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[Re_YM_0727.docx](https://mdr.nims.go.jp/filesets/745cf396-5e18-4c26-b0db-73cc6ba705a9/download)

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Yui Maejima, Ai Takabatake, Shin-ichi Takeda, [Hiroshi Fudouzi](https://orcid.org/0000-0003-1442-4667), [Keiki Kishikawa](https://orcid.org/0000-0002-7539-568X), [Daisuke Unabara](https://orcid.org/0000-0001-5347-7257), Tasuku Hamaguchi, [Koji Yonekura](https://orcid.org/0000-0001-5520-4391), [Takuma Kureha](https://orcid.org/0000-0003-4680-6440), [Hideyuki Mitomo](https://orcid.org/0000-0002-6748-7816), Kanae Ito, Keiichi Osaka, Kiyoshi Kanie, [Michinari Kohri](https://orcid.org/0000-0003-1118-5568)

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This document is the Accepted Manuscript version of a Published Work that appeared in final form in Langmuir, copyright © 2025 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.langmuir.5c03990.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Visualization of the Assembly Behaviors of Gel Layer-Coated Melanin Particles at the Air/Water Interface via Structural Color](https://mdr.nims.go.jp/datasets/77f07bd8-2a91-48b4-98b4-406c19cad5cb)

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Visualization of the Adsorption/Stabilization Processes of Gel Layer-Coated Melanin Particle Assemblies at the Air/Water Interface via Structural ColorYui Maejima,1 Ai Takabatake,2 Shin-ichi Takeda,3 Hiroshi Fudouzi,4 Keiki Kishikawa,5 Daisuke Unabara,6 Tasuku Hamaguchi,6 Koji Yonekura,6,7 Takuma Kureha,8 Hideyuki Mitomo,9 Kanae Ito,10 Keiichi Osaka,10 Kiyoshi Kanie,6,11 and Michinari Kohri*51 Department of Applied Chemistry and Biotechnology, Graduate School of Science and Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan2 MS Scientific Co. Ltd., 4-11-3, Kuramae, Taito-ku, Tokyo, 111-0051, Japan3 Takeda Colloid Techno-Consulting Co., Ltd., 16-23, Toyotsu-cho, Suita, Osaka, 564-0051, Japan4 National Institute for Materials Science, 1-2-1, Sengen, Tsukuba-Shi, Ibaraki, 305-0047, Japan5 Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan6 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577 Japan7 Biostructural Mechanism Laboratory, RIKEN SPring-8 Center, 1-1-1 Koto, Sayo, Hyogo, 679-5148 Japan8 Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki 036-8561, Japan9 Research Institute for Electronic Science, Hokkaido University, Kita 21, Nishi 10, Kita-Ku, Sapporo, Hokkaido 001-0021, Japan10 11 International Center for Synchrotron Radiation Innovation Smart, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, JapanCorresponding author*E-mail: kohri@faculty.chiba-u.jp  ABSTRACTSurface-modified particles and microgels are recognized for adsorbing at the air/water interface. This particle assembly behavior has been evaluated primarily under conditions where all the particles are adsorbed using a dilute dispersion. In this study, we visualized the adsorption/stabilization process of particle assemblies at the air/water interface in a concentrated dispersion through structural coloration. Melanin particles coated with a gel layer were prepared, and these aqueous dispersions were dried. Pronounced coffee ring patterns with almost no particle deposition in the center of the pattern were obtained from pristine melanin particle dispersions. On the other hand, the use of polymer gel-coated particles suppressed the coffee ring effect and resulted in particle deposition not only around the pattern periphery but also in the center, indicating that the assembly behavior was affected by the gel layer. The observation of structural color during the drying process revealed adsorption at the air/water interface and differences in the size and motion of the domain. Furthermore, it was demonstrated that the use of this gel-coated particle is effective for structural color coating with uniform visibility.Keywords: structural color, gel, adsorption, air/water interface, coffee ring effectINTRODUCTIONThe ordered periodic structures of colloidal particles show a structural color derived from optical interactions. 1, 2 Structural colors do not fade as long as the microstructure is maintained, which is an attractive feature for next-generation paints. 3, 4 Designing the structural color materials by controlling the particle assembly has been widely conducted. For example, the reflective wavelength and angle independence are tuned by the particle size and regularity of the particle arrangement. 5 The expansion distance between the particles reflects a higher wavelength color, and the low regularity of microstructures corresponds to a color with low angle independence. 6, 7 While the structural color produced by conventional polymer particles appears whitish due to light scattering, the color saturation is effectively increased by doping with scattering absorbers such as carbon black, iron oxide, and graphene. 8-11 As a scattering absorber, we use polydopamine (PDA), a mimic of melanin, a structural color component found in many animals and insects. 12, 13 Structural color materials obtained by the assembly of melanin particles with a PDA coating on the surface of core particles are highly visible even under normal ambient light. 14-17For applications such as structural color coatings, controlling drying patterns by assembling colloidal dispersions and elucidating their formation mechanisms are important issues. Many methods are being considered for assembling dispersed particles, including spray-coating, dip-coating, and the Langmuir‒Blodgett method. 