On-Demand Capture and Release of Organic Droplets UsingSurfactant-Doped Polypyrrole SurfacesWei Xu, Anthony Palumbo, Jian Xu, Youhua Jiang, Chang-Hwan Choi, and Eui-Hyeok Yang*

Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States

*S Supporting Information

ABSTRACT: In this paper, we demonstrate the controlledcapture and release of dichloromethane (DCM) droplets ondodecylbenzenesulfonate-doped polypyrrole (PPy(DBS)) sur-faces in an aqueous environment. The droplets captured onoxidized PPy(DBS) surfaces were released on-demand via areduction process at ∼0.9 V, with controlled release time anddroplet morphology. The release time of an entire droplet (2± 1 μL) was proportional to the thickness of the PPy(DBS)coating, increasing from 11.5 to 26.3 s for thicknesses rangingfrom 0.6 to 5.1 μm. The droplet-release time was also affectedby the redox voltages, and among the tested redox voltages, the fastest release was achieved at −0.9/0.1 V. The PPy(DBS)surfaces with larger thicknesses were more durable for the droplet capture and release. The droplets were more rapidly releasedfrom PPy(DBS) surfaces with increased surface roughness ratios, such as 6.0 s on a micropillared surface and 10.3 s on a meshedsurface, as compared to 14.6 s on the 1.8 μm thick PPy(DBS) surfaces coated on frosted-glass substrates (i.e., with randommicrostructures). The release of a single droplet was achieved by increasing the underwater oleophobicity of PPy(DBS) surfacevia O2 plasma treatment.

KEYWORDS: droplets, polypyrrole, adhesion, surfactants, microstructures

■ INTRODUCTION

The control of adhesion of liquid droplets on solid substrates hasbroad implications in surface cleaning, water treatment, micro-fluidics, biochemistry, and lab-on-a-chip devices.1−9 By thechanging of the surface chemistry and morphology, varioussurfaces have been developed for controlled liquid adhesion fromadhesive to repellant for applications including coating, printing,droplet manipulation, and water repellency.10−14 Furtherdevelopments of smart surfaces have also resulted in on-demandadhesion control of liquid droplets on solid substrates usingexternal stimuli such as light,15 heat,16 electrical potentials,17

magnetic fields,18 or mechanical stress.19 Besides these methods,which are mainly based on the tunable wetting property of solidsubstrates, the on-demand control of liquid adhesion has recentlybeen explored through the change of the surface property ofliquid droplets (e.g., surface tension or surface polarity) usingpolypyrrole (PPy)-coated surfaces.20,21 Polypyrrole as aconjugated polymer has gained great attention in severalapplications, including actuators,22 supercapacitors,23 biosen-sors,24 and microfluidics,25 due to its good stability, biocompat-ibility, high conductivity, and mechanical property.26−29

Recently, there has been a growing interest in the study on theliquid adhesion and wetting on polypyrrole surfaces.20,21,30−33

For example, Chatzipirpiridis et al. reported the electrochemicalmanipulation of apolar solvent drops in aqueous electrolytes onpolypyrrole architectures by altering the surface polarity ofdroplets.21 In our previous work, we investigated thefundamentals of tunable wetting and liquid adhesion on

dodecylbenzenesulfonate-doped polypyrrole (PPy(DBS)) sur-faces.20,30−33 For example, Tsai et al. reported the tunablewetting mechanism of PPy(DBS) surfaces via redox for low-voltage droplet manipulation.30 Xu et al. demonstrated in situmanipulation of organic droplets between the pinned and slidingstates on PPy(DBS) surfaces in an aqueous environment, whichwas induced by the accumulation of surfactants desorbed fromPPy(DBS) on the droplet surface and the consequent decrease ofinterfacial tension and pinning force of the droplet during thereduction of the polymer.20 However, despite such progress, littlehas been explored on the capture and release of organic dropletsusing PPy(DBS) or other conjugated polymer surfaces with acapability to control the release process, such as release time andmorphology of released droplets, as well as a systematical studyof the roles of experimental parameters.34

Here, we demonstrate on-demand capture and release ofdichloromethane (DCM) droplets on PPy(DBS) surfaces withcontrolled release time and droplet morphology via the redoxprocess electrochemically manipulated upon low-voltage actua-tions. We elucidate the mechanism and study the effects ofPPy(DBS) coating thickness and redox voltage on the dropletcapture and release behaviors, including the release time,durability, and the number of droplets released. We furtherinvestigate the effects of surface morphology and wettability on

Received: March 16, 2017Accepted: June 16, 2017Published: June 16, 2017

Research Article

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© XXXX American Chemical Society A DOI: 10.1021/acsami.7b03787ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

the droplet capture and release process, demonstrating control-lable droplet capture and release for tailored applications.

