Mimicking the Wettability of the Rose Petal using Self-assembly of

Jul 23, 2013 - surfaces such as the rose petal. The raspberry particles were obtained by layer-by-layer self-assembly of 850 nm core polystyrene parti...
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Mimicking the Wettability of the Rose Petal using Self-assembly of Waterborne Polymer Particles A. M. Telford,† B. S. Hawkett,† C. Such,‡ and C. Neto*,† †

School of Chemistry, The University of Sydney, NSW 2006 Australia Dulux Group Australia, Clayton, VIC 3168, Australia

‡

S Supporting Information *

ABSTRACT: In this work, we present a novel platform to synthesize polymeric “raspberry” particles and explore their use to fabricate surfaces with low wettability and high water adhesion, resembling the properties of naturally occurring surfaces such as the rose petal. The raspberry particles were obtained by layer-by-layer self-assembly of 850 nm core polystyrene particles bearing surface carboxylate groups and corona nanoparticles of different sizes and polymers (polystyrene, poly(para-fluorostyrene) or poly(2,3,4,5,6penta-fluorostyrene), also bearing surface carboxylates), interleaved with positively charged poly(ally amine) hydrochloride. The raspberry particles were then bound together by covalent coupling of the carboxylate groups with the amine groups of the polymeric interlayer. The films produced by drop-casting the raspberry particles exhibited static water contact angles between ∼135° and ∼146°, depending on the nature of the corona, hysteresis of ∼135° and high adhesion of water droplets. Since all particles were synthesized from scratch by surfactant free emulsion polymerization in air and every step of the protocol was performed in water, this platform is up-scalable and environmentally friendly. KEYWORDS: self-assembly, superhydrophobic, raspberry particles, emulsion polymerization

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INTRODUCTION The mimicry of surfaces found in nature has become common practice to produce surfaces with controlled wettability. Particular interest has been dedicated to surfaces with very low wettability.1−3 A well-known example is the lotus leaf, with its self-cleaning ability, due to the ease with which water droplets can roll off its surface.4 Another example that has recently gained much attention is the rose petal. This, and other flower petals, exhibit very high water contact angles (>150°) but also high adhesion of water droplets.5,6 Both these types of surfaces exhibit intrinsic hydrophobicity, due to a waxy coating, along with particularly high roughness, due to a dual-scale hierarchical architecture, with features in the micrometric as well as the nanometric scales. The specific topography of these surfaces, such as the aspect ratio of their features and their relative distance, is responsible for the particular wetting states exhibited. For example, water droplets on the surface of the lotus leaf are believed to be in a Cassie−Baxter wetting state; that is, they sit on the features’ tips, minimizing the contact with the surface, and trapping air in the space between the features. This dramatically reduces the adhesion of the water droplet to the surface.1,7 In the rose petal, instead, water droplets are believed to partly penetrate at least one level of the hierarchical roughness, as in the Wenzel state, and hence are strongly pinned to the surface.6,8,9 Recently, the assembly of colloidal particles has been proposed as a route to manufacture films with dual-scale © XXXX American Chemical Society

roughness, similar to those found in the natural examples described above.10 Large core particles (a few hundred nanometers up to a few micrometers in diameter) have a corona, that is, a shell of one or more layers of smaller particles (approximately 10 times smaller),11−17 attached to the surface to produce hierarchical colloids that resemble raspberries, called appropriately “raspberry particles”. The colloidal route is very appealing because of its relatively low cost and up-scalability, when compared to other means of fabricating surfaces with dual-scale roughness, such as lithography or templating, which usually require sophisticated equipment and the application of a hydrophobic coating after the fabrication of the surface structures.2 Raspberry particles have been extensively used to prepare films that mimic the lotus leaf surface,11−15 all requiring postmodification of the particle films to minimize their surface energy, which limits their up-scalability and use outside the laboratory. On the other hand, to our knowledge, only two examples of the mimicry of the rose petal properties using raspberry particles exist in the literature.18,19 Both these examples are based on the polymerization of styrene particles on the surface of silica core particles chemically modified with vinyl anchoring groups. The nontrivial chemistry involved in Received: May 21, 2013 Revised: July 19, 2013

A

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Figure 1. Scheme of raspberry particle preparation and casting of a hydrophobic film. The raspberry particle fabrication process consists of three steps. (a) In step 1, core and corona particles with surface carboxylate groups are synthesized by emulsion polymerization. (b) In step 2, the core particles are coated with PAH by electrostatic interaction. (c) In step 3, the anionic corona particles are aggregated onto the surface of the cationic core particles and bound together by EDC coupling. (d) The resulting raspberry particles have dual-scale roughness. (e) The raspberry particles were drop cast from water to form rough films. (f) The films exhibit very high water contact angles.

