Liquid Marbles Supported by Monodisperse Poly(methylsilsesquioxane)

Article pubs.acs.org/Langmuir

Liquid Marbles Supported by Monodisperse Poly(methylsilsesquioxane) Particles Shigesaburo Ogawa,† Hirohmi Watanabe,† Liming Wang,§ Hiroshi Jinnai,†,∥,⊥ Thomas J. McCarthy,§ and Atsushi Takahara*,†,∥,⊥ †

Japan Science and Technology Agency (JST), ERATO Takahara Soft Interfaces Project, 744 Motooka, Nishi-ku Fukuoka 819-0395, Japan § Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003, United States ∥ Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ⊥ International Institute for Carbon Neutral Energy Research(WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: The preparation of model, well-controlled colloidal assemblies has been a central approach to understanding and optimizing the characteristics and functionality of complex colloidal dispersion systems. This approach, which has created a significant literature and rather deep understanding for emulsions and foams, has yet to be established for the liquid marble (water-in-air) motif. In this article we report the preparation of well-controlled liquid marbles using monodisperse micron-size particles of poly(methylsilsesquioxane) (PMSQ). The low cohesive nature of the stabilizing particles, their narrow size distribution, and their hydrophobicity permit the formation of liquid marbles containing a particulate monolayer with a hexagonally close-packed (HCP) structure. The “cleaning process” by rolling of liquid marbles under a flow of air on a hydrophobic substrate was useful to obtain the monolayer structure. Moreover, the monolayer structure was only obtained from liquids with high surface energy, whereas the others were not useful even though multilayered structure was formed from liquids that have intermediate surface energy.

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INTRODUCTION Colloidal particles with micrometer and submicrometer diameters have significant technological impact because they can adsorb strongly to liquid/vapor and liquid/liquid interfaces. The assembled structures stabilize various dispersion systems, notably emulsions and foams.1,2 Among these systems, the liquid-in-air assembly is unique because traditional small surfactants do not function to maintain this structure. The liquid-in-air assemblies include “dry water” (water-in-air powders) as well as what is termed a “liquid marble” which describes a monolithic liquid-in-air assembly in which one liquid droplet (usually water) is encapsulated by lyophobic particles.3−6 The typical size of liquid marbles ranges from 1 to 10 mm3 in volume, and they are spherical when the radius is less than the capillary length of the liquid. Because of the encapsulated structure, liquid marbles make solid−solid interfacial contact with solid objects and exhibit potentially useful behaviors such as low friction and low adhesion on various substrates.3,5,6 The abilities to float and to have their motion driven by external fields on liquid or solid surfaces are also unique characteristics of liquid marbles.3,6,7 The stability of these structures can be enhanced in some cases because evaporation is inhibited due to the encapsulation.8−11 Various applications for liquid marbles such as in microfluidics, in sensors, and as microreservoirs for long-term storage have been proposed.3,5,6,12−19 © 2014 American Chemical Society

The characteristics of liquid marbles can be tuned through the chemical and physical properties of the stabilizing particles. We have reported that the low surface energy fluoropolymer poly[2-(perfluorooctyl)ethyl acrylate] (PFA-C8) stabilizes various liquids including low-surface-tension liquids.20 Binks and co-workers reported that the intermediate hydrophobicity of solid particles is critical for providing stability.21−23 The simple geometry of a monolithic liquid marble appears very amenable to detailed studies; however, an understanding of liquid marble structure−property relationships based on surface and interface science has been impeded by the generally agglomerated structure of stabilizing particles.8−10,13,15,20,21,24,25 This increases the complexity of the wetting states. For example, the capillary forces between individual particles will impact the secondary structure of the aggregates, and this aggregate structure will affect both wetting behavior and cohesive interactions. Thus, in most cases, the simple description that can be made of a liquid marble containing a particulate monolayer of monodisperse particles is far from reality. Preparative methods for liquid marbles are neither straightforward nor reproducible, and whether or not reported structures are particle monolayers, multilayers, or a mixture of Received: May 4, 2014 Revised: July 13, 2014 Published: July 14, 2014 9071

