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Controlling the Structure of Supraballs by pH-Responsive Particle Assembly Takafumi Sekido, Sanghyuk Wooh, Regina Fuchs, Michael Kappl, Yoshinobu Nakamura, Hans-Jürgen Butt, and Syuji Fujii Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04648 • Publication Date (Web): 08 Feb 2017 Downloaded from http://pubs.acs.org on February 8, 2017

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Controlling the Structure of Supraballs by pH-Responsive Particle Assembly

Takafumi Sekido,†,§ Sanghyuk Wooh,*,‡,§ Regina Fuchs,‡ Michael Kappl,‡ Yoshinobu Nakamura,† Hans-Jürgen Butt,*,‡ Syuji Fujii*,†

†

Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology

5-16-1 Omiya, Asahi-ku, Osaka 535-8585, Japan ‡

Physics at Interfaces, Max Planck Institute for Polymer Research, Ackermannweg 10, D-

55128, Mainz, Germany

Key words: supraball, colloidal assembly, pH-responsive microparticle, superamphiphobic surface

ABSTRACT: Supraballs of various sizes and compositions can be fabricated via drying of drops of aqueous colloidal dispersions on super liquid-repellent surfaces with no chemical wastes and energy consumption. “Supraball” is a particle composed of colloids. Many properties, such as mechanical strength and porosity, are determined by ordering of colloidal assembly. To tune such properties, colloidal assembly needs to be controlled when supraballs are formed during drying. Here, we introduce a method to control colloidal assembly of supraballs by adjusting dispersity of the colloids. Supraballs are fabricated on superamphiphobic surfaces from colloidal aqueous dispersions of polystyrene microparticles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate] (PDEA-PS particles). Drying of dispersion drops at pH 3 on superamphiphobic surfaces leads to the formation of spherical supraballs with densely-packed colloids. The pH 10 supraballs are more oblate shaped and consist of more disordered colloids than pH 3 supraballs, caused by particle aggregates with random sizes and shapes in pH 10 dispersion. Thus, the shape, crystallinity,

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porosity and mechanical properties could be controlled by pH, which allows broader uses of supraballs.

■ INTRODUCTION Many particle fabrication methods on super liquid-repellent surfaces, e.g. superhydrophobic and superamphiphobic surfaces, have been introduced with various materials.1-8 Drops of water exhibit high receding contact angle (CArec > 150o) and low sliding angle below 10o on superhydrophobic surfaces by creating air cushions.9-14 This excellent water repellency is extended to nonpolar liquids, e.g. olive oils, decane and hexadecane, on superamphiphobic surfaces with low surface energy overhang structures.15-19 By using spherical liquid drops with high contact angles on such surfaces, spherical particles can be realized via various methods. For example when polymer powders are heated over their melting point on superamphiphobic surfaces, spherical polymer melt drops are formed. After cooling down, spherical polymer particles are obtained from the polymer melts.5 Alternatively, spherical polymer particles can be fabricated by polymerizing UV curable monomer drops with spherical shape on the superamphiphobic surface.5 Fabrication methods of mesoporous supraballs (also called supraparticles) have also been developed via drying colloidal dispersions on the super liquid-repellent surfaces.1-4 Particularly, on the superamphiphobic surfaces, various supraballs are easily realized with no energy consumption and chemical waste.4 By this process, moreover, the size, composition and architecture of supraballs can be simply controlled by varying the dispersion drop volume, concentration and type of colloids. Supraballs have allowed fundamental insights into different aspects of colloidal selfassembly20-23, and enabled various applications, e.g. photonic24-25 and phononic26 crystals and catalysts4, 27. Supraball consists of colloidal assembly. Therefore control of colloidal assembly is a crucial factor that determines various properties, such as photonic band gap, porosity and mechanical strength.28-30 Well-ordered colloidal assemblies have low porosity and are 2 ACS Paragon Plus Environment

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mechanically harder than disordered colloidal assemblies. Due to their periodic structure, these well-ordered colloidal assemblies are photonic and phononic crystals. On the other hand, disordered assemblies result in weaker structures with high porosity than well-ordered assemblies. Due to the high porosity and large surface area, supraballs with disordered assemblies are useful for catalyst applications, such as TiO2, SnO2 and ZnO photocatalyst supraballs.4 Even though various types of supraballs have been fabricated on the superamphiphobic surfaces, until now, there have been no established ways to vary ordering of colloidal assembly of supraballs without changing sizes and materials of colloidal particles.