■ RESULTS AND DISCUSSION

Droplet Capture and Release. Figure 1a illustrates themechanism of on-demand capture and release of organic dropletson PPy(DBS) surfaces in an aqueous electrolyte environment viaelectrochemical redox process, requiring relatively low voltage(<1 V) for the actuation. The PPy(DBS) surface turns into anoxidized state upon application of an oxidative voltage (e.g., 0.1V), in which doped DBS− molecules bond to the PPy chains viapolar sulfonic acid groups, while dodecyl chains protrude outfrom the polymer chains, constituting the surface layer (Figure1a-i).30 Because the dodecyl groups are hydrophobic (oroleophilic), the oxidized PPy(DBS) surface exhibits a state ofunderwater oleophilicity and high adhesion to organic droplets.Consequently, the organic droplets can be captured by theoxidized PPy(DBS) surface, even when the sample is placedupside down (Figure 1a-ii).The PPy(DBS) surface turns into a reduced state upon

application of a reductive voltage (e.g., −0.9 V). The DBS−

molecules reorient within the reduced PPy(DBS), exposing thehydrophilic (or oleophobic) sulfonic acid groups at theoutermost surface. This change reduces the interfacial tensionbetween the PPy(DBS) surface and aqueous medium, causing

the reduced polymer surface to be more underwater oleophobic.Furthermore, a minute amount of DBS− molecules is desorbedfrom the PPy(DBS) matrix during reduction, and therefore, aportion of desorbed DBS− molecules, which are near the dropletboundary, accumulate at the interface between the organicdroplet and the aqueous electrolyte (Figure 1a-iii).20 Theaccumulated DBS− molecules, acting as surfactants, result in adecrease of interfacial tension between the organic droplet andaqueous electrolyte. Once the decreased interfacial tension canno longer balance external forces (e.g., gravity), the dropletelongates, thinning in its middle section (Figure 1a-iv), and aportion of the droplet falls downward (Figure 1a-v).35,36

Subsequently, the portion of the droplet remaining on thePPy(DBS) surface often falls in a similar way, forming additionaldrops (Figure 1a-vi). After the release of the entire droplet, thearea of PPy(DBS) surface, which was initially in contact with thedroplet, is then exposed to the electrolyte and likewise reducedby the reductive voltage. The reduced PPy(DBS) surface canthen be switched back to a high-adhesion state upon applicationof oxidized voltages.Figure 1b shows a demonstration of the on-demand capture

and release of a dichloromethane (DCM) organic droplet on aPPy(DBS) surface immersed in an aqueous environment (0.1 MNaNO3). The PPy(DBS) surfaces were prepared by coating thepolymer on Au/Cr-coated frosted glass via the electrodeposition

Figure 1. (a) Schematic illustration of the mechanism of the on-demand capture and release of an organic droplet on a PPy(DBS) surface in an aqueouselectrolyte environment via the redox process. (b) A demonstration of the on-demand capture-and-release process of a DCM droplet on a PPy(DBS)surface in 0.1MNaNO3. (i) ADCMdroplet was dispensed on anO2-plasma-treated glass slide; (ii) the PPy(DBS) surface oxidized at 0.1 V was loweredto contact the DCM droplet; (iii) the droplet was captured by the oxidized PPy(DBS) surface; (iv) −0.9 V was then applied to reduce the PPy(DBS)surface; (v−vii) the droplet was gradually elongated, developing a thin cylindrical neck; (viii) the droplet segregated into two portions, and the lowerportion fell from the PPy(DBS) surface; and (ix−xv) the remaining portion of the droplet on the PPy(DBS) surface repeated deformation and releasedseveral satellite drops in a similar manner. The entire droplet was released in 4.2 s. (c) The scanning electron microscope (SEM) images of thePPy(DBS) surface used for the test in panel b, which was electrodeposited on Au/Cr-coated frosted glass (electrodeposition voltage: 0.7 V; surfacecharge density: 300 mC/cm2). The top image shows the top view with a tilting angle of 45°, and the bottom image shows the cross-section view.

