Triblock Cylinders at Fluid–Fluid Interfaces - ACS Publications

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Triblock Cylinders at Fluid−Fluid Interfaces Sung-Min Kang,† Ankit Kumar,‡ Chang-Hyung Choi,† Kwadwo E. Tettey,‡ Chang-Soo Lee,*,† Daeyeon Lee,*,‡ and Bum Jun Park*,‡,§ †

Department of Chemical Engineering, Chungnam National University, Daejeon 305-764, South Korea Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States § Department of Chemical Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, South Korea ‡

S Supporting Information *

ABSTRACT: We present the interactions and assembly of triblock cylinders at oil−water and air−water interfaces. ABA-type triblock cylinders with different block ratios and surface wettabilities are prepared using a micromolding method. These triblock cylinders at fluid−fluid interfaces induce complex interface deformation depending upon their relative block ratio and the surface wettability. It is observed that triblock cylinders generate octapolar interface deformation at the air−water interface, whereas the same cylinders cause quadrupolar deformation at the oil−water interface. Consequently, the interactions and assembly behavior of these triblock cylinders at each fluid interface strongly depend upon the nature of the interface deformation.

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INTRODUCTION Interactions and assembly behaviors of solid particles (i.e., colloids) confined at liquid−liquid interfaces represent an active area of research for areas in a number of industrial applications, including stabilizers or dispersants in emulsion systems (e.g., paints, inks, cosmetics, and pharmaceuticals) and flocculants or coagulants in separation processes (e.g., oil separation, oil spill remediation, and wastewater treatment).1−4 Recent studies have shown that particle shape and surface chemistry play a critical role in determining their lateral interactions at multiphasic fluid interfaces, which, in turn, have a significant impact on the physical properties of the fluid interface.5−20 Chemically homogeneous but geometrically anisotropic particles, such as ellipsoids, dumbbells, and cylindrical particles, for example, have shown to interact with each other via longranged capillary attractions to minimize the total surface free energy of the system.10−14,21,22 The source of these attractions is capillary forces between particles dominantly caused by quadrupolar interface deformation around each non-spherical particle.23−25 More recently, higher order interface deformation has been realized by imparting chemical heterogeneity to geometrically anisotropic particles.7−9 So-called Janus cylinders with amphiphilic nature have been shown to adopt tilted configurations at fluid−fluid interfaces (oil−water and air− water interfaces) and induce hexapole-like or quasi-quadrupolar interface deformation.7,8 Similarly, numerical calculations showed that Janus ellipsoids with a tilted orientation at an oil−water interface also induce hexapolar interface deformation.9 The shape of the interface deformation of these amphiphilic Janus particles can be attributed to the geometric and chemical properties of the particles that consequently © 2014 American Chemical Society

determine the orientation behaviors of the interfacial particles. Such complex interface deformation directly affects the pair interaction behaviors between two particles, which highly depend upon the relative alignment of the particles as they approach one another.7−9 The structure of multi-particle assemblies made of these Janus cylinders is non-deterministic, which is in sharp contrast to deterministic assemblies generated from chemically homogeneous ellipsoids and cylinders.12−14 Although several studies have investigated the interfacial behavior of particles with two faces of opposite wettability (Janus particles), little understanding on the interactions between particles with multiple wettability domains currently exist. To deepen and broaden our understanding of the interfacial behavior of complex particles with chemical and geometrical heterogeneity, we examine the behavior of particles with three faces at fluid−fluid interfaces in this report. In particular, we investigate the interparticle interactions and assembly of cylinders with three domains of different lengths and wettabilities at air−water and oil−water interfaces. Our study shows that the assembly behaviors of these triblock cylinders strongly depend upon the wettability of the particle surface as well as the geometry of the cylinders.