■ EXPERIMENTAL SECTION Synthesis of PDEA-PS particles. Poly[2-(diethylamino)ethyl methacrylate] (PDEA) homopolymer synthesized by solution polymerization in isopropyl alcohol (IPA) was used as a colloidal stabilizer for dispersion polymerization of styrene. The isopropyl alcohol (IPA) solution (293.8 g, 10.21 wt%) of PDEA and IPA (2.66 L) were mixed in 5 L round-flask, and bubbled nitrogen gas for 30 min at room temperature. Under a nitrogen stream, a mixture of styrene monomer (300 g, 2.88 mol), which was purified through an aluminium oxide column, and the initiator 2,2'-azodiisobutyronitrile (3.0 g, 1.83 mmol) was injected to the flask with constant stirring of 250 rpm at 70 ˚C. After 24 h reaction, styrene monomers were polymerized in the presence of PDEA colloidal stabilizer, forming PDEA-PS particles. After cooling down to room temperature, the synthesized particles were purified by centrifugation (3,000 rpm for 15 min) and re-dispersion in IPA (1.21 kg), repeated 4 cycles. And then particles were purified again by centrifugation (4,500 rpm for 15 min) and re-dispersion in deionized water (2.72 kg), repeated 4 cycles, using the centrifuge (Hitachi, CF16RXII type centrifuge with a Hitachi T15A 36 rotor).


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Preparation of superamphiphobic surfaces. Superamphiphobic surfaces were prepared by soot deposition method.18 Approximately 10 µm thick soot layers were deposited on substrates (wafer and glass) followed by forming thin silica shell (20 ~ 40 nm) by chemical vapor depositon of tetraethyl orthosilicate and ammonia for 24 h. After calcination at 500 ˚C for 4 h, the surfaces were modified with fluorocarbon molecules (trichloro(1H,1H,2H,2Hperfluorooctyl)silane) to reduce the surface energy. Supraball fabrication on superamphiphobic surfaces. Aqueous dispersions of PDEAPS particles (15 µL, 10 wt%) were dropped on superamphiphobic surfaces. pH values were adjusted by adding HCl and NH3 aqueous solutions. Drying drops of the PDEA-PS particle dispersions on the superamphiphobic surfaces were carried out in a closed glass container at 21 ˚C and 0.1 MPa. After ~ 2 h, supraballs were formed with PDEA-PS particle assemblies from the dried dispersion drops. Stereo microscope (TG300PC; Shodensha, Osaka, Japan) fitted with a digital system (Shimadzu Moticam 2000, Kyoto, Japan) and halogen lamp (12 V, 10 W) were used to monitor the drying behaviour of dispersion drops. Roundness of the droplets and supraballs were determined by using an ImageJ software (National Institute of Health, USA). Regarding the height, the distance between the highest point of the droplets/supraballs and the surface of the superamphiphobic substrate was measured. Interface fixation process. The fixation for visualizing the air/water interface of dispersion drops was performed by following the method established by N. Vogel et al..31 First, the aqueous dispersion drops of PDEA-PS particles (concentration, 10 wt%) were dropped on superamphiphobic surface in a closed glass container. After 15 min, the ethyl 2cyanoacrylate monomer (0.7 g) placed in a petri dish on a hotplate (50 ˚C) was put into the container. The monomers evaporated and polymerized at the air/water interface of the dispersion drops in the container for 120 min (pH 3 dispersion) and 30 min (pH 10 dispersion), respectively. The polymerization of cyanoacrylate generating polycyanoacrylate (i.e. 4 ACS Paragon Plus Environment