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process (Figure 1c) (fabrication details are described in theExperimental section). The random microstructures on thefrosted glass surface can effectively improve the adhesionbetween the PPy(DBS) film and the substrate, preventing thecoated PPy(DBS) film from being peeled off during redox. Thesurface roughness ratio (i.e., the ratio of actual area of the solidsurface to the projected area) of the PPy(DBS) surface coated onfrosted glass is 1.09 (the measurement method of surfaceroughness ratio is explained in the Experimental section). Duringthe droplet capture-and-release process (the configuration of theexperimental setup is shown in Figure S1), a DCM droplet wasfirst dispensed on an O2-plasma-treated glass slide submerged inaqueous electrolyte, showing a high contact angle of ∼160°(Figure 1b-i). The O2 plasma treatment increased the surfacehydrophilicity and underwater oleophobicity of the glass slide,thereby increasing the contact angle. The PPy(DBS) surfaceoxidized at 0.1 V was then lowered to contact the DCM dropleton the glass slide (Figure 1b-ii). Following contact, thePPy(DBS) surface was raised, and the droplet was captured bythe oxidized PPy(DBS) surface due to strong adhesion (Figure1b-iii). The captured droplet was transported with the oxidizedPPy(DBS) surface to a new location without falling. A reductivevoltage of −0.9 V was then applied to the PPy(DBS) surface(Figure 1b-iv), causing the captured droplet to graduallyelongate, developing a thin cylindrical neck with a decreasedcontact area with the PPy(DBS) surface (Figure 1b-v−vii) due tothe decrease of interfacial tension between the organic dropletand aqueous electrolyte against gravity (Density of DCM: 1.33g/cm3). The decrease of the PPy(DBS)-droplet contact area (asa result of reduced interfacial tension between the PPy(DBS)surface and aqueous medium) also contributed to the dropletelongation due to mass conservation.37 The droplet fullysegregated into two portions in ∼3 s (Figure 1b-viii). Thelower portion of the droplet was released from the PPy(DBS)surface under gravity, forming the primary falling drop (i.e., thefirst falling drop), while the other portion remained on thePPy(DBS) surface. The droplet was usually broken in the middlesection, not from the interface between PPy(DBS) surface anddroplet. This behavior indicates that the balance of the interfacialtension (i.e., between the organic droplet and the aqueouselectrolyte) and the external forces (e.g., gravity) dominates themechanics for the release of droplet. After the release of theprimary drop, the remaining portion of the droplet on thePPy(DBS) surface repeated deformation and released severalsatellite drops in a similar manner until there was virtually noorganic liquid remaining on the PPy(DBS) surface (Figure 1b-ix−xv). After a few seconds (∼4.2 s for this specific example), thecaptured DCM droplet was fully released from the PPy(DBS)surface, forming multiple drops on top of the glass slide. In thiswork, we have tested DCM droplets with volumes ranging from0.7 to 7.3 μL (∼10× difference in volume) for the on-demandcapture and release of droplets (Figure S2).The gradual deformation of the droplet during the release

process indicates the decrease of interfacial tension between theorganic droplet and the aqueous electrolyte due to theaccumulation of the surfactants at the interface. As shown inFigure 2, the change of the interfacial tension was measured byanalysis of the profile of the deformed pendant droplet; theinterfacial tension was only measured after the droplet exhibitedan apparent deformation within measurement accuracy of agoniometer. The initial interfacial tension between the DCMdroplet and the aqueous electrolyte (γOW) was 27.8 mN·m−1,measured using a pendant-droplet method.38,39 After 0.5 s of

reductive voltage (−0.9 V) application, the interfacial tensionwas significantly decreased to 5.2 mN·m−1 and continued todecrease to 1.9 mN·m−1 before the release of the droplet. Thisresult confirms the decrease of the interfacial tension between theorganic droplet and the aqueous electrolyte, induced by thegradual accumulation of the surfactants desorbed from PPy-(DBS) on the droplet surface,20,32,33 as explained in Figure 1a.Due to the decrease of interfacial tension, the droplet cannot beheld as before by interfacial tension against the gravitationalforce, which results in the release of the droplet. The portion ofthe droplet remaining on the PPy(DBS) surface wassubsequently released in a similar manner. The effect of DBS−

molecules on the change of interfacial tension was furtherconfirmed by the PPy surface that was not doped with DBS−

molecules.20 The DCMdroplet on an “undoped” PPy surface didnot show any change in shape during a redox process.Figure 3a shows an example of the capture and release of DCM

