhongli

320, Taiwan. E-mail: [email protected] b

Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan.

E-mail: [email protected]

ABSTRACT The motion of droplets on typical surfaces is generally halted by contact line pinning associated with contact angle hysteresis. In this study, it was shown that on a zwitterionic sulfobetaine silane (SBSi) coated surface, aqueous drops with appropriate solutes can demonstrate the hysteresis-free behavior, whereas a pure water drop shows spontaneous spreading. By adding solutes such as polyethylene glycol, 2(2-butoxy ethoxy) ethanol, or sodium n-dodecyl sulfate, an aqueous drop with a small contact angle (disappearance of spontaneous spreading) was formed on SBSi surfaces. The initial drop shape was readily relaxed back to a circular shape (hysteresis-free behavior), even upon severe disturbances. Moreover, it was interesting to observe the self-propulsion of such a drop on horizontal SBSi surfaces in the absence of externally provided stimuli. The self-propelled drop tends to follow a random trajectory and the continuous movement can last for at least 10 mins. This self-propelled random motion can be attributed to the combined effects of the hysteresis-free surface and the Marangoni stress. The former comes from the total wetting property of the surface while the latter originates from surface tension gradient due to fluctuating evaporation rates along the drop border.

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1. INTRODUCTION The generation and manipulation of drop movements are required for various chemical, biological, and technological applications.1-4 The motion of a drop on a surface is often driven by gravity and pressure difference. It can also be achieved by the surface forces resulting from wettability gradient and surface tension gradient, so called the Marangoni stress.2-4 However, drop movement is generally hindered by contact line pinning associated with contact angle hysteresis (CAH).5,6 Recently, droplet microfluidics have been developed as a multipurpose tool owing to its various benefits including that minute droplet volumes ranging from pico-liter to nano-liters are applied, low amount of reagent is required, and drop by drop control can be employed.7,8 Moreover, these tiny drops can be considered as lab-on-a-chip or nanolabs and have advantages of reducing interfacial contact with the surface, thus minimizing unavoidable surface adsorption and contamination especially for the systems containing biomolecules.7-10 The ability to transport liquid droplets rapidly and precisely on a surface should overcome resistive forces arising from the contact line pinning, in addition to the viscosity associated with fluid motion.4,11 The wettability gradient surfaces are related to the interfacial tension differences between the solid-gas and solid-liquid phases and have been employed to guide the motion of drops for coalescence and splitting.4,12 The motion can be attributed to an imbalance of forces acting at the contact line, resulting in the driving force along the direction of decreasing contact angle or increasing wettability.12 Fabrication of surfaces having sufficiently large wettability gradient to overcome the resistance caused by CAH13,14 is one of the strategies. Varying the solute concentration or temperature can also yield the surface tension gradient (liquid-gas phases), required for the Marangoni stress.15-19 For instance, when a solution of two liquid components with different surface tensions and vapor pressures was employed to form drops on the corona-discharge-cleaned glass slide, the drop motion was induced by the vapor emitted from the neighboring drop owing to non-uniform 2


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concentrations associated with non-uniform evaporation rates.16 The self-propulsion of swimming droplets in an oil-surfactant medium arising from the Marangoni flow induced by a surface-tension gradient has also been observed. The surface tension is maintained through specific chemical reactions such as the hydrolysis of the surfactant.18 Moreover, self-propulsion of liquid drops has been reported on a superhydrophobic surface with the ratchet-like structure.20 However, the liquid drop should be in the Leidenfrost state. That is, the drop levitates due to vapor generation when placed on a hot surface having a temperature greater than the boiling point of the liquid.20,21 The vapor propels the drop in the direction fixed by the ratchet. Although the hydrophobic layer was employed to reduce the surface temperature, this technique still requires temperatures as high as 100 oC.20 Self-induced motion of a liquid droplet is of keen interest as it does not require an external force for liquid transport and can result in unexplored applications.18,20 CAH is the origin of the resistance to the motion of a drop and is closely related to contact line pinning which hinders the motion of contact line owing to surface roughness or chemical inhomogeneity.11-13,22 Surfaces with negligible CAH can be achieved by fabricating superhydrophobic surfaces or lubricant infused surfaces. Drops tend to roll on the former but slide on the latter.23-27 Because of abundant air pockets on the superhydrophobic surface, contact line pinning is significantly reduced.23-25 In contrast, owing to direct contact of the drop with the thin oil phase on an oil-infused surface, nearly hysteresis-free behavior has been observed.28,29 Recently, the induced motility of two-component drops on corona-cleaned glass slides was reported to demonstrate the hysteresis-free characteristic.16 Since a pure water drop spreads spontaneously on a corona-cleaned glass, this suggests that a total wetting surface with a positive spreading coefficient can provide surfaces with negligible CAH.16 It is well-known that the flamed glass or plasma treated glass surfaces demonstrate the total wetting property for pure water drops.16,30 However, they suffer the drawback of long-term stability. Lately, surface modification has been used to obtain materials with 3


