Double Hydrophilic Janus Cylinders at an Air–Water Interface

Article pubs.acs.org/Langmuir

Double Hydrophilic Janus Cylinders at an Air−Water Interface Bum Jun Park,†,§,# Chang-Hyung Choi,‡,§ Sung-Min Kang,‡ Kwadwo E. Tettey,† Chang-Soo Lee,*,‡ and Daeyeon Lee*,† †

Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States Department of Chemical Engineering, Chungnam National University, Daejeon 305-764, South Korea

‡

S Supporting Information *

ABSTRACT: Colloidal particles spontaneously attach to the interface between two immiscible fluids to minimize the interfacial area between the two phases. The shape and wettability of particles have a strong influence on their configuration and interactions at fluid−fluid interfaces. In this study, we investigate the behavior of asymmetrically hydrophilic Janus cylinders (or double hydrophilic Janus cylinders with two different hydrophilic regions) trapped at an air−water interface. We find that these double hydrophilic Janus cylinders with aspect ratios of 0.9, 1.2, and 2.4 adopt both end-on and tilted configurations with respect to the interface. Our numerical calculations show that the coexistence of these configurations is a result of multiple energy minima present in the attachment energy profile that can be represented as a complex energy landscape. Double hydrophilic Janus cylinders with tilted orientations induce hexapolar interface deformation, which accounts for the pair interactions between the particles as well as the nondeterministic assembly behaviors of these particles at the interface.

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to multipolar interface deformation around each particle.32 The resulting assembly behaviors are nondeterministic, which is strikingly different from those found in chemically homogeneous particles with shape anisotropy.33−39 To date, most studies in this area have focused on studying the interfacial behavior of Janus particles with amphiphilic properties; that is, the two sides of the particles have opposite wetting properties. One interesting class of Janus particles that have not been extensively studied is particles with asymmetric hydrophilicity. Such particles are analogous to the so-called double hydrophilic block copolymers (DHBCs) that comprise two hydrophilic segments.40 These DHBCs behave as common hydrophilic polymers in aqueous media and do not spontaneously assemble to form micelles like typical amphiphilies. The surface activity in these DHBC polymers, however, can be activated in the presence of appropriate solid− liquid or liquid−liquid interfaces. Based on this property, DHBCs have been utilized to enhance the stability of colloids, template nanoparticle synthesis, and control the growth of inorganic crystals.41−43 In the same token, double hydrophilic Janus particles with two hydrophilic regions could provide unique opportunities as colloid surfactants of which the surface activity can be selectively controlled by environmental stimuli (e.g., fluid, substrate, pH, and temperature). Such sensitivity to the environmental conditions potentially provides an opportunity for a wide range of novel applications, such as generation

INTRODUCTION Colloidal particles spontaneously attach to the interface between two immiscible fluids to minimize the interfacial area between the two phases.1−3 The surface activity and behavior of particles at fluid−fluid interfaces have important implications in various industrial applications such as emulsion stabilization, froth flotation, oil recovery, and wastewater treatment. Assembly of solid particles at the fluid interfaces has also led to the generation of new materials such as colloidosomes and microreactors for biofuel conversion.4−8 In addition, the interactions and assembly of colloids at fluid−fluid interfaces provide a rich model system to study the fundamental physics of colloids under two-dimensional confinement.9−23 Particles with chemical anisotropy, also known as Janus particles, exhibit unique behaviors at fluid−fluid interfaces.24 Amphiphilic Janus spheres at an oil−water interface spontaneously expose their hydrophobic and hydrophilic regions to the apolar and polar fluid phases, respectively, to minimize their interface attachment energy.25−28 Because of their strong tendency to attach to fluid−fluid interfaces, Janus spheres can be used as colloidal surfactants to produce thermodynamically stable emulsions.29 Recently, it was shown that anisotropy in the shape of amphiphilic Janus particles results in diverse configurations, assemblies, and interactions at fluid−fluid interfaces. Unlike Janus spheres, nonspherical amphiphilic particles at the interfaces can adopt multiple configurations (i.e., upright and tilted) due to the presence of a secondary energy minimum in the attachment energy profile.30,31 The interparticle interactions between these nonspherical amphiphilic particles depend strongly on their lateral alignments due © 2013 American Chemical Society

