Article pubs.acs.org/Langmuir
Colloidal Assembly in Leidenfrost Drops for Noniridescent Structural Color Pigments Che Ho Lim, Hyelim Kang, and Shin-Hyun Kim* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea S Supporting Information *
ABSTRACT: Noniridescent structural color pigments have great potential as alternatives to conventional chemical color pigments in many coloration applications due to their nonbleaching and color-tunable properties. In this work, we report a novel method to create photonic microgranules composed of glassy packing of silica particles and small fraction of carbon black nanoparticles, which show pronounced structural colors with low angle-dependency. To prepare isotropic random packing in each microgranule, a Leidenfrost drop, which is a drop levitated by its own vapor on a hot surface, is employed as a template for fast consolidation of silica particles. The drop randomly migrates over the hot surface and rapidly shrinks, while maintaining its spherical shape, thereby consolidating silica particles to granular structures. Carbon black nanoparticles incorporated in the microgranules suppress incoherent multiple scattering, thereby providing improved color contrast. Therefore, photonic microgranules in a full visible range can be prepared by adjusting the size of silica particles with insignificant whitening.
1. INTRODUCTION
The amorphous structures of colloids have been prepared in a form of thin films by three distinctive ways, which suppress their crystallization: coassembly of binary colloids,16,17 screening of interparticle repulsion,18,19 or rapid concentration of particles.20 However, the film format is incompatible for further processing to make coating solutions containing structural color pigments. To overcome the limitation of the films, water-in-oil emulsion drops have been used as templates to make microgranules composed of glassy colloidal structures. The electrostatic charges on the colloidal surface are screened by salt ions, thereby yielding random packing of colloids from each drop through its consolidation.19 However, emulsion-templating method has a limited throughput due to slow consolidation and entails a complex washing step. Moreover, delicate control of chemical composition of salt and surfactant is required to effectively screen colloidal repulsion and stabilize emulsions. Therefore, the development of facile and pragmatic methods to produce noniridescent photonic pigments containing isotropic colloidal structures remains an important challenge. Here, we report a novel strategy to create glassy packing of monodisperse colloids in a form of microgranules by employing Leidenfrost drops as a rapidly shrinking template; drops floating over a hot surface by insulating vapor film are called Leidenfrost drops.21−24 An ethanolic suspension of silica
Structural colors are developed by regular nanostructures through constructive interference of selected wavelength of light. Therefore, the colors never fade as long as nanostructure remains undestroyed and can be tuned by adjusting interscatter distance or refractive index in the nanostructures. Such superior properties of structural colors to conventional chemical colors have provided great opportunity in many practical applications including color pigments, reflection-mode displays, and colorimetric sensors. To create structurally colored materials, monodisperse colloidal particles have been utilized to form crystalline structures. A periodic modulation of refractive index in the colloidal crystals yields photonic band properties,1−5 thereby strongly reflecting a certain wavelength of light. In particular, colloidal crystals that have stop bands in visible range exhibit strong iridescent structural color;1−5 the iridescence is contributed from variation of optical path length depending on angle.5−9 However, most applications of structural color pigments require consistent colors regardless of viewing angles, thereby severely limiting the utility of colloidal crystals.10 The angle dependence of optical path length can be highly reduced in glassy packing of monodisperse colloids. Although constructive interference is suppressed in such amorphous structures, thereby leading to low reflectivity, noniridescent structural color can be developed;11−15 there is a trade-off in low angle dependency and high reflectivity between crystalline and glassy structures.15 © 2014 American Chemical Society
Received: June 3, 2014 Revised: June 24, 2014 Published: July 2, 2014 8350
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Preparation of Crystalline Dome. To make a crystalline dome, an aqueous suspension of silica particles with diameter of 256 nm is dropped on the superhydrophobic surface, and the drop is incubated at room temperature for 6 h to slowly evaporate water. The superhydrophobic substrate is prepared by the colloidal templating method reported elsewhere.26
particles is dropped on a hot metal surface, forming a Leidenfrost drop by instant formation of vapor film under the drop. The drop randomly migrates on the surface and shrinks, while maintaining its spherical shape, thereby concentrating silica particles in the drop. As most of the ethanol is depleted within tens of seconds, their vapor no longer supports the weight of the drop, and, therefore, the drop sits on the hot surface at the last moment, leading to very fast final consolidation of colloidal structure; this makes the resultant microgranule buckle at its top surface. Such fast concentration of colloids strongly suppresses crystallization of silica particles, thereby leading to the formation of amorphous structures.4,20 Although the glassy packing of silica particles displays pale color due to random scattering, the color contrast can be significantly enhanced by incorporating carbon black nanoparticles.13,19,20 This enables the preparation of structural color pigments in full visible range by simply adjusting the size of silica particles; the reflectance peak can be positioned in a wavelength range between 400 and 700 nm. This Leidenfrost drop-guided formation of glassy colloidal structures obviates any complex fabrication step and produces photonic microgranules in a short time, thereby providing a pragmatic way to create isotropic photonic pigments.