Table 1. Reagents Quantities Employed in the Emulsion Polymerization in Air of the Different Corona Particles (PSs, PSL, P1FS, P5FS) monomer [g] (concn [wt %]) V501 [g] (concn [wt %]) NaOH [g] (concn [wt %]) NaHCO3 [g] (concn [wt %]) AMA-80 [g] (concn [wt %]) water [g]

PSs

PSL

P1FS

15.000 (31.92) 0.125 (0.27) 0.036 (0.08) 0.050 (0.11) 0.525 (1.12) 31.250

7.450 (29.82) 0.025 (0.10) 0.008 (0.03) 0.025 (0.10) 0.175 (0.70) 17.300

7.500 (31.92) 0.063 (0.27) 0.018 (0.08) 0.025 (0.10) 0.263 (1.12) 15.625

P5FS 3.750 0.031 0.009 0.013 0.131 7.813

(31.92) (0.27) (0.08) (0.10) (1.12)

and produces polystyrene latexes with narrow size distribution with high reproducibility.

this kind of approach, together with the nonpolymeric nature of the core particles, limits its up-scalability. In this work, we describe for the first time an approach to produce large quantities of entirely polymeric raspberry particles, which produce coatings with low wettability and high adhesion, in a simple and robust manner. Stable dispersions of intrinsically hydrophobic raspberry particles were produced via a fully self-assembled protocol entirely conducted in water. Our approach consists of three steps, as illustrated in Figure 1: (1) core polystyrene microparticles (850 nm in diameter) were synthesized bearing negatively charged carboxylate residuals for stabilization in water (Figure 1a); (2) the anionic core particles were decorated with cationic poly(ally amine) hydrochloride (PAH) using the layer-by-layer assembly technique (Figure 1b); (3) the core particles were decorated with corona polymer particles, (made of polystyrene, poly(parafluorostyrene) or poly(2,3,4,5,6-penta-fluorostyrene), also bearing carboxylates) via controlled heterocoagulation driven by electrostatic interactions (Figure 1c), leading to raspberry particles (Figure 1d). The wettability of the films drop-cast from the raspberry particle dispersions could be tuned by changing the monomer used to synthesize the corona particles (styrene, para-fluorostyrene, penta-fluorostyrene), and required no additional surface modification. Notably, the particles used as building blocks for the raspberry assemblies were all produced in large quantities from scratch, by emulsion polymerization, using a method recently developed by the authors,20 which does not require lengthy deoxygenation steps,

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MATERIALS AND METHODS Preparation of Cationic Core Particles. Polystyrene (PS) core particles with negatively charged surface carboxylate groups were prepared by surfactant-free emulsion polymerization, as reported in another manuscript.20 The PS particles were then coated with a layer of positively charged poly(allylamine) hydrochloride (PAH), similarly to the protocol employed in the layer-by-layer deposition of polyelectrolytes.21 Briefly, a 4 wt % dispersion of core PS particles in water was added dropwise to a 5 × 10−2 monoM* solution of PAH (Mn = 15000 g mol−1) in NaCl 0.55 M, pH 6 (final NaCl concentration of 0.5 M) (*monoM refers to the absolute concentration of monomer allylamine; it is used because of the large molecular weight dispersity of the PAH used). The volume ratio between the particle dispersion and the PAH solution was 1:10. The dispersion was stirred overnight and then washed either by dialysis against water (membrane cutoff 50 KDa) or by centrifugation/redispersion in water. Preparation of Anionic Corona Particles. PS, poly(parafluorostyrene) (P1FS) and poly(2,3,4,5,6-penta-fluorostyrene) (P5FS) corona particles were prepared by emulsion polymerization in air. The quantities of reagents used are summarized in Table 1. Briefly, monomer, buffering salt (NaHCO3), and water (Milli-Q) (minus 5 g retained to dissolve the initiator) were B

