How Wide and High can Polyhedral Liquid Marbles be Fabricated?

Liquid marbles (LMs) are liquid droplets in gaseous phase, which are generally millimeter size, coated by solid particles. In this study, how wide and high LMs can be fabricated is investigated. Polyhedral LMs are fabricated using polymer plates with millimeter to meter sizes as a stabilizer and water as an inner liquid. Rectangular LMs with widths exceeding 1 m, the largest width ever reported, can be successfully fabricated. The height of the LMs is found to be subject to restriction by the hydrostatic pressure and cubic LMs with heights of up to 5 mm, which is the maximum height limit for LMs stabilized with nano/micrometer‐sized particles, are fabricated. Reduction of the hydrostatic pressure by changing the LM shape from cube to pyramid and introduction of particle‐stabilized bubble into the LM enabled the increase of height of the LM up to 9.8 mm, the highest height ever reported. Investigation using non‐aqueous liquids as an inner liquid confirmed that the longer the capillary length, the higher the maximum possible height of LMs. Finally, the polyhedral LMs are demonstrated to function as a microreactor for silver mirror reaction and chemical oxidative polymerization, resulting in the formation of Janus polymer plates.


Introduction
Liquid marbles (LMs) are millimeter-sized liquid droplets in a gaseous phase coated by solid particles. [1]The liquid core does not make contact with the supporting substrates where the LM is placed, due to the solid particle protective layer on its surface.Therefore, the inner liquid can be shielded from outside DOI: 10.1002/admi.2023010294][5][6][7][8][9][10][11][12][13] Furthermore, the LMs can be disintegrated by applying external stimuli, including pH, temperature, magnetic field, and mechanical force, resulting in the release of the internal liquid. [5]Due to these characteristics, research on LMs has yielded a number of intriguing applications, such as miniature reactors, [6,9,[14][15][16][17][18][19][20][21][22] sensors, [23][24][25][26][27] carriers of materials, [4,[28][29][30][31][32] microfluidics [33] and powdered pressure-sensitive adhesives. [34]espite the intriguing nature and applicability of LMs, there are still certain issues that must be resolved in order to increase the diversity of LMs as a useful system.The LMs are often stabilized with solid particle aggregates that have widesize distributions and ill-defined shapes.The particle layers on the LMs are opaque because the particle aggregates, whose sizes range from a few micrometers to a few hundred micrometers, scatter visible light, making it difficult to see the inner liquids clearly from the outside.The LMs generally have near-spherical and oblate shapes, both of which are thermodynamically stable in the presence of gravity, and shape designability of the LMs would be an important step toward the realization of functional systems.
We have previously shown that it is possible to fabricate transparent LMs with polyhedral shapes, including cubic and cuboid, by using millimeter-sized polymer plates as a stabilizer. [35,36]In addition, a rectangular LM with a length of more than 1 m has been successfully fabricated by joining multiple near-spherical polyhedral LMs.Here, some interesting questions arise: how large can LMs be fabricated in the planar direction using only six polymer plates without merging multiple LMs and what is the maximum achievable height of LMs?The heights of the LMs fabricated using water as an inner liquid so far are ≈ 3-5 mm, [34,37,38] which are comparable and/or a little bit higher compared to the capillary length of the water (≈2.7 mm), and LMs much higher than the capillary length of the internal liquid are susceptible to be formed.In this study, we investigated how large LMs can be fabricated in the planar and height directions using polymer plates as the LM stabilizer and water as the inner liquid.Additionally, we investigated how high LMs can be formed using nonaqueous liquids.Furthermore, we demonstrated the application of the polyhedral LMs as a microreactor for chemical reactions, resulting in the formation of Janus polymer plates.

Polymer Plates as an LM Stabilizer
Transparent poly(ethylene terephthalate) (PET) plates with square, rectangular, and equilateral triangle shapes were used as an LM stabilizer.All plates are monodispersed in shape and size, as confirmed by optical photography, stereo, and scanning electron microscopy (SEM) studies (Figure 1; Figures S1 and S2, Supporting Information).Square-shaped plates have side lengths (s) ranging from 1 to 100 mm and a thickness (t) of 55-71 μm, respectively.Square-shaped plates with a side length of 1000 mm have a thickness of 40 μm.The rectangular plates have a vertical length (s 1 ) of 2 mm, a width (s 2 ) of 2-1000 mm, and a thickness (t) of 40-72 μm.The equilateral triangle-shaped plates have side lengths (s) ranging from 2 to 16 mm and a thickness (t) of 57-73 μm, respectively (Table S1, Supporting Information).The original PET plates were relatively hydrophilic; a static contact angle of water was 78 ± 1°.Therefore, stable LMs could not be formed using the original plates, and only meta-stable LMs were formed, which could be disintegrated by the application of weak mechanical stress.To make the PET plates an effective LM stabilizer, hydrophobization of the plates was conducted with trichloro(1H,1H,2H,2H-heptadecafluorodecyl)silane. [35]or NeverWet NEO (Supporting Information).The static contact angles of water increased to 118 ± 3°(silane coupling agent) and 101 ± 4°(NeverWet NEO), indicating successful surface modification.Gravity dominates over the particle-particle attractive force for the millimeter-sized PET plates, therefore the PET plates did not form aggregates in the air phase.

