Microcapsules Fabricated from Liquid Marbles Stabilized with Latex

Feb 28, 2014 - School of Environmental and Life Sciences, The University of Newcastle, University Drive, Callaghan New South Wales 2308, Australia. §...
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Microcapsules Fabricated from Liquid Marbles Stabilized with Latex Particles Kazuyuki Ueno,† Sho Hamasaki,† Erica J. Wanless,‡ Yoshinobu Nakamura,†,§ and Syuji Fujii†,* †

Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology 5-16-1 Ohmiya, Asahi-ku, Osaka 535-8585, Japan ‡ School of Environmental and Life Sciences, The University of Newcastle, University Drive, Callaghan New South Wales 2308, Australia § Nanomaterials Microdevices Research Center, Osaka Institute of Technology 5-16-1 Ohmiya, Asahi-ku, Osaka 535-8585, Japan S Supporting Information *

ABSTRACT: Millimeter- and centimeter-sized “liquid marbles” were readily prepared by rolling water droplets on a powder bed of dried submicrometer-sized polystyrene latex particles carrying poly[2-(diethylamino)ethyl methacrylate] hairs (PDEA-PS). Scanning electron microscopy studies indicated that flocs of the PDEA-PS particles were adsorbed at the surface of these water droplets, leading to stable spherical liquid marbles. The liquid marbles were deformed as a result of water evaporation to adopt a deflated spherical geometry, and the rate of water evaporation decreased with increasing atmospheric relative humidity. Conversely, liquid marbles formed using saturated aqueous LiCl solution led to atmospheric water absorption by the liquid marbles and a consequent mass increase. The liquid marbles can be transformed into polymeric capsules containing water by exposure to solvent vapor: the PDEA-PS particles were plasticized with the solvent vapor to form a polymer film at the air−water interface of the liquid marbles. The polymeric capsules with aqueous volumes of 250 μL or less kept their oblate ellipsoid/near spherical shape even after complete water evaporation, which confirmed that a rigid polymeric capsule was successfully formed. Both the rate of water evaporation from the pure water liquid marbles and the rate of water adsorption into the aqueous LiCl liquid marbles were reduced with an increase of solvent vapor treatment time. This suggests that the number and size of pores within the polymer particles/flocs on the liquid marble surface decreased due to film formation during exposure to organic solvent vapor. In addition, organic−inorganic composite capsules and colloidal crystal capsules were fabricated from liquid marbles containing aqueous SiO2 dispersions.

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INTRODUCTION Liquid marbles,1−4 which are typically millimeter-sized water droplets stabilized by adsorbed particles at gas−liquid interfaces, have attracted increasing attention in view of their potential applications in cosmetics,5,6 transport and microfluidics,1,7−10 miniature reactors,11,12 personal and health care products,13 sensors,14−16 accelerometers,17 and gas storage.18,19 These liquid-in-gas dispersed systems are usually prepared using relatively hydrophobic particles that adsorb at the gas− liquid interface. Most of the literature is concerned with surface-modified lycopodium powder,20,21 hydrophobic silica particles,22−25 carbon black,26,27 carbon nanotubes,28 or small organic molecule powder,29 and increasingly, organic polymer particles30−40 as liquid marble stabilizers. In principle, such synthetic organic polymer particles should be particularly attractive for preparing liquid marbles, since they can be readily designed with specific surface chemistries (and hence wettability) using various functional monomers. Thanks to this advantage, it is possible to confer stimulus-responsive character: a wider variety of stimulus-responsive characters can be introduced using organic polymer particles compared to inorganic particles. For example, it has been reported that © 2014 American Chemical Society

stimulus-responsive liquid marbles, which can be disrupted by external stimuli, such as pH and temperature, can be fabricated using polymer particles.32,34,36−39 One of the other attractive features of polymer particles is its film forming ability, as has been widely deployed in the paint, adhesive, and paper industries.41 Film formed from polymer particles adsorbed to the water droplet surface leads to reduced pore size between particles and within flocs, and the evaporation speed of the liquid marble internal water can be suppressed.40 Herein, we describe the preparation of liquid marbles stabilized with polymer particles and then fabrication of polymeric capsules by solvent vapor treatment (Scheme 1). The resulting liquid marbles and polymeric capsules were extensively characterized in terms of their surface/inner morphology using a digital camera, a high speed camera, scanning electron microscopy (SEM), and Brunauer−Emmett−Teller (BET) surface area analysis. The rate of water evaporation (or absorption) from (or by) the liquid marbles Received: January 29, 2014 Revised: February 28, 2014 Published: February 28, 2014 3051