Received: December 6, 2012 Revised: January 17, 2013 Published: January 18, 2013 1841

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number limit (NRe = (2Rvρ)/μ ≪ 1, where ρ and μ are the density and viscosity of the fluid, respectively), a linear relationship between the interaction force (F ∼ rβ−1) and the drift velocity (v = dr/dt) can be assumed.37 From the two relationships of log r ∼ α log t and log t ∼ (2 − β)log r, the power law exponent (β) of the interaction force can be related to the value of α, resulting in β = 2 − 1/α. Optical Profilometry. To visualize the interface profiles of double hydrophilic Janus cylinders at the air−water interface, we use optical profilometry (Zygo, NewView 6K). Profilometry is based on white light interferometry in which an interferometric objective with a 20× magnification vertically scans a sample. The resulting three-dimensional interferogram is then rendered by the detection of interferogram intensities.

of Pickering emulsions in which the shape and structure can be reversibly tuned under external stimuli. In this paper, we present the interfacial behavior of asymmetrically hydrophilic Janus cylinders (or double hydrophilic Janus cylinders) at an air−water interface. The configuration of individual Janus cylinders at the air−water interface is investigated using experimental and theoretical approaches. These double hydrophilic cylinders adopt configurations that have not been previously observed in homogeneous and amphiphilic particles at fluid−fluid interfaces. We show that the configuration of double hydrophilic Janus cylinders is described by a complex energy landscape. Subsequently, we study the assembly and interactions of double hydrophilic Janus cylinders at the air−water interface and demonstrate that hexapolar deformation of the interface around each particle leads to diverse assemblies.

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RESULTS AND DISCUSSION We use a recently reported micromolding method to prepare Janus cylinders with three different aspect ratios, AR = 0.9, 1.2, and 2.4.44 The aspect ratio of a Janus cylinder is defined as a ratio of the length (L) and the diameter (2R) of the cylinder, in which L is the sum of the lengths of strongly polar (LSP) and weakly polar (LWP) regions, AR = (LSP + LWP)/2R = L/2R. For AR = 2.4 particles, we study both symmetric (AR = 2.4S; LWP/ LSP ≈ 1) and asymmetric (AR = 2.4A; LWP/LSP ≈ 2) particles (see the detailed geometry in Table 1). The wettability of each region of the particles is characterized by three-phase contact angles: θSP ≈ 32° for the strongly polar region and θWP ≈ 80° for the weakly polar region. Janus cylinders are placed at the air−water interface by placing a drop of particle suspension in a mixture of water and isopropyl alcohol (1:1 v/v). Double hydrophilic Janus cylinders at fluid−fluid interfaces exhibit a unique configuration behavior that has not been observed in either homogeneous cylinders or amphiphilic Janus cylinders with opposite wettabilities (i.e., apolar and polar) on the two sides. Optical microscopy of these particles, regardless of their aspect ratio, shows that they adopt two different orientations: upright and tilted, as shown in the third column in Figure 1. Similar behaviors have been observed in our recent report involving amphiphilic Janus cylinders at an oil−water interface and were attributed to the existence of a secondary energy minimum in the attachment energy profile.32 Direct observations of double hydrophilic cylinders using the so-called gel trapping method,49,50 however, reveal that the upright cylinders have very different configuration compared to amphiphilic Janus cylinders at the oil−water interface. As shown in the scanning electron microscopy (SEM, FEI Quanta 600 FEG ESEM at 3 kV) images of double hydrophilic Janus cylinders partially embedded in PDMS slabs in Figure 1, the particles with different aspect ratios (AR = 0.9, 1.2, 2.4S, and 2.4A) exhibit coexistence of end-on and tilted configurations. Note that the visible portion of each particle in the SEM images in Figure 1 was originally immersed in water and the other invisible portion embedded in the PDMS was exposed to air. The frequencies of end-on and tilted configurations in both AR = 0.9 and 1.2 particles are approximately 20% and 80%, respectively (black and blue solid bars in Figure 2d). Among the end-on particles, the upright end-on geometry with an orientation angle of θr = 0° is dominantly observed (∼93% for AR = 0.9 and ∼85% for AR = 1.2 in Figure 2d), and only a small fraction adopts the inverse end-on geometry with θr = 180° (green bars in Figure 2d). In the case of the high aspect ratio particles with AR = 2.4S and 2.4A, the frequency of the tilted configuration is slightly higher (∼90%) than that of AR = 0.9 and 1.2 particles (red and purple solid bars in Figure 2d), whereas no particles with the inverse end-on configuration are