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diameter. Conventional emulsion polymerization with surfactant cannot achieve sizes much larger than a couple of hundred nanometers in diameter in one step, while maintaining low size dispersity. The surfactant-free approach had the additional advantage of reducing the time required for latex purification, as salts are removed much more rapidly than surfactants during dialysis. The particles obtained had narrow size distribution and surface carboxylate groups, arising from the N,N′-azobis(cyanovaleric acid) initiator residues. The mean diameter of the core particles, as determined by DLS in MES 12.5 mM, was 854 ± 9 nm, with polydispersity index (PDI) of 0.04 ± 0.02. Shown in Figure 2 is a typical SEM micrograph of the prepared

mixed in a 100 mL (PSs) or 50 mL (other particles) round bottomed flask. The flask was sealed with a rubber septum. The headspace inside the flask was then quickly purged with nitrogen for 10 s. No deoxygenation of the emulsion was used. The flask was immersed in an oil bath at 70 °C for 5 min, with stirring. In the meanwhile, initiator (N,N′-azobis(cyanovaleric acid), V501), basic salt (NaOH), and the remaining 5 g of water were mixed in a glass vial. After being heated in the oil bath for 1 min, the initiator solution was injected in the roundbottom flask through the rubber septum, using a syringe. The reaction mixture was stirred (600 rpm) at 70 °C for 24 h. The synthesized corona particles bore negatively charged carboxylate groups on their surface. Assembly of Raspberry Particles. The raspberry particles were assembled by adaptation of the stepwise heterocoagulation method developed by Okubo et al.22 A dispersion of corona particles was prepared in 2-(N-morpholino)ethanesulfonic acid (MES) buffer, 25 mM, pH 6. The nonionic surfactant TERIC 17A10 (Huntsman, TX) was also added in the concentration of 0.1 wt %. The solid content of the latex was selected so as to have roughly 1000 times excess corona particles relative to core particles. The same volume of a 1 wt % dispersion of cationic core particles in water was added dropwise to the corona particles, and the dispersion was mixed at room temperature for 20 min. The dispersion was then heated at 70−80 °C for 1 h. After cooling at room temperature, the coupling agent 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), in combination with N-hydroxysuccinimide (NHS), was added. EDC and NHS (50 equivalents relative to the total concentration of COOH surface groups) were dissolved in MES buffer 25 mM (same volume as that of latex mixture), added dropwise to the particles, and left to react overnight, with gentle agitation. The raspberry particles obtained were separated from the coupling byproducts, the surfactant, and the excess corona particles by centrifugation/ redispersion in water. Particle Characterization. The size and ζ potential of the particles were determined by dynamic light scattering (DLS, Zetasizer, Malvern, U.K.) as averages over three to five measurements. The morphology of the particles was characterized by scanning electron microscopy (SEM, Zeiss Ultra Plus). The thickness of the films was measured by optical microscopy. Film Preparation and Characterization. The clean raspberry particle dispersions contained approximately 4 wt % solids (same as the initial concentration of core particles). 50 μL of the dispersion were drop-cast onto a glass slide, which had been cleaned in an air−plasma chamber, and dried at 80 °C on a hot plate, so as to rapidly obtain a homogeneous film. The wettability of the films was assessed by water contact angle measurements (WCAM, KSV CAM200). Static WCAs were measured with a 5 μL droplet of Milli-Q water (resistivity = 18.2 MΩ cm), after equilibration for 2 min. Advancing contact angles were measured by increasing the volume of a water droplet from 10 to 25 μL, at 0.1 μL s−1. Receding contact angles were measured by decreasing the volume of a water droplet from 25 to 0 μL, at 0.2 μL s−1. Dynamic WCAMs were recorded with 496 ms frame length.

Figure 2. Typical SEM micrograph of PS core particles prepared by surfactant-free emulsion polymerization in air. The scale bar is 500 nm.