3. RESULTS AND DISCUSSION Colloidal Assembly in a Leidenfrost Drop. An ethanolic suspension of silica particles forms a Leidenfrost drop when it is dropped on a hot stainless steel surface of which the temperature is much higher than the boiling point of ethanol. As the suspension drop approaches the surface, the ethanol molecules instantly vaporize from the bottom of the drop and form a vapor film between the drop and the surface. The film supports the suspension drop, and, therefore, the drop randomly migrates over the surface without contacting the surface. The drop maintains its spherical shape when its radius is smaller than the capillary length of approximately 1 mm, which corresponds to a few microliters in volume; the capillary length can be estimated by (γ/ρg)1/2, where γ is a surface tension, ρ is a density of the suspension, and g is a gravitational acceleration. During the migration, the ethanol continuously vaporizes, making the drop shrunken and the silica particles concentrated; the evaporation is relatively slow in comparison with evaporation of a drop deposited on a hot surface with direct contact because the vapor film insulates the drop from the surface.24 Although the ethanol dominantly vaporizes from the bottom surface of the drop at which temperature is highest,23 random migration of the drop makes isotropic shrinkage, thereby leading to concentration of the silica particles near entire drop interface. The ethanol is almost depleted from the drop in tens of seconds, and the vapor film no longer supports the drop. Therefore, the drop sits on the surface, which leads to very fast vaporization of remaining ethanol through top surface of the drop. This makes that resultant colloidal aggregates buckle on their top surface. The overall formation process of a microgranule composed of randomly packed silica particles is schematically illustrated in Figure 1a and shown in movie S1 in the Supporting Information. Typically, 7 μL of ethanolic suspension containing 1 wt/wt % silica particles with diameter of 190 nm is dropped on the surface with temperature of 230 °C, which forms a spherical drop with radius of 1.18 mm in the beginning. The drop continuously shrinks and becomes fully dried in 20 s, producing a microgranule composed of random aggregate of silica particles. The microgranule is indented on its top surface due to buckling at the last stage of drying, thereby providing a bowlshaped structure as shown in Figure 1b and c. The resultant microgranule displays a pale bluish color as shown in Figure 1b; there is no spackling color pattern, which typically appears in crystal packing of colloids.27,28 Silica particles in whole volume of the microgranule are randomly packed to form glassy structure except the outermost layer at which the particles are slightly ordered into hexagonal or square arrays in local area as shown in Figure 1d and e. This random packing is attributed to fast concentration of the particles. The drop with radius of 1.18 mm is fully dried to form a microgranule with radius of 289 μm in 20 s. The average rate of radial shrinkage is therefore as high as 44.5 μm/s. By contrast, maximum diffusion distance of silica particles dispersed in ethanol for 20 s is only 10.4 μm by assuming free diffusion of the particle, where we assume
2. EXPERIMENTAL SECTION Preparation of Silica Suspension. Monodisperse silica particles with diameters of 190, 256, and 295 nm are synthesized by two steps: the two-phase method for the seed formation and the modified Stöber method for the particle regrowth.25 To prepare seed particles, 6.75 mL of cyclohexane (99.5%, Sigma-Aldrich) containing 8.25 mL of tetraethylorthosilicate (TEOS, 99.999%, Sigma-Aldrich) is loaded on 103.5 mL of aqueous solution of 6.61 mM L-arginine (≥98%, SigmaAldrich). The TEOS molecules slowly diffuse from the cyclohexane layer into the aqueous solution, forming monodisperse silica nanoparticles with diameter of 22 nm in 12 h. The seed nanoparticles are further subjected to regrowth step. To prepare silica particles with diameter of 256 nm, 3 mL of aqueous suspension of silica nanoparticles is mixed with 20 mL of ethanol and 1.5 mL of ammonium hydroxide solution (28.0−30.0% NH3 basis, SigmaAldrich). Subsequently, 4 mL of TEOS is injected into the mixture at a volumetric flow rate of 0.8 mL/h using a syringe pump; during the injection, the mixture is stirred. After 6 h, resultant silica particles are washed with ethanol several times, and their final concentration is set to be 1 wt/wt %. In some experiments, carbon black nanoparticles with average diameter of 23 nm (HIBLACK 420B, Korea Carbon Black Co., Ltd.) are