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“superglue”) was initiated at the interface upon contact with water by nucleophiles (e.g. water molecules). As monomers were supplied continuously via the gas phase, the polymerization was propagated into the dispersion drops from the surface, which created the polycyanoacrylate films. Nanoindentation. Nanoindentation measurements were performed with a standard-force MFP NanoIndenter (Asylum Research, Santa Barbara, CA, spring constant k = 2,390 N/m) equipped with a flat-end diamond indenter (diameter d = 652.74 µm, Synton-MDP AG, Nidau). Indentations were performed in load-controlled mode with a maximum load of 100 µN and a loading rate of 20 µN/s. Thermal drift was measured and corrected for each indentation. At least 10 supraballs of pH 3 and pH 10 fixed to glass slides by means of a small amount of epoxy glue were tested in order to get an estimation of the standard deviation of the mechanical properties. All load-displacement curves were analyzed using a self-written LabVIEW software. The elastic modulus for each particle was determined by fitting the first 10 – 20 % of the approach curve using the Hertz model after setting the onset of measurable contact force between indenter and supraball as zero indentation. The particle radius for each particle was obtained from optical images before indentation measurement. We found relatively large standard deviations for the modulus values, which can be attributed to the surface roughness of supraballs. In the Hertz theory, one assumes a perfect sphere with no surface roughness. However supraballs were not perfectly round and had considerable surface roughness. Since we only indented with a force of only 100 µN to stay within elastic deformation of the supraballs and therefore had a small indentation depth (~ 200 nm) (Figure S3), surface roughness cannot be ignored. Only elastic modulus values equal to, or larger than, 10 MPa contributed to the mean elastic modulus values for pH 3 and pH 10 particles to avoid any error arising from the resolution limit of the measurement setup.


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■ Results and Discussions PDEA-PS Supraball Fabrication on Superamphiphobic Surface. Here, we introduce a method to control an ordering of colloidal assembly of supraballs by varying the interaction between the colloidal particles. Supraballs were fabricated on soot-templated superamphiphobic surfaces by drying sessile drops of colloidal dispersion (Figure 1a).4, 18 The soot-templated superamphiphobic surface has a porous structure with overhangs, which leads to water contact angles over 150°. Therefore spherical supraballs are successfully formed after evaporation of dispersion drops on the superamphiphobic surface as the contact line of dispersion drops can freely move while it evaporates.4 As colloidal particles, we used monodisperse polystyrene microparticles carrying poly[2-(diethylamino)ethyl methacrylate] at their surface, acting as colloidal stabilizer (PDEA-PS particles), synthesized by dispersion polymerization in isopropanol (IPA) (Figure 1b).32 The PDEA-PS particles had a numberaverage diameter of 2.2 µm (coefficient of variation, 2%) and the PDEA content in one particle was estimated to be 2.66 wt% by nitrogen elemental microanalysis. As PDEA is well soluble in IPA but not in PS, PDEA is expected to mainly exist at the PDEA-PS particles surface. This is confirmed by the fact that PDEA-PS particles exhibit a pH-responsive dispersion-flocculation behavior: At and below pH 6, the PDEA-PS particles show high colloidal stability due to the positively charged PDEA leading to electrosteric repulsion.32-33 On the other hand, at and above pH 8, PDEA becomes neutral and hydrophobic by deprotonation. Thus, PDEA-PS particles flocculate in water at pH ≥ 8 due to a loss of electrosteric repulsion and hydrophobic attraction (Figure 1c and Figure S1). To investigate the effect of colloidal interaction between the dispersed particles on the ordering of particle assembly, we used the PDEA-PS colloidal dispersions of pH 3 with well-dispersed particles and pH 10 with aggregated particles.