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desired wettability. A surface grafted with zwitterionic functional groups exhibits the highly hydrophilic feature and has sparked interest31-33 owing to its antifouling properties, that resist the non-specific protein adsorption and adhesion of bacteria.34-36 Therefore, such zwitterionic surfaces have been explored for various applications in the field of bio-medical sciences.36-38 For example, materials used for bio-implants suffer the problems such as bio-adhesion, tissue inflammation, and infection. Therefore, sulfobetaine based zwitterionic surfaces have been employed for making the implants biocompatible.38-40 In addition, these antifouling zwitterionic surfaces also demonstrate technological applications such as protein separation, self-cleaning, anti-fog, and oil-water separation.34,41-43 Although zwitterionic surfaces based on carboxybetaine silane show partial wetting, the sulfobetaine silane (SBSi) surfaces exhibit the total wetting property for both water and hexadecane.31 In this work, an SBSi surface has been fabricated and aqueous drops with a small contact angle were developed on such a total wetting surface by adding appropriate solutes. The hysteresis-free behavior was then examined based on the experiments of the contact line relaxation subject to severe disturbances. Surprisingly, an aqueous drop exhibited the self-propelled random motion on a SBSi horizontal surface in the absence of external stimuli. Furthermore, the vapor-mediated motion of an aqueous drop has also been performed by placing an adjacent pure water drop. Finally, the mechanism based on the hysteresis-free behavior and the Marangoni stress has been proposed to explain the self-propulsion.

2. EXPERIMENTAL SECTION

2.1. Materials. (N,N-dimethylaminopropyl)trimethoxysilane (DMASi) was procured from Gelest Inc. (USA), sodium n-dodecyl sulfate (SDS, 99%) from Alfa Aesar (UK), dextran (500 k) from Alfa Aesar (Canada), anhydrous ethanol (99.5%) from Echo Chemical Co. (Taiwan), 1,3-propane sultone (99%) from Sigma-Aldrich Co. (USA), non-ionic surfactant Aquet from 4


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Bel-Art products (USA), polyethylene glycol (PEG, 20 K) and polyvinyl alcohol (PVA, 31 K) from Fluka (Switzerland), polyvinyl pyrrolidone (PVP, 99%), triethylene glycol (TEG, 99%) and 2-(2-butyl ethoxy)ethanol (BDG, 99%) from Acros Organics (USA). All the analytical grade chemicals were used as such without further purification. The glass slides were bought from Yancheng Guanghui Medical Products Factory (China).

2.2. Fabrication of SBSi surfaces. The chemical SBSi has to be synthesized as reported.31 In brief, 5 g of (N,N-dimethylaminopropyl)trimethoxysilane and 3 g of 1,3-propanesultone were dissolved in 25 ml acetone and the mixture was kept under N2 for 6 hrs at room temperature. After filtration and washing with acetone, a white powder of SBSi was obtained. The resultant powder was cold dried under vacuum for 6 hrs. The SBSi surface was prepared on a glass slide. The glass slide was washed thoroughly and then treated by O2-plasma. The plasma treatment introduced hydroxyl groups (OH) on the surface of a glass slide. This pretreated glass was then immersed in an aqueous ethanolic solution of SBSi. The ethanolic SBSi solution was made by dissolving 0.1 g of SBSi powder in an aqueous ethanolic solution (1:2, v/v). After the hydrolysis and condensation reaction for 12 hrs, the modified glass slide was washed by ethanol so as to remove any unreacted monomer and the slide was then annealed at 80 oC for about 1 hr. The fabricated SBSi surface can be used repeatedly and large number of drops (more than 500) were tested on each surface. More than 50 separate SBSi surfaces were used.