added into the ethanolic silica suspension with a weight ratio to silica particles of 0.2, 0.5, and 1.7 wt/wt %. To prepare silica particles with diameter of 190 and 295 nm, the amount of TEOS injected is controlled to be 2.1 and 9.9 mL, respectively. Consolidation of Silica Particles in Leidenfrost Drop and Characterization. The ethanolic suspension of silica particles is micropipetted and dropped on a stainless steel dish of which temperature is controlled by a hot plate (RET control/t IKAMAG safety control, IKA), where a gap between the dish and the hot place is filled with silicone oil (KF-96-3000CS, Shin-Etsu Silicone) to remove insulating air film. The drop lift time is recorded to measure Leidenfrost point of ethanolic suspension. After complete consolidation, the colloidal aggregate is observed by an optical microscope (Nikon, L150) equipped with a camera (Nikon, DS-5M) and scanning electron microscope (SEM, FEI Co., Nova230) after OsO4 coating. Photo images and movies of the drops and granules are taken by digital cameras (EOS 20D with a lens of EF 100 mm f/2.8 Macro USM, Canon or Nex-5 with a lens of SEL1855, Sony). The reflection spectrum of the granular structure is measured by a fiber-coupled optical spectrometer (Ocean Optics, USB4000) mounted to the optical microscope with 20× lens and a field stop. 8351
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Figure 2. Influence of plate temperature on lift time of 7 μL drop on the plate, where a pure ethanol (denoted with ●) and 1 wt/wt % ethanolic silica suspension (denoted with red ▲) are used. Both liquids spread over the plate and rapidly vaporize when the temperature is lower than 180 °C; this is referred to as transition boiling. For the plate temperature larger than 190 °C, both liquids form a Leidenfrost drop and slowly vaporize; this regime is referred to as film boiling.
as film boiling. Lifetime decreases as plate temperature further increases. The Leidenfrost point of pure ethanol is close to that of ethanolic suspension. However, lifetime of the Leidenfrost drop of pure ethanol is longer than that of the ethanolic suspension as shown in Figure 2. We attribute this to larger density of ethanolic suspension than that of pure ethanol. Although the density difference is small at the beginning because of low concentration of silica particles in suspension, the difference becomes significantly large as ethanol vaporizes. The drop containing a high concentration of silica particles sits on the plate at the last stage before ethanol is fully vaporized, which leads to very fast vaporization of residual ethanol from the drop. Coloration in Glassy Packing of Silica Particles. Colloidal crystals prepared by slow concentration of particles in emulsion templates possess long-range order and display iridescent structural color.2,4,5 By contrast, amorphous structures, prepared by rapid concentration, only have shortrange order and show noniridescent structural color; the shortrange order is contributed from consistent interparticle distance, which enables the optical resonance at selected wavelength of light.13,14,19,20 Therefore, glassy packing of silica particles prepared by the Leidenfrost drop can exhibit structural color of which wavelength is tunable by adjusting the size of silica particles. To study this, we use three different sizes of silica particles of 295, 256, and 190 nm in diameter to make microgranules as shown in Figure 3a−c. Although microgranule composed of 190 nm silica particles shows a relatively clear bluish color, the others show obscure reddish and greenish color, where all microgranules are aligned to be dimple-side down. This whitening is caused by incoherent and multiple scattering in whole visible range, which reduces color contrast;13,19,20 the whitening effect is less significant in small size of scatters, thereby providing relatively clear blue color in microgranule made of 190 nm silica particles. Reflectance spectrum measured from each microgranule has a clear peak as shown in Figure 3d, where the spectrum is measured only from the top surface of microgranule using a field stop; peak wavelengths, λmax, are 668, 572, and 420 nm for constituting silica particles with diameter of 295, 256, and 190 nm,
Figure 1. (a) Schematic illustration showing the formation of glass packing of colloids confined by a Leidenfrost drop. (b,c) Optical microscope (OM) image (b) and scanning electron microscope (SEM) image (c) of a photonic microgranule composed of random packing of silica particles with diameter of 190 nm. (d,e) SEM images showing colloidal arrays on the microgranule surface (d) and its crosssection (e).
