Underwater Oil Droplet Splitting on a Patterned Template - Langmuir

Nov 9, 2017 - Underwater Oil Droplet Splitting on a Patterned Template. Xiaolong Yang†‡ , Xin Liu†, ... *E-mail: [email protected]...
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Article Cite This: Langmuir 2017, 33, 13522-13529

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Underwater Oil Droplet Splitting on a Patterned Template Xiaolong Yang,†,‡ Xin Liu,† Dennis W. Hess,‡ and Victor Breedveld*,‡ †

Key Laboratory for Precision and Non-Traditional Machining Technology of the Ministry of Education, Dalian University of Technology, Dalian 116023, People’s Republic of China ‡ School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Underwater oil droplets stretched and pinned by dual-dot oleophilic patterns on a superoleophobic substrate have been split into two nearly equal-volume daughter droplets using an underwater superoleophobic blade at substantially lower cutting speeds than reported in previous studies. A “liquid exchange model” based on Laplace pressure-driven liquid transport has been proposed to explain the mechanism of the underwater droplet split process. The dependence of droplet geometrical shape (curvature) and liquid properties (surface tension, viscosity) on the critical cutting speed that allows equalvolume split was investigated. Results demonstrate that critical cutting speed increases with increased curvature and surface tension of the split droplet, and decreases with increased droplet viscosity, which agrees with the proposed model. The ability to reproducibly split a single bulk oil droplet into daughter droplets with nearly equal volume facilitates the development of new functions for underwater microreactors.

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INTRODUCTION Handling small liquid volumes in the form of droplets is critical for applications in biomedicine,1−4 energy,5−8 nanoparticle synthesis,9 sensors,10 and lab-on-a-chip devices.11−14 To enable fully functional droplet-scale processes, several key manipulations must be performed with precision: droplet storage,1,2,15−19 transport,8,10−12,20−27 mixing,3,9,28,29 and splitting.1,24,30−37 Droplet manipulation techniques have been investigated intensively in recent years. For instance, Li et al. reported that a bulk droplet can be split into tiny microdroplet arrays with uniform volumes by sliding a larger droplet across a patterned superhydrophobic−superhydrophilic surface. This droplet splitting method can be used to isolate cells and produce single cell arrays for biological analysis.37 Levkin et al. also utilized superhydrophobic−superhydrophilic patterned surfaces to spontaneously seed and store microdroplet arrays. These stored microdroplet arrays can serve as microscopic vessels to culture cells for high-throughput cell screening applications.1 Balu et al. presented a new lab-on-paper device by printing high hysteresis patterns on superhydrophobic papers using a desktop printer; droplet manipulations of storage, transfer, and mixing were thus achieved.29 Li et al. reported an efficient way to split a single bulk droplet into multiple sample droplets of well-defined volumes by using a high hysteresis multidot array printed on a low hysteresis superhydrophobic paper. This adhesion-based droplet splitting method is reproducible and can be applied to quantitative colorimetric biomedical tests, although it is limited to creating well-defined sample droplets from a larger reservoir drop.31 © 2017 American Chemical Society

Bormashenko et al. split droplets on a superhydrophobic surface by using a superhydrophobic scalpel at high blade speeds to cut droplets into two equal parts.32 Yanashima et al. reported that a droplet pinned on two separate wire loops can be stretched into a water cylinder and split into two parts using a superhydrophobic sheet as a knife and a superhydrophobic substrate as a table.33 The cutting speeds in this latter experiment appear to be quite low, but were not quantified explicitly; furthermore, the vertical motion of the blade is controlled manually and the blade movement can be adjusted in real time by hand to avoid droplet shifting during the splitting process. The studies discussed above focused on liquid-in-air systems; due to evaporation, such systems are not suitable for volatile liquids or processes that require long droplet storage times. More recently, studies have focused on liquid−liquid systems, mostly with oil droplets surrounded by an aqueous phase. The work on underwater superoleophobic surfaces by Jiang et al. has propelled research on the control of oil wetting properties of underwater materials for self-cleaning38−42 and oil/water separations.43−48 Inspired by underwater superoleophobic materials and in-air droplet manipulation research, underwater micro oil droplet manipulations are also attracting attention because of potential applications in underwater microreactors and multiphase microfluidic devices.49−61 For instance, Jiang et Received: October 16, 2017 Revised: November 7, 2017 Published: November 9, 2017 13522