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longer deposited material, the cleaning process was considered complete. Langmuir−Blodgett (LB) Film Preprataion. Langmuir monolayers of PMSQ particles were prepared at the air−water interface as follows: A methanol dispersion of PMSQ particles was spread onto the subphase, and the particulate layer was then compressed by moving a barrier with a constant speed of 0.05 mm/s. The particle monolayer was transferred from the water surface to substrates with a speed of 0.05 mm/s at a constant value of about 80% of the respective collapse pressure (πc) of the LB film. Surface pressure during the compression was measured using a Wilhelmy device. During the experiments, the temperature of the subphase was maintained at 20 °C using a circulating water bath under 65% of relative humidity. The transferred LB films were dried on a hot plate at 120 °C for 12 h to remove adsorbed water. Because the glass transition temperature (Tg) of PMMA is approximately 120 °C, the heating also caused the embedding of the particulate layer into the PMMA film. As a result, mechanically stable films were obtained that prevented the disordering of the structure during contact angle measurements. Note that no differences in TGA curves of PMSQ particle were observed before and after heating at 120 °C. This indicates that the chemical properties of materials remain intact during the thermal treatment. In contrast, for the OTS-modified film, the LB layer was easily broken even by drying the substrate with a gentle nitrogen blow. Contact angle measurements of these samples were difficult.

these states is unclear from most of the literature. Certain reports describe systems using monodisperse particle assemblies, but in one of these, multilayer formation is evident in the data,26 and the other uses particles with a size range (40−500 μm) that does not form robust liquid marbles.27 With the goal of helping establish such a methodology, we report the preparation of liquid marbles using a widely available (commercialit is used in cosmetic applications) microparticle source.

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EXPERIMENTAL SECTION

Materials. Poly(methylsilsesquioxane) (PMSQ) particles, TOSPEARL 2000B (Lot 12DJPA307), was purchased from Momentive Performance Materials Inc. To remove a trace of adsorbed water on the surface, particles were heated at 120 °C for 24 h before use. 1Ethyl-3-methylimidazolium tetrafluoroborate (EmimTFB) was purchased from Kanto Chemical Co. Inc. 1-Ethyl-3-methylimidazolium methyl sulfate (EmimMS) was purchased from Sigm-Aldrich Co. LLC. These ionic liquids were dried at 120 °C under vacuum for 2 h before use. Other chemicals used in this study were used as received. Si(111) substrates were purchased from Matsuzaki Seisakusho Co., Ltd. Measurements. Scanning electronic microscopy (SEM) was performed using a JCM-5000 Benchtop SEM, NeoScope (JEOL) after Au sputtering. The Fourier transform infrared (FT-IR) absorption spectrum was recorded on a VERTEX 70 (Bruker Co., Ltd.) using a germanium attenuated total reflectance element. Thermogravimetric analyses (TGA) were carried out on a Pyris 1 TGA (PerkinElmer Inc.) under ambient atmosphere. Langmuir− Blodgett (LB) films were obtained using USI-3-22 (USI Co., Ltd.) LB film deposition equipment. During the process, the isothermal curve of surface pressure (π)−occupied area (A) of the particle at water/air interface was monitored by the Wilhelmy method. Contact angle measurements were performed using a drop shape analysis system DSA 10Mk2 (KRÜ SS GmbH). Optical images were recorded by a MSX-500Di (SCHOTT MORITEX Corp.), a Nicon ECLIPSE LV100 (Nikon Co.), and a drop shape analysis system DSA 10Mk2 (KRÜ SS GmbH). The image of confocal laser scanning microscopy (CLSM) was obtained using a LSM510META (Carl Zeiss Co., Ltd.) with excitation laser of 543 nm. Substrate Preparation. Oxidization of silicon substrates was done as follows: First, Si(111) substrates were soaked in piranha solution (sulfuric acid and hydrogen peroxide H2SO4/H2O2 = 7:3 (v/v)) for 1 h. (Caution: piranha solution is highly corrosive and reacts violently with organic matter!) Then, the soaked substrate was rinsed thoroughly with purified water and dried in a N2 flow. Second, vacuum-ultraviolet radiation of 172 nm in wavelength was performed on the substrates using a Xe2 excimer irradiation unit (UER20-172 V, Ushio Co. Ltd.) for 15 min. Poly(methyl methacrylate) (PMMA) spin-coated substrate was obtained by spin-coating of a PMMA in ethyl lactate (6.0 wt %) on oxidized Si substrates at 2000 rpm for 60 s. The samples were then baked at 120 °C for 30 min. The film thickness was estimated as about 190 nm by ellipsometry. n-Octadecyltrimethoxysilane (OTS) monolayer modified substrate was formed on an oxidized Si substrates via condensation reaction of OTS using acetic acid as the catalysis. Oxidized silicon substrates were covered in 50 mL of distilled toluene containing 210 μL of OTS with a small amount of acetic acid. After reaction for 12 h, the sample was rinsed with toluene, hexane, ethanol, methanol, and water and dried by a N2 flow. The thickness of OTS monolayers was estimated as about 3.3 nm by ellipsometric measurements. Preparation of Liquid Marbles. Liquid droplets were applied to a powder bed of PMSQ particles with a microliter syringe. A liquid marble was prepared by gentle rolling of liquid on a powder bed of PMSQ particles. To remove excess (greater than monolayer coverage) particles, the as-prepared marble was transferred to an OTS-modified silicon wafer and gently rolled using a stream of air. A trail of PMSQ particle was initially left behind the rolling marble. After the marble no