droplets on a PPy(DBS) surface coated on a frosted glasssubstrate during the cyclic redox process (surface charge densityfor PPy(DBS) electrodeposition: 300 mC/cm2). Droplets of 2±1 μL were used in the cyclic capture and release process forconsistency. The release time of the primary falling drop (i.e., thetime taken for the first portion of droplet to be released from thesubstrate) during each cycle was mainly between 2.2 and 11.0 s,whereas at some rare and random cycles, the release time wasshorter than 1 s or longer than 20 s (Figure 3b). The averagerelease time of the primary drop was 7.3 s, and the total releasetime for the entire droplet (the time that the entire droplet isreleased) was 11.5 s on average (Figure 3b inset) for thisexample. The contact angles of captured DCM droplets on theoxidized PPy(DBS) surface gradually increased with addition ofredox cycles; as shown in Figure 3c, the contact angle increasedfrom 67° to 120° after 16 redox cycles, which indicates that thePPy(DBS) surface in an oxidized state gradually changed towardhigher underwater oleophobicity. There are several factors thatwe consider for this change of the wetting property of PPy(DBS)surfaces during the cyclic redox process. First of all, as is well-known, PPy(DBS) undergoes volume changes during redox.40

Liu et al. reported the increase of surface roughness of thePPy(DBS) in a reduced state as a result of volume change.41 Theincrease of surface roughness could cause the change of wettingproperty of a PPy(DBS) surface by the formation of a Wenzel orCassie−Baxter state.42,43 In addition, the influx of water from theaqueous electrolyte into the polymer and the accumulation of

Figure 2. Decrease of interfacial tension of a pendant DCM droplet (in0.1 M NaNO3) captured on a PPy(DBS) substrate upon the applicationof a reductive voltage of −0.9 V.

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surfactant dopants desorbed from the polymer onto the polymersurface during the redox process could also increase theunderwater oleophobicity of a PPy(DBS) surface.44,45 Mean-while, we observed that the contact radii of the droplets on theoxidized PPy(DBS) surface were reduced (Figure 3c), and thenumber of falling drops formed during the release decreasedfrom approximately 5 to 2 after 16 redox cycles (Figure 3d). Thenumber of drops formed during the release is related to thechange of contact radii of droplets on the substrate. According toHarkins and Brown,35,46,47 the actual volume percentage of adrop falling from a capillary is correlated with the capillary radius.As shown in Figure 1, the falling of droplets from the PPy(DBS)surface is similar to the situation in which the droplet falls from acapillary, and the droplet contact radius on the PPy(DBS)surface is analogous to the capillary radius. In this example, thedroplet contact radius, normalized by the cube root of volume,decreased from approximately 0.54 to 0.26 after 16 redox cycles(Figure 3c), and as a result, the volume percentage of the primaryfalling drop to the entire droplet increased from ∼60 to ∼90%(Figure 3d), which agrees with Harkins and Brown’s study. Thenumber of falling drops formed during the release processconsequently decreased with the increase of volume percentageof falling drops to the entire droplet.Effects of Coating Thickness and Redox Voltage. We

furthermore studied the droplet-release time and durability ofPPy(DBS) surfaces (i.e., capability of maintaining the dropletcapture and release functionalities), along with the effects ofPPy(DBS) coating thickness and redox voltages during multipleredox cycles. Within each redox cycle, the PPy(DBS) surface was

first oxidized for the droplet capture and then reduced for thedroplet release as illustrated in Figure 1. The thickness of thePPy(DBS) coating was controlled by the surface charge densityduring the electrodeposition process.20,32 As shown in Figure 4a,by using three different surface charge densities of 100, 300, and1000 mC/cm2, the PPy(DBS) surfaces were coated with anaverage thickness of approximately 0.6, 1.8, and 5.1 μm,respectively. The average release time of the primary drop andthe total release time increased with the coating thickness (i.e.,6.3, and 11.5 s on the 0.6 μm thick samples; 10.6 and 14.6 s on the1.8 μm samples; and 20.9 and 26.3 s on the 5.1 μm samples)(Figure 4b). We attribute this change in release time to slowerdesorption of DBS− molecules from thicker PPy(DBS) surfaces,which has been reported elsewhere through an EDS study to thecontent change of DBS−molecules in PPy(DBS) duringmultipleredox cycles.32 However, the PPy(DBS) surfaces with thickercoating showed better durability. For example, the droplets werecaptured and released for an average of 39 cycles on the 5.1 μmthick samples, compared to 2 cycles on the 0.6 μm thick samplesand 14 cycles on the 1.8 μm thick samples. After these redoxcycles, the captured droplet would not be released from thePPy(DBS) surface, even it elongated under reduction (FigureS3). The better durability of thicker PPy(DBS) coatings isattributed to the abundance of DBS− molecules within thevolume of the PPy(DBS) film, wherein DBS− molecules may bedesorbed during prolonged redox cycles.According to the cyclic voltammetry (CV) of the electro-

polymerized PPy(DBS) surface (Figure 4c), the reduction wascompleted at−0.9 V, while the oxidation was completed at 0.1 V.