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Drying Behaviors of PDEA-PS Particle Dispersion Drops. To investigate supraball formation, drops of 15 µL of PDEA-PS particle dispersions were placed on the superamphiphobic surfaces and water allowed evaporation. The shrinking drops were monitored by a stereo microscope from the top and a side-camera (Figure 2). From the sideimages, we characterized the contact angle, contact diameter, and radius of dispersion drops as a function of evaporation time (Figure 3a). Contact angles of pH 3 dispersion drops over 150o were almost constant during evaporation (Figure 3b). The edge of contact line moved continuously and the contact diameter (diameter of contact line) decreased (Figure 3c) in parallel with decreasing drop radius and height (Figure 3d,e). Therefore drops made from pH 3 dispersions resulted in nearly spherical supraballs with roundness of 0.90 (Figure 3f). The roundness was calculated using equation (1): Roundness =

[] [ ]

(1)

where [Area] and [Major axis] are the projected drop area and the largest diameter observed in the side-camera images, respectively. A roundness value of 1.0 would indicate a perfect sphere and values smaller than 1 would indicate an increasing deviation from perfectly round shape. The roundness was calculated by using ImageJ software (National Institute of Health, USA).34 In case of the pH 10 dispersion drops, contact diameter was somewhat increased and contact angle was decreased at the evaporation time of 10 ~ 20 min (Figure 2b and Figure 3). Since the PDEA-PS particles were hydrophobic at pH 10, during evaporation, they accumulated at the air/water interface of the drop and formed a shell. This shell delayed shape changes and maintained the shape of drop for roughly 15 min. The total volume of drop kept gradually decreasing by evaporation of water that generated a stress in the shell. To release the stress, after typically 10 ~ 20 min, the shell deformed and the shape of drop slightly flattened. As a result the contact diameter increased and wrinkles formed on the surface of the drop (images of 16 min in Figure 2b, Figure 3c,d, and Movie S2 and S4). Due to the larger 7 ACS Paragon Plus Environment

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contact diameter, pH 10 dispersion drops formed more anisotropic oblate supraballs (roundness: ~ 0.78) than pH 3 drops. This anisotropic drying of pH 10 drop by the shell formation on the air/water interface is much like a deflating balloon with rigid shell. Similar evaporation behaviors of drops with shells were reported by Velev and Gradzielski.35-36 Positions of PDEA-PS Particles in Aqueous Dispersion Drops. To visualize the position of PDEA-PS particles at the air/water interface of the drops during the drying process, we used ethyl 2-cyanoacrylate monomer to “freeze” the interface (Figure 4).31 The particle dispersion drops on the superamphiphobic surfaces were place in a closed chamber which also contained ethyl 2-cyanoacrylate monomers (Figure S2). Monomer evaporated and got in contact with the air/water interfaces of dispersion drops and polymerized to polycyanoacrylate (“superglue”). It has been confirmed that the polymerization occurs at air/water interface first, and then proceeded into the aqueous phase.31 Therefore, the PDEA-PS particles near the surface of the drop were trapped by superglue films via polymerization. In the pH 3 dispersion drop, a closed polycyanoacrylate formed at the interface (Figure 4a). Below this interfacial layer, PDEA-PS particles that presented close to the interface but still fully embedded (Figure 4b). We can therefore conclude that particles were fully immersed in the aqueous phase (top scheme of Figure 4e). At pH 10, however, the polycyanoacrylate and PDEA-PS particles co-existed at the surface of the drop. Thus, PDEA-PS particles were partially protruding from the air/water interface, forming a three-phase-contact line (Figure 4c,d). The contact angle of the particles at the air-water interface was estimated to be 46˚ (bottom scheme of Figure 4e).37 This observation verifies that PDEA-PS particles prevailed at the surface of dispersion drops because of the hydrophobicity of PDEA at pH 10. As the pure water surface is partially replaced by the more hydrophobic PDEA-PS surface, this will lead to a lower effective surface tension and an increase in wetting radius. Furthermore, as the drop shrinks by evaporation, the particles that are trapped at the interface become closer and closer