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Keyence) was used. The shutter speed was set at 2 ms and a frame rate of 500/s was utilized. Scanning Electron Microscopy (SEM). The dried liquid marbles were placed on an aluminum stub and sputter-coated with gold using a Au coater (SC-701 Quick Coater, Elionix, Japan) in order to minimize sample-charging problems. SEM studies were conducted using a VE8800 instrument (Keyence, Japan) operating at 12 kV. BET. The specific surface area of PDEA-PS particles before and after toluene vapor treatment (4 min) was measured by the Brunauer− Emmett−Teller (BET) method. Nitrogen sorption experiments were carried out using a BET apparatus (Gemini VII 2390, Shimadzu, Japan). Mass Change of Liquid Marble. The liquid marbles were kept under static conditions, and the mass change due to water evaporation and/or condensation was monitored using an electric analytical balance (AB135-S/FACT, Mettler Toledo). The accuracy of weight is in the order of 0.01 mg. The experiments were performed at 22 °C under a range of relative humidity (RH).

Scheme 1. Fabrication of Polymer Capsule, Hollow Microsphere, and Colloidal Crystal Capsule by Solvent Vapor Treatment of Liquid Marbles Stabilized with Polymer Particles

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RESULTS AND DISCUSSION The PDEA-PS particles used in this study were prepared by dispersion polymerization in IPA using a PDEA-based macroinitiator as an inistab, as described elsewhere (see the SI).36,37 The PDEA component is soluble, and the PS component is insoluble in IPA; therefore, the PDEA should be present on the PS latex surface and effectively operate as a colloidal protective layer to form a stable dispersion of the PDEA-PS particles in the medium.41,42 The weight-average molecular weight and polydispersity (Mw/Mn) of the macroinitiator were measured to be 17 800 and 1.13 by GPC, respectively. The number-average molecular weight (Mn) and degree of polymerization were estimated to be 23 560 and 63 by 1H NMR, respectively, which accorded with calculated values (Mn, 22 700; degree of polymerization, 60). Prior to purification, the milky-white IPA dispersion contained excess free PDEA-based macroinitiator and its byproducts. After the fifth centrifugation-redispersion cycle using IPA, it was confirmed by ultraviolet−visible spectroscopy that the concentration of nonadsorbed free PDEA in the IPA phase was negligible. After four further centrifugation-redispersion cycles using deionized water, the IPA medium was completely replaced with aqueous medium. After centrifugal washing, the purified latex was freeze-dried and ground to obtain a fine white powder (SI Figure S2a). SI Figure S2b shows an SEM image of the dried PDEA-PS particles, from which a number-average diameter and a coefficient of variation were estimated to be 430 nm and 16%, respectively. Elemental microanalysis indicated that the percentage mass of PDEA loading in the PDEA-PS particles was 2.4% by comparing its nitrogen content to that of the PDEA homopolymer synthesized by free radical polymerization: N = 0.18% (PDEA-PS particles) and 7.55% (PDEA homopolymer). The composition of the PDEA-PS particles was determined to be PS homopolymer and PDEA-block-PS polymer in a weight ratio of 85.7:14.3. An SEM image at lower magnification confirms that the near-monodisperse spherical PDEA-PS primary particles formed large polydisperse flocs (Dn 16 ± 16 μm) (see SI Figure S2 inset). Individual liquid marbles were prepared by rolling a droplet of deionized water/aqueous salt solution over the dried PDEAPS powder. This powder immediately coated the droplet, rendering it both hydrophobic and nonwetting. These “liquid marbles” clearly have significant surface roughness, which suggests that they are coated with particle flocs as observed in SEM images (SI Figure S3), rather than just a monolayer.36

was characterized gravimetrically, and the relationship between morphology and rate of the liquid marble mass change is discussed. Moreover, organic−inorganic composite capsules and colloidal crystal capsules were prepared from liquid marbles containing aqueous dispersions of SiO2 particles.