PS core particles. The particles were spherical in shape and had fairly uniform size. The roughness on the surface of the particles derives from the peculiar nucleation mechanism involved in surfactant-free emulsion polymerization, as recently confirmed by our group.20 The core PS particles were functionalized with positively charged amine groups by deposition of the positively charged poly(allyl amine) hydrochloride (PAH). This approach is equivalent to one cycle of the layer-by-layer deposition of polyelectrolytes.21 The deposition conditions were finely tuned to obtain a thick PAH coating on the carboxylated core particles, in order to reverse their surface charge, while maintaining colloidal stability throughout the deposition: (i) The PS particles were mixed with PAH at pH 6.1, as at this pH the carboxylic acid groups were mainly deprotonated, providing particle stability, and exerting a strong attractive force toward the almost fully ionized PAH chains. (ii) The mixing was performed in 0.5 M NaCl. In the absence of salt, polyelectrolytes with high, opposite charge adsorb onto each other in very thin layers, which provide little charge inversion.23 Increasing the ionic strength guaranteed the formation of a thick, loopy PAH layer, the charge of which dominated the overall surface charge of the particles, as confirmed by ζ potential data in MES 12.5 mM. (iii) In order to avoid linking many core particles with one PAH chain, the dispersion of particles was slowly added to a concentrated solution of PAH.

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RESULTS Synthesis of the Polystyrene Core Particles. Surfactantfree emulsion polymerization in air was used to prepare spherical polystyrene (PS) core particles of ∼850 nm in C

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Table 2. Average Size and Size Distribution Derived by DLS Measurements or SEM Measurements, and ζ-potential of the PS Corona Particles Obtained by Emulsion Polymerization in Air, As Well As of the Raspberry Particles with PSs and P1FS Coronas sample name

composition

diam. [nm]

PDI (DLS)

PSs PSL P1FS P5FS RP PSs RP P1FS

polystyrene polystyrene poly(para-fluorostyrene) poly(penta-fluorostyrene) raspberry particles with PSs corona raspberry particles with P1FS corona

105.5 ± 0.9 (DLS) 279.7 ± 2.6 (DLS) 108.1 ± 1.5 (DLS) 144.4 ± 0.7 (DLS) 1100 ± 50 (SEM) 1000 ± 100 (SEM)

0.03 ± 0.02 0.05 ± 0.03 0.10 ± 0.01 0.120 ± 0.002

ζ-potential in MES 12.5 mM [mV] −39.2 −39.5 −40.0 −27.2

± ± ± ±

0.9 0.3 0.8 0.4

was monitored by SEM. Shown in Figure 3a is a SEM micrograph of raspberry-like assemblies formed by

Under these conditions, the negatively charged carboxylates on the surface of the particles interacted with the positive charges on the PAH and the surface of the particles was coated entirely with the cationic polymer. The surface charge was inverted, as demonstrated by measurement of the ζ-potential in MES 12.5 mM, which changed from −55.0 ± 0.9 mV for the plain PS core particles to +28.5 ± 0.8 mV after the addition of the PAH layer. The PAH layer remained stable on the surface after repeated washing with Milli-Q water by centrifugation and redispersion, and upon sonication. The mean diameter and PDI of the modified particles, as determined by DLS in MES 12.5 mM, were respectively 886 ± 10 nm and 0.04 ± 0.02. The small (4%) increase in size is consistent with the formation of a thick PAH layer (swollen in water) and, together with the low PDI, indicates that no interparticle cross-linking occurred. Synthesis of the Polystyrene Corona Particles. Emulsion polymerization in air was used to prepare spherical corona particles of different sizes and polymers. All the particles were decorated with carboxylate surface groups, arising from the N,N′-azobis(cyanovaleric acid) initiator residues. The composition, size and ζ-potential of the particles prepared are summarized in Table 2. The particles were all stable in water (storage medium), and ζ-potential measurements showed that they were all stable and negatively charged in the buffer used for assembly (MES final concentration 12.5 mM, pH 6). Assembly of Raspberry Particles with 105 nm Polystyrene (PSs) Corona. The assembly of the raspberry particles consisted of three straightforward steps: (i) mixing of the core and corona particles in presence of a thermoresponsive surfactant, (ii) increasing coverage of the core particles by the corona particles (PSs in this first instance) by heating above the cloud point of the surfactant, and (iii) covalently binding the particles in the assembly by EDC/NHS coupling. (i) The amino-functionalized core particles were mixed with carboxylated corona PS particles (PSs) of diameter 105 nm, with an excess of corona particles (approximately 1:1000), in the presence of a thermoresponsive surfactant (TERIC 17A10) and buffer (MES, final concentration 12.5 mM, pH 6). The surfactant, a commercial block copolymer composed of 17 ethylene units (average) and 10 ethylene oxide units, was necessary to prevent the random coagulation of the particles into large aggregates. The use of a collapsible surfactant was inspired by the work of Okubo et al.,22 though in our work, the pH was fixed at the beginning of the assembly process, and thus did not need to be further adjusted. When no buffer was used, no assembly of particles was observed. With buffer, after stirring for 20 min, the particles began to gradually assemble, and the process