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packed. Finally the available surface for a spherical or oblate shape would no longer suffice and the outer shell starts to buckle as can be seen in Figure 2b at a drying time of 16 min. Orderings of PDEA-PS Particle Assemblies. After drying, PDEA-PS supraballs were characterized by scanning electron microscopy (SEM). Well-ordered particle assemblies consisting of crystallites of typically 60 µm diameter separated by grain boundaries were observed on the surface of supraballs fabricated at pH 3, named pH 3 supraball (Figure 5a-c). In contrast, the surface of supraball fabricated at pH 10 (pH 10 supraball) showed less ordered particles (Figure 5g-i). Crystalline regions were much smaller than the one observed on pH 3supraballs. The same tendency in the degree of orderings was noted inside the supraballs, as characterized by cross-sectional SEM images (Figure 5d and j) that were taken after breaking the supraballs. Not only the near surface region (Figure 5f) but also in the center region (Figure 5e) of the pH 3 supraballs, the PDEA-PS particles formed well-ordered assemblies, while the center (Figure 5k) and near the surface region (Figure 5l) of pH 10-supraball were all made up of disordered PDEA-PS particle assembly. Partially, ordered assemblies were observed on the surface of pH 10 supraballs (Figure 5h). They formed by capillary attraction due to evaporation of water at the end of the evaporation process. The capillary attraction forced the particles into denser structure. Light reflection patterns on the surface of drop gives a hint of this particle reorganization step during evaporation. Right before the end of the evaporation (evaporation time: ~ 81 min), light reflection patterns appeared and became clear on the surface of pH 10 drop (Movie S5). It confirmed that particles reorganized and partially assembled on the surface of pH 10 drop at the end of evaporation process. From those results, controlling particle assembly on supraballs by tuning external stimuli (pH) of dispersions was studied for the first time. The distinctive difference in the degree of ordering between pH 3 and pH 10 suparballs is attributed to the different interactions between the colloids and their surface properties (Figure 6). At pH 3 the PDEA-PS particles are well dispersed and repel each other and are free to adjust their position (Figure 6a) until the end of 9 ACS Paragon Plus Environment

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evaporation. Therefore they have time to find their thermodynamically favored position and crystallize. Since they are so well dispersed, shell formation does not occur. In case of pH 10 dispersion, the PEDA-PS particles adsorbed at the air/water interface causes buckling, and in combination with the increase of contact radius eventually leads to non-spherical morphology. In addition, the PDEA-PS particles form aggregates already in the dispersion due to hydrophobicity of PDEA-PS particles in the deprotonated state (Figure 6b). By evaporation, the aggregates are packed each other, however, due to the random sizes and shapes of aggregates, disordered particle assemblies are realized. Porosities and Mechanical Properties. The degree of ordering of particle assembly is directly related to porosity and mechanical property of supraballs. Porosities of supraballs were calculated by using the volume ratio of particles in dispersion, initial volume of particle dispersion and measured total volume of supraballs after evaporation (Table 1). We assume that the density of PDEA-PS particle is 1.04 g/cm3 and the shape of supraballs is ellipsoid. Then, the volume ratio of particles in dispersions is 0.0962 since the concentration of PDEAPS particles is 10 wt% in dispersions. By the volume ratio of particles and the initial volume of particle dispersion (15 µL), the volume of PDEA-PS particles is known to be approximately 1.44 µL. Total volumes of pH 3 (1.96 ± 0.01 µL) and pH 10 (2.6 ± 0.04 µL) supraballs are calculated with height and radius of supraballs measured from the side-view images (averaged by 4 images per each). Porosity (Φ) is Φ = VV / VT, where VV is the volume of void-space and VT is the total volume of supraball. Therefore calculated porosities of pH 3 and pH 10 supraballs were 0.27 ± 0.01 and 0.44 ± 0.01, respectively. The packing factors (PF = 1 - Φ) of pH 3 and pH 10 supraballs are approximately 0.73 and 0.56, respectively. Especially, the packing factor of pH 3 supraball (0.73) is close to the complete hexagonal close-packed structure (PF: 0.74)38. The porosity of pH 10 supraball is ~ 63 % higher than that of pH 3 supraball. This higher porosity is attributed to the loosely packed disordered assembly of PDEA-PS particles of pH 10 supraballs. 10 ACS Paragon Plus Environment