18-20 The self-assembly of colloidal particles by solvent evaporation is used as a convenient assembly process, and the shape, wettability, and surface potential of the particles are associated with their assembled structure. 21, 22 In addition, the viscosity, surface tension, and boiling point of the solvent are also essential factors. 23, 24 The properties of both particles and solvents influence the deposit morphology and particle packing formation. Generally, when colloidal particles dispersed in a solvent are dropped onto a substrate, they move toward the edge of the droplet due to capillary flow caused by the solvent evaporation gradient, forming a coffee ring pattern. 25, 26 To reduce the coffee ring effect, many strategies have been proposed, such as adding surfactants and other additives to the dispersion. 27, 28 Previous research has shown that the relaxation of the capillary force during the solvent evaporation process and the adsorption of particles to the air/water interface are important for constructing uniform colloidal particle assembly patterns. 29-31 The key is that the solvent evaporates while the particles are adsorbed at the air/water interface, which prevents the coffee ring effect. 32The monolayer formed at the interface by adsorption is transferred to a substrate and analyzed. 33, 34 Micron-sized particles can be observed directly under an optical microscope, and the particle arrangement behavior at the interface is evaluated individually for each particle. For example, the particle adsorption and tunable crystallization at the air/water interface induced by surfactants have been observed using optical microscopy. 35 Poly-N-isopropyl acrylamide (PN) microgels are widely known to adsorb at air/water or oil/water interfaces; many studies have been conducted to investigate their behavior. 36, 37 Recently, there have also been reports of real-time observations of the deformation process of microgel particles adsorbed to the air/water interface. 38, 39 These particle adsorptions at the air/water interface are evaluated mainly using a dilute dispersion solution of particles, and some papers mention diffraction grating-type structural coloration formed by particle monolayers at the interface. 40, 41 On the other hand, constructing multilayer particle assemblies from highly concentrated submicron-sized particle dispersions is necessary to create structural color materials based on Blagg diffraction. However, submicron-sized particles are too small to be observed individually under an optical microscope. Furthermore, in highly concentrated systems, only a portion of the particles adsorb at the air/water interface, with most of the particles remaining in the solvent. Thus, the particle assembly process in the concentrated dispersions is complex and difficult to observe. Further effort is essential to understand the adsorption behavior of submicron-sized particles at the air/water interfaces.In this study, we focused on the structural color domains that appear in the concentrated dispersions. The adsorption and assembly behaviors at the air/water interface of submicron-sized particles were directly investigated based on the vivid structural color exhibited by melanin particle assemblies. To assemble particles at the interface effectively, PN gel layer-coated melanin (M) particles were prepared via seeded polymerization. We prepared particles with different sizes of M particles and thicknesses of the PN shell layer (Figure 1). The particles were designated MX@PNY (X = 1–4, Y = 1–3) particles. The adsorption and assembly behaviors at the air/water interface were evaluated by comparing the drying patterns of the MX and MX@PNY particles. The MX@PNY particles formed a drying pattern with mitigated coffee ring effect, demonstrating the potential for facilitating structural color coatings with uniform appearance.Figure 1. Preparation of MX and MX@PNY particles.EXPERIMENTAL SECTIONMaterials.Styrene, N,N'-methylenebis(acrylamide) (BIS), ammonium persulfate (APS), potassium persulfate (KPS), sodium dodecyl sulfate (SDS), and tris(hydroxymethyl)aminomethane (Tris) were obtained from Kanto Chemical Co., Inc. (Tokyo, Japan). Dopamine hydrochloride (DA) was obtained from Sigma‒Aldrich Japan Co., LLC. (Tokyo, Japan). N-isopropylacrylamide (NIPAM) was obtained from Tokyo Chemical Industry (Tokyo, Japan). A Millipore Simplicity UV system provided deionized water with an 18.2 MΩ·cm resistance. NIPAM was purified by recrystallization using a hexane/toluene mixture. All other chemicals and solvents were of reagent grade and were used as received.Measurements.Optical microscopy images and movies were obtained with a digital microscope (VHX-X1F; Keyence). Video recording of samples under an optical microscope was carried out using a video microscope (Mitutoyo VMU-H) equipped with an infinity corrected objective lens (Mitutoyo Plan Apo 10X), a light source unit (HOYA-SCHOTT HL150 Cold light; FA-TSUKUMO), and a display unit (LCD-700VH; CENTURY). Reflection spectra were measured with a microscope spectrophotometer (MSV-370; JASCO) with a spot size of 50 μm and a mirror as a reference. Infrared absorption spectra were obtained using an attenuated total reflection Fourier transform infrared (ATR–FTIR) spectrometer (FT/IR 4700; JASCO). Transmission electron microscopy (TEM) images were obtained using transmission electron microscopy (H-7650; Hitachi). Scanning electron microscopy (SEM) images of the samples were obtained using a scanning electron microscope (JSM-6510A; JEOL). Particle purification was performed with a centrifuge (Suprema 21; TOMY). The grain boundaries were obtained using Wolfram Mathematica 11.Preparation of MX particles.The MX particles were prepared by modifying a previously reported method. 