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MATERIALS AND METHODS

Triblock cylinders consisting of three building blocks (ABA) are prepared using a micromolding method (Scheme 1).26 The ultraviolet (UV)-curable precursor for the A blocks is a mixture of 1 mL of Received: September 18, 2014 Revised: October 23, 2014 Published: October 24, 2014 13199

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Scheme 1. Schematic of Preparation of Triblock Cylinders Using the Micromolding Method

trimethylolpropane triacrylate and 50 μL of Darocur 1173. The B block is composed of 700 μL of polyethylene glycol diacrylate (Mn = 575), 300 μL of pentaerythritol tetracrylate, and 50 μL of Darocur 1173. The A-block mixture with an appropriate amount of ethanol (see Table S1 of the Supporting Information) is introduced first on the polydimenthylsiloxane (PDMS, Sylgard 184 silicone elastomer kit, Dow Corning) micromold with pre-designed cylindrical wells, prepared using soft lithography. This mixture is polymerized under UV irradiation for 2 min. Subsequently, the B-block mixture in ethanol is also added to the cylindrical wells of the micromold and subsequently polymerized under UV irradiation. Note that the amount of ethanol for each block is determined, depending upon the target ratio of the three blocks (see Table S1 of the Supporting Information). Finally, the A-block mixture without ethanol is added and polymerized to form triblock cylinders. The triblock cylinders embedded in the micromold are recovered using isopropyl alcohol (Figure 1). After the

the triblock particles (70 μm in length and 30 μm in diameter), we believe that IPA does not significantly affect the experimental measurements. n-Decane (Acros Organics) is used as the oil superphase. All chemicals are purchased from Sigma-Aldrich, unless otherwise noted. The three-phase contact angle is determined using a sessile drop on planar polymer films, prepared with the same precursor solutions.7,8,28,29 The measured contact angles are θA‑aw ≈ 83°, θA‑ow ≈ 129°, θB‑aw ≈ 32°, and θB‑ow ≈ 62°, where the subscripts A and B denote A and B blocks and aw and ow indicate the air−water and oil− water interfaces, respectively.7,8 To monitor pair interactions between two triblock cylinders at the interfaces, a high-speed camera (Phantom, version 7.1) is used to capture an image sequence at a frame rate of 300 frames/s.12 A particle tracking method is used to obtain their trajectories and the corresponding center-to-center separation (r) as a function of time.30,31 A gel trapping method is used to visualize the particles and the interface deformation around them.32 The air−water or oil−water interfaces containing 2 wt % Gellan (Phytagel) in the aqueous subphase are formed in a pre-heated oven (50−60 °C), and subsequently, the particles are spread at the interfaces. After the sample is cooled at room temperature, the gelation of the subphase occurs within a couple of minutes. This sample is used for optical profilometry (Zygo, NewView 6K), which visualizes the shape of the interface profiles around individual particles. To prepare a scanning electron microscopy (SEM, FEI Quanta 600 FEG ESEM at 3 kV) sample, the superphase is replaced with a PDMS precursor and cured for 48 h at room temperature. The PDMS slab with embedded particles is separated from the gel surface.

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RESULTS AND DISCUSSION We use two types of triblock cylinders with block length ratios of ABA′ = LA/LB/LA′ ≈ 0.21:0.66:0.13 (type 1) and 0.39:0.42:0.19 (type 2), as shown in Figure 1. The geometry of the type 2 particles is more asymmetric (LA ≠ LA′) compared to that of the type 1 particles. The aspect ratio of these particles is kept constant at AR = L/(2R) ≈ 2.4, which is defined as the ratio of major and minor axes. When type 1 triblock cylinders are spread at an air−water interface, they adopt horizontal orientation. On the basis of the attachment energy calculation,33,34 the lowest energy minimum in the attachment energy occurs at the orientation angle θr = 90° (see Figure S1 of the Supporting Information), consistent with the experimental result. The interface around each cylinder is found to be deformed in an octapolar geometry;

Figure 1. Two types of triblock cylinders with different length ratios. The microscope images are taken when the particles embedded in the PDMS micromold are recovered using isopropyl alcohol. The scale bar is 100 μm. particles are washed several times using isopropyl alcohol, they are redispersed in 1:1 (v/v, %) of water and isopropyl alcohol (IPA). These particles are spread at an air−water or oil−water interface to study pair interactions and assembly behaviors. Note that such measurements are performed about 30 min after adding the particle solution to the interface, ensuring that convective flows become negligible. The spreading solvent (i.e., IPA) may affect the particle wettability via IPA adsorption onto the porous and rough particle surface.27 However, IPA-induced wettability change becomes negligible as the particle size becomes large. Considering the dimensions of 13200