METHODS

Materials. Janus cylinders are prepared by using a micromolding method.44 Precursor mixtures consisting of two different polar regions are 900 μL of trimethylolpropane triacrylate, 100 μL of laury acrylate, 50 μL of Darocur 1173, and an appropriate amount of ethanol for the weak polar region (mixture 1), and 700 μL of polyethylene glycol diacrylate (Mn = 575), 300 μL of pentaerythritol tetracrylate, and 50 μL of Darocur 1173 for the strong polar region (mixture 2), respectively.45 Mixture 1 is introduced on the polydimethylsiloxane (PDMS) micromold with cylindrical wells fabricated by using soft lithography. Ethanol is allowed to evaporate during UV irradiation (365 nm) for 2 min. Subsequently, mixture 2 is added to the micromold, which is UV irradiated for another 2 min. Isopropyl alcohol is used to recover the Janus cylinders embedded in the PDMS mold. The dimensions of the obtained Janus particles are shown in Table 1. All chemicals are purchased from Sigma-Aldrich unless otherwise noted.

Table 1. Dimensions of Janus Cylindersa AR

2R (μm)

L = LSP + LWP (μm)

LSP (μm)

0.9 1.2 2.4S 2.4A

41 41 28 28

37 49 68 68

16 21 34 23

Superscripts S and A indicate geometrically symmetric (LWP/LSP ≈ 1) and asymmetric (LWP/LSP ≈ 2) shapes, respectively. a

Contact Angle Measurements. To measure the three-phase contact angle, we use a planar polymer sample prepared on a glass substrate.21,46 For the weakly polar film, mixture 1 with ethanol is placed on a glass substrate and covered with a PDMS slab. The mixture is polymerized under UV (365 nm) exposure for 2 min. A water droplet (2 μL) is gently placed on the polymer surface in air (Supporting Information Figure S5a), and the three-phase contact angle is measured by using a goniometer. For the strongly polar region, the film is prepared by using a similar method but in the absence of ethanol. In this case, we use an inverted method on a glass substrate in which the inverted film substrate is submerged in water, and an air bubble is placed on the film surface as shown in Supporting Information Figure S5b. Pair Interactions. To measure the pair interactions between two Janus cylinders, a small number of particles dispersed in 1:1 v/v % of water and isopropyl alcohol are spread at an air−water interface. A high speed camera (Phantom V7.1) is used to capture an image sequence at a frame rate of 100 frames/s while two adjacent particles approach each other and make contact at t = tmax. Using a particle tracking method,47,48 the particle trajectories and the corresponding center-to-center separation (r) are obtained as a function of time, r ∼ (tmax − t)α. Because the particle behaviors are in the low Reynolds 1842