42, 43 Briefly, core polystyrene (PS) particles were synthesized by emulsion polymerization. Styrene (300 g, 2.88 mol) and SDS (2.14–5.36 g, 7.4–18.6 mmol) were dispersed in 3,000 mL of water and deoxygenated by purging with nitrogen for 1 hour. After that, KPS (2.50 g, 9.2 mmol) was dissolved in the aqueous solution, and the polymerization was initiated by heating to 70 °C under a nitrogen atmosphere. The obtained PS core particles (10.0 g) and Tris (12.1 g, 100 mmol) were dispersed in deionized water (980 mL), and DA (1.00 g, 5.37 mmol) in 20 mL of water was added for 20 min. The dispersion was stirred for 20 h at room temperature. The resulting MX particles were purified 5 times by centrifugation (12,000 rpm for 30–90 min) and redispersion.Preparation of MX@PNY particles.The MX particles (0.24 g), NIPAM (0.15–0.45 g, 1.33–3.99 mmol), and BIS (7.2–21.6 mg, 0.047–0.140 mmol) were dispersed in deionized water (120 mL), and the solution was purged with argon. After 15 min, the reaction mixture was heated to 65 °C, and an initiator solution (24 mg of APS dissolved in 3 mL of water) was added. After 5 h, the particles were separated and purified repeatedly by centrifugation (12,000 rpm for 30–90 min) and redispersion.The hydrodynamic diameter (Dh) and ζ-potential measurements.Dynamic light scattering (DLS) measurements were performed using Zetasizer Nano ZS (Malvern Panalytical). The Dh value of samples was measured using a dilute particle aqueous dispersion (0.01 wt%) and was calculated using the Stokes–Einstein equation with the diffusion coefficient obtained from the correlation function. The ζ-potential of the samples was measured in particle dispersions (0.01 wt%) with 5 mM NaCl solution. The electrokinetic phenomena of the particles were analyzed using Smoluchowski's equation.Laser microscope observations.The three-dimensional (3D) shape and height information of the pellet samples were analyzed using a laser microscope (VK-X200; Keyence). Pellet samples were obtained by dropping a water dispersion of particles (15 wt%, 2 μL) onto a glass substrate and drying it overnight in a humidity-controlled chamber (STU-1; as one) at a temperature of 25 °C and a humidity of 60 %.Time domain nuclear magnetic resonance (TD-NMR) measurements.The proton relaxation time of the samples was measured using a TD-NMR system (MagnoMeter XRS; Mageleka). The T2 relaxation time was determined using the CPMG method with an aqueous particle dispersion (3 wt%).Cryo-TEM measurements and image processing.An aqueous particle dispersion (5 wt%) was deposited onto a glow-discharged holey carbon TEM grid (R2/2 Cu, 200 mesh; Quantifoil). Excess solvent was removed using filter paper in a semi-automated vitrification plunger (EM GP2; Leica) under condition of 95% relative humidity and 25 °C. The sample was then rapidly vitrified in liquid ethane. Cryo-TEM measurements were conducted at temperatures below 100 K using a cryo-TEM system (CRYO ARM 300II system; JEOL) operated at an acceleration voltage of 300 kV. Images were acquired at a nominal magnification of ×40,000, corresponding to a pixel size of 1.24 Å, using a combination of SerialEM 44 and Digital Micrograph (Gatan). The acquired images were processed using RELION for contrast transfer function (CTF) correction and two-dimensional (2D) averaging. To measure the thickness of the formed gel layer, image contrast was adjusted using ImageJ (FIJI), and 120 points were randomly selected along the particle surfaces for measurement.Ultra-small-angle X-ray scattering (USAXS) measurements.USAXS measurements were performed at the beamline BL19B2 of SPring-8 under the following conditions: wavelength 0.7Å, exposure time 180 s, camera length 41 m. Samples placed in a 2 mm diameter borosilicate glass capillary (WJM-Glas) were measured using a sample transport automated system at room temperature. 45 As the scattering is isotropic with respect to all radial directions, two-dimensional images were circularly integrated using FIT2D in order to obtain one-dimensional (1D) scattering profiles.RESULTS AND DISCUSSIONPreparation and characterization of the particles.Four MX particles (X = 1–4) were prepared by PDA coating the surface of PS core particles with diameters of 187, 232, 260, and 170 nm (Figure S1). Monodispersed MX@PNY particles (Y = 1–3) were obtained by constructing a PN gel layer with different thicknesses on the surface of the MX particles. The synthesis conditions and characterizations of the resulting particles are summarized in Table 1. IR measurements revealed a broad peak at 3,200–3,500 cm-1 and an absorption band at 1,627 cm-1 due to the presence of amide groups, also indicating the construction of PN gel layers on the melanin particles (Figure S2). Under conditions where the amount of BIS was kept constant (3.5 mol% of NIPAM), the amount of NIPAM was varied to change the thickness of the PN gel layer. The PN gel layer in the prepared MX@PNY particles was not evaluated by TEM observation due to the low contrast between the melanin core and the PN gel layer. However, the diameter of dry state (Ddry) as measured in the low magnification TEM images were greater than those of the MX particles, suggesting the coating of the PN layers (Figure S3). To directly evaluate the PN gel layer in water, we conducted cryo-TEM observation of the M1 and M1@PNY particles (Figures 2 and S4a). While the cryo-TEM image of M1 particles without gel layer showed smooth particle surface, a swollen PN gel layer was clearly observed on the surface of the M1@PNY particles. The thickness of the gel layer was quantified by