How Large can LMs be Fabricated in Planar direction?
Generally, an LM can be fabricated by rolling a droplet on a dry particle powder bed and covering it with particles.Unfortunately, it is difficult to fabricate LMs larger than the bed size by using this method.Therefore, in this study, cuboid LMs were fabricated by adsorbing PET plates, as a stabilizer, to the droplet surfaces by using tweezers or fingers rather than moving the droplets on the particle bed.Specifically, a controlled amount of water was first added onto a square-shaped PET plate placed on the hydrophobic substrate, and the other square-shaped PET plate with the same size was adsorbed onto the top surface of the water droplet.Next, four rectangular PET plates with a vertical length of 2 mm and a width the same as the square-shaped PET plate were adsorbed onto the naked sides of the water droplet sandwiched between the two square-shaped PET plates.As a result, cuboid LMs with a height of 2 mm and widths from 5 to 1000 mm were successfully fabricated (Figure 2).To fabricate cuboid LMs, it was necessary to apply mechanical pressure to droplets sandwiched between two PET plates from the top PET plate using finger(s) to spread the water to the edge of the bottom PET plate before the side PET plates were adsorbed to the water.This is because the water contact angle on the hydrophobic PET plate was 118 ± 3°, and it is difficult for the water droplet to wet and spread on the surface of the PET plate, spontaneously.Noteworthily, the internal liquids did not leak out from the sides or corners of the LMs even when slight pressures were applied from the top surface of the LMs with a finger for all sizes of cuboid LM systems.To determine the pressure at which the LM was destroyed, various pressures were applied to the center of the top PET plate of the cuboid LM with a height of 2 mm and a width of 100 mm (Figure S3, Supporting Information).As a result, the cuboid LMs could survive up to the pressure of 33.0 Pa.From the above results, it is expected that the LMs can theoretically be as large as required in width, if the height does not exceed the capillary length of internal liquid.The LMs stabilized with the (milli)meter-sized PET plates showed excellent transparency (Figure 2).The air-PET plate and water-PET plate interfaces scatter visible light (refractive indices of air, water, and PET are 1.0003, [39] 1.33, [39] and 1.57. [40]) The (milli)metersized PET plates have smaller specific surface areas compared to the typical nano-and micrometer-sized LM stabilizers, thus less area is available for scattering of light and the transparency of the LM is high.Aqueous solution of Rhodamine B (20 μL, 0.5 wt%) was introduced from the corner of the 1000 mm-sized cuboid LM, and the LM was allowed to stand for 36 h to observe the diffusion of the dye in the LM.The dye diffusion was limited to an area within ≈100 mm from the position where the dye was introduced.This indicates that the internal liquid in the LM has low fluidity.
To observe the position of PET plates at the air-water interface, ethyl-2-cyanoacrylate (ECA) vapor treatment was applied to the 2 mm-sized cubic LMs, followed by observation of the LMs using a stereomicroscope and SEM after evaporation of internal water. [41,42]Slight space existed between the PET plates adsorbed on the water droplet, where a poly(ethyl-2-cyanoacrylate) (PECA) film was formed via anionic polymerization of ECA by contacting with bare air-water surface (Figure S4, Supporting Information).Cross-sectional stereomicroscope and SEM studies confirmed that the PECA film connected the two PET plates, and only one face of the PET plate was in contact with the internal liquid.The air-water-plate three-phase contact line was pinned at the edge of the plates, which explains why the LMs were stable on a supporting solid substrate: the plates on the LM surface created a gap between internal liquid and the substrate.Based on the adsorption position of the PET plates at the air-water interface, the adsorption energy of one square plate (ΔG) can be calculated using Equation (1). [35] where  WS is water-solid interfacial tension,  AS is air-solid interfacial tension and  AW is air-water interfacial tension.The adsorption energy of the 2 mm square PET plate was calculated to be −3.72 × 10 13 kT.The negative value of ∆G means that the PET plate is spontaneously adsorbed to the droplet.The adsorption energies of the surfactants at a molecular level are several kT -several tens of kT, indicating that the adsorption energies of the (milli)meter-sized PET plates are significantly high, resulting in irreversible adsorption and no flipping over on the droplet surface at room temperature.