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EXPERIMENTAL SECTION

Materials. The chemicals used for the synthesis of polystyrene particles carrying poly[2-(diethylamino)ethyl methacrylate] hair (PDEA-PS particles) are the same as those used in our previous study. 36,37 Styrene, α,α′-azobisisobutyronitrile (AIBN), 2(diethylamino)ethyl methacrylate (DEA, 99%), isopropanol (IPA, 99%), toluene (≥99.0%), dichloromethane (≥99.0%), tetrahydrofuran (≥99.0%), ammonium solution (28%), sodium hydroxide (≥98%), and potassium sulfate (K2SO4, ≥ 99.0%) were purchased from SigmaAldrich. 2,2′-Azobis[2-methyl-N-(2-hydroxyethyl)propionamide] (VA086) and lithium chloride (LiCl, 99.0 > %) were purchased from Wako Chemicals, Japan. Phenolphthalein was purchased from Kanto Chemical. Gellan gum (KelcogelF) and Bindzil 2040 (20 nm, 40 wt % aqueous dispersion) were kindly donated from CPKelco, U.S.A. and Eka Chemicals AB, respectively. Snowtex (MP-2040, 180 μm, 40 wt % aqueous dispersion) was purchased from Nissan Chemical Industries, Ltd. Hypresica TS (N3N, 11 μm, powder) was purchased from Ube Exsymo Co., Ltd. Milli-Q water (Millipore Corp., MA, U.S.A.) with a specific resistance of 18.2 × 106 Ω·cm was used in all experiments. Preparation of Liquid Marbles. Water droplets were deposited onto the dried PDEA-PS particle powder bed using a micropipet (Nichipet EX, Nichiryo: 2 to 20 μL, or Micropipet, Eppendorf research: 200 to 1000 μL). By gently rolling the aqueous droplet on the powder bed, the liquid was entirely encapsulated by the PDEA-PS powder, resulting in a liquid marble. The volume of the dispensed water droplet was varied from 15 to 1000 μL. Fabrication of Polymer Capsules from Liquid Marbles. The liquid marbles sitting on a polypropylene mesh with a 1 mm diameter pore size was placed on an M-shaped rigid aluminum foil to facilitate vapor exposure to the entire marble surface (Supporting Information, SI, Figure S1). Then, the sample was placed in a closed glass container (volume: 5.70 × 10−5 m3, base area: 9.57 × 10−4 m2) containing 3 mL of toluene. After some period of time, the liquid marble was taken out to use for further experiments. The temperature and the humidity during this process were 25 °C and 30 ± 5% relative humidity, respectively. Characterization of the Liquid Marbles. Digital Camera. Photographs of the samples were taken with a digital camera (G700SE, Ricoh). High-Speed Camera. For observation of the motion of liquid marbles, a high speed camera (VW-6000, Keyence) equipped with a zoom lens (VH-Z200, Keyence) and a free angle stand (VW-S200, 3052

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Figure 1. (a, b, e, f, i, j) Digital photographs and (c, d, g, h, k, l) SEM images of PDEA-PS particle-stabilized liquid marbles (a, e, i) before and (b−d, f−h, j−l) after evaporation of water. Liquid marbles (a−d) without toluene vapor treatment and (e−l) with toluene vapor treatment for (e−h) 1 min and (i−l) 4 min. Parts (d, h, l) are magnified images of (c, g, k), respectively. Insets are highly magnified images of liquid marble surfaces.

Figure 2. (a) Weight loss percentage of liquid marbles as a function of evaporation time: (Δ: RH 58−63%) bare water droplet and liquid marbles without toluene vapor treatment (○: 21−26 RH%; □: 58−63 RH%; ◊: 81−87 RH%). The experiments were conducted at 24.2−25.4 °C. (b) Weight loss percentage of the liquid marbles without and with toluene vapor treatment as a function of evaporation time (toluene vapor treatment time: ○, 0 min; ●, 1 min; ■, 2 min; ◆, 3 min; ▲, ▼, 4 min). The experiments were conducted at 24.2−25.4 °C at (○, ●, ■, ◆, ▲) 58−63 RH% and (▼) 85−89 RH%.

Table 1. Summary of Shell Thickness, Evaporation Rate and Time for Complete Evaporation for Bare Water Droplet and Liquid Marblesa Without And With Solvent Vapor Treatment toluene vapor treatment time (min) 0 0 0 0 1 2 3 4 4 0 4 0 4 a

shell thickness (μm) bare water 151 151 151 112 77 58 34 34 151 34 151 34

± ± ± ± ± ± ± ± ± ± ± ±

24 24 24 11 12 10 7 7 24 7 24 7

RH (%) 58−63 21−26 58−63 81−87 58−63 58−63 58−63 58−63 85−89 53−56 53−56 50−54 50−54

salt

evaporation rate (10−4 g min−1)