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the interface rises on the two flat sides (red in Figure 2a and the interface profiles in Figure 2b), whereas the interface is

oil−water interfaces.7,8 Similarities in the interaction scaling behavior found for the triblock particles with octapolar deformation and the amphiphilic cylinders with hexapolar deformation suggest that the capillary interaction between cylindrical particles is governed by local interface deformation around the tip regions as the particles approach in the tip-to-tip alignment. The scaling exponent of the far-field interactions between pairs of type 1 triblock cylinders strongly depends upon their lateral alignments. When two triblock cylinders at the air−water interface approach in a staggered side-to-side alignment (see Figure 3), the measured value of α is approximately 0.175 when

Figure 2. Capillary interactions between two type 1 triblock cylinders at the air−water interface when the two particles approach tip-to-tip. (a) Profilometry image shows octapolar interface deformation around a type 1 particle. (b) Magnitude of interface deformation in the crosssection in panel a. (c) Separation, r, between the two particles versus time, tmax − t. The inset is the corresponding log−log plot. Microscope image examples are shown on the top. The scale bar is 100 μm.

depressed around the sides of the A and A′ blocks (blue in Figure 2a and the profile in Figure 2b). The interface rises slightly around the sides of the B block, as shown in panels a and b of Figure 2. The magnitude of the interface deformation around the two end surfaces is similar to each other and approximately 2 times larger than that of the negative deformation around the sides of the A and A′ blocks (Figure 2b). The octapolar deformation of the air−water interface around the type 1 cylinders induces attractive interactions between the particles. This capillary attraction is attributed to reduction of the surface free energy because of the overlapping of the interface deformation around individual particles trapped at the interface as they approach each other.24,25 Separation (r) between two approaching particles as a function of time (t) can be used to reveal the nature of interactions (U) between these particles. When r = (tmax − t)α (where tmax denotes the time when the two particles come in contact) is fitted to the obtained particle trajectory, the pair potential as a function of the separation distance (U ∼ r−β, where β = 2 − 1/α) can be obtained using a previously described procedure (see the Supporting Information for detailed information).12 As shown in Figure 2c, when two type 1 particles approach each other in a tip-to-tip alignment with negligible rotational motion of the particles, the interactions can be best described with α = 0.144 ± 0.004 and β ≈ −5 (i.e., U ∼ r−5). This scaling behavior for these cylinders with the octapolar interface deformation suggests that their interaction decays significantly slower than the case when two spherical particles with symmetric octapolar deformation are interacting with each other (theoretically β = −8 for two interacting spheres with octapolar deformation).35 Also, it should be noted that, for homogeneous cylinders with quadrupolar deformation at the air−water interface, β has been found to be about −4 when they approach in the tip-to-tip alignment,14 indicating that the asymmetry of the octapole around the type 1 triblock cylinders has a significant impact on the interaction between pairs of cylinders. Interestingly, the U ∼ r−5 dependence is similar to the recently reported results of two tilted amphiphilic cylinders with hexapolar interface deformation interacting with each other at the air−water and

Figure 3. Capillary interactions between two type 1 triblock cylinders at the air−water interface when the particles approach side-to-side. Two curves represent two different particle pairs. Example snapshots are shown on the top. The scale bar is 100 μm. The inset is an SEM image example of PDMS replica. The scale bar is 50 μm.