Figure 3. Capture and release of DCM droplets on a PPy(DBS) sample (surface charge density for electrodeposition: 300 mC/cm2) during multipleredox cycles (redox voltages: 0.1 V for oxidation (O) and −0.9 V for reduction (R)). (a) Durability, (b) the release time of the primary drop and totalrelease time for the entire droplet, (c) the change of contact angle and contact radius (normalized by the cube root of droplet volume) of the dropletscaptured on the oxidized PPy(DBS) surface over the redox cycles, and (d) the change of the number of falling drops and the volume percentage of theprimary falling drop to the entire droplet over the redox cycles.

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The increase of the current with the oxidative voltage higher than0.6 V indicates the occurrence of side reactions such as cross-linking, overoxidation, and the reaction of monomers with waterto form amide groups.30,48 To this end, we compared threedifferent combinations of reduction and oxidation voltages (i.e.,−0.9/0.1, −0.9/0.6, and −0.6/0.1 V) to investigate the effect ofredox voltages on the droplet capture and release behaviors(Figure 4d). The average release times of the primary drop andthe total release time on the samples operated at−0.9/0.1 V were10.6 and 14.6 s, respectively (surface charge density forelectrodeposition: 300 mC/cm2). When the oxidative voltagewas increased to 0.6 V, the release time of the primary drop andthe total release time increased to 17.4 and 24.5 s, respectively.However, when the reductive voltage was lowered from −0.9 to−0.6 V, the release time of the primary drop and the total releasetime were also increased to 18.2 and 25.5 s, respectively. Weattribute this to a stronger repulsive force that −0.9 V applied onthe negatively charged DBS− molecules than −0.6 V, facilitatingthe desorption of DBS− molecules from PPy(DBS). Accordingto this result, a relatively higher reductive voltage (−0.9 V) and alower oxidative voltage (0.1 V) would cause a shorter releasetime. The durability was similar (i.e., 12 to 14 cycles) on thosesamples operated at different redox voltages, confirming that thethickness should be the primary factor determining the durabilityof PPy(DBS) surfaces.Effects of Surface Morphology and Wettability. We

further studied the effect of surface morphology on dropletcapture and release behaviors. In addition to the PPy(DBS)

samples coated on frosted glasses with random surfacemicrostructures, we also tested PPy(DBS) samples with well-regulated micropillar and meshed structures (Figure 5). ThePPy(DBS) micropillar structures were 7.8 μm in diameter and6.0 μm in height, fabricated by electrodepositing PPy(DBS) onAu/Cr-coated silicon micropillar structures (Figure 5a-i). ThePPy(DBS) mesh structures with square pores of ∼149 μm inwidth were made by electrodepositing PPy(DBS) on Au/Cr-coated stainless steel meshes (Figure 5b-i). The surface chargedensity during electropolymerization was 300 mC/cm2 for bothsamples; details of the fabrication process are explained in theExperimental section.Figure 5a-ii shows the capture and release process of DCM

droplets on PPy(DBS)-coated micropillar structures. Thedroplet showed a smaller contact angle of ∼52° in the oxidationstate compared to that on the PPy(DBS) surface coated onfrosted glass. This smaller contact angle indicates that the DCMfilled gaps between micropillars and formed an underwaterWenzel state,42 as illustrated in Figure 5a-iii. Upon reduction, therelease of the droplet started in 3.8 s and completed in an averageof 6.0 s. We note that the droplet release time from thePPy(DBS) surface with micropillar structures was significantlyshorter than that on the PPy(DBS)-coated frosted glass (averagerelease time of the primary drop: 10.6 s; average total releasetime: 14.6 s) (Table 1). We attribute this difference to a largersurface roughness ratio (i.e., the ratio of actual area of the solidsurface to the projected area) of the micropillar samples (2.47)compared to that of the frosted glass (1.09) (the measurement

Figure 4. (a) Thicknesses of PPy(DBS) surfaces fabricated with different surface charge densities during electropolymerization. (b) Release time anddurability of the PPy(DBS) samples with different thicknesses (redox voltages: −0.9/0.1 V). (c) Cyclic voltammogram (CV) of PPy(DBS) in 0.1 MNaNO3 solution (20 mV/s). (d) Release time and durability of the PPy(DBS) samples reduced and oxidized at different voltages (−0.9/0.1, −0.9/0.6,and −0.6/0.1 V; surface charge density for electrodeposition: 300 mC/cm2).