time for complete evaporation (min) 125 85 130 315 135 140 160 195 530

LiCl LiCl K2SO4 K2SO4

1.250 1.787 1.224 0.515 1.148 1.122 0.969 0.812 0.295 −0.956 −0.662 1.370 1.121

110 135

Volume of bare water droplet and aqueous phase of liquid marbles, 15 μL. 3053

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treatment, aqueous liquid marbles became translucent from an original opaque white color (Figure 1e,i). This is because the roughness of the liquid marble surface was decreased, and the air voids among the particles/flocs became smaller, whereby reducing the intensity of light scattering. Interestingly, macroscopic cracks were often observed on the liquid marble surface during/after toluene evaporation from the liquid marble, which was also reported in the case of poly(lactic acid) powderstabilized liquid marbles after solvent treatment.40 This is attributed to the shrinkage of the film during the polymer dissolution−solvent evaporation procedure resulting from insufficient coating of the water droplet with PDEA-PS particles. Five min of toluene vapor treatment resulted in the formation of a weak polymeric capsule, which can be easily broken when the polymer capsule on the polypropylene mesh was placed on the desk and water leaked out from the capsule (SI Figure S5). Toluene vapor treatment greater than ca. 35 min result in leakage of water from the polymer capsule on the polypropylene mesh during treatment and the water wet the mesh. The polymer coating on the water droplet formed after toluene vapor treatment (>ca. 35 min) is concluded to be too thin or discontinuous, and thus unable to keep water inside of the liquid marbles and the gravitational force on the water results in marble leakage. That is, an appropriate duration of solvent vapor treatment is thus necessary to maintain the polymer shell structure. Liquid marbles prepared on the powder bed using water droplets up to 250 μL water could be transformed into polymer capsules, which retained their shape after drying, following the toluene vapor treatment. For small volumes (15 μL), the liquid marbles are almost spherical, but greater deformation is observed as the droplet volume is increased. Gravitational forces cause the liquid marbles to deviate from their ideal spherical shape. For liquid marbles with a volume above 300 μL, the integrity of the film formation of PDEA-PS particles at the air−water interface was compromised and did not prevent capsule collapse after drying. SEM studies on the liquid marble surface confirmed that the granular shape of the adsorbed PDEA-PS flocs was deformed to be smoother through the solvent exposure, and a film was formed on the liquid droplet surface (Figure 1). The original submicrometer-sized PDEA-PS particle shape disappeared after 1 min of toluene vapor treatment, and the micrometer-sized roughness originating in the surface flocs became smoother through this polymer dissolution−solvent evaporation process. Laser microscopy studies confirmed that the surface roughness (Ra) of the PDEA-PS powder before (19.2 μm) and after 1 min toluene vapor treatment (19.3 μm) was unchanged, although it became significantly smoother after a total of 4 min treatment (14.8 μm) (SI Figure S6). The resolution of the laser microscope is around 1 μm, and it is thus unable to detect any deformation of the primary particles. The decrease in Ra after 4 min is therefore attributed to deformation of the micrometer-sized PDEA-PS flocs. SEM investigation of the inner shell morphology confirmed that PDEA-PS particles had lost their submicrometer-sized morphology even after 2 s of solvent exposure indicating the start of film formation (SI Figure S7). Any phase separation of the PDEA-PS polymers after the toluene vapor treatment at the air−water interface may be accessible using high resolution transmission electron microscopy and is the subject of our ongoing investigations. The thickness of the shell layer decreased from 112 ± 11 down to 34 ± 7 μm with an increase of toluene vapor treatment