measuring 120 points each M1@PN1, M1@PN2, and M1@PN3. The average thicknesses were estimated as 3.9 ± 0.9 nm, 7.8 ± 1.8 nm, and 12.5 ± 2.9 nm, respectively (Figure S4b).Figure 2. Cryo-TEM images of (a) M1, (b) M1@PN1, (c) M1@PN2, and (d) M1@PN3 particles. White circles indicate contrast from the carbon support film. Magnified views of the boxed particles are shown to the right of each panel. Table 1. Characterization of the prepared particles. sample name NIPAM [g] Ddrya [nm] CVa [%] Dh 25°Cb [nm] PDIb Dh 45°Cb [nm] PDIb α25°Cc [–] α45°Cc [–] ζ-potentialsb [mV] M1 – 191 2.1 195 0.002 195 0.012 1.1 1.1 –46.4 M1@PN1 0.15 193 1.6 210 0.002 205 0.022 1.3 1.2 –32.4 M1@PN2 0.30 196 1.8 218 0.009 208 0.022 1.4 1.2 –20.0 M1@PN3 0.45 197 2.1 250 0.020 210 0.026 2.0 1.2 –10.3 M2 – 237 2.6 237 0.013 237 0.015 1.0 1.0 –55.1 M2@PN3 0.45 240 3.1 301 0.002 250 0.033 2.0 1.1 –9.8 M3 – 264 2.1 262 0.002 263 0.035 1.0 1.0 –52.4 M3@PN3 0.45 266 2.3 303 0.002 275 0.070 1.5 1.1 –29.4 M4 – 175 2.8 175 0.005 174 0.002 1.0 1.0 –56.8 M4@PN3 0.45 179 2.0 203 0.002 183 0.010 1.5 1.1 –18.1a Determined from TEM images of 50 particles. b Measured by DLS in water. c Calculated by eq (1).The hydrodynamic diameter (Dh) of the particles obtained was measured using DLS at 25 °C. The Dh 25°C values of the bare MX particles (X = 1–4) were not significantly different from the Ddry values. On the other hand, the Dh 25°C values for the MX@PNY particles were greater than the Ddry values, indicating that the PN layers had swollen in water. The swelling rate (αt°C) at t°C was calculated using the following eq (1): 46αt°C=Dh t°C 3Ddry3       (1)The α25°C, the swelling rate at 25 °C, gradually increased with an increasing amount of the PN layer coating. In particular, the α25°C was high at approximately 2.0 for M1@PN3 particles prepared with 0.45 g of NIPAM. PN is a well-known polymer that has a lower critical solution temperature (LCST). 47 In chemically cross-linked PN gels, a volume phase transition phenomenon occurs in which the gel volume shrinks discontinuously at a specific temperature, and this behavior corresponds to the LCST. 48 We thus performed DLS measurements while changing the temperature to measure the temperature dependence of Dh. As shown in Figure S5, the Dh of the M1@PN3 particles gradually decreased between 25 and 35 °C. A significant decrease was observed at 40 °C, and almost no change was observed above 45 °C. The Dh values were measured at 45 °C, where the particles shrink sufficiently, and α45°C values were calculated (Table 1). The α45°C of the MX@PNY particles decreased to approximately 1.2 regardless of the thickness of the PN layer, indicating that the PN layer shrank above the LCST. While the ζ-potentials of the MX@PNY particles decreased with increasing PN gel layer thickness, the particles were well dispersed, suggesting that they were dispersed by surface potential and steric stabilization.Drying patterns of the particle dispersions.Drying patterns of particle-dispersed droplets were prepared under conditions of 25 °C temperature and 60 % relative humidity, and particle assembly behavior was investigated. The prepared dispersions (15 wt%, 2 µL) were dropped and dried on a glass substrate. The pellet samples created from each particle exhibited vivid colors (Figure 3a). SEM measurements revealed a regular periodic arrangement of particles on the surface of the drying pattern (Figure S6). Furthermore, reflection measurements of the pattern surface showed a reflection spectra corresponding to blue, indicating that the vivid colors observed in the drying pattern are structural colors derived from colloidal crystals (Figure S7). 42 As shown in Figure 3a, the MX particles formed a distinct coffee ring pattern. 25, 26 In contrast, M1@PN3 particles with a thick PN gel layer reduced the coffee ring effect. The drying patterns of the M1@PN1 and M1@PN2 particles also showed this trend, indicating the gel layer was affected the particle assembly. Height information of the samples was analyzed by laser microscopic observations (Figures 3b and 3c). The 3D image and height distribution of the M1 dried pattern showed the presence of a distinct ring, approximately 100 μm in height, due to the coffee ring effect. The thickness at the center of the M1 pattern was almost negligible, suggesting the presence of only a few particles. Due to particle localization, color appeared only at the outer periphery of the pattern, resulting in an uneven structural color. Even in the drying pattern of M1@PNY (Y = 1–3), while the ring structure was still observed, the height of the center varied greatly. As the thickness of the PN layer increased, the height of the pattern at the center of the sample increased to approximately 30 to 50 μm. The drying behavior of particles with a thick PN gel layer was predominantly influenced by the PN gel layer. The thick shell layer was more dominant than the hard core in determining drying behavior, suggesting that it tends to make the drying pattern more uniform. It is noteworthy that the uniformity of structural color in the pattern improves as the thickness of the PN layer increases. When the thickness of the melanin particle assembly layer exceeds 10 μm, structural color can be clearly observed. 