the particles are far apart from each other (beyond the dashed line in Figure 3). The corresponding value of β is about −3.7, which represents a slower decay in the pair interaction than the case of the tip-to-tip approach (Figure 2). The value of β ≈ −3.7, in fact, resembles quadrupolar capillary interactions between two spherical particles.25,35 Also, the alignment dependence of the power law exponents is similar to the case of homogeneous ellipsoids, for which the value of β is found to be ∼ −3 for the side-to-side approach and ∼ −4 for the tip-totip approach.12 For small separations, the rotational motion of the triblock cylinders becomes significant and affects the interactions, resulting in the nonlinear power law behavior seen in Figure 3.7,14 A geometric factor in triblock cylinders alters the shape of the interface deformation and, therefore, their lateral interactions. As shown in panels a and b of Figure 4, it is found that the type 2 triblock cylinders with higher geometric asymmetry compared to the type 1 particles at the air−water interface induce asymmetric octapole interface deformation, resembling the geometry of the type 2 particle. We find that the interaction between pairs of particles in the tip-to-tip approach is similar to that found for the type 1 particles in Figure 2; that is, the power law exponents in the long-range separations are measured as α ≈ 0.142 and β ≈ −5 for the tip-to-tip approach in Figure 4c. This similarity in the power law exponents of the type 1 and type 2 particles in the case of the tip-to-tip approach clearly indicates the importance of local interface deformation around the tip regions. Interestingly, despite the highly asymmetric interface deformation around the side region of the type 2 particles compared to the case of the type 1 particles, the scaling behavior of the pair interaction in the side-to-side approach is found to be α ≈ 0.171 and β ≈ −3.8 (Figure 4d), 13201

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angle of the A- or A′-block surface at the oil−water interface (θA‑ow ≈ 129°) is greater than that at the air−water interface (θA‑aw ≈ 83°), and subsequently, the effect of the polar B block likely diminishes, such that the overall interface deformation becomes negative along the sides. This quadrupolar shape in panels a−c of Figure 5 is quite similar to the deformation induced by homogeneous cylinders.14 The measured values of the power law exponents are α ≈ 0.16 and β ≈ −4.25 and are independent of the alignments between two particles, which is consistent with the interactions between homogeneous cylinders with quadrupolar deformation at the air−water interface.14 This is another strong piece of evidence that the octapolar character that existed at the air−water interface disappears when the type 1 triblock cylinders are placed at the oil−water interface. The shape of the interface deformation around individual particles directly affects their assembly behaviors. As shown in Figure 6, the assemblies consisting of multiple type 1 triblock Figure 4. Interface deformation and capillary interactions between type 2 triblock cylinders with ABA′ ≈ 0.39:0.42:0.18 at the air−water interface. (a) Profilometry image of asymmetric interface deformation. The scale bar is 100 μm. (b) Magnitude of the interface deformation of the cross-section in panel a. (c and d) Separation as a function of time while two particles approach in (c) tip-to-tip and (d) side-to-side alignments. The scale bar is 100 μm.

which are also similar to those obtained from the type 1 particles in Figure 3. We believe that further quantitative analyses and numerical calculations are warranted to systematically characterize the scaling behaviors that depend upon location interface deformation and particle alignments. For the tip-to-tip approach, it is not observed that any particular direction is preferred in the pair assemblies (e.g., A−A, A−A′, and A′−A′ assemblies). This lack of orientation preference in the interaction scaling behavior of the tip-to-tip approach can be attributed to the fact that the magnitude of the interface deformation around the two tips is quite similar, as shown in Figure 4b. The shape of the interface deformation around the type 1 triblock cylinders changes significantly when these particles are placed at an oil−water interface. As shown in panels a−c of Figure 5, the interface deformation is observed to be quadrupolar; that is, the interface rises around the end regions and is depressed along the sides of the cylinders. The contact

Figure 6. Snapshots of assemblies of type 1 triblock cylinders at the (a) air−water or (b) oil−water interfaces. The scale bar is 200 μm. (c) Statistics of pair assemblies. The total number of particle pairs analyzed is 66 for the air−water interface and 84 for the oil−water interface.

cylinders at the air−water and oil−water interfaces dominantly exhibit linear structures, which are similar to the assembly of homogeneous cylinders at the air−water interface.14 The staggered side-to-side assembly is only found at the air−water interface, indicating that the octapolar interface deformation affects the particle assembly at the air−water interface. The fact that no staggered side-to-side assembly is observed for the same cylinders at the oil−water interface is another strong indicator that these particles lose octapole-like characters when they are at the oil−water interface. Around 14% of particle pairs exhibit the angled assembly, in which a portion of the positive and negative deformations around the A or A′ blocks of the two particles simultaneously overlaps each other. The frequency of these assemblies is observed to be approximately the same at the air−water and oil−water interfaces. Such angled geometry has also been observed in the case of amphiphilic cylinders with hexapole-like interface deformation.7,8 Only a small number of T-shaped assembly is observed, likely because the positive deformation along the sides of the B block is relatively small compared to the negative deformations along the sides of the A