These liquid marbles remained intact after transfer onto a glass slide (see Figure 1a), PET film, paper substrates, or a planar air−water surface. Figure 1a,b shows the morphological changes of the liquid marble (15 μL) upon drying. The PDEA-PS particles were irreversibly adsorbed at the air−water interface, and the total surface area of the liquid marble remained constant. Thus, the liquid marble adjusted its surface-to-volume ratio by undergoing deformation. By comparison, a bare water droplet sitting on PDEA-PS powder evaporated while keeping a near-spherical shape (SI Figure S3), which indicates that the air−water interfacial area decreased with a decrease of water volume. From the mass of dried liquid marble (determined gravimetrically after complete drying of 15 μL liquid marble) and the surface area of the water droplets used for their preparation, we estimate that the liquid marble coating consisted of a layer of 142 particles, which corresponds to an estimated thickness of approximately 61 μm. In order to investigate the microscopic morphology, SEM studies were conducted (Figure 1). The PDEA-PS particles, which were adsorbed as a floc on the surface of the liquid marble can be clearly observed, and the thickness of the liquid marble particulate shell is 151 ± 24 μm, as determined from the SEM images of the dried liquid marble. For this estimation of PDEA-PS shell thickness, a dried liquid marble was prepared using 2 wt % aqueous gellan solution, which helped to keep a near-spherical shape during drying (SI Figure S4, because it was difficult to estimate shell thickness from a dried liquid marble with a flattened morphology (e.g., Figure 1b). The evaporation of liquid from liquid marbles has attracted significant recent interest.26,43−45 The rate of water evaporation from the liquid marbles was estimated gravimetrically under various relative humidities (Figure 2a and Table 1). The evaporation rate was found to be approximately linear, as previously reported by Laborie et al. for liquid marble coatings consisting of much larger 40−500 μm PS particles at RH50%.43 The evaporation rate of the liquid marble was marginally slower than that of a bare water droplet sitting on the PDEA-PS powder bed. The inner water (15 μL) completely evaporated after 130 min for the liquid marble, and the time for complete evaporation of the bare water droplet (15 μL) was 125 min. These results indicate that the PDEA-PS powder shell slightly retards water evaporation, as expected. PDEA-PS particles and flocs adsorbed at the air−water interface of the water droplet decreased the bare air−water interfacial area from which water can evaporate. The observed slight reduction in evaporation rate is in accord with that reported by Laborie et al., based on the thickness of the multilayered shell.43 An increase of relative humidity of the atmosphere decreased the water evaporation rate of the liquid marble, as reported previously for graphite26 and PTFE45 powder stabilized liquid marbles. There have been several recent reports of attempts to fabricate microcapsules from the liquid marbles.40,46−48 Chin et al. demonstrated the fabrication of microcapsules by interfacial polymerization of ethyl-2-cyanoacrylate on the surface of the liquid marbles.48 Film formation of the particles at the liquid marble surface is an alternative approach toward fabrication of microcapsules.40 Film formation is one interesting character of polymer particles, which cannot be easily attained using inorganic particles at low temperature (near room temperature), and PDEA-PS particles become film forming by exposure to toluene vapor, which can plasticize and dissolve both PS and PDEA components. After the toluene vapor 3054

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10−4 g/min for 1 min, 1.122 × 10−4 g/min for 2 min, 0.969 × 10−4 g/min for 3 min, and 0.812 × 10−4 g/min for 4 min toluene vapor treatment. A liquid marble after solvent vapor exposure for 4 min showed a long-term stability up to 530 min in 85−89 RH%; that is 4 times longer than the untreated counterpart in 58−63 RH%. The reduction in pore volume within the adsorbed PDEA-PS particles/flocs by the solvent vapor treatment thus suppressed the water evaporation. The color of the polymer capsule after water evaporation was significantly more opaque white compared with that of the polymer capsule containing water, which is attributed to the greater difference in refractive index (and hence scattering of visible light) between air and PS compared to water and PS. It is interesting to ask whether solvents other than toluene can be used to prepare polymeric capsules from the liquid marbles via vapor treatment. Dichloromethane vapor was also found to effectively form polymeric capsules from liquid marbles, but tetrahydrofuran (THF) did not, as the liquid marbles in this case readily lost their spherical shape and leaked water onto the underlying mesh) after exposure to THF vapor for around 1 s. THF can be dissolved in water at any weight ratio, and the liquid marble adsorbed sufficient THF to decrease the surface tension of the water droplet and for the PDEA-PS particles/ flocs to wet the liquid surface. Consistent with this behavior are the measured surface tensions of water after 5 min vapor treatment with toluene, dichloromethane, and THF at 70.11, 71.58, and 63.51 mN/m, respectively. Next, the effect of the introduction of water-soluble salts in the aqueous phase on the rate of mass change of the liquid marble in various atmospheres was investigated. Figure 4 shows

time (Figure 3 and Table 1). The significant decrease of the shell thickness indicates that the solvent vapor treatment is

Figure 3. SEM images of the shell of the dried liquid marbles stabilized with PDEA-PS particles after toluene vapor treatment: (a) 1 min, (b) 2 min, (c) 3 min, and (d) 4 min.