2.3. Fabrication of flamed glass. The glass slide was thoroughly washed with surfactant (Aquet) and then with deionized water. The slide was then dried by a jet of nitrogen. One side of the dried glass slide was burned under the flame for about 20-30 seconds. The flamed glass became a total wetting surface but its hydrophobicity grew significantly after about 30 minutes. 5


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2.4. Wettability characterization and observation of the drop motion. The wetting behavior of the SBSi coated substrate was characterized by employing Dataphysics DSA10-MK2 (Kruss, Germany) contact angle measurement system. The automated micro-syringe system released a liquid droplet having the volume of 5 µl on the substrate surface. A charged coupled device (CCD) camera was used for recording the shape of the droplets. These measurement were performed under ambient condition and finished within few seconds to eradicate the effect of evaporation on the contact angle measurements. The standard deviation of the contact angles is ± 2o. The top-view of drop shape restoration and drop motion (self-propelled and induced) of aqueous drops on horizontal SBSi and flamed glass surface were recorded by a CCD camera (IMAGINSOURCE, DFK 23U274). The wetting area of the drop on a horizontal surface was determined by Image J software. The standard deviation of the wetting area is less than 0.1 mm2. These observations were recorded under ambient conditions and relative humidity (RH) of about 50 %.

3. RESULTS AND DISCUSSION The total wetting surfaces including flamed glass slides and zwitterionic sulfobetaine silane surfaces were fabricated and the wetting behavior of water droplets containing various solutes was observed. It was found that the water drop containing PEG or BDG was able to exhibit self propulsion on a total wetting surface. These observations demonstrate important propulsion characteristics that are useful in applications of digital microfluidics for sensing and transport.

3.1. Contact angle hysteresis-free surface and the shape restoration On a total wetting surface, a pure drop will spread spontaneously and the characteristic 6


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size of the contact line will grow as long as the evaporation can be neglected.27,44 However, if an aqueous drop with an additive such as PEG was placed on this total wetting surface, a flat-shaped droplet with a very small contact angle was formed. This consequence reveals that this total wetting surface for pure water drops becomes a superhydrophilic surface for the PEG-containing water drop. Consider a droplet (5 µl) of 1 wt% aqueous PEG (20 K) solution placed on a SBSi coated glass surface. The flat circular shape developed very soon and the respective wetting area and contact angle (CA) were about 40 mm2 and 8o, respectively. Note that a pure water or hexadecane drop on an SBSi surface tends to expand its wetting area with time when evaporation is negligible.27 When a liquid drop is placed on typical surfaces, its shape reaches the final state quickly and cannot be disturbed easily because of contact line pinning.6 For example, when the static drop is disturbed by external stimuli such as gravity or vibration, it is quite difficult to move the contact line owing to CAH.6,23 The CAH is generally determined via inflation/deflation approach and the tilted plane method.23,45 For an aqueous PEG drop (10µl) on the SBSi surface, the removal of a very small amount of liquid volume (e.g. 2 µl) leads to the immediate shrinking of the drop area. Also, the drop shape can be easily altered by using a pipette tip. Moreover, as the SBSi surface was inclined slightly (e.g., tilt angle 1o), the aqueous PEG drop starts to move owing to gravity. These behaviors are distinctly different from that of a drop on a typical horizontal surface, where the contact line motion is difficult due to contact line pinning. These consequences indicate the existence of negligible CAH in this system. When an aqueous PEG drop was placed on a SBSi surface, spread-withdrawal behavior was observed and this process was completed within a short period. However, if one disturbs the initial shape of the drop to acquire an irregular shape with a larger area, the contact line of the drop will relax back to a circular shape gradually. We have observed the shape restoration of multiple droplets and the same trend in the time evolution of circularity was obtained. 7