How Large can LMs be Fabricated in Height Direction?
First, the relation between the internal liquid volume and the height of the LM was examined for the LMs stabilized by submicrometer-sized polytetrafluoroethylene (PTFE) particles, which have been widely used as commercially available LM stabilizer, [43][44][45][46] as a general model of LMs (Figure 3; Figure S5, Supporting Information).The LMs were fabricated by dropping water droplets (volume, 1-1331 μL) on the PTFE particle pow-  der bed placed on a surface-roughened perfluoroalkoxy alkane (PFA) petri dish, followed by rolling them on the bed (Figure S6, Supporting Information).When the internal liquid volumes were 1-51 μL, the LM height increased with an increase of internal liquid volume while keeping a spherical or near-spherical shape.The capillary length of water is ≈ 2.7 mm, and for LMs with a height equal to or lower than this length (1-51 μL internal liquid volume), the shape is (near)spherical because the effect of surface tension is more dominant than gravity.On the other hand, when the internal liquid volumes were above 64 μL, the LM started to deform to an oblate spheroid shape, and the height of the LM increased with an increase of the internal liquid volumes up to 517 μL.Compared to the spherical LM, the oblate spheroid LM showed a lower rate of increase in the height direction in relation to the increase in the internal liquid, and preferentially grows in the planar direction.Furthermore, when the internal liquid volumes were larger than 517 μL, the heights of the LM kept constant at 5.0 ± 0.1 mm, and the LM only became larger in the planar direction with increasing internal liquid volume.The change of the LM shape in relation to the increase in the internal liquid volume was similar to those in the previously reported studies using poly(vinylidene fluoride) particles, [47] polystyrene particles, [24] and poly(perfluoroalkyl ethyl acrylate) particles [38] as a stabilizer, and the maximum heights were almost the same.
Next, the relationship between the internal liquid volume and the height of the LM was investigated in the cubic LMs system.Cubic LM was fabricated by dropping a water droplet on one PET plate placed on a hydrophobic supporting substrate and then adsorbing the other five PET plates on the side and the top of the droplet, in this order.As a result, cubic LMs with one side length between 1 and 5 mm could be fabricated (Figure 4a).The cubic LM with one side length of 2 mm could slide and roll on the solid substrate (Movies S1 and S2, Supporting Information), suggesting high handling performance.On the other hand, the internal liquid leaked out from the bottom of the LM in the cases of LMs with side lengths of ≥ 6 mm (Figure 4a).These results indicate that both the cubic LM and the conventional LM exhibit a maximum height of ≈ 5 mm.The height limit of the LM should be due to the capillary length of water (2.7 mm): the effect of gravity becomes dominant compared to the surface tension of the droplet above the capillary length.In the cubic LM system, when the height of the LM exceeds the capillary length, the effect of gravity increases, and the hydrostatic pressure on the bottom of the LM increases, leading to leakage of water from the bottom of the LM.Therefore, the relationship between the hydrostatic pressure on the bottom PET plate of a cubic LM and the formability of the LM was investigated.Hydrostatic pressure (Pa) was calculated based on Equation (2) (Table 1).
where V is volume of internal liquid,  is density of internal liquid, g is gravitational acceleration and S is area of large face of the PET plate.The hydrostatic pressure for the 5 mm-sized cubic LM, which enabled the fabrication of a stable LM, was 48.9 Pa, and that for the 6 mm system, which showed leakage of the internal liquid, was calculated to be 58.7 Pa.
In order to reduce the hydrostatic pressure, a particlestabilized bubble was introduced into the internal liquid of a cubic LM, and the apparent density of the internal liquid was decreased.(Note that no LM could be prepared using an aqueous bubble stabilized with molecular surfactant, namely sodium dodecyl sulfate because the aqueous solution of surfactant wets the PET plates.)The introduction of bubble (diameter, 2.1 mm; volume, 5 μL), stabilized by polystyrene (PS) particles (diameter, 40 μm) adsorbed at the air-liquid surface, into the internal liquid of the LM led to an increase in the maximum height of the cubic LM (bubble) up to 6 mm (Figures 3 and 5a-d).In the 6 mm-sized LM system, the hydrostatic pressure decreased from 58.7 to 57.3 Pa by the introduction of the particle-stabilized bubble.Therefore, the lower hydrostatic pressure suppressed the leakage of the internal liquid from the bottom of the LM, allowing the fabrication of a higher LM.As a control experiment, when the particle-stabilized bubble was introduced into the spherical LM stabilized with PTFE particles, the height of the LM increased from 5.0 to 5.3 mm (Figure 3).The height of the LM stabilized with PET plates increased 1.2 times from 5 to 6 mm when the bubble was introduced into the LM, whereas the height of the LM stabilized with PTFE particles increased 1.06 times and the rate of increase in height was lower.In the PET plate system, the entire PET plate adsorbed on the top area of the LM was lifted up by the buoyancy force.On the other hand, in the PTFE particle system, only the PTFE particles at the top of the bubble are lifted locally.Therefore, the spherical LM could not grow to the height direction more efficiently than the cubic one.
Changing the shape of the LM from cube to pyramid is another method to increase the maximum height of the LM: the hydrostatic pressure was lowered by decreasing the internal  liquid volume while keeping the bottom area constant.Recently, the pyramid structure has also been observed in particlestabilized emulsion systems. [48]The pyramidal LM was fabricated by adsorbing a square PET plate with a side length of 9 to 14 mm on the bottom of the water droplet and four equilateral triangle PET plates with a side length equal to that of the square PET plate on the side of the droplet.As a result, well-defined stable LMs with a side length of up to 12 mm and a height of up to 7.9 mm could be fabricated (Figures 3 and 4b).The pyramidal LM with one side length of 2 mm could slide and roll on the solid substrate (Movies S3 and S4, Supporting Information).In the case of the pyramidal LM with a side length of 13 mm, one side PET plate slipped and shifted, causing the shape to collapse partially, but the internal liquid did not leak out and the height of the LM was maintained at 8.2 ± 0.3 mm.On the other hand, in the case of a side length larger than 14 mm on one side, the internal liquid leaked out from the bottom of the LMs.Hydrostatic pressures calculated for the pyramidal LMs of 13 mm on one side without leakage and 14 mm on one side which showed leakage were 27.7 and 32.3 Pa, respectively (Table 2).