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Furthermore, the elastic moduli of supraballs were measured by a nanoindenter using a flat punch (Table 1 and Figure S3). For calculation of elastic modulus, we assumed a spherical shape for supraballs and used the Hertz model to correlate measured force and deformation. The elastic moduli of pH 3 and pH 10 supraballs were 62 ± 27 MPa and 39 ± 24 MPa, respectively, which were averaged by values from 10 supraballs per each. Due to the close-packed structure, pH 3 supraballs showed higher elastic moduli than pH 10 supraballs.

■ CONCLUSIONS In conclusion, the degrees of ordering of supraballs formed by evaporating drops of PDEA-PS colloidal dispersions on superamphiphobic surfaces depend on pH. Spherically shaped supraballs with a high degree of crystalline closed-packed ordering and high rigidity were fabricated from the pH 3 dispersion. More oblate shaped supraballs with dispordered microparticles and a porosity of ≈44% were formed at pH 10. By tuning the pH, ordering of colloidal assembly and thus controlling the properties of the supraballs were achieved. This stimulus-responsive ordering control concept, which allows tunable mechanical, electrochemical and optical properties of suprastructures, opens a door for wider uses of various functional supraballs.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional information on materials, experimental details and movies. Figure S1: pH-responsive dispersion/aggregation behavior of the PDEA-PS particles; Figure S2: experimental setup for PDEA-PS particle fixation by polymerization of ethyl 2-cyanoacrylate monomers; Figure S3: force-displacement curves of 11 ACS Paragon Plus Environment

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nanoindentation; Movie S1: drying of pH 3 PDEA-PS particle dispersion drop (15 µL, 10 wt%) on superamphiphobic surface (top view, speed: X 256); Movie S2: drying of pH 10 PDEA-PS particle dispersion drop (15 µL, 10 wt%) on superamphiphobic surface (top view, speed: X 256); Movie S3: drying of pH 3 PDEA-PS particle dispersion drop (15 µL, 10 wt%) on superamphiphobic surface (side view, speed: X 256); Movie S4: drying of pH 10 PDEA-PS particle dispersion drop (15 µL, 10 wt%) on superamphiphobic surface (side view, speed: X 256); Movie S5: drying of pH 10 PDEA-PS particle dispersion drop (15 µL, 10 wt%) on superamphiphobic surface (drying time of 80 ~ 83 min, top view, speed: X 8).

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Author Contributions §

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS This work was supported by JSPS-DAAD Bilateral Joint Research Projects, Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant Number JP16H04207) and Scientific Research on Innovative Areas “Engineering Neo-Biomimetics (No. 4402)” (JSPS KAKENHI Grant Numbers JP15H01602 and JP25120511), “New Polymeric Materials Based on Element-Blocks (No.2401)” (JSPS KAKENHI Grant Numbers JP15H00767 and JP25102542) and “Molecular Soft Interface Science (No. 2005)” (JSPS KAKENHI Grant Number 23106720). This work was also supported by the ERC for the advanced grant 340391-SUPRO (H. J. B.) and the Key Research Program (SPP 1486 PiKo “particles in Contact”) Grant 12 ACS Paragon Plus Environment

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KA1724\1 (M. K. and R. F.). S.W. thanks the Alexander von Humboldt Foundation for a postdoctoral fellowship. T.S. thanks the Osaka Institute of Technology for support for study abroad program.