temperature of ethanol drop as 70 °C; diffusivity, D, can be estimated by the Stokes−Einstein equation, kT/3πηd, and diffusion length can be estimated by (Dt)1/2, where k is a Boltzmann constant, T is temperature, η is viscosity of ethanol, and d is diameter of silica particle. Therefore, the particles are rapidly concentrated near the drop interface in a course of shrinkage, thereby leading to glassy packing of the silica particles. Unlike particles in bulk volume, the particles in the outermost layer are guided by the smooth interface, which leads to the partial ordering. Influence of Temperature on Drop Lift Time. A Leidenfrost drop can be formed by instant formation of vapor film on the bottom of drop. Therefore, there is a minimum temperature of surface that can create a Leidenfrost drop; the minimum temperature is referred to as the Leidenfrost point. To evaluate the Leidenfrost point of ethanolic suspension of silica particles, we investigate the influence of plate temperature on the lifetime of the drop of ethanolic suspension as shown in Figure 2. We drop a 7 μL suspension on the plate of which temperature is controlled in a range between 170 and 230 °C. The drops spread on the plate and rapidly vaporize within 2 s when the plate temperature is lower than 180 °C; this regime is referred to as transition boiling.22−24 By contrast, drops levitate and randomly migrate, while maintaining their spherical shape when the plate is heated above 190 °C. Formation of insulating vapor film between the drops and plate significantly slows vaporization and therefore increases lift time more than 10 times; this regime is referred to 8352
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Figure 3. (a−c) OM images of photonic microgranules composed of silica particles with diameters of 295, 256, and 190 nm, respectively. (d) Reflectance spectra of three different photonic microgranules shown in (a)−(c).
respectively. However, peak intensity is less than 1.2 times of background intensity for microgranules composed of 295 and 256 nm silica particles due to undesired random scattering, providing a whitish appearance under ambient light. Color contrast can be dramatically enhanced by incorporating carbon black nanoparticles into the glass packing of silica particles. The carbon black nanoparticles reduce unexpected incoherent multiple scattering across the visible range.13,19,20 To investigate the influence of carbon black on coloration and find its optimum concentration, we disperse carbon black nanoparticles with diameter of 23 nm into the ethanolic suspension containing 1 wt/wt % silica particles with diameter of 295 nm at controlled concentrations; the weight fraction of the carbon black nanoparticles to silica particles is set to be 0.2, 0.5, and 1.7 wt/wt %. With the suspensions, microgranules are prepared from Leidenfrost drops with initial volume of 7 μL, as shown in Figure 4a−d. As a fraction of carbon black nanoparticle increases from 0 to 1.7 wt/wt %, the central red dot becomes clearer, and its surrounding becomes darker as shown in Figure 4a−d. Internal arrangement of silica particles is shown in the inset of Figure 4a−d. The particles maintain their glassy packing structure, although some of the carbon black nanoparticles are aggregated. For further increase of carbon black loading, carbon black nanoparticles form larger aggregates, which separate silica particles in uncontrolled fashion and therefore disturb the formation of consistent interparticle distance. The change of color contrast is also confirmed with reflectance spectra as shown in Figure 4e. Without carbon black particles, peak intensity relative to background is only 1.2. The value increases to 1.46, 1.77, and 2.11 for fractions of carbon black of 0.2, 0.5, and 1.7 wt/wt %, respectively; peak wavelength remains unchanged. This enhancement is caused by suppressed incoherent multiple scattering.