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Supporting Information and our previous reports.30,31 We minimize the attachment energy (ΔE/kBT) as a function of the vertical displacement (dv, distance between the center of mass of a cylinder and the fluid interface in Figure S1 in the Supporting Information) and orientation angle (θr) as shown in the two-dimensional energy landscape for Janus cylinders with AR = 1.2 and 2.4A in Figure 2a and b, respectively (The plots for AR = 0.9 and 2.4S are available in Figure S2a and b in the Supporting Information). The minimum attachment energy (ΔEmin/kBT) is defined as the lowest energy at a constant value of θr, as shown in Figure 2c, which corresponds to pink dots in Figure 2a,b. For particles with AR = 0.9 and 1.2, the primary (or equilibrium) and secondary (or metastable) energy minima indicate the coexistence of the upright end-on and tilted (θr = 28° for AR = 0.9 and 38° for AR = 1.2) configurations, respectively (circles and squares in Figure 2c). In contrast, for the high AR particles with AR = 2.4S and 2.4A, the primary and secondary energy minima are found at the tilted (θr = 79° for AR = 2.4S and 77° for AR = 2.4A) and upright end-on configurations, respectively (triangles and diamonds in Figure 2c). Notably, the tilted angles obtained from the calculations show an excellent agreement with the experimental values obtained from the SEM image analysis: 31° ± 2°, 46° ± 5°, 78° ± 3° and 77° ± 5° for AR = 0.9, 1.2, 2.4S and 2.4A, respectively (Figure 1). In general, double hydrophilic Janus cylinders with a high aspect ratio tend to adopt configurations with a bigger θr to increase the displaced interfacial area between the two phases (i.e., the interface area displaced by the presence of a particle), whereas shorter ones tend to increase the surface area of preferential wetting (i.e., polar in contact with water; end-on configuration). The experimentally observed frequencies of the upright endon and tilted configurations are in excellent agreement with the numerical calculations. Previously, we proposed that the orientation angle (θr,b) of the energy barrier located between the primary and secondary energy minima can be used to estimate the probability of each configuration, given by Pupright = θ r,b/180° and Ptilted = (180° − θr,b )/180°.30,31 These expressions are based on the assumption that the initial orientation of adsorbed particles to the interface is random and that they subsequently rotate to find either equilibrium or metastable energy state. The obtained values of θr,b in Figure 2c are 18−20° for all ARs, and the corresponding probabilities show good agreement with the experiments, as shown in Figure 2d. Notably, the expressions for the two configurations do not consider the presence of a third energy minimum, which would result in the inverse end-on configuration in Figure 2a−c. When a particle with initially inverse orientation (θr = 180°) comes in contact with the air−water interface, these particles rotate to minimize the attachment energy as they sink into the aqueous phase. This continuous rotation during attachment process is described by the green arrows in Figure 2a and b. In order for double hydrophilic Janus cylinders to adopt the inverse end-on configuration, particles would have to follow the attachment process described by the orange arrows, which are energetically unfavorable. Therefore, the majority of particles dominantly adopt either the upright end-on or tilted configurations, which are experimentally observed in Figure 2d. Previous studies on double hydrophilic block copolymers (DHBCs) have shown that the surface activity in these polymers can be activated by the presence of liquid−liquid or liquid−solid interfaces.40 Inspired by these phenomena observed in DHBCs, we test how double hydrophilic Janus

Figure 1. Configurations of double hydrophilic Janus cylinders at the air−water interface. The first and second columns are SEM images of Janus cylinders embedded in PDMS slabs prepared by the gel trapping method. Arrows indicate the location of the wettability separation line (WSL). The schematic representations show the side view of particle configurations at the air−water interface where four colors represents four different particle-fluid surfaces: weakly polar surface in air (black), weakly polar in water (red), strongly polar in air (blue; rarely shown), and strongly polar in water (cyan). The third column shows the corresponding optical microscopy images. The scale bars are 50 μm.