The hydrostatic pressures on the PET plate at the bottom of the cubic LM and pyramidal LM were compared at the volumes at which the internal liquid leaked out.The hydrostatic pressures of the cubic LM (6 mm on one side) and pyramidal LM (14 mm on one side) were 58.7 and 32.3 Pa, respectively, indicating that the pyramidal LM collapsed at lower hydrostatic pressure than the cubic one.The reason for this should be the difference in contacting configuration between the PET plates adsorbed on the sides of the LM and the supporting substrate where the LMs were placed.The PET plates adsorbed on the sides of a cubic LM contact their side (thickness) areas with the supporting substrate in a plane manner (2D), whereas the PET plates adsorbed on the sides of a pyramidal LM contact their edges with the substrate in a line manner (1D).Therefore, when the side PET plate of the LM slides down by hydrostatic pressure, the friction between the PET plate and the substrate is lower in the pyramidal LM system than in the cubic LM system, and the pyramidal LM should collapse at lower hydrostatic pressure.The correlation between the thickness of PET plate and the formability of cubic and pyramidal LMs was investigated (Figures S7-S13, Supporting Infor-mation).In the cubic LM system, the maximum heights of LMs were 4, 5, 6, and 6 mm for the PET plates with thicknesses of 16, 38, 75, and 125 μm, respectively, and tended to increase as the plate thickness increased.On the other hand, in the pyramidal LM system, the maximum height of the LM was constant at ≈ 7.8 mm for all thicknesses (Figure S13, Supporting Information).The number of adjacent PET plates per one PET plate should also play some role for formability/stability of the LMs; one plate can contact four and three plates in the cases of cubic and pyramidal LMs.The plate contacting larger numbers of plates should experience higher friction.
From the above results, it was shown that the change of the LM shape from cube to pyramid and the introduction of particlestabilized bubbles into the internal liquid is effective in fabricating higher LMs.The introduction of particle-stabilized bubbles into the pyramidal LM led to the formation of pyramidal LMs (bubbles) with a side length of 14 mm and heights up to 9.8 mm (Figure 3).In the case of the 15 mm pyramidal LM (bubble), the height of the LM was 9.1 ± 0.1 mm, which was lower than that of the 14 mm-sized LM (Figure 5e-h), although the internal liquid did not leak out.This is due to the partial collapse of the shape of the LM caused by the slip and displacement of the PET plates adsorbed on the side.The hydrostatic pressures for the pyramidal LMs with a side length of 14 mm before and after the introduction of particle-stabilized bubbles were 32.2 and 32.0 Pa (Tables 1  and 2).These results indicate that a combination of the introduction of bubbles and the shape change of the LMs can lead to the formation of LMs with heights that greatly exceed the capillary length of water.
Mechanical integrity of LMs was evaluated based on drop fall test.The LMs with cubic and pyramidal shapes were dropped from prefixed heights varied between 5 and 100 mm to the solid substrate (PFA petri dish) (Figure S14, Supporting Information).The outcomes of this test were classified as "survived", if the LM was able to keep its 3D shape without water leakage and as "destroyed", if the water leaked from the LM.The cubic and pyramidal LMs with one side length of 2 mm could survived up to 5 cm height but were destroyed at and above 7.5 cm.The cubic LMs with one side length of 5 mm were destroyed at and above 0.5 cm.These results indicated that the larger the LMs, the weaker the mechanical integrity.The gap distance between polymer plates adsorbed at the water droplet surface (bare airwater interface) also affects the mechanical integrity (Figure S15, Supporting Information).This gap distance could increase by increasing the volume of inner water droplet, and the mechanical integrity of the LMs decreased if the gap distance increased.The cubic LMs stabilized with 2 mm PET plates (inner liquid, 15 μL), which had longer gap distances, were destroyed at and above 5 mm, although those (inner liquid, 8 μL), which had shorter gap distances, could survived up to 5 cm.This should be due to the increased possibility for the inner liquid to contact with the substrate.Evaporation rate of water from the LMs was evaluated gravimetrically at 25, 26 °C and a relative humidity of 18-21%, and was compared with that of bare water droplet (the same water volume with the LM) placed on superhydrophobic substrate [35] (Figure S16, Supporting Information).The evaporation rate determined for the cubic LMs with one side length of 2 mm (1.3 × 10 -6 g s −1 ) was lower than that determined for the bare water droplet (2.3 × 10 -6 g s −1 ).This is because the LM was covered by the plates and the area of bare air-water interface, where water can evaporate, is smaller than that of the bare water droplet, which accorded with previous studies. [43,49,50]We also studied the morphological changes of the LMs during evaporation of the inner liquid (Figure S17, Supporting Information).Optical photography studies confirmed that no/one or two plates on the water droplet moved inside the cubic LM with keeping the 3D structure during the evaporation of water.It is interesting to note that the LMs sometimes could keep 3D structure even after complete evaporation of water.These results were strikingly different from those observed for the general LMs stabilized with nano-/micrometersized particles. [51,52]wrinkles appeared on the surface of the LMs during the evaporation of water because the volume of the LM decreased while the surface area remained constant, and became "flatten ball" morphology after complete evaporation of water.