Figure 4. (a−d) OM images of photonic microgranules composed of 295 nm silica particles and four different fractions of carbon black nanoparticles to silica particles: (a) 0 wt/wt %, (b) 0.2 wt/wt %, (c) 0.5 wt/wt %, and (d) 1.7 wt/wt %. Inset shows internal structure of photonic microgranules. (e) Reflectance spectra of the four photonic microgranules in (a)−(d). Inset images show top surfaces of the photonic microgranules, where reflectance spectra are measured.
The carbon black nanoparticles loaded in the Leidenfrost drop also influence the overall shape of microgranule. Although all microgranules are prepared by a 7 μL suspension on the plate with temperature of 230 °C, the diameter of microgranule slightly increases as a fraction of carbon black nanoparticle increases; the diameter of microgranule without carbon black is 578 μm, whereas the diameter with 1.7 wt/wt % carbon black is 631 μm. The increase of diameter entails the decrease of height, and, therefore, carbon black makes the microgranule structures become slightly flattened. We attribute this to adsorption of carbon black nanoparticles at the air−ethanol interface. The carbon black nanoparticles are hydrophobic and prefer to be adsorbed at the interface to reduce interfacial energy. The adsorption partially immobilizes the interface and induces buckling of structure at earlier stage, thereby making more flattened structure. The suppression of undesired whitening enables the formation of photonic microgranule in a full visible range. By employing silica particles with diameters of 295, 256, and 190 nm, we prepare photonic microgranules with red, green, and blue color as shown in Figure 5a−c, where a fraction of carbon black nanoparticles are maintained at 1.7 wt/wt %. By comparison with Figure 3a−c where no carbon black is 8353
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Figure 5. (a−c) OM images of photonic microgranules composed of three different diameters of silica particles and a constant fraction of carbon black nanoparticles to the silica particles, 1.7 wt/wt %: (a) 295 nm, (b) 256 nm, and (c) 190 nm. Inset image shows the top surface of each photonic microgranule. (d) Reflectance spectra of the three photonic microgranules shown in (a)−(c).
incorporated, the color contrast is much enhanced. Full optical microscope images showing change of appearance of photonic microgranules depending on the fraction of carbon black nanoparticles are shown in Figure S3 of the Supporting Information, where enhancement of color contrast and increase of microgranule diameter can be clearly observed. Noniridescent Structural Color. Amorphous structures of colloids show noniridescent structural color due to their isotropic morphology unlike crystalline structure. Photonic microgranules prepared by Leidenfrost drops are composed of glassy packing of silica particles, and therefore they are expected to exhibit angle-independent structural color. To evaluate their optical properties, especially for angle dependence, we prepare photonic microgranule composed of 256 nm silica particles and 1.7 wt/wt % carbon black nanoparticles as shown in Figure 6a. For a comparison, we prepare crystalline counterpart on superhydrophobic surface with the same silica particles used for the amorphous microgranule;5 an aqueous colloidal suspension of 256 nm silica particles is dropped on superhydrophobic surface, and water molecules are allowed to slowly evaporate at room temperature, providing a crystalline dome as shown in Figure 6b. The amorphous and crystalline structures show dramatic differences in their appearance. Amorphous structure shows weak greenish color from the top center, while crystalline structure shows strong yellow reflection at the top center and violet glaze around the central dot. Angular dependence of their reflectance spectra is evaluated by optical microscope; reflectance spectrum from local area of photonic dome is measured as the measurement spot is moved from the center to the edge of the dome as shown in Figure 6c, where displacement from the center is normalized with dome radius, x = l/L, to denote a position of measurement.15 The reflectance peak position and peak width remain almost unchanged for the
Figure 6. (a,b) OM images of (a) a photonic microgranule composed of 256 nm silica particles and (b) a crystalline dome composed of the same particles. (c) Schematic illustration of measurement of reflectance spectrum using optical microscopy, where a field of measurement is displaced from the center of photonic structure to investigate angle dependence of the spectrum; displacement is denoted with l, and radius of the structure is L. (d,e) Reflectance spectra and OM images of (d) the photonic microgranule and (e) crystalline dome, which are obtained at five different positions of x = 0, 0.2, 0.4, 0.6, and 0.8, where x is defined as l/L.