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Figure 1a shows the typical evolution of the circularity of an aqueous PEG drop with time and the corresponding movie S1 has been provided in the Supporting Information. Here, the circularity is defined as the ratio of the square of the perimeter to the product of 4πand the area. For a circle, the circularity is unity. The drop relaxes from the irregular shape having circularity of about 2.0 to a nearly circular shape (circularity ≈ 1.06). The relaxation from the irregular shape to a more circular shape was generally achieved within approximately 30 seconds. The phenomenon of contact line pinning was absent. Soon after the shape restoration, the drop exhibited self-propulsion behavior. As a result, the circularity continued to fluctuate owing to the shape fluctuation associated with the moving drop. Evidently, the shape restoration is closely related to surface tension and CAH-free surface. Figure 1b demonstrates the variation of the wetting area of the relaxing drop with time and the corresponding movie S2 has been provided in the Supporting Information. The circularity for this drop has been shown in the inset of Figure 1b. The evaporation within first 60 seconds was negligible, therefore the decrease of the wetting area was mainly caused by the shape relaxation. However, during self-propulsion, the wetting area may continue to decrease slowly due to evaporation. Note that Figures 1a and 1b are from different experiments. The errors in those measurements for Figures 1a and 1b are less than 2 % for the circularity and less than 0.1 mm2 for wetting area. The same experiment was repeated on the flamed glass slide and similar behavior was observed. The time evaluation of the circularity observed on flamed glass was similar to that on the SBSi surface, as demonstrated in Figure S1.

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Fig. 1a. The time evolution of the circularity of 5 µl 1 wt % aqueous PEG drop on a SBSi surface. Scale bar 5 mm.

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Fig. 1b. The time variation of the wetting area for 5 µl 1 wt % aqueous PEG drop on a SBSi surface. Scale bar 3.5 mm. Inset figure demonstrate the time evolution of circularity for this drop.

3.2. Self-propelled random motion Drop motion on a typical solid surface is generally hindered by CAH and its movement can be driven by external stimuli.7,46,47 An aqueous drop containing additives such as PEG on a total wetting surface exhibits negligible CAH, and thus the drop is anticipated to move easily because of no contact line pinning. In the present study, soon after the spread-withdrawal of the aqueous PEG drop on a horizontal SBSi surface, the continuous movement of the drop was indeed observed. It should be noted that this drop starts to move 10


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on its own in the absence of any external stimuli. By observing the trajectory of the drop, it was found that the motion was not confined to a particular direction but followed a random path. Figure 2 shows a typical example of the time variation of the drop position for an aqueous PEG drop (5µl) placed on the SBSi surface. Another trajectory for the drop random motion was depicted in the inset of Figure 2. All trajectories were found to be different and the drop moved in random directions. The drop seems to perform a two-dimensional Brownian motion (see Supporting Information movie S3). The self-propelled random motion can last for approximately 600 s. Because of evaporation, the drop shrunk and eventually stopped moving. This self-propulsion behavior was general and repeatable. In Supporting Information Figure S3, six trajectories of 1 wt % PEG (20 k) drops (5 µl) on a horizontal SBSi surface have been shown. By analyzing the trajectories for more than 10 experiments, the mean square displacement (MSD) of the self-propelled drops has been evaluated. Figure 3 shows the plot of MSD against the lag time and a straight line has been obtained. The diffusion coefficient was determined from the slope and two-dimensional relationship was employed (MSD ∼ 4Dt). This consequence suggests that drop motion was like a two-dimensional random motion (similar to normal diffusion)48-49 and the diffusivity is about 0.1 mm2/s. The diffusion coefficient depends on the additive. For example, the random motion of the drop containing PEG was slower than that of the drop containing BDG.

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Fig. 2. The self-propelled random motion of 5 µl 1 wt % PEG (20k) drop on a SBSi surface with time. Inset figure show the random motion for another 5 µl 1 wt % PEG (20k) drop. Scale bar 3.5 mm

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70 60 50 2

MSD (mm )

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40 30 20 10 0 0

20

40

60

80

100

120

140

t (s) Fig. 3. The plot of mean square displacement (MSD) against lag time for the trajectories of various 1 wt % aqueous PEG drops on a horizontal SBSi surface.