DOI: 10.1021/acs.langmuir.7b03604 Langmuir 2017, 33, 13522−13529

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Figure 1. (a) Schematic of the underwater superoleophobic template with dual-rail oleophilic tracks; left two insets are in-air images of water droplets on textured and smooth areas, while right two insets are underwater images of 5 μL DCM droplets on the textured and smooth areas, respectively, SEM images of (b) polished and (c−e) oxidized edges of the copper foils at different magnifications; (f) DCM and (g) DIM droplets underwater using the sliding and splitting process; insets in (g) are underwater images of 5 μL DIM droplets on the textured and smooth areas, respectively The distance between parallel tracks was 3.37 mm in (f) and 2.61 mm in (g); the track width was 0.62 mm in (f) and 0.59 mm in (g).

operations are difficult to implement because they require very high blade speeds and complex manual operations. In this work, we propose that an underwater oil droplet that is stretched on a patterned oleophilic template is much easier to split than a spherical droplet on a uniform superoleophobic surface. A new droplet cutting system with patterned templates and an underwater superoleophobic cutting blade driven by a linear actuator was developed to investigate the influence of blade speed, stretched droplet curvature, and liquid properties (surface tension and density) on the volumes of two daughter droplets after splitting. This research can be applied to the design of new components for droplet manipulation in underwater microreactors and other lab-on-a-chip devices.

al. reported that frosted glass is superhydrophilic in air but is superoleophobic underwater. They utilized frosted glass surfaces as a “mechanical hand” to capture and transfer oil droplets for mixing process.60 Chen and co-workers showed an effective method to fabricate patterned glass with controllable oil adhesion using a femtosecond laser.56 Wu et al. also utilized femtosecond laser ablation to fabricate patterned nickel substrates; the irradiated area was textured and exhibited underwater superoleophobicity, while the nonirradiated smooth area showed high oil adhesion. By fabricating a smooth Yshaped track surrounded by a textured surface, the ability to create anisotropic oil droplet adhesion for guiding and mixing oil droplets along a surface underwater was demonstrated.54 Yu et al. added additional complexity by creating a threedimensional underwater superoleophobic tunnel on an aluminum substrate using high rate wire electric discharge machining; the low-hysteresis tunnel can guide oil droplets when tilted only 3°.55 Song et al. presented a multistep wettability patterning method that combines electrochemical etching, fluoroalkylsilane surface modification, and masked plasma treatment. Underwater superoleophobic surfaces with wedge-shaped superoleophilic patterns on aluminum can spontaneously transport oil droplets.49 These studies have generated insight into several key underwater oil droplet manipulation processes, but methods for reproducible, quantitative splitting of oil droplets underwater still present a significant challenge. As noted above, although droplet splitting in air has been studied previously,1,24,30−37 it is unclear how this knowledge transfers to liquid−liquid systems where the density and viscosity of both the surrounding medium and the droplets must be considered. Furthermore, the in-air droplet cutting

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EXPERIMENTAL SECTION

Materials. Copper foil (0.127 mm thick, annealed, 99.9%) was obtained from Alfa Aesar Co., Ltd. (Haverhill, MA, United States). Sharpie ultrafine point permanent mark pens (tip diameter 200 μm, black) were purchased from a local office supply store (Office Depot). Dichloromethane (DCM, 99.5%) and diiodomethane (DIM, 99%) were obtained from Sigma-Aldrich Co., Ltd. (St. Louis, MO, United States). Sodium bicarbonate (NaHCO3) and ammonium persulfate ((NH4)2S2O8) were purchased from Amresco (Solon, OH, United States). All reagents are ACS grade and were used as received. Fabrication of the Cutting Blade. The edge of a sheared copper foil (40 × 15 mm) was polished using #1000 sandpaper to remove debris and eliminate uneven sharp edges. The polished copper foil was then cleaned in acetic acid for 1 min62 before immersion in an aqueous solution of NaHCO3 (0.1 M) and (NH4)2S2O8 (0.02 M) for 24 h to grow surface nanorod cluster structures.63 After rinsing with acetone and drying in air, the oxidized copper foil is underwater superoleophobic and can be used as an underwater oil droplet cutting blade. 13523