49 Sufficient assembly of particles in the center of the pattern contributed to the uniformity of the appearance of structural color. We conducted the same experiment using M2 and M3 particles with different core sizes and confirmed similar results (Figures 3a and S8). The subsequent experiments utilized particles with significantly different drying patterns: M1 and M1@PN3.Figure 3. (a) Optical microscope images of pellet samples of prepared particles obtained by dropping an aqueous dispersion (15 wt%, 2 µL) onto a glass substrate and drying overnight at 25 °C and 60 % relative humidity. (b) 3D images of M1 and M1@PNY (Y = 1–3) pellets obtained by laser microscopy. (c) Height information corresponding to the position of the sky-blue lines in the 3D image shown in (b).Drying patterns were prepared from M1 and M1@PN3 gel particle dispersions at 45 °C (Figure 4a). While the drying pattern of the M1 particles prepared at 45 °C was similar to that at 25 °C, a coffee-ring pattern was observed. An increase in drying temperature typically promotes particle entrapment near the surface due to the rapid decrease in the air/water interface, leading to relatively uniform deposition. 29, 30, 50 The number of particles in the center of the M1 dispersion patterns slightly increased. In contrast, the M1@PN3 particles displayed a different pattern than when they were dried at 25 °C. Interestingly, the patterns for the M1 and M1@PN3 particles at 45 °C were similar, showing a typical coffee-ring pattern. This is also clear from the 3D images and height distribution of the pellets (Figures 4b–4d). Deposition at the contact line was more dominant than interface capture. 51 As mentioned earlier, at temperatures above the LCST, the PN layer of M1@PN3 particles shrinks and becomes a hard colloidal particle, making adsorption to the air/water interface probably difficult. These results indicate that water-containing layers on particle surfaces are crucial for achieving uniform drying patterns. The proton spin‒spin relaxation time (T2) of aqueous solutions containing dispersed particles was measured using TD-NMR to enhance these discussions. The relaxation number Rno, which indicates the mobility of solvent molecules, is a dimensionless parameter defined by eq (2): 52-54Rno=T2wT2s-1        (2)where T2s and T2w are the spin‒spin relaxation times of protons in the suspension and its dispersing solvent (water), respectively. Water in contact with or adsorbed onto the particle surface reacts differently to magnetic field changes than bulk water in a free state. Therefore, the value of T2 becomes shorter as the number of water molecules constrained at the topmost surface of the particle or taken up in the surface layer increases, so the Rno value increases according to eq (2). While it is possible to interpret this situation as an increase in wettability (hydrophilic in this case), swelling and shrinkage of the surface layer were also observed in this experimental system. It may be too simplistic to view this merely as a change in “hydrophilicity.” More experiments are needed to address this, and such studies are underway. Figure 4e shows the Rno values for the M1 and M1@PN3 particles (particle concentration: 3 wt%). The values below the LCST of 25 °C were compared. The Rno value of the M1 particles, which was approximately 0.48, increased to approximately 0.62 for the M1@PN3 particles, indicating water molecules are constrained near the particle surface due to water penetration into the PN layer. At 45 °C, the Rno values for the M1 and M1@PN3 particles are nearly identical, at approximately 0.9. The PN layer shrinks at temperatures above the LCST, pushing out the water, and the Rno values of the two particles become almost the same. These results are consistent with the previously described swelling ratio measurements at 25 and 45 °C. The Rno values at 45 °C were greater than those at 25 °C. This is likely because the relaxation time increased due to various factors, including enhanced particle diffusion and reduced system viscosity, leading to an increase in the Rno values.Figure 4. (a) Photographs of pellet samples containing M1 and M1@PN3 particles. A dispersion (15 wt%, 2 µL) was dropped onto a glass substrate and dried at 45 °C and 60 % relative humidity. (b) 3D images of the prepared pellets shown in (a). (c) Height information corresponding to the position of the sky-blue lines in the 3D image shown in (b). (d) Rno values at 25 and 45 °C for the M1 and M1@PN3 particles.Visualization of the assembly behavior of the particles at the air/water interface.We observed the M1 and M1@PN3 particle dispersions using a digital microscope to investigate their drying behavior. Figure 5a shows that pristine M1 particles, lacking PN gel layers, assembled from the droplet edge and formed a typical coffee ring pattern because of hydrodynamic flow toward the droplet edge. 25, 26 Since the reflected wavelengths caused by colloidal crystal structures change with the center-to-center distance between the nearest particles, we evaluated the behavior of particle assemblies by observing changes in structural color. 55 Focusing on the droplet edge of the M1 particle dispersion, the structural color shifted from red to green to blue, revealing particle assembly from the three-phase contact line due to capillary flow. 56 To better visualize the behavior of the particles at the air/water interface, we examined the area around the contact line, the boundary between regions exhibiting structural color, and those boundaries that did not (Figure 5b, Movie S1). As water evaporation progresses, vivid green structural colors can be seen, indicating that the distance between the particles had decreased. 