In addition to the additive PEG polymer, the self-propelled random motion was also observed for drops with low molecular weight additives such as 2(2-butoxyethoxy) ethanol (BDG), whose chemical structure is similar to that of PEG monomer. A typical trajectory for an aqueous BDG drop has been shown in Figure 4a. Moreover, drops with triethylene glycol (TEG) additive, which has a hydroxyl end group instead of CH3 in BDG, also demonstrate the random motion, as shown in Figure 4b. Interestingly, another polymeric additive such as polyvinyl pyrrolidone (PVP) also demonstrates self-propelled random motion (see Figure 5a). Note that polymers PEG and PVP are not surface-active solutes. When the additive was changed to surfactant sodium n-dodecyl sulfate (SDS), the drop still exhibited the

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self-propelled random motion, as shown in Figure 5b. The short-term self-propulsion speed (instantaneous velocity) of aqueous drops containing various additives has been estimated using a temporary linear trajectory and was found to be around 0.025-0.050 mm/s. For example, the mean speed for PEG was around 0.028 mm/s and the standard deviation is about 0.002 mm/s. However, there exists no obvious relation between the solution surface tension and self-propulsion speed of aqueous drops. It should be noted that only some solutes including powders (PEG, PVP, and SDS) and liquid (BDG and TEG) are useful for self-propelled random motion. Additives such as poly-saccharides, mono-saccharides, salts, acids, bases, and some surfactants do not demonstrate the self-propelled random motion. For example, the droplets with added poly-hydroxyl group rich polymers such as polyvinyl alcohol (PVA) and dextran fails to show self-propelled random motion. The water drops containing these solutes spread spontaneously for a short period of time (less than 20 s) on SBSi surfaces. However, the contact line motion stops and the wetting area remains unchanged as shown in Figure 6. This phenomenon is referred to as self pinning on a total wetting surface due to solute addition.50

Fig. 4. The self-propelled random motion of (a) 5 µl 1 wt % BDG drop and (b) 5 µl 1 wt %

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TEG drop on SBSi surfaces.

Fig. 5. The self-propelled random motion of (a) 5 µl 1 wt % PVP (3.5k) drop and (b) 5 µl 1cmc SDS drop on SBSi surfaces.

Fig. 6. The time variation of the wetting area for aqueous drop of (a) 5 µl 1 wt % PVA (31 k) and (b) 5 µl 1 wt % dextran (500 k) on SBSi surfaces. According to the behavior of spontaneous spreading of a pure water drop on the SBSi surface, one has a positive spreading coefficient defined27 as S = γsg – ( γsl + γlg). Here γsg, γsl, 15


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and γlg represent the interfacial tension between solid–gas, solid–liquid, and liquid–gas phases, respectively. However, the addition of solutes such as PEG and BDG leads to the formation of drops with contact angles (less than 10o), indicating a negative value of S associated with partial wetting. Because γsg remains the same under the above two conditions, the change in S has to come from γsl and γlg. Since the addition of those solutes (PEG or BDG) results in a decrease of γlg, the negative value of the spreading coefficient must be attributed to a significant increase in γsl. From the thermodynamic viewpoint, this consequence reveals that unfavorable interactions exist between the solute and substrate surface. It can be realized by the result, γsl (BDG) - γsl (water) = S + [γlg (water) - γlg (BDG) cos θ] > 43 mN/m, from the spreading coefficient for pure water and the contact angle for pure BDG. The self-propelled random motion has been demonstrated for aqueous drops having a 5 µl volume and 1 wt % solute concentration (except SDS) on horizontal SBSi surfaces. At 1 wt % concentration, random motions are still displayed as the drop volume varies from 1 to 40 µl. This reveals that the random motion is insensitive to drop size. However, the self-propulsion speed decreases with increasing drop volume. Another question is the influence of the solute concentration on the random motion. Spontaneous spreading was observed for pure drops such as those of water and TEG on a SBSi surface. However, a static shape has been observed for a pure BDG drop with CA of about 20o. No random motion was observed for these pure drops. This consequence indicates that there exists a concentration range for aqueous drops to exhibit the random motion. The self-propelled random movement of a BDG drop was always observed for 0.1 to 35 wt % of BDG solute but ceases beyond this concentration range. Also, for the PEG solute, this concentration range varies from 0.1 to 20 wt % (see Figure S2). Thus, a large concentration range is valid for achieving the self-propelled random motion for an aqueous drop. 3.3. Proposed mechanism Shape restoration and self-propelled random motion of aqueous drops containing solutes 16