photonic microgranule prepared from the Leidenfrost drop as the value of x is changed from 0 to 0.8 as shown in Figure 6d; the peak shift is less than 3 nm, and the peak broadens only for x = 0.8 due to low reflection at high angle of incident beam on the surface of the granule. In addition, optical microscope images in the inset, taken at the positions where the spectra are measured, show the same greenish color. By contrast, the crystalline dome shows peak shifts as large as 49 nm for the same displacement to the photonic microgranule as shown in Figure 6e. The peak red-shifts for x = 0.2 and 0.4 and blueshifts for x = 0.6 and 0.8; we attribute the blue shift to inclined incident light on the (111) plane of face-centered cubic structure, which is formed along the outermost layer, while the red shift we attribute to contribution from crystal planes other than the (111) plane.5,27 The color change is also discernible with optical microscope images in the inset. 8354
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and outdoor paintings, which do not require fine granules. In addition, we expect that the size of photonic granules can be reduced to tens of micrometer by employing smaller droplets and lower concentration of silica particles. The color tunability in entire visible range and noniridescence, as well as facileness of fabrication procedure and easiness of further processing of granular structure, will provide new opportunities as alternatives to chemical or toxic dye materials in many practical coloration applications.
Angle dependence of structural color of photonic microgranules is further studied by the naked eye. To see the influence of angle for specular reflection, we illuminate the photonic microgranules with a directional spot light (Led Lenser P4BM, Led lnser) at a distance of 30 cm at three different angles, while maintaining sample position and camera angle (EOS 20D with a lens of EF 100 mm f/2.8 Macro USM, Canon); the angle between incident light and direction of camera view is controlled to be 10°, 45°, and 90°. Because granules have a curved surface, specular reflection is dominant for this observation with these three different angles. Three distinct photonic microgranules, composed of 295, 256, and 190 nm silica particles and constant loading of 1.7 wt/wt % carbon black nanoparticles, are used. The photonic microgranules show a low degree of color variation for the angle change, as shown in Figure 7. A slight blue shift of the color is
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ASSOCIATED CONTENT
S Supporting Information *
SEM images of silica particles and carbon black nanoparticles; series of OM images of photonic granules composed of three different silica particles containing carbon black nanoparticles at four different loadings; and a movie clip showing formation and shrinkage of a Leidenfrost drop containing silica and carbon black particles. 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 This work was supported by the Midcareer Researcher Program (2014R1A2A2A01005813) through an NRF grant funded by the MEST and the International Collaboration grant (no. Sunjin-2010-002) funded by the MOTIE (Korea).
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REFERENCES
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Figure 7. Photographs of three different photonic microgranules composed of 295, 256, and 190 nm silica particles from the leftmost, where the angle between incident light and direction of view is controlled to be 10°, 45°, and 90°; all microgranules contain 1.7 wt/wt % carbon black nanoparticles.
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4. CONCLUSIONS We have demonstrated a novel and facile strategy to create photonic microgranules with noniridescent structural colors. Using a Leidenfrost drop as a confining geometry, silica particles are rapidly concentrated, which provides glassy packing of particles in each microgranule after complete drying. Although the amorphous structure exhibits strong incoherent multiple scattering, thereby appearing white, incorporation of carbon black nanoparticles in the photonic granules enables the suppression of the undesired whitening, thereby enhancing color contrast. Therefore, the photonic microgranules in a full visible range can be prepared by simply adjusting the size of the silica particles. The isotropic nature of glassy packing of particles in the photonic microgranule makes the structural color angle-independent, unlike anisotropic crystalline structures. We confirm this noniridescence by reflectance spectrum analysis and direct observation. Photonic granules with several hundred micrometers of diameter can be used for signboards 8355
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dx.doi.org/10.1021/la502157p | Langmuir 2014, 30, 8350−8356