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method of surface roughness ratio is described in theExperimental section). A larger effective surface area wouldfacilitate greater desorption of DBS− molecules near the dropletboundary, accumulating more DBS− molecules on the interfacebetween the organic droplet and aqueous electrolyte. Theabundance of DBS−molecules on the interface further lowers theinterfacial tension, resulting in a faster release of the droplet.However, because the droplet was in an underwater Wenzel stateon the oxidized micropillared PPy(DBS) surface with adecreased contact angle, the droplet had an increased contactradius with the PPy(DBS) substrate than that on the PPy(DBS)surface on the frosted glass (e.g., the contact radius normalizedby the cube root of volume was 0.58 for the droplet shown inFigure 5a-ii). Therefore, a larger portion of a droplet remained onthe substrate after the release of the primary droplet during therelease process, as discussed in Figure 3. The average volumepercentage of the primary falling drop to the initial entire droplet

was∼56% (Table 1), which was lower than that of the PPy(DBS)coated on frosted glass with random microstructures (∼78%).Because a larger volume of the droplet remained on the surface,there were more drops continually released while the reductivevoltage was applied. For example, there were approximately fivedrops formed in average during the release process from themicropillared PPy(DBS) surface, while there were approximatelythree drops from the surface coated on the frosted glass withrandom roughness.The capture and release process of DCM droplets on a

PPy(DBS) mesh (Figure 5b-i) is shown in Figure 5b-ii. Thedroplet on PPy(DBS) mesh showed a higher contact angle of122°with a smaller contact radius (the contact radius normalizedby the cube root of volume was 0.26 for the droplet shown inFigure 5b-ii) than the micropillared surface. The droplet on themesh showed a higher contact angle because the aqueouselectrolyte filled the pores between the mesh structures, asillustrated in Figure 5b-iii, resulting in the formation of anunderwater Cassie−Baxter state.43 The average release time ofthe primary drop on the PPy(DBS) mesh was 9.2 s, and the totalrelease time was 10.3 s, which were also shorter than that on thePPy(DBS) coated on frosted glass. We attribute the reducedrelease time to the increased surface roughness ratio (2.67) of thePPy(DBS) mesh (both sides of the surface are counted inestimating surface roughness ratio) and, hence, a larger effectivesurface area for the desorption of the DBS− molecules, asdiscussed for the case on the PPy(DBS) surface with micropillarstructures. However, because the contact radius of the dropletwith PPy(DBS) mesh decreased with the increase of contactangle, fewer DBS− molecules would accumulate on the dropletsurface compared to that on themicropillared PPy(DBS) surface.Therefore, the release time was longer on a PPy(DBS)mesh thanthat on the micropillared PPy(DBS) surface, even though thesurface roughness ratios of the two samples were similar. Inaddition, because the contact radius became smaller, the dropletdetached with a larger volume, compared to the case of thePPy(DBS) coated on frosted glass. The average volumepercentage of the primary falling drop to the entire droplet onthe PPy(DBS) mesh was 88% (Table 1).We also investigated the effect of surface wettability on the

capture and release of droplets on PPy(DBS) substrates. Toavoid the influence of surface microstructures, the PPy(DBS)coatings deposited on flat Cr/Au-coated silicon substrates wereused in this test (surface charge density for electrodeposition:1000 mC/cm2; redox voltages: −0.9/0.1 V). The PPy(DBS)samples were treated with 25 WO2 plasma for 2 min (the detailsare included in the Experimental section). Figure 6a shows thesurface morphology of the PPy(DBS) surface before and afterthe treatment. Although there was no significant difference inmicroscale surface morphology, the PPy(DBS) surface after thetreatment showed a change of nanoscale surface morphologywith the formation of tiny nanodots. Due to the change of surfacemorphology and surface chemistry after the O2 plasmatreatment, the PPy(DBS) surface became underwater oleopho-bic with a contact angle of 142° (Figure 6b). With such a largecontact angle and a small contact radius, the droplets were usuallyreleased as a single droplet (or two droplets, in which the volumepercentage of the primary falling drop was over 99%). However,it should be noted that the total release time for the droplets onthe plasma treated surface was ∼41 s, which was longer than thaton untreated surfaces. This is due to the small contact radius of adroplet on the plasma-treated PPy(DBS) surface, resulting in alower amount of surfactants desorbed from PPy(DBS) near the

Figure 5. On-demand capture and release of DCM droplets on (a)PPy(DBS) micropillars and (b) PPy(DBS) mesh in an aqueouselectrolyte environment (0.1 M NaNO3). (i) Surface morphology ofPPy(DBS) substrates, (ii) the droplet-release process, and (iii) theschematics of the DCM droplet on PPy(DBS) substrates.