Figure 3. SEM micrograph of raspberry particles formed by controlled heterocoagulation of PSs corona particles and 850 nm core particles at (a) room temperature and (b) 70 °C. The scale bars are 500 nm.

controlled heterocoagulation of cationic core particles and anionic corona particles at room temperature, in the presence of 0.1 wt % TERIC 17A10 surfactant. At room temperature, the assembly was incomplete, and although the negatively charged corona particles covered most of the surface of the positively charged core particles, the coverage was relatively sparse. This is noticeable in Figure 3a, where parts of the underlying core surface are visible in between the corona particles. D

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(ii) The assembled particles were then heated at the cloud point of the surfactant (70−80 °C) for 60 min. The heating induced some coagulation visible to the eye, but the aggregates were easily redispersed by brief sonication. After the heat treatment, the resulting raspberry particles were stable, and the core particles were fully covered with the corona particles (Figure 3b). (iii) In the last step, the core and corona particles forming the raspberry assemblies were covalently bound together by using EDC coupling.24 The chemical reaction was first investigated in a model system (see the Supporting Information), where ∼900 nm PS particles with carboxylate surface groups were modified with a short, water-soluble diamine (2,2′-(ethylenedioxy)bis(ethyldiamine)). This simple system could be easily characterized by DLS and Raman infrared spectroscopy. The data indicated that a moderate conversion of the surface carboxylate groups was indeed achievable in water. Once we had ascertained that the coupling was possible, the carboxylates on the core and corona particles in the raspberry particles were covalently bound to the intermediate polyamine layer (PAH). In the final protocol, 50 equivalents of EDC and NHS (relative to the total number of COOH groups on the particle surface) in MES buffer 25 mM, pH 6, were added, and the dispersion was left to react overnight at room temperature. The mixture was finally washed repeatedly by centrifugation and redispersion in water, in order to remove the excess corona particles, coupling reagents, and surfactant. Shown in Figure 4

Figure 5. SEM images of particles after self-assembly to produce raspberry architectures with different coronas: (a) P1FS, (b) P5FS, and (c) PSL. The scale bars are 500 nm.

raspberry particles obtained. The different corona particles assembled differently on the core particles. The P1FS corona particles packed densely around the core, in more than one layer, in a similar way to the PSs particles (Figure 5a). The slightly larger P5FS corona particles packed less densely on the cores and produced a corona that exposed some of the underlying surface (Figure 5b). The much larger PSL corona particles did not assemble onto the surface of the core particles at all (Figure 5c). A possible reason for these observations is discussed in the next section. Wetting Properties of Films of Raspberry Particles. The raspberry particles with PSs, P1FS and P5FS coronas were used to prepare films on glass slides, to assess their wettability. The films were obtained by drop-casting from water dispersions free of residual surfactant. The average thickness of the cast films, as measured by optical microscopy on sections of the films, was 50 ± 8 μm (see the Supporting Information). Summarized in Table 3 are the data for static, advancing, and Table 3. Water Contact Angle Data for Films Formed by Different Particles, and for a Control Flat Film of PS SpinCast from Toluene films PS film (no charged residues) PS cores (∼ 850 nm) raspberries PSs corona raspberries P1FS corona

static CA [deg]

adv./rec. CA [deg]

CA hysteresis [deg]

92 ± 1

95 ± 1/82 ± 2

13 ± 2

118 ± 4 135 ± 1 146 ± 2

144 ± 9/< 14a 161 ± 2/< 24a 161 ± 2/< 28a

>130 >137 >133

a

Lowest angle recordable before the droplet contact line started to recede. Substantial droplet deformation made it impossible to measure the exact receding WCA.