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Figure 2. (a) Front and (b) side views of the experimental configuration for controllable underwater droplet splitting.

159.5 ± 2.7° and the smooth area is oleophobic with a DCM contact angle of 124.6 ± 6.9°. However, the underwater receding contact angle of DCM on the smooth area is very low (36.2 ± 3.3°) and, as a result, the smooth area can easily be wetted by oil (oleophilic). The oxidation method can thus endow the copper foil with excellent superoleophobicity and achieve underwater superoleophobic/oleophilic patterned surfaces when combined with ink patterning technology. Using this approach, an underwater superoleophobic surface with dual-rail oleophilic tracks was fabricated on copper foil, with a superoleophobic blade placed in the middle of the dual tracks at the point of track separation (Figure 1a). The blade surface was rough, with clusters of nanorod structures, but the sharp edge of the cutting blade was not affected by the hierarchical micro/nanoscale structures when compared with the polished edge (Figure 1b−e). Underwater oil droplets bridging the dual-rail oleophilic track can easily slide along the track due to the strongly anisotropic sliding angles on the patterned substrate. As shown in Figure 1f, when the substrate was tilted at 36°, a 30 μL dichloromethane (DCM) droplet slid along the tracks, impinging onto the superoleophobic edge of the cutting blade. Due to the high momentum of the droplet, it deformed from the midpoint and broke into two smaller droplets that continued to slide individually along the two tracks. Similar results were observed for DIM droplets. (See Movie S1). The sliding and splitting process is driven by gravity without the need for additional power input. After initial assembly of the cutting blade between the tracks, further blade adjustments are not needed during the cutting operation. As a result, the sliding and splitting process is efficient and suitable for highthroughput splitting of microliter-sized droplets. However, the cutting speed (i.e., droplet sliding speed) is difficult to control in this process and initial fabrication of the superoleophobic template with dual-rail oleophilic track and central cutting blade is complicated. In fact, placement of the blade perfectly centered between the tracks is extremely difficult, and the location of initial impact of the blade with the sliding droplet critically affects the relative sizes of daughter droplets after splitting. To study the droplet splitting in greater detail, a new configuration was developed in which the blade speed and point of impact can be controlled independently, as depicted in Figure 2. The underwater superoleophobic cutting blade was mounted on the carriage of a linear ball bearing, which had one surface attached to a supporting frame and the other to a linear actuator in order to achieve precise, reproducible vertical motion. Inspired by the experiments of Bormashenko et al., who successfully split water marbles and droplets on a