How High can Cubic LMs be Fabricated using Non-Aqueous Liquid as the Internal Liquid?
Liquids with surface tensions between 72.5 and 26.1 mNm −1 and densities between 3.32 and 0.76 kgm −3 were utilized to fabri-cate cubic LMs (Table 3).When glycerol, ethylene glycol, benzyl benzoate, and n-hexadecane, which have densities similar to water, were used as the internal liquid, cubic LMs with maximum heights of 4, 3, 2, and 1 mm could be fabricated, respectively.In the case of n-tetradecane as the internal liquid for the 1 mm size system, the liquid wetted the PET plate and LM could not be fabricated.These results clearly indicate that the lower the surface tension value (in other words, the lower the capillary length), the lower the maximum height of LMs that can be fabricated, for liquids with similar values of the density.Here, the capillary length ( −1 ) was calculated using Equation (3).
where  is density of internal liquid, g is gravitational acceleration and  is surface tension of internal liquid.Furthermore, the maximum height of LMs were investigated using liquids with similar surface tension values and different densities (Table 3).
When ethylene glycol was used as the internal liquid, LMs with heights of up to 3 mm were fabricated.In the case of 1,2,3tribromoethane, which has a higher density than ethylene glycol, the maximum height was 2 mm, and the maximum height of the LM was 1 mm for diiodomethane, which has an even higher density.These results indicated that the maximum height of the LMs decreased as the density increased.This is rationalized by the reduction in capillary length as the density of the liquid increases and the droplet is more strongly affected by gravity force.