Table 1. Capture and Release Behaviors of DCM Droplets onVarious Microstructured PPy(DBS) Samples

samplesroughness

ratio

releasetime ofprimarydrop (s)

totalreleasetime(s)

volumeof

primarydrop (%)

numberof fallingdrops

PPy(DBS) onfrosted glass(randommicrostructures)

1.09 10.6 14.6 78% 3.3

PPy(DBS)micropillars

2.47 3.8 6.0 56% 5.3

PPy(DBS) mesh 2.67 9.2 10.3 88% 2.1aSurface charge density for PPy(DBS) electrodeposition is 300 mC/cm2 and redox voltages are −0.9/0.1 V.

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droplet boundary that accumulate on the interface between thedroplet and aqueous electrolyte, causing the change of interfacialtension.

■ CONCLUSIONSWe have demonstrated the on-demand capture and release oforganic (dichloromethane) droplets on PPy(DBS) surfaces in anaqueous environment via a low-voltage redox process withcontrolled release process. The on-demand droplet capture andrelease are facilitated by the tunable interfacial tension of theorganic droplet with the surrounding aqueous electrolyte uponthe application of redox voltages, which is induced by thedesorption of surfactants from PPy(DBS) surfaces. The timerequired for the droplet to release is proportional to the thicknessof the PPy(DBS) coating and is also affected by the redox voltage.The durability of PPy(DBS) surfaces is mainly dependent on thecoating thickness. The droplets are released more quickly on thePPy(DBS) surfaces with increased surface roughness ratios dueto a larger effective surface area for the desorption of DBS−

molecules near the droplet boundary, which results in theaccumulation of more DBS− molecules on the interface betweenthe organic droplet and aqueous electrolyte (facilitating a fasterdecrease of the interfacial tension). The average release time ofthe primary drop (i.e., the first falling drop during the releaseprocess) and the total release time of the entire droplet are 3.8and 6.0 s on PPy(DBS) micropillar samples and 9.2 and 10.3 s onPPy(DBS) mesh samples, respectively, compared to the releasetime of the primary drop of 10.6 s and total release time of 14.6 son the PPy(DBS) surfaces coated on frosted glass substrates.Therefore, the PPy(DBS) surfaces with a large surface roughnessratio, such as micropillar and mesh structures, can be used to trapoil from water and, in the meantime, achieve quick self-cleaningfor water treatment applications. However, the PPy(DBS)surfaces with increased underwater oleophobicity, indicated by

a higher contact angle, enhance the efficacy of the droplet releaseby forming fewer drops during release. On the PPy(DBS)surfaces treated with O2 plasma, the droplets can be released as asingle droplet, compared to approximately three drops on thePPy(DBS) samples coated on frosted glass. The obtainedknowledge on the mechanisms of the droplet capture and releasewould be applicable to droplet transportation and droplet-basedmicroreactors.

■ EXPERIMENTAL SECTIONFabrication of PPy(DBS) Surfaces. PPy(DBS) thin films were

coated via electropolymerization on various substrates, including Cr/Au-coated frosted glass (Micro Slides, VWR, Radnor, PA), flat siliconwafer (WRSMaterials, San Jose, CA), micropatterned silicon wafer, andstainless steel mesh (Mesh 100 × 100, McMaster-Carr, Robbinsville,NJ). The Cr (10 nm) and Au (30 nm) coatings were deposited ondifferent substrates by using an e-beam evaporator (Explorer 14, DentonVacuum, Moorestown, NJ). The PPy(DBS) films were then electro-polymerized on the Cr/Au-coated substrates. During the electro-polymerization, the substrates were submerged in a solution consistingof 0.1 M pyrrole (reagent grade, 98%, Sigma-Aldrich, St. Louis, MO)and 0.1 M sodium dodecylbenzenesulfonate (NaDBS) (technical grade,Sigma-Aldrich, St. Louis, MO) as the working electrode. A saturatedcalomel electrode (SCE) (Fisher Scientific Inc., Pittsburgh, PA) and aCr/Au-coated silicon wafer were also submerged in the solution as thereference electrode and the counter electrode, respectively. Thedeposition of PPy(DBS) was carried out at 0.7 V versus SCE using apotentiostat (263A, Princeton Applied Research, Oak Ridge, TN).30