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such as PEG or PVP on the SBSi surface reveal the absence of contact line pinning. As demonstrated in Figure 7a, for an aqueous PVP drop on a SBSi surface, the wetting area was large initially which continuously decreases with time due to evaporation. Obviously, the contact line was not pinned and keeps shrinking. Note that the drop position changes because of self-propelled random motion. However, for the same drop on a glass substrate, the wetting area was initially small and remained unchanged with time (see Figure 7b). This clearly indicates contact line pinning and volume reduction due to evaporation was manifested by a decrease in the droplet height. Because of the hysteresis-free feature on the SBSi surface, those aqueous drops are ready to move if a certain driving force is applied. Since the drop moves autonomously on a horizontal SBSi surface, the driving force must originate from the drop itself. The water drop always suffers the problem of evaporation and its surface tension is dependent on the solute concentration.51 These additives are non-volatile and therefore the vapor pressure of those aqueous drops is reduced. Consequently, it is anticipated that the combined effects of water evaporation and surface tension lead to the drop motion. In general, the latter yields fluid motion only through the Marangoni stress which is related to the surface tension gradient. Since the random motion of the drop is driven by the inhomogeneity of Marangoni stress, it is very difficult to perform the rigorous theoretical analysis for the unsteady Marangoni flow associated with asymmetrical solute concentration or vapor pressure. Nevertheless, a simple quantitative estimation has been performed to evaluate the surface tension gradient

required for self-propulsion. The driving force is related to surface tension, Fc = γ s dl , where s denotes the unit vector tangent to the free surface and normal to the contact line C, and dl is the incremental arc length along C. The resistance to the motion of the drop with volume V on a CAH-free surface comes mainly from the viscous flow in the drop.

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There is no correlation between surface tension and motility in this work. In fact, surface tension of aqueous drops demonstrating motility falls within the same range as the one showing spontaneous spreading. For example, the aqueous drop of PVA (1 wt %) showing spreading has γlg = 50 mN/m, which is comparable to that of the aqueous BDG drop (1 wt %) demonstrating self-propulsion, γlg = 52 mN/m. So, the explanation based on the differences in surface tension is not feasible. As the drop moves with the velocity v, the rate of the work

done by the Marangoni stress Fc.v equals the viscosity dissipation μγ dV, where µ is the viscosity and γ the shear rate. The former is about ∆γlg.D.v while the latter is estimated as µ(v/H)2(HD2), where D and H represent the characteristic base diameter and height of the drop, respectively. The above balance yields ∆γlg

≈ µ(v/H)D ≈ 10-3 mN/m, which

corresponds to a very small surface tension gradient. This consequence indicates that the self-propulsion of a drop on a CAH-free surface can be achieved without resorting to surface active solutes whose surface tensions are very sensitive to their concentrations. Therefore, in the present study, the addition of solutes without surface activity can demonstrate the self-propulsion.

Fig. 7. The variation in wetting area with time for 5 µl 1 wt % aqueous PVP drop on 18


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horizontal (a) SBSi surface and (b) glass surface. The evaporation of an aqueous drop is generally proportional to the wetting area and therefore it is significant for aqueous drops on the SBSi surfaces. After the droplet has dried, a stains of solutes was often left and is referred to as “coffee ring”52-54 (see Figure 6b). Since the evaporation rate is higher near the contact line, edgeward capillary flow occurs to replenish the solvent during the drying process. Consequently, the solute molecules are transported towards the drop border and eventually deposited to form a ring-like structure.55,56 When one carefully examines the solute residue, the final deposition appears to be non-uniformly distributed along the border.53,57 This result simply suggests that the evaporation rate cannot be perfectly uniform at the border during the drying process. Since the solute concentration along the contact line is expected to be non-uniform, the surface tension gradient is established and the Marangoni stress appears.18,58 It should be noted that there always exists a surface tension gradient from the center to the edge due to the differences in solute concentration.16,58 However, there is no net Marangoni stress on the drop if this gradient is axisymmetric. In this study, the self-propelled random motion of an aqueous drop on a SBSi surface can be attributed to the combined effects of the hysteresis-free surface and the Marangoni stress. The former was provided by the total wetting property of the substrate. The latter arises from the non-uniform surface tension around the drop periphery. As shown in Figure 8, the evaporation rate of the solvent is faster at the border than at the center on the hydrophilic surface, leading to higher solute concentration near the contact line. However, at any instance, some fluctuations of the evaporation rate exists at the edge, resulting in a varying solute concentrations at the drop border (evaporation behavior), as illustrated in the top view of Figure 8. In fact, the non-uniformity of the non-volatile solute concentration which retards the evaporation of solvent thus induces varying evaporation rate. The variation of the solute concentration thus generates the surface tension gradient. The resulting Marangoni stress 19