The blade can also be rendered superhydrophobic by coating the oxidized copper foil with fluorocarbon film via plasma deposition.25 Fabrication of the Underwater and In-Air Patterned Template. Dual-dot patterns were drawn directly on a piece of clean copper foil (40 × 40 mm2) using the mark pen and a ruler stencil. The patterned substrate was then subjected to the same treatment as the cutting blade: oxidation in an aqueous solution of NaHCO3 (0.1 M) and (NH4)2S2O8 (0.02 M) for 24 h to siteselectively grow nanostructures. After cleaning in acetone to remove the ink from the pattern and air-drying, the patterned dots were smooth showing underwater oleophilicity, while the surrounding textured area exhibits underwater superoleophobicity. This underwater superoleophobic template with oleophilic dual-dot patterns can be used as the underwater “operation table” to stretch and support oil droplets for cutting. Two underwater patterned templates (P1 and P2) were fabricated with different center-to-center distances between dots. P1 is composed of two oleophilic dots with diameters of 2.33 mm and a center-to-center distance of 5.01 mm; P2 pattern is composed of two oleophilic dots with diameters of 2.23 mm and a center-to-center distance of 3.62 mm. An underwater superoleophobic template with oleophilic parallel line (dual-rail) tracks can be fabricated using the same method. In-air patterned templates can be obtained by drawing dual-dot patterns with hydrophilic ink directly on a superhydrophobic copper substrate using the marker pen. The in-air pattern (P3) is composed of two hydrophilic dots with diameters of 2.28 mm and a center-to-center dot distance of 5.04 mm on a superhydrophobic substrate. The superhydrophobic blade and substrate can be prepared by oxidation and subsequent plasma deposition of a thin fluorocarbon film, as we have demonstrated previously.25 Characterization. All static-contact-angle (SCA) measurements were performed by placing 5 μL droplets of selected fluids onto the sample substrates. The underwater oil SCAs were measured in an acrylic box (102 × 102 × 102 mm3, 3.2-mm-thick walls). All reported contact angle (CA) values are the average of five measurements. Droplet images and videos were captured and analyzed with a RaméHart CA goniometer (model 290). The morphology of the cutting blade surface was characterized by scanning electron microscopy (Hitachi SEM SU8010, Japan). The underwater droplet splitting process was recorded with a CCD camera (Lumenera Lu135, Canada) equipped with an APO lens (Leica Z6, German) and dedicated image acquisition software (Lucam Recorder). The volumes of the daughter droplets after splitting were measured by using image analysis software to detect the droplet contour and rotating the outline in AutoCAD software, assuming axial symmetry. Videos of droplet sliding and splitting processes were captured using an SLR camera equipped with an EFS 18-135 mm lens (Canon 700d, Japan).

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RESULTS AND DISCUSSION Droplet Sliding and Splitting. As shown in the insets in Figure 1a, the oxidized textured surface was superhydrophilic in air while the unoxidized smooth area is hydrophilic (left two insets in Figure 1a). When submerged in water, the textured area showed superoleophobicity with a DCM contact angle of 13524

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Langmuir superhydrophobic substrate in air,32 we first tried to split a 40 μL DCM droplet deposited on a superoleophobic template underwater using the developed system. The droplet was pinned to the substrate by a small oleophilic dot (diameter of 0.6 mm is smaller than droplet footprint) in order to prohibit the droplet from sliding during the cutting process. Image sequences from a representative cutting attempt in Figure 3a

upper limit of 1 denotes a perfectly equal split, while 0 means that the original droplet remained intact. The data in Figure 4 can be divided into three regimes. In regime I, for cutting speeds below 1.6 mm/s, the droplet cannot be effectively split (see images in Figure 5a); all experiments in this regime

Figure 3. Splitting a 40 μL DCM droplet pinned on an oleophilic dot at cutting speeds of (a) 9.4 and (b) 25.2 mm/s; scale bar in the first frames is 2.0 mm.

Figure 5. Time lapse images of splitting 40 μL DCM droplet: (a) at 1.6 mm/s; (b,c) at 3.7 mm/s; and (d) at 7.1 mm/s; scale bar in the first frames is 2.0 mm.

show that the droplet could not be split into two nearly equal volume daughter droplets at a cutting speed of 9.4 mm/s. Equal-volume splitting was observed at a much higher cutting speed of 25.4 mm/s, but sensitivity of the cutting process to blade alignment with the droplet made it very difficult to perform this process reproducibly. Using Bormashenko’s theory, the critical cutting speed for a 40 μL DCM droplet in water can be predicted to be 170 mm/s, which is much larger than 25.4 mm/s. The difference between these values can likely be ascribed to the pinning force exerted by the dots on the droplet in our experiment, which facilitates splitting. (See Movie S2). Splitting Process on a Dual-Dot Pattern. To improve the cutting process, the oil droplet was stretched and pinned on a dual-dot oleophilic pattern on the superoleophobic substrate (P1 pattern). The two dots of P1 pattern have an average diameter of 2.33 mm and center-to-center distance of 5.01 mm. This approach greatly enabled the reliable splitting of an oil droplet at much lower blade speeds. Figure 4 presents the volume ratio between daughter droplets as a function of blade speed for 40 μL DCM droplets that bridge a dual-dot pattern; it should be noted that the volume ratio is defined as the ratio of the volume of the smaller droplet to the larger one, so that the