effective in reducing the voids between the PDEA-PS particles/ flocs. Apparent densities of the liquid marble shell were calculated to be 2.81 × 10−1, 4.16 × 10−1, and 5.59 × 10−1 g/ cm3 after 1, 2, and 3 min toluene vapor treatment using the shell thickness determined by SEM studies and the mass of dry PDEA-PS particles adsorbed to the liquid marble surface (0.99 mg). The apparent density of the shell after 4 min vapor treatment was calculated to be 9.81 × 10−1 g/cm3, which is approaching that of PS homopolymer (1.04 g/cm3).49 (Note that 97.6% of PDEA-PS particles consist of PS, and the contribution from PDEA is negligible.) These results indicate that the air originally contained in the shell of the liquid marbles is expelled after 4 min vapor treatment. The specific surface area of PDEA-PS particles adsorbed at the liquid marble surface without toluene vapor treatment was determined to be 12.94 m2/g by BET measurement of dried liquid marble. This is similar to that of dried PDEA-PS bulk powder (11.91 m2/g). The PDEA-PS particle diameters calculated from these specific surface area values were 442 and 480 nm for PDEA-PS particles at the liquid marble surface and dried bulk powder, respectively, consistent with the Dn value determined using SEM images. This result indicates that the shape of PDEA-PS particles remained intact at the air− water interface of the liquid marble. The specific surface area of the liquid marble decreased to 7.71 m2/g after 4 min toluene vapor treatment, which is due to film formation. Assuming complete film formation of PDEA-PS particles at the liquid marble surface yielding a pure solid polymer coating, the specific surface area can be calculated to be 6.07 × 10−2 m2/g, which is 127 times smaller than the experimental result. This indicates that the liquid marble shell was still rough and microporous even after 4 min toluene treatment. Solvent vapor treatment prolongs the lifetime of the liquid marble (Figure 2b and Table 1). An increase in toluene vapor treatment time increased the time required for complete water evaporation (58−63 RH%): 130 min before treatment, 135 min for 1 min, 140 min for 2 min, 160 min for 3 min, and 196 min for 4 min toluene vapor treatment. The water evaporation rate decreased with an increase of toluene vapor treatment time (58−63 RH%): 1.224 × 10−4 g/min before treatment, 1.148 ×

Figure 4. Weight increase percentage of the liquid marbles containing saturated LiCl aqueous solution (○, ●) and humidity decrease of atmosphere (□, ■) as a function of time (○, □: without toluene vapor treatment; ●, ■: 4 min toluene vapor treatment). The experiments were conducted at 24.2−25.4 °C and 53−56 RH%.

the mass change of liquid marbles prepared using saturated LiCl aqueous solution with and without 4 min toluene vapor treatment. Interestingly, the mass of the liquid marbles increased with time, which has been never observed with liquid marbles containing pure water and aqueous solutions of surfactant50 and polymer.51 The relative humidity of the atmosphere in the enclosed balance chamber where the liquid marbles existed decreased through the experiment, which indicates the liquid marbles absorbed water vapor from the air and thus increased in mass. Saturated LiCl solution can reduce the relative humidity to 11 RH% at 25 °C,52 and the volume of 3055

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the liquid marble kept increasing to attain equilibrium of the relative humidity by water uptake shown in Figure 4. Further detailed studies on mass changes of liquid marbles containing aqueous salt solutions is ongoing in our research group. The rate of mass increase for the liquid marble without toluene vapor treatment is higher than that after vapor treatment. Once more, this is because of the low vapor transmission rate through the PS film shell on the droplet surface. In the case of saturated K2SO4 aqueous solution used as aqueous phase, the liquid marble mass decreased with an increase of time (SI Figure S8). In theory, the evaporation could continue until the relative humidity of the atmosphere reaches 97 RH% at 25 °C, if sufficient water is present in the marble.52 The mass decrease ceased after 110 min for the untreated liquid marble, which is shorter than that for the liquid marble exposed to 4 min of toluene vapor treatment (135 min). It should be noted that K2SO4 precipitation would ultimately have occurred during the evaporation of water from these liquid marbles, however, this did not affect the liquid marble stability. Differences in vapor transmission rate through the marble shell can be also demonstrated using pH indicator (SI Figure S9). When the liquid marble prepared using aqueous NH3 solution was placed on a planar air−water interface containing phenolphthalein, the underlying water started to appear pink within 17 s. However, it took 31 s for the water to be colored in the case of a liquid marble that had received 1 min of toluene vapor treatment. Liquid marbles prepared using nonvolatile aqueous NaOH solution did not change the color of the underlying aqueous phenolphthalein solution even after over 1 h. These results indicate that basic NH3 vapor coming out through the marble shell and contacting with the planar air− water interface can change the underlying water pH, but that there is no direct contact between the basic water within the liquid marble and the planar air−water interface. In a parallel set of experiments, there was a difference in coloration time for liquid marbles containing phenolphthalein with and without toluene vapor treatment for 4 min when exposed to a gaseous NH3 atmosphere (SI Figure S10): 1 s for nontreated liquid marble compared with 30 s for a vapor-treated liquid marble. These results indicate a possible use of these liquid marbles as a gas sensor. The effect of PDEA-PS particles/flocs size and size distribution on both the evaporation rate and lifetime of the liquid marbles merits further investigation. The floc size distribution will vary according to the size of the primary PDEA-PS particles and thus the attractive van der Waals force holding the flocs together, plus the energy input during drying and grinding which may subsequently affect the liquid marble evaporation rate considerably. In order to investigate the effect of film formation at the liquid marble surface on the deformability of liquid marbles, the following dynamic experiment was conducted at room temperature: a liquid marble was released 3 cm above a planar surface, and the collision with the surface was recorded with a high speed camera. The impinging kinetic energy per liquid marble (15 μL) was estimated to be 4.7 × 10−6 J, and the velocity of the colliding liquid marble was estimated to be 7.7 × 10−1 m/s. In the case of liquid marbles without toluene vapor treatment, the liquid marbles bounced on impact following their vertical descent and survived with their original volume preserved. High-speed digital photography confirmed that elastic deformation of the liquid marble occurred on contact with the paper (Figure 5). Interestingly, areas of bare air−water interface appeared as cracks in the surface coating of the