receding water contact angles (WCAs) on films of raspberry particles with PSs and P1FS coronas, together with reference data for a flat PS film prepared by spin-coating a solution of uncharged PS in toluene, and for a drop-cast film of PS core particles alone. The static WCA of the flat PS film was ∼92°, close to literature values,25 and the static WCA for films of ∼850 nm PS core particles, bearing carboxylate surface groups, was higher (∼ 118°). The main difference was in the WCA hysteresis, that is, the difference between advancing and receding CAs. Upon removal or addition of volume, the water droplet was able to move freely on the flat PS film (hysteresis of 13°), while it was strongly pinned onto the PS core particles (hysteresis of ∼130°). The hysteresis was also very high on films of raspberry particles with PSs and P1FS, that is, the raspberry particles with complete core coverage. The drop pinning caused such a deformation in the droplet that the receding WCA could only be estimated. A dramatic increase of the static WCA was observed on the raspberry particles film (with dual-scale roughness, >135°) compared with the PS core particles film (with microscale roughness only, 118°). Of the

Figure 4. SEM image of a raspberry particle after covalent bonding of the carboxylated core to PSs corona particles via the intermediate PAH layer by EDC coupling. The scale bar is 500 nm.

is a typical SEM image of one of the final raspberry particles, after covalent bonding of the carboxylated core and corona particles to the intermediate amino-rich PAH layer by EDC coupling. Assembly of Raspberry Particles with Different Coronas. The assembly process was repeated using the same 850 nm core particles, but using corona particles of different size and composition: poly(para-fluorostyrene) particles 108 nm in diameter (P1FS), poly(penta-fluorostyrene) particles 144 nm in diameter (P5FS), and polystyrene particles 280 nm in diameter (PSL). Shown in Figure 5 are SEM images of the E

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raspberry particles with a P1FS corona. As shown in Figure 6, the surface could be tilted at 90° (Figure 6c), or even inverted at 180° (Figure 6d), without detachment of the large droplet. This equates to an adhesion force of approximately 37.5 μN mm−2 (for an equivalent flat surface).

two raspberry particles (PSs and P1FS) used, the average static WCA was higher in the case of the more hydrophobic P1FS corona (146° vs 135°). Shown in Figure 6 are SEM images of

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DISCUSSION We anticipated that there would be two dominant and competing requirements in order to successfully synthesize polymer raspberry particles from building blocks of two different sized particles: the first was the need for both building block particles to be stable enough in solution to avoid uncontrolled aggregation and subsequent precipitation; the second requirement was that the particles actually did come together to form self-assembled raspberry particles when needed. In addition, the raspberry particles needed to be hydrophobic and still stable in aqueous dispersion in order to produce uniform coatings of low wettability. We were able to achieve our goals by synthesizing both building block particles from scratch and by finely tuning the surface properties of the particles, which counterbalanced these seemingly opposing forces. In the following is a critical summary of the main findings of our investigation. Synthesis of Charged Polymer Particles. We were able to tailor the size and surface properties of the polymer particles by synthesizing them from scratch using emulsion polymerization, a platform that is commonly employed in the coating industry, as it can produce large quantities of particles in one batch. Our newly developed emulsion polymerization technique20 was employed to produce spherical particles with the desired size (core particles around 850 nm in diameter, and corona particles around 100 to 280 nm in diameter), with narrow size distribution (PDI around 0.05 for the PS core and corona particles, slightly larger for the fluorinated corona particles), and stabilized by surface carboxylate groups, derived from the water-soluble initiator employed (azobis(cyanovaleric acid)). The size ratio of 1:10 corona/core (for PSs and P1FS) was chosen, as it has been shown to lead to surface coatings with the lowest water wettability.11−16 The surface of the particles was designed to possess significant charge so that electrostatic forces could be used both to stabilize the particles and to aggregate them. Control over Particle Self-Assembly. A combination of layer-by-layer deposition and controlled heterocoagulation was used to control the self-assembly of the core and corona particles into raspberry particles, to produce surfaces with controlled wettability. The negative carboxylate charge on the surface of the core particles was reversed by adsorbing the positively charged PAH, which, under the correct deposition conditions, completely coated the surface. The PS particles remained stable during the surface charge inversion, and the resulting positive particles were stable to washing and sonication. The cationic core and anionic corona particles were used as building blocks for the construction of the final raspberry-like architecture using controlled heterocoagulation. Unlike comparable systems where heterocoagulation by electrostatic interactions in water is exploited,26−28 in this case, the reaction could not be conducted in pure water, because oppositely charged particles mixed in pure water would coagulate very rapidly, and the mixed latex would readily flocculate, even with a 1:1000 ratio of core/corona particles. Attempts to reduce the range of action of the electrostatic attraction by mixing the