resulted in highly uneven daughter droplet volumes with relatively small error bars in the volume ratio. In regime II, for cutting speeds between 1.6 and 4.6 mm/s, droplet splitting is observed, but not reliably into equal volumes. The splitting process is unstable with large variations in volume ratio between repetitions. For instance, most experiments at 3.7 mm/s did not yield controlled droplet splitting (Figure 5b), but occasionally, (nearly) equal-volume splitting did occur (Figure 5c). In Figure 4, significant data scatter among individual experiments and large error bars in the average data characterize this transitional regime. Finally, when the cutting speed exceeds 4.6 mm/s (regime III), which can be regarded as the critical cutting speed, the droplet can be split into daughter droplets of nearly equal volumes with high reproducibility; data in this segment have small error bars. Influence of Cutting Speed and Liquid Properties on Splitting Process. The observations in Figure 4 highlight the fact that droplet breakup is a competition between (i) surface tension and adhesive forces, which drive restoration of the droplet shape, and (ii) deformation due to blade impact. If the droplet shape disturbance is slow, the droplet is able to restore its equilibrium shape, which is established by surface tension forces. In comparison to the cutting of an unpinned 40 μL droplet (Figure 2b), the stretched droplets can be split successfully at much lower blade speeds. Bormashenko et al. hypothesized that droplet splitting in air is possible when the cutting process is accomplished faster than the naturally occurring oscillation frequency of the droplet (Rayleigh frequency). For their experiments, the critical cutting speed Vcr for droplet splitting could therefore be derived as a function of geometrical shape (droplet radius R and droplet height H), effective interfacial tension (γeff), and density (ρ) of the droplet:32

Figure 4. Volume ratio of split drops as a function of blade speed when splitting a 40 μL DCM droplet bridging a dual-dot pattern (P1; dot diameters 2.33 mm and center-to-center distance 5.01 mm); solid squares represent results from individual experiments, while open symbols with error bars denote averages.

Vcr ≅

H H = τ π

2γeff ρR3

(1)

Using this equation, Bormashenko et al. reported that the estimated critical cutting speed for PTFE coated liquid marbles 13525

DOI: 10.1021/acs.langmuir.7b03604 Langmuir 2017, 33, 13522−13529

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Figure 6. Volume ratio of split droplets as a function of cutting speed when cutting droplets with (a) different curvature Cstr and (b) different liquid properties (γeff and μ); P1 and P2 have two oleophilic dots for underwater experiments (P1: diameters 2.33 and distance 5.01 mm; P2: diameters 2.23 mm and distance 3.62 mm), while P3 has two hydrophilic dots (diameters 2.28 mm, distance 5.04 mm) on a superhydrophobic substrate for inair experiments.

(R = 3.5 mm, H = 7.0 mm) is ∼110 mm/s. Indeed, cutting speeds as high as 200−500 mm/s allowed successful cutting, indicating that eq 1 offers an estimate of the velocity required. In our experiments, where the oil droplet is stretched underwater on a patterned template, the process is more complicated. The shape restoring force of the surface tension is much smaller for underwater oil droplets than an in-air water droplet, while the stretching further lowers the energy barrier needed for effective and controllable cutting. Direct application of eq 1 to the experiments in Figures 4 and 5 gives a predicted critical velocity of 50.2 mm/s, which is much higher than the observed value of 7.1 mm/s. It is worth noting that when using eq 1 to calculate the underwater critical velocity, γeff should be the interfacial tension between the interfaces of oil and water, which for DCM in water is 27.9 mN/m.64 Although a full model of the cutting process for stretched droplets is extremely complicated and beyond the scope of this paper, we propose a qualitative “liquid exchange model” to explain the critical cutting speed for stretched droplets, Vcr‑str. As is well established,65 the internal Laplace pressure of droplets is proportional to the droplet curvature C:

ΔP = 2γeff C

Note that a key difference between eqs 1 and 3 is the role of viscosity, which is absent in Bormashenko’s model. For highspeed, inertia-driven in-air cutting, this is understandable, but for our underwater experiments, droplet viscosity plays an important role by suppressing internal flow. It should be noted that the viscosity of the surrounding medium (i.e., water in our experiments) also plays a role in the process of droplet splitting, since water must be displaced to accommodate droplet deformation and breakup. However, the medium is constant in our experiments and the effects of the external viscosity effects are expected to be less pronounced than of the oil droplet, because the deformations and stresses in the bulk medium are smaller than in the confined droplet. The parameter ΔCdro is difficult to quantify experimentally, but intuitively explains why droplet stretching lowers the critical cutting speed. When a droplet is stretched on a patterned template, the curvature becomes anisotropic: in the elongated direction perpendicular to the cutting blade, Cstr is much lower than the unstretched equilibrium droplet, even though the total curvature is greater. As a result, ΔCdro caused by blade impact, which is by definition related to Cstr, will also be lower for a stretched droplet, thus reducing Vcr‑str. To verify the predictions of eq 3, the effects of stretching and liquid properties on droplet splitting were investigated. Figures 6a and 7 show the effect of Cstr on Vcr‑str by varying the degree

(2)

When the cutting blade penetrates a stretched droplet (initial curvature Cstr), the droplet surface on both sides of the blade willby chancehave slightly different curvatures. If the blade motion was stopped, liquid from the side with larger curvature (CL) would flow toward the side with smaller curvature (CS) due to the Laplace pressure difference between the two sides under the influence of the curvature difference ΔCdro = CL − CS, until the droplet breaks. If the cutting speed is high enough to accomplish the cutting process before significant liquid exchange can occur, the droplet can be split into two nearly equal volume droplets. If the cutting speed is too low, liquid exchange will create daughter droplets with different volumes. As a result, it can be inferred that the critical cutting speed in our case (Vcr‑str) must have a positive correlation with the flow rate of the liquid exchange process: fast flow requires high cutting speed. In addition to curvature difference ΔCdro, the effective surface tension γeff also drives liquid exchange (eq 2),12,66 while flow is hindered by the liquid viscosity μ. Vcr‑str can thus be predicted to display the following scaling: ⎛ 1⎞ Vcr ‐ str ∝ ⎜ΔCdro , γeff , ⎟ μ⎠ ⎝

Figure 7. Time lapse images of cutting underwater DCM droplets: (a) 22 μL droplet on P1 with Cstr of 0.217 mm−1 at Vcr‑str of 3.9 mm/s; (b) 40 μL droplet on P2 with Cstr of 0.331 mm−1 at Vcr‑str = 9.8 mm/s; scale bar in the first frames is 2.0 mm.

of droplet stretching by decreasing the droplet volume (increasing stretching, and thus decreasing curvature Cstr) and by decreasing the pattern spacing (and increasing curvature Cstr). The results clearly indicate a positive correlation between critical cutting speed Vcr‑str and Cstr. Figure 6b shows the dependence of critical cutting speed on liquid properties. First, the stretched droplet cutting was carried out for a water droplet

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or increasing the droplet density to flatten the droplet and suppress curvature. The effect of viscosity of the surrounding medium may also warrant further study, because it is expected to suppress splitting, but this quantity was kept constant in the underwater experiments reported here. Our underwater droplet splitting system can be applied to microreactors and can function as a distributor that can split a reacted bulk droplet into two nearly equal volume daughter droplets for further reaction or analysis.