Polyhedral LMs as a Microreactor
Droplet-based reaction vessels are anticipated to contribute to the advancement of green and sustainable chemistry as the scaledown of reaction vessels realizes a large reduction in the amount of reagents and reaction time.LMs are simple to fabricate and handle, therefore, they exhibit a promising potential for applications in the microreactor research field. [9,53]Here, we demonstrate how the polyhedral LMs could be used as microreactors for chemical reactions, namely silver mirror reaction and chemical oxidative polymerization.For the silver mirror reaction, cubic and pyramidal LMs were fabricated using a mixture of aqueous solution of ammonia and silver nitrate (surface tension value: 67.2 mNm −1 ) and aqueous solution of glucose (surface tension value: 73.4 mNm −1 ) as the internal liquid.The LMs were placed in a thermostatic chamber at 60°C for 20 min, resulting in formation of silver-colored LMs (Figure 6a-f).When a piece of paper with the letter "LM" printed on it was placed next to the LM, the letter "LM" was projected on the surface of the LM (Figure 6c,f).SEM, energy dispersive X-ray (EDX), and X-ray photoelectron spectroscope (XPS) studies confirmed that silver films with a number-average thickness of 0.56 ± 0.27 μm were formed only on one large surface of the PET plates which is in contact with the liquid phase, indicating the formation of metal-polymer Janus plates (Figure 6h-m; Figure S18-S21, Supporting Information).Similarly, chemical oxidative polymerizations could be conducted in the LMs.Chemical oxidative polymerization of pyrrole was carried out for 1 h in the 2 mm-sized cubic LM containing a mixture of aqueous solution of pyrrole and iron nitrate as an internal liquid.As the polymerization proceeded, the blackness of the LM increased, indicating the production of polypyrrole (PPy) (Figure 7a,b).Light scattering at the surface of LM could be significantly reduced compared to those fabricated using solid particles with dimensions of tens of nanometers to micrometers because the polyhedral LMs were stabilized with a millimetersized plate-shaped stabilizer.The internal liquid of the polyhedral LM could thus be observed with ease.After the polymerization, six PET plates adsorbed on the LM became black, and SEM observation indicated that submicrometer-sized roughness derived from PPy particles was introduced onto the PPy-coated surface (Figure 7g).EDX and XPS studies revealed that only one large surface of the PET plate in contact with the liquid phase was covered with PPy film with an average thickness of 0.13 ± 0.01 μm (Figure 7d-f,g,i; Figure S22, Supporting Information).On the other hand, the surface facing the air phase and not coated with PPy film, was smooth (Figure 7h).Similar to the PPy system, chemical oxidative polymerization of aniline in a cubic LM also resulted in the synthesis of a PET-polyaniline (PANI) plate with a 0.59 ± 0.14 μm thickness PANI film on only one surface that was in contact with the liquid phase (Figures S23 and S24, Supporting Information).These results indicated that the polyhedral LMs can be utilized as a microreactor and that position-selective introduction of functional materials onto the plate could be realized; the air-liquid interface on the LM surface functions as a field for introducing anisotropy, leading to the fabrication of Janus-type materials.The LMs stabilized by nanoparticle monolayer, called liquid plasticines, [8,[54][55][56][57] can also work as a transparent microreactor because of low visible light scattering at the surface of the LMs.The polyhedral LMs studied in this study have an advantage to be able to fabricate well-defined Janus-type materials with millimeter size, which is impossible using the liquid plasticines.