Figure 6. (a,b) SEM micrographs of films of raspberry particles dropcast from water dispersions, with (a) PSs corona particles and (b) P1FS corona particles. The scale bars are 4 μm. (c,d) Optical micrographs of a 30 μL water droplet on a film of raspberry particles with P1FS corona. The droplet maintained adhesion when the surface was tilted at (a) 90° and (b) 180°.

the films made from raspberry particles with (a) PSs and (b) P1FS coronas, exhibiting the expected hierarchical roughness. When wettability measurements were attempted on the raspberry particles with the P5FS corona, the water droplet rapidly penetrated the film. The reason for this is explored in the Discussion. The adhesion of a large 30 μL water droplet was tested on the most hydrophobic surface prepared, that is, a film of F

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most hydrophobic corona (i.e., P5FS), likely because of the incomplete coverage of the core, which exposed the underlying hydrophilic PAH layer. The nature of the polymer constituting the corona particles clearly influenced the wettability of the film, as raspberry particles prepared from corona particles based on monofluorinated monomer exhibited higher static WCA than those where nonfluorinated monomer was used in the corona particles. This effect is remarkable, as all corona particles had hydrophilic, charged carboxylate groups on their surface. The influence of the subsurface composition on the wettability of a surface has been recently investigated by Jacobs and co-workers.30 The group showed that long-range van der Waals forces, relevant at distances up to ∼100 nm,31 have a strong influence on the behavior of a surface. In the case of multilayered material, any layer up to a depth corresponding to the range of influence of van der Waals forces must be taken into account. In the films studied in this work, the electrostatic forces due to the outer carboxylated layer of the corona particles, which promote wettability, are likely to be counteracted by the van der Waals forces due to the hydrophobic material in the particles’ bulk. The overall outcome is the ability to reduce the wettability of the surface by using more hydrophobic corona particles. The contribution of the electrostatic forces is evident in the strong pinning of the water droplet to the surface. Strong water droplet adhesion has been observed also on PS material allegedly free of surface charge, such as vertically aligned PS nanotubes,32 and has been explained by the “gecko” effect arising from the very large number of pinning points available due to the peculiar morphology of such a surface. The surfaces studied here do contain many corona nanoparticles exposed to the water droplet, which could act as pinning points able to distort the three-phase contact line of a moving droplet. Nonetheless, the fact that a large WCA hysteresis could also be observed on films prepared exclusively of core PS particles, supports the hypothesis of a strong influence of the charged outer layer of the particles. In summary, the novelty and significance of this work are both in the approach and in the method. The approach aimed to be up-scalable and therefore employed only self-assembly principles and solution chemistry to fabricate the building blocks that formed the functional surfaces. The self-assembly approach is versatile because, in principle, it could be used for particles of any material, as long as they contain some surface charge. The emulsion polymerization methods used successfully produced monodisperse polymeric particles with the desired size and surface chemistry, without requiring lengthy deoxygenation steps; it is, therefore, more convenient than alternative polymerization methods for practical applications. In this work, we explored the use of the raspberry particles in producing nonwetting surface coatings. More generally, the raspberry particles prepared could have a variety of applications. For example, the peculiar architecture of these particles could be used for porous electrodes templating, as they can easily be dissolved after the metal deposition, due to their fully polymeric composition.26 The high surface area of raspberry particles could be exploited for applications where high light scattering is required, as large irregular particles scatter more light than spherical ones.33