observed. It is interesting to note that although tilted orientations were observed with amphiphilic Janus cylinders with opposite wettabilities at the oil−water interface, end-on configurations were not observed with such particles.32 In contrast, while end-on configurations have been observed with small aspect ratio homogeneous cylinders, these particles do not have two coexisting configurations.36 These observations highlight the unique interfacial behavior of these double hydrophilic Janus particles at the fluid−fluid interface. To understand the unique configuration behaviors of double hydrophilic Janus cylinders, we numerically calculate the attachment energy of these particles to the air−water interface by determining the contact area between each side of the Janus cylinders with the two fluid phases and also the displaced area at the air−water interface due to the presence of the particles.30,31 This attachment energy calculation enables the determination of the equilibrium and metastable configurations of the particles at the air−water interface as well as the probability of each configuration. For the detailed description of the numerical calculation, we refer the readers to the 1843

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Figure 2. Two-dimensional attachment energy (ΔE/kBT) landscape for (a) AR = 1.2 and (b) 2.4A as a function of dv/L and θr. (c) Minimum attachment energy (ΔEmin/kBT) versus θr. Inset shows the corresponding dv,min/L. (d) Comparison of configuration statistics of double hydrophilic Janus cylinders between experiments and calculations. The total number of particles in the experiments for AR = 0.9, 1.2, 2.4S, and 2.4A are 76, 242, 178, and 156, respectively.

Figure 3. Wetting transitions of double hydrophilic Janus cylinders from the air−water interface to the decane-water interface. (a−d) SEM images of Janus cylinders with AR = 1.2 at the air−water (a and c) and at the oil−water (b and d) interfaces. The thin arrows indicate the WSL location. (e) Schematic diagram for the upright end-on and the inverse end-on configurations. (f) Attachment energy profiles upon the wetting transition.

particles respond to the appearance of a new fluid−fluid interface. We change the superphase from air to oil by carefully adding oil atop the water phase. The wettability of these Janus particles are altered by the addition of oil (i.e., decane), in which the strongly (θSP ≈ 32°) and weakly polar (θWP ≈ 80°) surfaces at the air−water interface become polar (θP ≈ 62°) and apolar (θA ≈ 135°) regions at the oil−water interface, respectively.

Changes in the wettability of each region of Janus particles strongly affect their configurations at this new interface. We find that the upright end-on configuration (dv/L = −0.5) of Janus cylinders with AR = 1.2 at the air−water interface spontaneously transforms to the pinned-upright configuration (dv/L ≈ 0.08) upon the addition of oil, as shown in Figure 3a,b. In this pinned-upright configuration at the oil−water interface, the apolar and polar surfaces of the particle are fully exposed to 1844

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Figure 4. Lateral assemblies of double hydrophilic Janus cylinders. (a, b) Representative images of aggregates showing complex assembly behavior of Janus cylinders with (a) AR = 1.2 and (b) 2.4A. The scale bar is 200 μm. (c) Statistics of assembled geometries between two Janus cylinders in which the total numbers of particle pairs analyzed are 67 for AR = 1.2 and 83 for AR = 2.4A. (d, e) Representative images and corresponding schematic illustration of each pair-assembly geometry for (d) AR = 1.2 and (e) 2.4A.

their preferred fluid phases (i.e., apolar with oil and polar with water), and thus, the WSL becomes pinned to the interface. This out-of-plane transition is attributed to the decrease in the attachment energy, as indicated by two energy profile curves represented by open circles (air−water) and closed circles (oil−water) in Figure 3f. By transforming the configuration from the end-on upright to the pinned upright, the energy of the particle is lowered. For a few particles with the inverse endon configuration at the air−water interface, however, this vertical transition is unlikely to occur due to the unfavorable contact of the polar surface with the oil phase upon the addition of oil (Figure 3c,d). The corresponding energy penalty in Figure 3f (open squares at the air−water interface and closed squares at the oil−water interface) also indicates that such transition would be unfavorable. For Janus cylinders with initially tilted orientation (θr ≈ 46° ± 5°) at the air−water interface, the orientation angle upon addition of oil does not change significantly (θr ≈ 45°); the Janus cylinders, however, shift upward into the oil phase due to the induced amphiphilicity (Figure S3 in the Supporting Information). It is interesting to note that when these Janus cylinders with AR = 1.2 are directly dispersed at the oil−water interface, they dominantly adopt the pinned upright configuration (∼98%).32 This difference suggests that the orientation behavior of anisotropic Janus particles at the interface depends on their initial configuration upon contact with the interface and also the energy landscape it experiences as it submerges into the subphase. Assemblies of multiple double hydrophilic Janus cylinders at the air−water interface show complex structures indicative of nondeterministic assembly behaviors (Figure 4a,b), which are