57 In areas where water remained, relatively large domains of approximately 400 μm² appeared, regions where particles had assembled. These macrodomains exhibited a red structural color, suggesting loose particle assemblies with a greater distance between particles. Notably, the macrodomains exhibiting red structural color moved dynamically at the air/water interface, with some collapsing due to redispersion into the water.Figure 5. (a) Time series photographs of the drying process of the M1 particle dispersion (particle concentration: 10 wt%, 2 µL). The white dotted circle in the photo is the light source of the microscope reflected in the image. (b) Digital micrographs of the droplet surface enclosed by the white square in (a). Compared with the M1 particles, the M1@PN3 particle dispersion exhibited different drying processes (Figure 6a, Movie S2). Structural colorations were first noted at the edge of the droplet, where the preferential solvent evaporates, similar to M1 particles. However, distinct differences were observed, particularly toward the end of the drying process. The particles adhere to the air/water interface of the droplet, forming numerous small domains of approximately 100 μm² that exhibit a vivid red structural color. Unlike M1 particles, distinct domain motion was not observed in the M1@PN3 particles (Figure 6b). The microdomains of M1@PN3 particles emerged from various locations on the interface and gradually became clearer (Figure 6c, Movie S3), whereas only a few macrodomains of M1 particles appeared near the contact line. Interestingly, the microdomains seemed to be adsorbed and stabilized at the interface. The microdomains of the adsorbed particles at the air/water interface remained fixed and dried in place without influence from capillary flow. At high concentrations, PN microgel suspensions are known to become viscoelasticity. 58 The soft contact of the PN network causes interparticle interactions in bulk water, 59 leading to a slight increase in solution viscosity, which may have affected particle mobility. In M1@PN3 particles, a slight coffee ring effect was still observed, but the adsorption of particle assemblies occurred across the entire air/water interface. As a result, a relatively uniform drying pattern was formed, ensuring the uniformity of the structural color appearance.Figure 6. (a) Optical microscope images of the changes in the M1@PN3 particle dispersion droplet (10 wt%, 2 µL) over time during the drying process. The white dotted circle in the photo is the light source of the microscope reflected in the image. (b) Assembly behavior of M1@PN3 particles on the droplet surface enclosed by the white square in (a). (c) Optical micrographs of microdomain adsorption at the air/water interface.In this system, the particle concentration gradually increases from 10 wt% as water evaporates, forming pellet samples. The formation of structural color domains, i.e., particle assemblies, likely occurs not only at the air/water interface but also in the bulk phase. We thus performed USAXS measurements of aqueous dispersions of M4 and M4@PN3 particles at particle concentrations of 15, 20, and 30 wt% to investigate the particle assembly behavior in the bulk phase during the water evaporation process. Figures 7a and 7b show the 1D USAXS intensity distributions of aqueous dispersions of M4 and M4@PN3 particles at different concentrations, respectively. The 1D USAXS profiles of M4 and M4@PN3 particles were dominated by the form factor, P(q), for sphere shape models, 60 regardless of particle concentration. Here, M4 particles were described using a hard sphere model with a particle radius of 89.4 nm (Figure 7a), whereas M4@PN3 particles were described using a core–shell sphere model with a core radius of 87.5 nm and a shell thickness of 32.1 nm (Figure 7b). In both cases, the radius measured by DLS was found to be approximately 10 % larger than the radius determined by USAXS (Table 1). This is probably because excess liquid is dragged along with the sphere. 61,62 M4 particles were observed clear peaks at a particle concentration of 30 wt%, and M4@PN3 particles showed at particle concentrations above 15 wt%. Each Bragg peak was identified assuming diffraction from a face-centered cubic (FCC) crystal. The differences in the concentrations at which peaks appeared were due to particle size, and the volume fraction of particles at each concentration at which peaks appeared was consistent at approximately 40 % (Table S1). These results indicate that both M4 and M4@PN3 particles form particle assembly structures when their volume fraction reaches approximately 40 % in water, meaning that they form colloidal crystal domains in bulk solution.Figure 7. USAXS profiles of aqueous dispersions of (a) M4 and (b) M4@PN3 particles. The numbers in the figure indicate the indices of the FCC crystal structure.Figures 8a and 8b show the possible assembly mechanisms for the MX and MX@PN3 particles, respectively, based on their drying patterns, structural color changes, and assembly behaviors of particles in the bulk phase. Since the solvent evaporates from the edge of the water droplets, the structural color resulting from the arrangement of MX particles is visible near the contact line. Continued evaporation of water gradually increases the concentration of particles in the dispersion, leading to the formation of relatively large macrodomains with loosely packed particles in the bulk phase. The transport of particles to the contact line by capillary flow is more dominant than adsorption at the air/water interface. Therefore, macrodomains move dynamically at the interface, and their pronounced movement is visualized by structural color. Eventually, most particles and the macrodomains they create are moved to the edge, producing the typical coffee ring pattern. Even with MX@PN3 particles, structural coloration occurs as the particles are assembled at the edge of the droplet during the early stages of solvent evaporation. The subsequent particle assembly behavior differs greatly. The formation and growth of the crystal nuclei of the MX@PN3 particles in the dispersion occurred at various locations, resulting in multiple microdomains. 