Figure 5. Deformation of the liquid marble stabilized with PDEA-PS particles observed using a high speed camera: Liquid marbles (a) without toluene vapor treatment and (b) with 1 min toluene vapor treatment. The liquid marble was released from a height of 3 cm onto the black paper. Arrows indicate where cracks appeared on the liquid marble surface. The numbers in brackets indicate times after contact with the paper.

deformed liquid marble (see the arrow in Figure 5a: 4 ms after contact with the paper), and disappeared upon return to the spherical morphology (Figure 5a: 16 ms after contact with the paper). During this deformation of the liquid marble from the spherical morphology, the air−water surface area increased, but in the absence of sufficient time for significant particle rearrangement, instead areas of bare air−water interface appeared. When the liquid marble bounced on the paper, additional PDEA-PS particles/flocs sitting on the liquid marble, which were not adsorbed to air−water interface but simply adhered to the outer surface of the liquid marble, became detached from the marble. By comparison, when a liquid marble that had received 1 min toluene vapor treatment was allowed to fall onto the paper, it fractured upon impact and attached to the paper, Figure 5b. The PS polymer film formed on the liquid marble surface is not elastic but instead glassy at room temperature (note that the Tg of PS is near 100 °C49), and the liquid marbles that had received vapor treatment were unable to survive the impact with the planar surface. Careful visual inspection of the destroyed film-formed liquid marble confirmed many surface cracks in the PS film. When the vaportreated liquid marble bounced on the paper, few PDEA-PS particles/flocs became detached from its surface after contact with the paper, which indicated that the majority of the adhered PDEA-PS particles had film formed with the adsorbed particles. Elasticity change within the liquid marble shell was also confirmed by the pendant drop test wherein the marble volume was able to be changed through the addition or subtraction of liquid (Figure 6). The liquid marble without toluene vapor treatment hanging at the end of the pipet had an elastic shell and was deformed to a deflated sphere morphology by retracting the water inside the marble back into the pipet and to then regain its shape by adding water again. However, 1 min of toluene treatment did not change the shape of the liquid marble when the water was retracted from the polymeric capsule, which confirms the rigid film formation at the air− water interface. When it was attempted to return the water to the empty polymeric capsule, the water droplet instead appeared between the tip of the pipet and the polymeric capsule, (see the arrow in Figure 6b). The decrease of surface 3056

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Figure 6. Deformation of the liquid marble stabilized with PDEA-PS particles (a) without and (b) with toluene vapor treatment for 1 min. (a) The water inside the liquid marble was retracted and added, repeatedly. The shrunken−swollen behavior can be repeated at least five times. (b) The water inside the toluene-treated liquid marble was retracted and added. The arrow indicates where water came out of pipet.

Figure 7. (a) Digital photograph and (b, c, d) SEM images of dried liquid marble prepared using 180 nm-sized SiO2 particle aqueous dispersion as an aqueous phase: (b) surface, (c) cross section, and (d) inner surface of the dried liquid marble. Inset of (b) is a magnified image of the liquid marble surface. Inset of (d) is an FFT image.