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RESULTS AND DISCUSSION PMSQ particles are available from Momentive Performance Materials with a range of diameters and structures. After screening a number of their products, we chose TOSPEARL 2000B to use for the experiments described here owing to the narrow particle size distribution. This particle has been discussed in a book chapter.28 These materials are prepared by methods very similar to those used to prepare Stöber particles. Since agglomerates tend to form due to the cohesive properties of small particles, low cohesiveness as well as lyophobicity is indispensable for the fabrication of liquid marbles with particulate monolayers. These particles exhibit both of these properties. Their methylsilsesquioxane structure (CH3SiO3/2, Figure 1a) accounts for their hydrophobicity, and

Figure 1. Poly(methylsilsesquioxane) (PMSQ): (a) chemical structure; (b) SEM image.

their low cohesiveness may be due to their reported large negative surface charge. The low cohesive property is confirmed by SEM observations (Figure 1b). The particles also have a quite narrow particle size distribution and readily form dense 2D assemblies with obvious hexagonal closepacking (HCP) tendency. The diameter was 4.4 μm by number-average as confirmed by SEM measurement. The methyl-terminated structure is confirmed by FT-IR spectroscopy (see the Supporting Information, Figure S1). Liquid marbles were prepared by the encapsulation of various liquids with PMSQ particles by rolling the liquid droplet on powder beds of PMSQ particles (Figure 2a). Liquids possessing higher surface energy than ethylene glycol formed stable liquid marbles (see the Supporting Information, Table S1, and Figure 9072

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dissolved in the EmimTFB. The glass and portions of the PMSQ particles appear green because of the reflecting light. As indicated in Figure 2e, leakage of liquid to contact the substrate did not take place. Only a single layer of particles is observed at the solid/liquid interface (see the Supporting Information, Figure S2), indicating that the liquid marble is stabilized by a particulate monolayer. This monolayer structure was also observed in glycerol and EmimMS liquid marbles (see the Supporting Information, Figure S3). Precise mass measurements were conducted to determine the surface particle density for water liquid marbles, before and after the cleaning procedure. The mass of PMSQ on a 5 μL water liquid marble was determined using TGA (see the Supporting Information, Figure S4). The theoretical value was calculated assuming full coverage of PMSQ particles with HCP arrangement (see the Supporting Information, Figure S5). The number of particulate layers is calculated by the actual mass of PMSQ particle divided by the theoretical value. These values are 1.42 and 1.07 for before and after surface cleaning, respectively, as summarized in Table 1. It is apprent that a water liquid marble supported by essentially a single particulate monolayer was obtained after the surface cleaning procedure.

Figure 2. (a) Preparation of a liquid marble with a particulate monolayer. (i) Rolling a droplet on PMSQ particle’s bed. (ii) Surface cleaning of liquid marbles on clean OTS-modified substrate. (b−d) Optical micrograph images of a liquid marble: (b) whole image, (c) magnified image of the as-prepared surface, and (d) magnified image after surface cleaning. (e) Confocal laser scanning microscopic images of bottom area of the liquid marble (5 μL of EmimTFB containing 0.1 mg/mL rhodamine B dye). The observation position is depicted by the blue line in the z-axis.

Table 1. Constituent Data of Water Liquid Marble Obtained before and after Surface Cleaning before after

2b). We note that the stabilizing particle layers of as-prepared liquid marbles were multilayer or a mixture of monolayer and multilayer in structure. HCP structure, however, was evident in the layers on high surface energy liquids: water, glycerol, EmimMS, and EmimTFB (Figure 2c and Table S2 in the Supporting Information). This indicates that PMSQ particles spontaneously assemble into this structure during the preparation procedure.29 Subsequent to preparation, the nascent mixed monolayer/ multilayer particle-supported marbles could be treated (“cleaned”) to prepare an ordered monolayer structure (Figure 2a). This was accomplished by rolling on a hydrophobic substrate. An OTS-treated silicon wafer was found to work well (see the Supporting Information, Table S3). Because of the weak interactions between particles, excess (greater than monolayer coverage) adsorbed particles were readily removed by adhering to the substrate. Close-packed particulate monolayers formed and remained stable on the entire spherical surface (Figure 2d). These “cleaned” liquid marbles maintained their spherical shape and did not deposit additional particles upon rolling on clean OTS-treated silicon surfaces. This stability likely in part arises from the HCP-derived mechanical strength that can support unequal stresses.26 This cleaning procedure was effective for liquid marbles prepared with high surface energy liquids (see the Supporting Information, Table S2) but was not for liquids of lower surface energyeven those that would form liquid marbles. Direct observation of the layered structure was carried out using CLSM. To avoid the morphological changes that would occur with volatile liquids, a nonvolatile ionic liquid was used in these experiments. Figure 2e shows a CLSM image of a 5 μL PMSQ-supported EmimTFB liquid marble on an OTSmodified glass plate. The EmimTFB appears bright red in the luminescence image as a small amount of rhodamine B was