Figure 5. Effect of wettability of type 1 particles on the pair interactions at the oil−water interface. (a) Profilometry image and (b) corresponding interface height of the cross-section in panel a. The scale bar is 100 μm. (c) SEM image of a particle partially embedded in a PDMS slab. The scale bar is 50 μm. (d) Pair interactions and microscope image examples when two particles approach each other. The scale bar is 100 μm. 13202

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CONCLUSION We have shown that the geometrical and chemical characteristics of triblock cylinders strongly affect the interface deformation around individual particles and, in turn, lateral interactions between multiple particles at fluid−fluid interfaces. The observed shape of the interface deformation around individual triblock cylinders accounts for the scaling behavior of the measured capillary interactions between pairs of particles and, consequently, the corresponding assembly structures. We believe that this work provides clues to engineer the surface wettability of cylinders to induce diverse assembly structures beyond those observed in chemically homogeneous cylinders. Our future work focuses on fabricating various patchy particles that can be used as building blocks for directing preprogramed self-assembly structures.

or A′ blocks (panels b and c of Figure 2). We believe that these negative deformations hinder the approach of another particle with its tip aligned toward the side of the middle block (B block). Notably, these triblock cylinders exhibits quite different assembly behavior to the case of triblock particles prepared using a particle replication in non-wetting templates (PRINT) method.5 These particles with a rectangular parallelepiped shape, composed of hydrophilic−hydrophobic−hydrophilic blocks, dominantly organize into a side-to-side assembly. This assembly structure is attributed to a strong affinity of the two hydrophilic blocks to water that causes the particles to immerse exclusively in the water phase. In this wetting condition, it was suggested that the assembly in the side-to-side manner is more preferred to reduce effectively the surface free energy than the tip-to-tip assembly. The geometric factor on the particles affects their assembly behaviors at the air−water interface. Although the type 2 triblock cylinders with a highly geometrical asymmetry dominantly form linear structures in Figure 7a, which are

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ASSOCIATED CONTENT

S Supporting Information *

Additional information on scaling of the pair interaction force, attachment energy profiles of triblock cylinders at the fluid− fluid interfaces, and volume of ethanol used for the particle preparation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the American Chemical Society Petroleum Research Fund (ACS PRF), the National Science Foundation (NSF) CAREER Award (DMR1055594), the PENN MRSEC DMR11-20901 through the NSF, and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, Information and Communications Technology (ICT) & Future Planning (MSIP, NRF2014R1A1A1005727 and NRF-2011-0017322). The authors thank Prof. R. Carpick for the use of the optical profilometer.

Figure 7. Assemblies of type 2 triblock cylinders at the air−water interface. (a) Snapshot examples of the microstructure. The scale bar is 200 μm. (b) Statistics of pair assemblies among 96 particle pairs.

similar to those observed for the type 1 triblock cylinders (Figure 6a), we find clear differences in the assembly structures between the two types of triblock particles. For instance, the frequency of the staggered side-to-side assembly of the type 2 particles (Figure 7b) is substantially smaller than that for the type 1 particles (Figure 6c) because the magnitude of interface deformation along the sides of the A′ block is significantly smaller than that around the A-block sides, and in turn, the interface deformations along A- and A′-block sides do not match each other. Thus, A−A′ and A′−A′ staggered side-toside combinations are not highly preferred. Moreover, the frequency of A−A tip-to-tip assembly (∼21.9%) is twice as large as that of A′−A′ tip-to-tip assembly (11.5%), indicating that A−A assembly is more favorable than A′−A′ assembly, despite the fact that the magnitude of the interface deformation around the two end regions, A and A′, is similar to each other, as seen in Figure 4b. When two particles initially approach each other in the side-to-side alignment with a mirror image to each other, they eventually rearrange themselves to have tip-to-tip contact. During rearrangements, the rotation of the two cylinders toward the A−A would be more preferred than that toward the A′−A′ direction because of the larger interface deformation along the sides of the A block.

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REFERENCES

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