particles in NaCl 0.5 M, to compress the electrical double layer, or at pH 5, to reduce the charge density on the anionic particles, were not successful, and flocculation occurred all the same. In the case of 0.5 M NaCl solutions, homocoagulation of like particles probably took place, in combination with heterocoagulation.29 Control over the coagulation process was achieved only by introducing 0.1 wt % of the surfactant TERIC 17A10. This thermoresponsive surfactant, used at room temperature, achieved the fine balance of stabilizing the particles against random coagulation and yet allowing several corona particles to be attracted to the surface of the core particles, as seen in Figure 3a. To complete the surface coverage, the mixed latex was heated at the cloud point of the surfactant (approximately 70−75 °C, according to the manufacturer), in order to collapse the olygo(ethylene oxide) stabilizing chains and allow more corona particles to approach the surface of the core particles (Figure 3b). Surfactants with similar hydrophilic-lipophilic balance and similar lower critical solution temperature are expected to provide a similar effect. In the system discussed here, controlled heterocoagulation was also critically influenced by the ionic strength of the medium. When the reaction was carried out in water with surfactant only, little assembly was observed, even after heat treatment was used to collapse the surfactant. However, when MES buffer was added at concentrations above 12.5 mM, uncontrolled heterocoagulation took place, either during the heat treatment or, for MES concentrations above 50 mM, even at room temperature. The assembly of raspberry particles was the result of a delicate balance in the stability of the complex mixture of particles with different size and charge. The heterocoagulation process appeared to be dependent on the mass and size of the corona particles used. The small PSs and P1FS particles, of comparable mass and size, could efficiently cover the core particles to form raspberry particles with high coverage of the core. The moderately heavier P5FS particles (larger and denser than PSs or P1FS) only partially covered the core particles, while the much larger PSL particles did not assemble onto the core particles at all. The dependence of the corona population density on the size and mass of the corona particles was likely related to the higher packing density achievable with smaller particles and also to the higher probability of a smaller, lighter particle colliding with a large core particle without having enough inertia to detach. Finally, the raspberry-like assemblies produced by heterocoagulation were irreversibly fixed in their shape by using EDC coupling.24 Wettability of Films of Raspberry Particles. The synthesized raspberry particles formed stable dispersions in water and could therefore be cast into uniform films. The films of raspberry particles with different coronas exhibited different wettability behaviors. The films prepared using the raspberry particles with PSs or P1FS coronas exhibited very high WCAs (135° and 146°, respectively) and no observable water penetration through the film. The films made from raspberry particles with PSs or P1FS corona particles exhibited the expected hierarchical roughness when imaged by SEM (Figure 6a, b). Together with the dual scale of the raspberry particles, closely resembling that found on natural surfaces such as the lotus leaf or the rose petal, the films possessed a macro-scale roughness given by the drying process of the water-based dispersions. The effect of the macro-scale roughness variations among different samples was accounted for in the error in the WCA data. On the other hand, water could easily penetrate the surface of the coatings made with raspberry particles with the

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CONCLUSIONS Surfaces with low wettability and high water adhesion were prepared from waterborne, polymeric raspberry particles. The G

dx.doi.org/10.1021/cm4016386 | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

films prepared exhibited a dual scale roughness resembling that of natural occurring surfaces such as the rose petal. The hydrophobic raspberry particles were dispersible in water due to the charged carboxylate residuals on their surface. Films cast from these waterborne raspberry particles exhibited low wettability and high water adhesion. The peculiar wettability of the films prepared was interpreted as arising from the combination of van der Waals forces from the subsurface hydrophobic polymer, and the electrostatic forces from the surface charged carboxylates. Strong adhesion combined with the low wettability of the surfaces, mimicking the properties of the rose petal, could have the potential for applications such as spectroscopy on ultrasmall volumes, immobilization of microreactors on a surface, or for moving minute volumes of water with no loss.

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ASSOCIATED CONTENT

S Supporting Information *

(i) Details on the model reaction used to assess the efficiency of the EDC coupling of 2,2′-(ethylenedioxy)bis(ethyldiamine) to PS particles with surface carboxylate groups; (ii) details on film thickness determination by optical microscopy. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the Australian Research Council and DuluxGroup Australia for funding. The authors also acknowledge the facilities, and the scientific and technical assistance, of the Australian Microscopy and Microanalysis Research Facility at the Australian Centre for Microscopy and Microanalysis, The University of Sydney. Finally, the authors acknowledge the Vibration Spectroscopy Facility at the School of Chemistry, The University of Sydney.

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