Conclusion
In conclusion, cuboid LMs with a height of 2 mm and side lengths from millimeters to meter size could be fabricated by adsorbing six hydrophobic PET plates (square and rectangle shapes) on the water droplet surface.It was suggested that the LM can be fabricated as large as possible in the planar direction if the height does not exceed the capillary length of the internal liquid (water: 2.7 mm). Cubic LMs with heights up to 5 mm, comparable to the capillary length of water, could be fabricated by adsorbing six square-shaped PET plates on the water droplets.By changing the shape of the LM from a cube to a pyramid, and reducing the hydrostatic pressure on the bottom plate of the LM, LMs with heights of up to 7.9 mm could be fabricated.Moreover, the maximum height of the LM can be increased up to 9.8 mm by introducing a particle-stabilized bubble into the internal liquid of the pyramidal LM.Furthermore, it is possible to use nonaqueous solvents as the internal liquid, and it was confirmed that the higher the surface tension value and the lower the density of the liquid (in other words, the longer the capillary length), the higher the maximum height of the LMs that can be fabricated.Moreover, the polyhedral LMs were demonstrated to work as a microreactor for silver mirror reaction and chemical oxidative polymerizations to synthesize Janus-type plates.Internal liquids of the LMs could be observed clearly thanks to the low light scattering of the plate stabilizer.][60][61][62][63] The LMs fabricated in this study are expected to be of great interest not only for microreactors but also for gas sensors and other applications [55,64] because of the ease of observation and handling of the internal liquid.
Preparation of Polymer Plates: Transparent PET film (pristine thickness, 38 ± 1 μm; Lumirror38T60, Panac Co., Ltd.) was cut into square, triangle or rectangle shape using a cutting machine (SDX1200, CNZ0531, Brother Industries., Ltd.) fitted with thin fabric auto blade (Brother Industries, Ltd.) and replacement blade for automatic adjustment (CADXBLDQ1, Brother Industries, Ltd.).The transparent PET film was adhered onto the low tack adhesive mat (Brother ScanNCut SDX Low tack adhesive mat CADXMATLOW12, Brother Industries, Ltd.), and was cut into the shape designed using a software (Brother CanvasWorkspace).After cutting out, the PET film was peeled off from the low tack adhesive mat using a spatula.The low tack adhesive mat with PET plates was placed in the glass bottle (5-128-25, As One Co.) containing 300 mL toluene, and the PET plates were then peeled off from the low tack adhesive mat by sonicating in an ultrasonic bath (Branson Co.) for 30 min.The PET plates were transferred to a 50 mL laboran screw tube bottle (9-952-09, As One Co.) or 450 mL glass bottle (5-128-04, As One Co.), and then were washed three times with ethanol.Then, the plates were placed in a fume hood to remove ethanol for one day, followed by drying in a reduced-pressure dryer for one day.Square PET plates with side lengths of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 20, 50 and 100 mm, equilateral triangle PET plates with side lengths of 2, 4, 9, 10, 11, 12,13, 14, 15 and 16 mm and rectangle PET plates with vertical length of 2 mm, the widths of 5, 10, 20, 50 and 100 mm were prepared.Square and triangle-shaped PET plates with various thicknesses were prepared using PET films (Lumirror6CF53, 5 ± 1 μm; Lumirror75T60, 77 ± 3 μm and Lumirror125T60, 123 ± 3 μm).PET film (thickness, 40 ± 1 μm; LumirrorT60 3-2159-02, As One Co.) was cut into a rectangular shapes with a vertical length of 2 mm and width of 1 m, and square shape with 1 m on each side, using a utility knife.
Hydrophobization of Polymer Plates: Hydrophobization was conducted using a hydrophobic silane coupling agent beside square-shaped plates with side lengths of 100 and 1000 mm, as reported previously. [35]The pristine PET plates were sonicated in ethanol for 10 min and the supernatant was discarded.This cycle was repeated 3 or 4 times.The washed PET plates were first dried in an ambient atmosphere for 24 h and then left under a vacuum overnight.The dried PET plates (plate used as stabilizer for cubic LM: 10 g, plate used as stabilizer for rectangular LM: ≈ 10 plates) were dispersed in n-hexane (40 mL), followed by the addition of trichloro(1H,1H,2H,2H-heptadecafluorodecyl)silane (40 μL).During the reaction time of 30 min, the dispersion was shaken every 5 min for ≈ 20 sec to ensure homogeneous surface modification of the PET plates.Subsequently, the modified PET plates were rinsed three times with n-hexane.The hydrophobized PET plates were first dried in the atmosphere for 24 h and then left under vacuum overnight.Hydrophobization of the squareshaped plates with side lengths of 100 and 1000 mm were conducted using NeverWet NEO.Observation of PET plates were conducted using a digital camera (Tough TG-6, Olympus Co.), stereomicroscope (STZ-161-TLED-1080M, Shimadzu Co.), and a scanning electron microscope (SEM, Keyence Co., VE-8800, 12 kV).
Fabrication of Particle-Stabilized Bubble: PNVP (1.50 g) was added to deionized water (50 g) containing PS particles (40 μm in diameter, 36.5 g) and stirred for 6 days using a magnetic stirrer.Then, the PS particles with PNVP adsorbed on their surfaces were obtained by removing free PNVP dissolved in the water by media displacement with deionized water.Next, the PS particles with PNVP adsorbed on their surfaces were introduced into deionized water and kept stationary for ≈ 30 min to prepare a particle sedimentation layer at the bottom of the glass bottle.When an air bubble (5 μL) was introduced into the sedimentation layer using a syringe needle, a bubble covered by particles adsorbed on the air-water interface floated up from the sedimentation layer to the surface of the water.During floatation of the bubble, excess particles that were not adsorbed on the air-water interface were removed and settled down at the bottom of the glass bottle.As a result, a bubble (2.1 mm in diameter) stabilized with a monolayer of particles adsorbed at the air-water interface was formed (Figure 5a inset).This particle-stabilized bubble was stable and did not disrupt at least for 3 h in the internal liquid of LM.
Fabrication of LMs Containing an Air Bubble Stabilized with Polymer Particles: Cubic LMs: Controlled amount of water (≈5 μL less than the final volume of the LM) was first added onto a square-shaped PET plate placed on the hydrophobic substrate.Next, four square-shaped PET plates with a width the same as the bottom square-shaped PET plate were adsorbed onto the naked sides of the water droplet.Then, the PS particle-stabilized bubble (5 μL) was introduced from the top surface area (bare air-water interface without PET plate) using a spatula.Finally, the top surface area was covered with a square-shaped PET plate.
Pyramidal LMs: Controlled amount of water (≈5 μL less than the final volume of the LM) was first added onto a square-shaped PET plate placed on the hydrophobic substrate.Next, the PS particle-stabilized bubble (5 μL) was introduced to the water droplet using a spatula.Then, four equilateral triangle-shaped PET plates with a width the same as the bottom square-shaped PET plate were adsorbed onto the naked sides of the water droplet.
Silver Mirror Reaction in LM: An aqueous ammoniacal solution of silver nitrate was prepared by adding an aqueous solution of ammonia (250 μL, 28.0 wt%) to an aqueous solution of silver nitrate (deionized water 10 g, silver(I) nitrate 0.2 g).Aqueous solution of glucose (deionized water 1.0 g, D(+)-glucose 0.3 g) was prepared.Cubic LMs with side lengths of 2 and 4 mm and pyramidal LMs with side lengths of 4 and 12 mm were fabricated using a mixture of ammoniacal silver nitrate solution (surface tension value: 67.2 mNm −1 , du Noüy method) and aqueous solution of glucose (surface tension value: 73.4 mNm −1 , du Noüy method) with a volume ratio of 3:1 as the internal liquid.Next, the LMs were placed in a thermostatic chamber at 60°C for 20 min.PET-Ag composite plates obtained after the silver mirror reaction were washed by dipping them in water and swaying slowly, and then dried in the air atmosphere for 24 h.The PET-Ag composite plates were observed using a stereomicroscope, EDX (Hitachi High-Tech Co., Ltd.) without Au coating, and SEM observation was carried out after Au coating.The Ag film was partially peeled off from the plate by scraping with the edge of a spatula, and the thickness of the Ag film was measured by SEM observation of the cross-section of the film.
Chemical Oxidative Polymerization in LM: Cubic LM with side length of 2 mm was fabricated using a mixture of Py solution (deionized water 5.0 g, Py 0.05 g) and Fe(NO 3 ) 3 solution (deionized water 5.0 g, Fe(NO 3 ) 3 •9H 2 O 0.7 g) at a volume ratio of 1:1 as the internal liquid.The LM was placed at room temperature (22.3°C) for 1 h to carry out the chemical oxidative polymerization of Py.The resulting PET-PPy composite plates were washed by dipping them in water and swaying slowly and then dried in the air atmosphere for 24 h.The chemical oxidative polymerization of ANI was conducted in the same manner using the mixture of APS-HCl solution (deionized water 5.0 g, HCl 100 μL 0.5 molL −1 , APS 0.3 g) and ANI-HCl solution (deionized water 5.0 g, HCl 100 μL 0.5 molL −1 , ANI 0.1 g) at a volume ratio of 1:1 as the internal liquid.