63 Adsorption of the microdomains formed at the interface occurs because the total free energy decreases as the contact area between air and water is partially replaced by microdomains. 32 Two cases are expected for the appearance of structural colored microdomains at interfaces: assembly of particles into nuclei generated near the interface, and adsorption of microdomains generated in the bulk phase to the interface. Importantly, observation of structural colors revealed that the adsorbed microdomains remained nearly stationary, probably stabilizing the interface. Thus, the transport of particles to the contact line by capillary flow was suppressed. This characteristic effect prevents the assembly of particles and microdomains at the droplet edge, allowing for a uniform drying pattern.Figure 8. Proposed assembly processes for (a) MX and (b) MX@PN3 particles.The surface of the pellets was evaluated using SEM after the water had completely evaporated (Figure 9). Most of the particles on the pellet surface exhibited colloidal crystal structures, and many grain boundaries were observed (Figures 9a and 9b). Grain boundaries are narrow interfaces that separate adjacent regions with different crystal orientations in polycrystalline materials. The grain boundaries were identified by the colormap of 6 values using SEM images (Figures 9c and 9d). 64 6 is the angle formed between the x-axis and the nearest particles. The color map of 6 shows nearly the same color in identical grains; grain boundaries are characterized by coloration. The grain size on the surface of the M1 pellet was 20~ µm2. On the other hand, the surface of the M1@PN3 pellets contained many small grains, approximately 5 µm2 in size. The difference in grain size is probably due to the stability of the particles in water. Owing to the high ζ-potential of –46.3 mV, the repulsion between M1 particles is strong. Thus, the number of crystal nuclei is small, and each colloidal crystal grows large, resulting in a large grain size. The low ζ-potential (–10.3 mV) of the M1@PN3 particles, along with the presence of the PN gel layer, promoted the nucleation of the particles in water. 65 This phenomenon induces the formation and crystallization from various locations, resulting in small grain sizes. 66,67 The good correlation between the size of the domains observed at the air/water interface, which showed the red structural color, and the grain size observed after drying supports this explanation. Digital microscope images of the pellet surfaces also confirmed the presence of large grains in the M1 pellet and small grains in the M1@PN3 pellet (Figures 9e and 9f). The structural color uniformity of the M1@PN3 pellets was lower than that of the M1 pellets because of the presence of several stacking structures. Based on these results, we propose that the grain size in the dry state reflects the domain size observed at the air/water interface.Figure 9. SEM images of the surfaces of the (a) M1 and (b) M1@PN3 pellets. (c) and (d) SEM images of (a) and (b) with overlaid colormaps of the grain boundaries, respectively. Optical microscope images of the surfaces of the (e) M1 and (f) M1@PN3 pellets.Demonstration of uniform structural color coating. A particle dispersion (10 wt%, 40 µL) was dropped onto a hydrophilic glass substrate (10 mm × 10 mm). The structural color layer obtained by dropping and drying MX particles had areas where the color was not visible due to a lack of particles caused by the coffee ring effect and areas where vivid structural colors were observed (Figure 10a). On the other hand, the MX@PNY particles spread over the entire surface of the substrate, and bright structural color coatings formed (Figure 10b). A structurally colored coating with a uniform appearance was easily achieved by simply dropping the dispersion and drying it at room temperature. The reflection spectrum of the adjusted structural color coating substrate showed peaks at wavelengths corresponding to the apparent colors (Figure 10c). The reflectance for the MX coatings were high at approximately 50 %. The presence of the PN layer caused a change in the refractive index, reducing the reflectance of the MX@PN3 coating to approximately 20 % but maintaining the vivid structural color. The location dependence of the reflection spectra was measured to evaluate the unevenness of the structural color coating (Figure S9a). A broad peak was observed where the M2 particles were lacking on the substrate, indicating color irregularity due to the formation of the coffee ring pattern. In contrast, a location-independent structural color was observed in the reflection spectrum of the M2@PN3 particles (Figure S9b). These results also support the uniformity of the layer prepared with MX@PNY particles. Furthermore, the Δλ/λmax value, an indicator of the regularity of the colloidal crystals, was less than 0.15 in all the cases, indicating that high-quality colloidal crystals were obtained (Figure 10d). 