noticeably different from those observed in geometrically anisotropic but chemically homogeneous particles, such as ellipsoids and cylinders.35,37,38 Such homogeneous high aspect ratio particles prefer to assemble into either side-to-side or tipto-tip assemblies (in some cases, they assembled into both sideto-side and tip-to-tip assemblies).38 These deterministic assembly behaviors have been attributed to a symmetric quadrupolar meniscus deformation around each particle leading to highly anisotropic capillary attractions between neighboring particles.35,37 To better understand the observed complex assembly behavior, we study pair assemblies of double hydrophilic Janus cylinders at the air−water interface. Janus cylinders form several representative assembly structures, as shown in Figure 4c−e: angled head-to-head (I/II−I/II), linear head-to-head (I− I), head-to-side (I−III), reverse side-to-side (II−II), and parallel side-to-side (II/III−II/III), where each Roman numeral denotes a particular region of a double hydrophilic Janus cylinder, as indicated by the schematic illustration in Figure 4c. The assembly frequencies for AR = 1.2 Janus cylinders are evenly distributed among I/II−I/II, I−I, II−II, and I−III geometries, whereas, in the case of AR = 2.4A, the pair assemblies of linear head-to-head (I−I) and parallel side-to-side (II/III−II/III) are more frequently observed than the other pair assemblies (Figure 4c−e). Interestingly, tail-to-tail (IV− IV), side-to-tail (II−IV), and angled tail-to-tail (III/IV−III/IV) assemblies are rarely found for both particles. The observed diversity in the assembly structures suggests the presence of multiple local energy minima in the interparticle potential that govern the lateral assembly between particles. We will discuss these observations more in detail below. It is interesting to note 1845

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Figure 5. Capillary interactions between two tilted Janus cylinders with (a) AR = 1.2 and (b) 2.4A. Inset in each panel is the corresponding log−log plot. Microscopy images on the top of each panel show examples of approaching particles.