Figure 1 .
Figure 1.a) Optical microphotographs and b) schematic diagrams of typical PET plates prepared using a cutting plotter.(i) Square, (ii) rectangular, and (iii) equilateral triangle shapes.

Figure 2 .
Figure 2. Optical photographs of cuboid LMs.The LMs were fabricated using two square PET plates (top and bottom) and four rectangular PET plates (side).Size in the plane direction: a) 5, b) 10, c) 20, d) 50, e) 100, and f) 1000 mm.Height: 2 mm.An inset of Figure 2a is an optical photograph of the LM taken from diagonally above.

Figure 4 .
Figure 4. Optical photographs of a) cubic LMs stabilized with square PET plates with a side length of (i) 1, (ii) 2, (iii) 3, (iv) 4, (v) 5, (vi) 6, (vii) 7 and (viii) 8 mm.(i-v) LMs maintained their cubic shape.(vi-viii) The Cubic shape was broken and the internal liquid leaked out.b) Pyramidal LMs stabilized with one square and four equilateral triangle PET plates with a side length of (i) 9, (ii) 10, (iii) 11, (iv) 12, (v) 13, and (vi) 14 mm.(v) Shape deviated from pyramid but the internal liquid did not leak out.(vi) The pyramidal shape was broken and the internal liquid leaked out.

Figure 5 .
Figure 5. Optical photographs of LMs containing a particle-stabilized bubble (5 μL).The side lengths of cubic LMs are a) 4, b) 5, c) 6, and d) 7 mm.The side lengths of pyramidal LMs are e) 13, f) 14, g) 15, and h) 16 mm.g) Shape deviated from the pyramid but the internal liquid did not leak out.d,h) LM shapes were broken and internal liquids leaked out.

Figure 6 .
Figure 6.Silver mirror reaction in polyhedral LMs.Optical photographs of silver mirror reaction in (a-c) 4 mm-sized cubic and (d-f) 4 mm-sized pyramidal LMs, a,d) immediately and b,c,e,f) 1 h after the start of the reaction.g) Schematic representing PET-Ag Janus plate showing the direction of observation.h) Stereomicroscopy, i,k-m) SEM and j) EDX images of PET-Ag Janus plates: k) Ag-coated surface, l) PET surface, m) Ag layer cross-section.

Figure 7 .
Figure 7.Chemical oxidative polymerization of pyrrole in 2 mm-sized cubic LM.Optical photographs of chemical oxidative polymerization a) immediately and b) 1 h after the start of polymerization.c) Schematic representing PET-PPy Janus plate showing the direction of observation.d) Stereomicroscopy, e, g-i) SEM and f) EDX images of PET-PPy Janus plates: g) PPy-coated surface, h) PET surface, i) PPy layer cross-section.

Table 1 .
The volume of internal liquid, height, and hydrostatic pressure on the bottom PET plate of the cubic LMs with and without a particle-stabilized bubble in a liquid phase.

Table 2 .
The volume of internal liquid, height, and hydrostatic pressure on the bottom PET plate of the pyramidal LMs with and without a particle-stabilized bubble in the liquid phase.Shape deviated from the pyramid but internal liquid did not leak out; b) LM shapes were broken and internal liquids leaked out.

Table 3 .
Internal liquids for cubic LMs and their surface tension, density, capillary length, maximum height of LM, and hydrostatic pressure at maximum height LM.