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induces motion of the drop. When the solute concentration at a particular position on the border becomes higher, the evaporation rate is depressed, and therefore, the edgeward flow towards this point weakens. Consequently, less solute will be carried to this location and the solute concentration will not grow monotonously. Owing to this compensation mechanism, the surface tension at the border fluctuates around a mean value and its fluctuating non-uniformity leads to random motion of the drop. In case of other solutes, that do not show random motion, a possible explanation is that they are unable to generate a strong enough surface tension gradient along the contact line (the Marangoni force). That is, uneven evaporation rates (similar to the coffee ring effect) and surface tensions have to be induced around the drop periphery by the suitable solute.

Fig. 8. Schematic representation for the side and top views of an aqueous additive drop on SBSi surface. The non-uniform evaporation resulting in Marangoni stress.

3.4. Self propulsion on the other total wetting surface

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The self-propulsion behavior of an aqueous drop has been observed on a horizontal SBSi surface and this surface can be classified into the total wetting surface owing to the spontaneous spreading of pure water drops. One may wonder whether the random motion of aqueous drops can be seen on other total wetting surfaces. The well-known total wetting surface for pure water is the flamed glass. Unlike SBSi surfaces which can be stable at the ambient condition for at least one month, the flamed glass loses its total wetting characteristics after a short duration (i.e. 30 min.). In this work, the behavior of water drops containing PEG or BDG has been examined on the horizontal flamed glass. Figure 9 shows the typical trajectories of these drops on the flamed glass. Evidently, the trajectories for these drops also follow random paths. A comparison between these trajectories seems to reveal that the BDG drop moves faster than the PEG drop. These experimental observations on both SBSi and flamed glass surfaces indicate that the total wetting property of the surface must be one of the key requirement for aqueous drops to move effortlessly.

Fig. 9. The trajectory for self-propelled random motion of (a) 5 µl 1 wt % PEG drop and (b) 5 µl 1 wt % BDG drop on a flamed glass surface. Recently, the motion of two-component droplets on corona-discharge-cleaned glass 21


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slides has been reported.16 This substrate has a positive spreading coefficient due to the complete spreading of pure water and propylene glycol (PG). The mixture of water and PG forms a drop with a small contact angle on this surface. It was suggested that this drop was kinetically stabilized by an evaporation induced surface tension gradient. Two droplets of the same concentration deposited on a clean glass surface can attract each other through long-range interaction. They are not subject to pinning and they move towards each other, followed by coalescence. This phenomenon was explained by the consequence of the Marangoni stress, which came into existence via the water vapor emitted from the neighboring drop. That is, the motion of the two-component drop was induced by the vapor gradient which caused a local increase in relative humidity and thereby lowered the local evaporation rate on the adjacent portions of the two drops. In this study, we also examined the long-range interaction between two drops on a SBSi surface. An aqueous PEG drop was placed near a drop of pure water and the induced motion was observed. As shown in Figure 10, the aqueous PEG drop moved towards the static water drop (pink color) and finally the two drops coalesced (movie S4). Since the vapor-induced motion was also attained on our SBSi surface, one can conclude that the SBSi surface exhibits the same total wetting characteristics as the clean glass that can be employed for vapor-mediated sensing.

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Fig. 10. (a-c) The snapshots for the induced motion of 5 µl 1 wt % PEG drop (transparent) placed near a water drop (pink color) on horizontal SBSi surface, (d-e) the drop coalescence and (f) resultant mixture drop. It has been reported16 that the pre-requisite for the two-component droplets to display mobility and interactions on a total wetting surface is that one of the chemical components possesses high surface tension and high vapor pressure than the other. This model holds true when water droplet containing solid additives such as PEG, PVP, and SDS (solid state) were placed on SBSi surfaces and flamed glass. However, if solid additives such as PVA, dextran, and glucose were used, both the drop spreading with/without self-pinning was observed. This consequence reveals that this model may not be valid for solid additives. The above studies have shown that an aqueous drop containing appropriate solutes on a total wetting surface can be induced to move by applying the water vapor gradient from a neighboring drop. However, in this work, a single drop was able to exhibit the self-propelled random motion in absence of the water vapor gradient. That is, the self-propelled behavior on the hysteresis-free surface appears without resorting to long-range drop interactions. Both the vapor-induced and self-propelled motions are driven by the Marangoni stress associated with surface tension 23