that the assembly behavior of the high aspect ratio cylinders bears some resemblance to that of homogeneous cylinders; that is, a large fraction of the particles assemble into either tip-to-tip or side-to-side assemblies.35,37,38 On a first approximation, the observed similarities can be attributed to the large tilt angle (θr = 77°) for AR = 2.4A particles, which is quite close to perfectly horizontal configuration of homogeneous cylinders (θr = 90°). We also study the interactions between two tilted double hydrophilic Janus cylinders at the air−water interface, which could provide critical information on the physical mechanism behind the attractive interactions. The interaction force can be obtained by using Stokes′ drag force in the low Reynolds number limit (NRe ≪ 1) when two particles approach each other due to capillarity, Finter ≈ −Fdrag.37 As shown in Figure 5, the center-to-center separation (r) between the two particles is measured as a function of time prior to contact at t = tmax. Assuming a linear relationship between the drift velocity (v = dr/dt) and the drag force (Fdrag), the power law exponent (α) obtained from fitting the curves using r = (tmax − t)α in Figure 5 provides insights into the nature of the interaction force (Finter ∼ rβ‑1; β = 2 − 1/α).37 The obtained values of β averaged over more than 10 pairs of cylinders are −4.9 ± 0.6 for AR = 1.2 and −5.0 ± 0.6 for AR = 2.4A,51 which are statistically smaller than that observed in quadrupolar capillary interactions between interface trapped particles (β = −4).22,23,35,37 Notably, our results indicate that neither the particle aspect ratio nor the relative alignments of two particles during approach significantly affects the values of β except for a particular case of tailto-tail approach which we discuss next. When the strongly polar tails of two particles face each other during approach, pair interactions between these two particles significantly deviate from the observed power law behaviors in Figure 5. As shown in the snapshots in Figure 6, when two AR = 1.2 particles approach each other with the strongly polar tails facing each other, they undergo significant in-plane rotation and rearrange themselves toward the head-to-side (I−III) assembly (see the Supporting Information for the corresponding movie). The corresponding plot of r versus tmax − t in a log−log scale in Figure 6 cannot be described by a single exponent power law. Instead, the separation profile is discontinuous at tmax − t = 0.67 and 0.13 s, representing a sudden change in the nature of interactions, indicated by arrows in Figure 6. The first discontinuity occurs during the transition from II−IV to I/ II−III/IV alignments, and the second one is found to occur during the rotation from I/II−III/IV to I−III. Note that the transient alignments observed during approach (i.e., III/IV−

Figure 6. Pair interactions between two particles (AR = 1.2) approaching initially with their strongly polar regions facing each other. The scale bar is 50 μm. The corresponding movie is available in the Supporting Information.

III/IV, II−IV, I/II−III/IV) prior to the final contact in Figure 6 are rarely found in the final pair assemblies in Figure 4c. We postulate that these rearrangements stem from the interaction between interface deformation around the two particles.52 The diverse pair assemblies and the determined power-law behavior in the pair interactions suggest the presence of complex interface deformation around tilted double hydrophilic Janus cylinders at the air−water interface. We visualize the interface deformation using the optical profilometry of geltrapped cylinders.53 The shape of the interface deformation around a tilted Janus cylinder with AR = 1.2 and 2.4A shows an asymmetric hexapolar deformation as shown in Figure 7a,b. The hexapolar deformation also can be seen around Janus cylinders that are embedded in PDMS slabs, which replaced the air phase over the gelled water phase (SEM images in Figure 1846

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Figure 7. Profilometry images of asymmetrically hexapolar interface deformation around tilted double hydrophilic Janus cylinders with (a) AR = 1.2 and (b) 2.4A. The white arcs indicate the location of the WSL. The scale bars are 50 μm. Insets in the bottom left are the SEM images of PDMSembedded cylinders prepared by the gel trapping method. The scale bar in each SEM image is (a) 40 and (b) 50 μm. Insets in the bottom right are the profilometry images for two attached particles. Note that the particles in the profilometry and SEM samples are partially embedded in 2 wt % Gellan gel in the aqueous phase and the PDMS slab, respectively. (c and d) Height of the interface deformation corresponding to the dashed lines (A, B, and C) in panels (a) and (b).

assemblies in Figure 4c results from the fact that these assemblies would not lead to a substantial reduction in the interface area upon particle contact. Especially, the reduction in interfacial area upon contact would be very small for AR = 1.2 particles because a large portion of tilted cylinders is submerged in the water phase. In this configuration, two Janus cylinders with the tail-to-tail assembly make contact in the water phase. Such contact geometry underneath the interface suggests that the excess interfacial area between the two particles remains, which likely induces in-plane rotation of particles to achieve a bigger reduction in the interface area. The asymmetrically hexapolar deformation also affects the lateral interactions between two tilted Janus cylinders. Notably, for homogeneous spherical particles, a theoretical study has shown that their pair interaction with a symmetric hexapolar interface meniscus around each particle scales as Uhexa = ∫ Fhexa dr ∼ r−6 (β = −6).22,23 This hexapole−hexapole interaction decays faster than the interactions between particles with quadrupole−quadrupole deformations (Uquad ∼ r−4) and between two particles with quadrupole−hexapole deformations (Uquad‑hexa ∼ r−5). The observed values of the power-law exponents for the interactions between two tilted Janus cylinders (i.e., β = −4.9 ± 0.6 for AR = 1.2 and −5.0 ± 0.6 for AR = 2.4A in Figure 5) indicate that these interactions are governed by the hexapole characters, as found in Figure 7. Finally, we note that we do not observe any significant interface deformation around the particles with the end-on configuration, which explains the negligible lateral capillary interactions that we observe between the upright cylinders.