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Figure 1. (a) Scheme of supraball fabrication through drying aqueous PDEA-PS particle dispersion drop on soot-templated superamphiphobic surface. (b) Scanning electron microscope (SEM) image of the PDEA-PS particles. Inset is a scheme of PDEA-PS particle. The scale bar indicates 5 µm. (c) Scheme of pH-responsive dispersion/aggregation behavior of the PDEA-PS particles.


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Figure 2. Top and side-view pictures of drying (a) pH 3 and (b) pH 10 aqueous dispersion drops (15 µL) of PDEA-PS particles on superamphiphobic surfaces (temperature: 21.8 ˚C, humidity: 48.6%-68.3 %). Dispersions of pH 3 and pH 10 were adjusted by adding HCl and NH3 aqueous solutions, respectively. The droplets were observed from top (top images) and side (bottom images). The scale bars indicate 1 mm.


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Figure 3. Typical drying behavior of aqueous dispersion drops of PDEA-PS particles (temperature: 21.8 ˚C, humidity: 48.6%-68.3 %). (a) Schematic of dimensions of dispersion drops on the superamphiphobic surface. (b) Time evolution of contact angle, (c) normalized contact diameter (I/Io), (d) normalized radius (r/ro), and (e) normalized height (h/h0) of the PDEA-PS particle dispersion drops as a function of evaporation time. (f) Time evolution of roundness which was determined from images by the side-camera. Green and black circles indicate pH 3 and pH 10 dispersions, respectively.


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Figure 4. Surface (a,c) and cross-section near surface (b,d) images after fixation process with PDEA-PS particles aqueous dispersion drops (10 wt%, 15 uL) of pH 3 (a,b) and pH 10 (c,d). Images were characterized by SEM after polymerizing ethyl 2-cyanoacrylate at the air/water interface (see Method for experimental details). The superglue (polycyanoacrylate) formed either a continuous layer that grew into the water phase and embedded fully dispersed particles (a,b) or formed a film between protruding particles (c,d). The scale bars indicate 1 µm. (e) Schematics of near surface of the PDEA-PS particles dispersions of pH 3 (top) and 10 (bottom).


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Figure 5. SEM images of supraballs fabricated by using pH 3 (a-f) and pH 10 (g-l) dispersions (15 µL). (a-c) show the surface at increasing magnification, which is highly ordered for the pH 3 case. (d) is a cross-sectional image of the same supraball. (e) and (f) are high magnification images of surface and central region as marked by white and yellow box in (d). (g-i) surface of pH 10 supraball at increasing magnification, revealing only short range order. Cross-sections (j-l) reveal the disordered state of the bulk for the pH 10 supraballs. Scale bars indicate 100 µm (a,d,g,j), 20 µm (b,h), and 5 µm (c,e,f,i,k,l).


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Figure 6. Schematic illustration of fabrication of supraballs with different ordering of particle assembly. Supraballs with highly ordered and disordered particle assemblies were created by (a) pH 3 and (b) pH 10 aqueous dispersion drops, respectively, where particle were either fully dispersed or partially pre-aggregated.


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Table 1. The porosity and elastic modulus of supraballs fabricated by drying pH 3 and pH 10 aqueous dispersion drops (15 µL) on the superamphiphobic surfaces Volume of PDEA-PS particlesa (µL)

Total volume of supraballb (µL)

Porosity (Φ)

Packing factor (1 - Φ)

Modulusc (MPa)

pH 3 supraball

1.44

1.96 ± 0.01

0.27 ± 0.01

0.73

62 ± 27

pH 10 supraball

1.44

2.60 ± 0.04

0.44 ± 0.01

0.56

39 ± 24

Sample

a

Volumes of PDEA-PS particles were calculated from the initial concentration (10 wt%) and volume (15 µL) of dispersion. bFor estimating total volumes of supraballs (4 supraballs per each), shape of supraballs were assumed as ellipsoid. cEach modulus were averaged with 10 supraballs.


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Table of Contents (ToC)


Controlling the Structure of Supraballs by pH-Responsive ... - DOIs

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