liquid (mg)

particle (mg)

wt % of particle

layer number

5.0 5.0

0.074 0.058

1.48 1.16

1.42 1.07

The effective surface energy (γeff) was evaluated by the puddle height of large marbles (see Supporting Information, Figure S6).21,24,31 The calculated γeff value of the liquid marble was 78.4 ± 1.9 mN m−1, and the value was higher than that of bare water (72.0 mN m−1). Because the energy of capillary interaction between floating particles was negligible for 4.4 μm PMSQ particles, the capillary attraction of immersion forces between particles at the air−water interface may impart the increase of the γeff value.21,30,31 Water evaporation behavior for water liquid marble was also investigated. The evaporation kinetics of the liquid marble was similar to that of a water droplet on a hydrophobic substrate (Figure S7). The apparent prevention of water evaporation was not observed similar to the water mable made by monodispersed polystyrenc beads with a size range 40−500 μm.27 We emphasize that particulate monolayers were not obtained from all liquids that formed liquid marbles. The cleaning process is not effective for liquid marbles prepared with relatively low surface energy liquids: diiodomethane, 1,1,2,2tetrabromoethane, and ethylene glycol (see the Supporting Information, Table S2). The liquid marbles remained unbroken with particulate multilayers. This indicates that the formation of the multilayer structure stabilized these liquid marbles. To clarify the difference between the high- and low-energy liquids that formed liquid marbles, the solid/liquid interface structure was more carefully investigated by observing the liquid behavior on Langmuir−Blodgett films of PMSQ particles. Optical microscopy images revealed that the PMSQ particles form LB films with HCP structure substratesimilar to that on liquid marbleson the surface of a PMMA (see the Supporting Information, Figure S8). Wetting behavior of liquids on the LB film was evaluated. Figure 3 shows the relationship between surface energy of liquids and the contact 9073

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particulate monolayer-stabilized liquid marbles will be useful in studies that require identifiable chemistry and topography such as adhesion and friction of surfaces with various chemistry and topography.

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ASSOCIATED CONTENT

S Supporting Information *

Characterization of PMSQ particles, liquid marble formation, the effect of surface cleaning, confocal laser scanning microscopy observation, calculation of number of particulate layer of water liquid marble, effective surface tension of water marble, and evaporation of water inside liquid marble and LB film properties. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 3. Contact angles of 5 μL drops of variable surface energy liquids on a PMSQ/PMMA LB substrate.

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angle. Pronounced differences in contact angle were observed for the two classes of liquids. The drastically lower contact angles of the low surface tension liquids, relative to those of the high surface tension liquids, indicate that liquid penetrated between the PMSQ particles to contact with PMMA substrate that has much lower surface energy than air (see Supporting Information, Table S3). This suggests that PMSQ particles, functioning as a monolayer, are not capable of supporting liquid marbles with these liquids. The increase in topography accompanying agglomeration is required to render a high enough contact angle to support liquid marbles in these cases. That is, those liquids reach the outer layer by penetration of monolayer, but the further capillary penetration must be blocked by the multilayers, at where the liquid contact PMSQ particle with a flat meniscus.32 This explains why the cleaning process is not effective with liquid marbles formed from these low surface tension liquids.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.T.). Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS Monodisperse PMSQ particles (commercially available TOSPEARL 2000B) are useful for the preparation of particulate monolayer-supported liquid marbles. Three classes of liquids were identified as illustrated in Figure 4: those with surface

Figure 4. Different surface energy liquids lead to different structures with PMSQ particles.

tensions too low to form liquid marbles, those with surface tensions sufficiently high to form marbles supported by aggregates of PMSQ particles, and those with high surface tension that can be stabilized by a particulate monolayer of PMSQ. A study with LB monolayers of PMSQ particles revealed that the liquids of intermediate surface energy penetrated the particulate monolayer while those of high surface energy did not. We expect that monodisperse 9074

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