in air, using a pattern of two hydrophilic dots on a superhydrophobic substrate (P3; dot diameter 2.28 mm and center-to-center distance 5.04 mm). In this case, Cstr of the water droplet is almost identical to the stretched underwater DCM droplet; water has a higher viscosity (0.89 mPa·s at 20 °C) than DCM (0.41 mPa·s at 20 °C) by about a factor 2, and γeff for water in air (72.0 mN/m) is roughly 2.5 times larger than for DCM in water (27.9 mN/m). These two effects counteract each other according to our hypothesis, but it is no surprise that the surface tension dominates, resulting in a higher critical cutting speed for the stretched water droplet in air than the DCM droplet underwater (Figure 6b, see also Movie S3). When comparing the DCM droplet with diiodomethane (DIM), γeff for DIM in water (48.6 mN/m67) is ∼1.7 times larger than for DCM in water (27.9 mN/m), but the viscosity is much greater for DIM (2.6 mPa·s at 20 °C) than for DCM (0.41 mPa·s). Furthermore, the large density of DIM flattens the 40 μL droplet, which suppresses curvature Cstr of the stretched DIM droplet. As a result, experiments show that DIM droplets underwater can be split at very low blade speed (Figure 6b; see also Movie S4). It should be noted that Bormashenko’s model (eq 1) would have predicted a higher critical cutting speed for DIM than for DCM, because viscosity is not taken into account in that model.

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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.7b03604. Movie S1: splitting of DCM and DIM droplets underwater using the sliding and splitting process (AVI) Movie S2: splitting of a DCM droplet adhered to a single oleophilic dot on a superoleophobic substrate underwater (AVI) Movie S3: splitting of a water droplet stretched by a dualdot hydrophilic pattern on superhydrophobic substrate in air (AVI) Movie S4: splitting of a DIM droplet stretched by a dualdot oleophilic pattern on a superoleophobic substrate underwater (AVI)

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CONCLUSIONS Underwater oil droplets stretched and pinned by dual-dot oleophilic patterns on superoleophobic substrates can be easily split into two nearly equal volume daughter droplets with high reproducibility using a superoleophobic blade. The velocity of the cutting blade was varied to quantitatively determine the critical cutting velocity, which varies significantly depending on droplet stretching and liquid properties (surface tension and viscosity). In the droplet splitting process, as the blade penetrates the droplet, the droplet surfaces on both sides of the blade willby chancehave slightly different curvatures, which drives the liquid from the side with larger curvature flow toward the side with smaller curvature. The liquid flow rate increases with the increased droplet curvature and surface tension and decreases with increased viscosity of the stretched droplet; this phenomenon is similar to a liquid transport process between two reservoirs connected by a channel. If the cutting speed is high enough to accomplish the cutting process before significant liquid flow can occur, the droplet can be split into two nearly equal volume droplets. Therefore, the critical cutting speed that allows equal-volume splitting has a positive correlation with curvature and surface tension but a negative correlation with the liquid viscosity of the stretched droplet; this prediction is in agreement with the experimental results. The lowest critical cutting velocity in our experiments, 1.6 mm/ s for a stretched diiodomethane droplet, is to the best of our knowledge the slowest reported droplet cutting speed in an automated, controlled blade cutting process. To decrease the critical cutting speed even further, greater droplet stretching would be beneficial, but pinning highly deformed droplets on the patterned template is nontrivial. The droplet will want to detach from the pattern when stretched too severely. Using flexible and/or deformable substrates like paper and polymers could enable extensions of dot separation after placement of the droplets. Alternatively, splitting would be facilitated by lowering the interfacial tension between the droplets and surrounding liquid phase, for example, through the addition of surfactants,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 1-(404)8944780. ORCID

Xiaolong Yang: 0000-0002-2324-6172 Victor Breedveld: 0000-0001-9108-7137 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the help of Dr. Won Tae Choi for SEM characterization. This work was financially supported by National Basic Research Program of China (Grant No. 2015CB057304); X.L.Y. thanks the China Scholarship Council for providing an opportunity to work at Georgia Tech as a joint PhD Student.

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REFERENCES

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DOI: 10.1021/acs.langmuir.7b03604 Langmuir 2017, 33, 13522−13529

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DOI: 10.1021/acs.langmuir.7b03604 Langmuir 2017, 33, 13522−13529