7a,b). The air−water interface partially covers the end surface of the weakly polar region leading to the interface depression (i.e., negative deformation denoted I(−) in Figure 7). Positive deformation (II) and negative deformation (III) occur along the two sides of weakly polar surface and the WSL, respectively (Figure 7). The air−water interface is partially pinned at the WSL of AR = 1.2 cylinders, and the strongly polar side is completely submerged under water (Figure 7a and also see Figure S4 in the Supporting Information). The pinning of the air−water interface along the WSL (a white arc in Figure 7a) leads to a positive pole denoted by IV. In the case of AR = 2.4A cylinders, a small region of the strongly polar side protrudes into the air phase and a meniscus forms on the strongly polar surface, causing positive deformation, IV(+). In short, the resulting shapes of the interface deformation for both particles are asymmetric hexapoles. The asymmetric hexapolar interface deformation accounts for the observed diversity in the pair assemblies, and multiple combinations of asymmetric hexapolar meniscus around each particle lead to the nondeterministic assembly behavior. For particles with AR = 1.2 and 2.4A, pair assemblies involving contacts with Regions I, II, and III are favored over those involving Region IV, as seen in Figure 4 (also see the snapshots of particle rearrangements in Figure 6). This biased assembly behavior is likely related to the relatively small interface deformation around the strongly polar region (IV), as shown in the interface profile in Figure 7c,d. The magnitude of interface deformation around this region for AR = 1.2 and 2.4A is ∼2−4 and 5 times smaller than those around the other poles (I, II, and III), respectively. The absence of tail-to-tail (IV−IV), sideto-tail (II−IV), and angled tail-to-tail (III/IV−III/IV) 1847

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CONCLUSIONS In summary, we have studied the interfacial behavior of asymmetrically hydrophilic Janus cylinders at the single particle and multiple particle levels. Analogous to double hydrophilic block copolymers, we observe that the surface configuration of these double hydrophilic Janus cylinders at the air−water interface can be controlled by replacing air with oil. We also find that their configuration behavior at the air−water interface can be described by a complex energy landscape with multiple energy minima. Tilted configuration of Janus cylinders leads to asymmetric hexapolar interface deformation, which has significant influence on the lateral interaction and assembly between the particles. We believe that this study will prompt further theoretical studies including quantitative calculations of pair interactions as well as multibody interactions between various types of anisotropic particles associated with the asymmetric multipolar interface deformation. Moreover, it is envisaged that these double hydrophilic Janus particles can be used as a model system to systematically understand complex capillary interactions between particles at fluid−fluid interfaces.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed method of the attachment energy calculation, additional plots for 2D attachment energy landscapes, the wetting transition of tilted particles upon addition of oil, a SEM image of PDMS-embedded AR = 1.2 cylinders, three-phase contact angle measurements, and a movie showing particle rearrangements during assembly. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.-S.L.); [email protected]. edu (D.L.). Present Address #

Department of Chemical Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do, 446-701, South Korea. Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the American Chemical Society Petroleum Research Fund (ACS PRF), NSF CAREER Award (DMR-1055594), the PENN MRSEC DMR11-20901 through the NSF, the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST, No. 2011-0017322). We thank Prof. R. Carpick for the use of the optical profilometer.

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REFERENCES

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