roughness of the liquid marble by toluene vapor treatment can be also observed in the pendant droplet experiment. Finally, organic−inorganic composite capsules and colloidal crystal capsules were fabricated from liquid marbles containing aqueous SiO2 dispersions. In this study, SiO2 particles with 11 μm, 180 nm, and 20 nm diameters were used. All the toluenetreated liquid marbles containing SiO2 particles (15 μL; 20 wt % for 11 μm SiO2, and 40 wt % for 180 and 20 nm SiO2 particles) kept their near-spherical shape after complete water evaporation, as observed for the toluene-treated liquid marbles prepared using pure water. For the 11 μm-sized SiO2 particle system, cross-sectional SEM images of the capsule confirmed the existence of the SiO2 particles within the polymer capsule: the SiO2 particles had primarily settled to the bottom of the capsule during the water evaporation and were only partially adsorbed to side and upper inner surfaces of the capsule (SI Figure S11). Using the 180 nm- and 20 nm-sized SiO2 particles, which are mainly controlled by Brownian motion rather than gravity, composite capsules with a uniform double shell layer consisting of a PS film outer shell and SiO2 nanoparticle inner shell were fabricated. SEM studies indicated that a smooth inner surface could be observed for 180 nm-sized SiO2 particles system (Figure 7c). Figure 7d shows a highly magnified image of the inner surface shown in Figure 7c: near perfect hcp arrays of SiO2 particles were observed. A fast Fourier transform (FFT) analysis of the magnified SEM image confirmed that these SiO2 particles are exquisitely ordered (inset of Figure 7d). After removal of the PS particle shell by heat treatment at 600 °C in air, a colloidal crystal capsule was obtained (Figure 8). The surface of the capsule had craters of submicrometer to a few tens of micrometers in size, attributed to PDEA-PS flocs intruding into the inner water phase at the surface of the original liquid marble. Disordered SiO2 particles were observed at the capsule surface (Figure 8b). The disordered structure and rough surface scattered visible light and the capsule appeared white in color. The cleaved capsule confirmed a single cavity in the capsule (Figure 8c) and the cross section SEM image indicated formation of a colloidal crystal structure (Figure 8d). The blue color observed from inside the capsule is attributed to this colloidal crystal structure. In the absence of toluene vapor treatment, the liquid marbles collapsed to form nonspherical

Figure 8. SEM images of hollow colloidal crystal capsules fabricated by calcination of dried PDEA-PS particle-stabilized liquid marble prepared using 180 nm-sized SiO2 particle aqueous dispersion. (a, b) Outer surface and (c, d) cross section of the colloidal crystal capsule shell. Insets in (a, c) are digital photographs of outer and inner surfaces of the colloidal crystal capsules, respectively.

PDEA-PS particle-coated SiO2 particle aggregates (SI Figure S12). In the case of the composite capsule fabricated using the 20 nm SiO2 particles, the inner SiO2 particle layer was broken into fragments in the size range 40 to 600 μm after drying and breaking the capsule (SI Figure S13). The glass-like cleavage of the inner silica layer is indicative of close-packed silica nanoparticles. A self-supporting SiO2 capsule was again fabricated by removal of the PS component from the composite capsule by heat treatment at 600 °C.

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CONCLUSIONS In summary, millimeter- and centimeter-sized “liquid marbles” with aqueous volumes varying between 15 and 1000 μL were readily prepared by rolling water droplets on the dried PDEAPS particle powder bed. The liquid marbles stabilized with flocs of the PDEA-PS particles adsorbed at the air−water interface were deformed to deflated spherical geometry after water evaporation. The rate of water evaporation decreased with an 3057

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increase in the relative humidity of the atmosphere. Introduction of LiCl salt into the liquid marbles led to water adsorption, which led to a weight increase of liquid marble. The liquid marbles can be transformed into polymeric capsules containing water by exposure to solvent vapor. The polymeric capsules kept their shapes even after complete water evaporation, which confirmed that a rigid polymeric film was successfully formed. Water evaporation/adsorption rate from/ into the liquid marble was reduced with an increase of solvent vapor treatment time, which indicated the number and size of pores among particles/flocs on the liquid marble surface decreased due to film formation. Moreover, organic−inorganic composite capsules and colloidal crystal capsules were fabricated from liquid marbles containing aqueous SiO2 dispersions. Provided that their mean lifetimes can be extended significantly by further formulation optimization, the liquid marbles described in this study are promising candidates for potential applications in various fields as smart capsules for the delivery of water-soluble actives and microfluidics and lab-on-achip devices.

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ASSOCIATED CONTENT

S Supporting Information *

Details on PDEA-PS particles, liquid marbles, polymeric capsule, composite microsphere, and colloidal crystal capsule. 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 We thank Miss Reiko Fukui for her assistance with the toluene vapor treatment setup. CPKelco (U.S.A.) is thanked for kind donation of the Gellan gum (Kelcogel F). Eka Chemicals AB is thanked for kind donation of the Bindzil 2040. This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Engineering Neo-Biomimetics”, “New Polymeric Materials Based on Element-Blocks” and “Molecular Soft-Interface Science” from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, and the Australian Academy of Science and the Japanese Society for the Promotion of Science on the Researcher Exchanges Program. We also thank the Osaka Institute of Technology for support for K.U.

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