68 While the optical micrographs of the samples revealed low color uniformity (Figure 9f), saturated colors were observed with the naked eye (Figure 10b). These results show that using particles with a gel layer on the surface effectively results in uniform structural color coatings without equipment.Figure 10. Photographs of structural color coatings with (a) MX and (b) MX@PNY particles formed on glass substrates. (c) Reflection spectra and (d) λmax and Δλ/λmax values for the samples shown in (a) and (b).CONCLUSIONThe adsorption of particle assemblies at the air/water interface, an important process for reducing a coffee ring pattern, has been successfully visualized through structural coloration under concentrated dispersion conditions. In the drying process, the structural color domains of the MX particles drastically moved. On the other hand, the MX@PNY particles were adsorbed and stabilized at the air/water interface during solvent evaporation. The adsorbed domain at the interface showed minimal movement as the solvent evaporated, the drying patterns with mitigated coffee ring effect were formed. This study presents a new perspective on the use of structural color to investigate the assembly behavior of particles in dispersions. Furthermore, we also provided a method of gel coating the particle surfaces, efficiently producing a uniform colloidal crystal structure simply by dropping the dispersions on a substrate. This method is likely to be widely applicable to materials other than melanin particles, such as metal nanoparticles. 69 Since the coffee ring effect should be prevented for applications such as inkjet printing, sensors, and coatings, this achievement is expected to be applicable to various fields.ASSOCIATED CONTENTSupporting InformationThe Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXX.· Additional experimental and characterization details including IR spectra, TEM images, cryo-TEM images, 3D images, SEM images, and micro-reflectance spectra of the particles created, as well as data on the temperature dependence of Dh (PDF). · Video of the area near the contact line during drying of the M1 particle dispersion: Movie S1 (MP4).· Video of the area near the contact line during the drying of the M1@PN3 particle dispersion: Movie S2 (MP4).· Video of the microdomain adsorption at the air/water interface of the M1@PN3 particle dispersion: Movie S3 (MP4).AUTHOR INFORMATIONCorresponding AuthorMichinari Kohri – Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan https://orcid.org/0000-0003-1118-5568; E-mail: kohri@faculty.chiba-u.jpAuthorYui Maejima – Department of Applied Chemistry and Biotechnology, Graduate School of Science and Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522, JapanAi Takabatake – MS Scientific Co. Ltd., 4-11-3, Kuramae, Taito-ku, Tokyo, 111-0051, JapanShin-ichi Takeda – Takeda Colloid Techno-Consulting Co., Ltd., 16-23, Toyotsu-cho, Suita, Osaka, 564-0051, JapanHiroshi Fudouzi – National Institute for Materials Science, 1-2-1, Sengen, Tsukuba-Shi, Ibaraki, 305-0047, Japanhttps://orcid.org/0000-0003-1442-4667Keiki Kishikawa – Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522, Japanhttps://orcid.org/0000-0002-7539-568XDaisuke Unabara – Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, 980-8577 JapanTasuku Hamaguchi – Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577 Japanhttps://orcid.org/0000-0003-0876-7700Koji Yonekura – Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577 Japan, Biostructural Mechanism Laboratory, RIKEN SPring-8 Center, 1-1-1 Koto, Sayo, Hyogo, 679-5148 Japanhttps://orcid.org/0000-0001-5520-4391Takuma Kureha – Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki 036-8561, Japanhttps://orcid.org/0000-0003-4680-6440Hideyuki Mitomo – Research Institute for Electronic Science, Hokkaido University, Kita 21, Nishi 10, Kita-Ku, Sapporo, Hokkaido 001-0021, Japanhttps://orcid.org/0000-0002-6748-7816Kanae Ito – Keiichi Osaka – Kiyoshi Kanie –Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577 Japan, International Center for Synchrotron Radiation Innovation Smart, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japanhttps://orcid.org/0000-0003-1477-6515NotesThe authors declare no competing financial interests.ACKNOWLEDGMENTSY. M. acknowledges the support of the Japan Science and Technology Agency (JST) Grant in Aid for SPRING (No. J24HJ00045). M. K. acknowledges the support of the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant in Aid for Scientific Research (B) (No. JP23H02018) and the Transformative Research Areas “Materials Science of Meso-Hierarchy” (No. JP24H01697). This work was supported by the National Institute for Materials Science (NIMS) Joint Hub Program (2024-042). The authors thank Professor Hiroyuki Ohshima of the Tokyo University of Science for his helpful discussions. The authors thank Hikaru Takahashi (Chiba University), Chikako Tsuda (NIMS), and Kaori Terui (NIMS) for their assistance with the experiments. This research was partially supported by the Research Support Project for Life Science and Drug Discovery (Basis for Supporting Innovative Drug Discovery and Life Science Research (BINDS)) from AMED under Grant No. JP24ama121006 and performed under the collaboration of the Advanced Imaging and Modeling Center for Soft-materials (AIMcS) of Tohoku University. This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices (MEXT) (Nos. 20251002 and 20255002). USAXS measurements were performed at BL19B2 of SPring-8 (proposal No. 2025A1728).REFERENCES(1) Vogel, N.; Utech, S.; England, G. T.; Shirman, T.; Phillips, K. R.; Koay, N.; Burgess, I. B.; Kolle, M.; Weitz, D. A.; Aizenberg, J. Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies. Proc. Natl. Acad. Sci. USA 2015, 112, 10845–10850. DOI: 10.1073/pnas.1506272112(2) Kolle, M.; Lee, S. 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DOI: 10.1021/acsami.5c02956Table of Contents1image3.jpegimage4.jpegimage5.jpegimage6.jpegimage7.jpegimage8.jpegimage9.jpegimage10.jpegimage11.jpegimage1.jpegimage2.jpeg