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gradient caused by the evaporation of water. The former was caused by the vapor gradient induced by an external source near the drop, while the latter was originated from the fluctuations in the evaporation rate along the contact line of the moving drop. These observations suggest that the SBSi surface having long-time stability can be employed for drop motions (self or induced) in applications of droplet based devices. Although our work has some similarities to the study on vapor-mediated motility,16 the differences between the two are summarized as follows: (1) The droplet on a horizontal SBSi surface always demonstrates a self-propulsion behavior without the application of any external force, while the motion of the droplet on corona-discharge-clean glass was induced by a neighboring droplet.16 (2) The fabricated SBSi surface is highly stable and can sustain its total wetting property for at least one month, while the corona treated glass16 loses its total wetting characteristics after a short period of time. As a result, the same experiments can be repeated on the same SBSi surface after few weeks. (3) While both mechanisms are based on the Marangoni stress, our case is a result of uneven evaporation rates along the drop periphery, while the induced motion was generated by vapor-mediated interactions that arise from another neighboring drop. (4) The addition of a small amount of solid solutes (e.g. 0.1 wt %) can result in self-propelled random motion, while the induced motion requires the addition of another liquid component. The pre-requisite16 for the second liquid component to show the induced motion is not valid for our solid solutes. 4. CONCLUSIONS An SBSi surface has been fabricated and a pure water drop on this surface exhibits spontaneous spreading. However, an aqueous drop with a small contact angle (less than 10o) can be formed if an appropriate solute was added, such as BDG, TEG, PEG, PVP and SDS. An aqueous PEG drop on the SBSi surface demonstrates the spread-withdrawal behavior within a short period of time. The wetting area can be easily enlarged by disturbing the drop shape and it will relax back to a circle gradually, suggesting the absence of contact line pinning. 24


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That is, the SBSi surface with the total wetting characteristic exhibits negligible CAH, which can be verified by the sliding motion of an aqueous PEG drop (5 µl) on a slightly inclined surface (the tilted angle 1o). After the shape relaxation, it was interesting to observe the continuous movement of an aqueous drop on a horizontal SBSi surface. This drop propelled on its own in the absence of any external stimulus. Moreover, the motion of the drop follows a random trajectory, similar to the two-dimensional Brownian motion. The long-range interaction between two drops on the SBSi surface has also been examined. The induced motion of an aqueous PEG drop towards the static water drop was observed. Evidently, the moving drop was free from contact line pinning, which is always encountered on typical surfaces. Note that only some solutes are useful for self-propelled random motion. It was suggested that the pre-requisite for a two-component droplet displaying mobility and interactions is that one of the chemical component possesses high surface tension and high vapor pressure than the other. This model can explain the results of PEG, PVP, and SDS but fail to account for the spreading behavior associated with the additives such as PVA, dextran, and glucose. In addition to SBSi surfaces, the self-propelled random motion was also observed on another high energy surface e.g. flamed glass. The presence of a total wetting surface and the addition of an appropriate solute are required for the effortless movement of an aqueous drop. Consequently, the self-propelled random motion of the drop on a total wetting surface can be attributed to the combined effects of the hysteresis-free surface and the Marangoni stress. The former is due to the total wetting property while the latter arises from the fluctuations in evaporation rates, leading to the surface tension gradient associated with the variation of the solute concentration along the drop periphery. The observations of self-propelled or induced drop motions indicate that the fabricated SBSi surface with long-time stability can find suitable applications for droplet-based devices.

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ACKOWLEDGMENTS The authors thank the Ministry of Science and Technology of Taiwan for financial support.

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Graphical Abstract

Self-Propulsion and Shape Restoration of Aqueous Drops on Sulfobetaine Silane Surfaces Vickramjeet Singha, Cyuan-Jhang Wua, Yu-Jane Shengb*, Heng-Kwong Tsaoa*

a

Department of Chemical and Materials Engineering, National Central University, Jhongli 320, Taiwan. E-mail: [email protected] b

Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan. E-mail: [email protected] 32


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