The surface charge densities during coating were set to 100, 300, and1000 mC·cm−2 to coat PPy(DBS) films with different thicknesses tostudy the effect of the thickness of PPy(DBS) films on droplet captureand release behaviors. The substrates coated with PPy(DBS) were thenrinsed with deionized (DI) water. The micropillar structured siliconsubstrates used for PPy(DBS) coating were fabricated throughphotolithography and the deep reactive ion etching (DRIE) process.49

The surface morphology of the PPy(DBS) coatings was characterizedvia scanning electron microscope (Auriga Small Dual-Beam FIB-SEM,Carl Zeiss, Jena, Germany). The surface roughness ratios (i.e., the ratioof actual area of the solid surface to the projected area) of PPy(DBS)samples coated on frosted glass, micropillars, and mesh were calculatedaccording to the SEM images. O2 plasma was used to treat thePPy(DBS) surfaces to change surface wettability. In this process, thesilicon substrates with PPy(DBS) coating (surface charge densitiesduring coating was 1000 mC·cm−2) were treated by using an inductivelycoupled plasma (ICP) etcher (HiEtch, BMR Technology Corp.,Anaheim, CA) with 25 W O2 plasma for 2 min. The O2 flow rate was30 sccm, and chamber pressure was 20 pa.

Droplet Capture and Release. The capture and release of organicdroplets on different PPy(DBS) surfaces was tested in an aqueouselectrolyte environment. In particular, a dichloromethane (≥ 99.8%,Sigma-Aldrich, St. Louis, MO) droplet was first placed on an O2-plasma-treated glass slide in a 0.1 M NaNO3 (≥99.0%, Sigma-Aldrich) solution.Droplets with volumes ranging from 0.7 to 7.3 μL have been tested.Droplets of 2± 1 μL were used for consistency during the cyclic dropletcapture and release process. The PPy(DBS) sample submerged in thesolution was oxidized by applying a positive voltage in a two-electrodeconfiguration, in which the PPy(DBS) sample was used as the workingelectrode and a platinummesh (13 mm × 35 mm) was connected as thecounter electrode. The oxidized PPy(DBS) sample was then placed incontact with the DCM droplet and moved upward to capture it. Afterthe droplet was captured, a negative voltage was applied to thePPy(DBS) surface to release the droplet. After the droplet was released,a new droplet was placed on the glass slide, and the PPy(DBS) samplewas tested for another redox cycle until the droplet could not becaptured or released any further. Three different combinations ofreductive and oxidative voltages including −0.9/0.6, −0.9/0.1, and−0.6/0.1 V were tested to study the effect of redox voltages on thedroplet capture and release behaviors. The entire capture-and-releaseprocess was monitored with a goniometer system (Model 250, Rame-́

Figure 6. On-demand capture and release of a DCM droplet on aPPy(DBS) surface treated with 25 W O2 plasma for 2 min. The surfacemorphology of PPy(DBS) substrates (a) before and (b) after the O2plasma treatment. (c) The droplet-release process.

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hart, Netcong, NJ). All solutions were prepared with Milli-DI water (>1MΩ·cm).

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.7b03787.

Figures showing a schematic illustration of the exper-imental setup, the release process of DCM droplets withdifferent volumes, and the degradation of PPy(DBS)surfaces for droplet capture and release. (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: eyang@stevens.edu. Phone: +1-201-216-5574.ORCIDChang-Hwan Choi: 0000-0003-2715-7393Eui-Hyeok Yang: 0000-0003-4893-1691NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported in part by National Science Foundationawards (grant no. ECCS-1202269) and an American ChemicalSociety Petroleum Research Fund (PRF no. 56455-ND5). Thiswork was also partially carried out at the Micro DeviceLaboratory funded with support from contract no. W15QKN-05-D-0011, Laboratories for Multiscale Imaging (LMSI) at theStevens Institute of Technology, and at the Center for FunctionalNanomaterials, Brookhaven National Laboratory, which wassupported by the U.S. Department of Energy, Office of BasicEnergy Sciences, under contract no. DE-SC0012704.

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