Wetting of Inkjet Polymer Droplets on Porous Alumina Substrates

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Wetting of inkjet polymer droplets on porous alumina substrates Haihua Zhou, Rui Chang, Elsa Reichmanis, and Yanlin Song Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03820 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 14, 2016

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Wetting of inkjet polymer droplets on porous alumina substrates Haihua Zhou1, 2, Rui Chang3, Elsa Reichmanis3, 4, 5, Yanlin Song1, 2 1

Key laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing

100190, China 2

Beijing National Laboratory for Molecular Science (BNLMS), Beijing 100190, China

3

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta,

GA 30332-0100, USA 4

School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-

0400, USA 5

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA

30332-0245, USA ABSTRACT The resolution of inkjet printing technology is determined by wetting and evaporation processes after the jet drop contacts the substrate. Here, the wetting of different pico-liter solubilized polymer droplets jetting onto one-end-closed porous alumina was investigated. The selected polymers are commonly used in inkjet ink. The synergistic effects of the hierarchical structure and substrate surface modification were used to control the behavior of polymer-based

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ink drops. A model which invokes the effect of surface tension was applied to calculate the amount of polymer solution penetrating into the pores. The calculation corroborates experimental observations and shows that the volume of polymer solution in the pores increases with an increase in pore radius and depth, resulting in less solution remaining on the substrate surface. The structure of the porous substrate, coupled with intrinsic polymer properties and surface modifications all contribute to the resolution that can be achieved via inkjet printing.

INTRODUCTION

Since the middle of the 20th century, inkjet printing has been gaining attraction for device fabrication in many areas including electronics, biosensors, micro fluidic devices, and high performance optical devices

[1-6]

, all of which require high resolution. The resolution of inkjet

printing technology is determined by the wetting and evaporation processes that occur when an ink droplet is jetted onto a solid surface. Several studies associated with controlling these three processes have been conducted [7-12]. Porous substrates are commonly used in inkjet printing such as paper and metal. The following factors will influence the wetting behavior: the intrinsic properties of the ink, the chemical contents and physical structure of the substrates. Many researches concerning the interactions between water and porous substrates have been conducted to control the wetting behavior

[13-17]

. For instance, Kalakkath et al. showed that the penetration

of a ceramic ink on a porous ceramic substrate (Al2O3/SiC) decreased as ink viscosity increased. The investigation showed that nozzle geometry, applied pressure, solution viscosity and surface tension of a ceramic ink can affect satellite drop formation as well as the resolution. Singh et al. found that the distribution of nano-pores on the alumina surface influences both equilibrium contact angle and evaporation dynamics of a water droplet

[18]

. Ran et al.

[19]

demonstrated that


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the wettability of water droplets on porous alumina changed dramatically from hydrophilic to hydrophobic upon increasing the pore diameter, while maintaining the pore interval and depth. Furthermore, transition between Wenzel and Cassie-Baxter states was achieved by increasing pore depth. Li et al. analyzed the phenomena from Ran’s work and found the transition between the Wenzel and Cassie-Baxter states originated from synergisms between capillary force, gravity and counter force from compressed air trapped within the pore

[20]

. These studies reported that

the porous structure has significant effect on the wetting behavior of water droplets. Chemical modification also plays an important role in controlling water droplet wetting behavior. Porous alumina

substrates

modified

with

hexamethyldisilazane

perfluorooctyltrichlorosilane have been investigated

[21-28]

(HMDS),

lauric

acid

or

. Notably, decreased surface free

energy derived from surface modification resulted in an increase in water contact angle. Concomitantly the wetting behavior of water droplets has been controlled by synergistic effects of the porous structure and chemical modification. While the previous studies focused on water or pure solvent droplets, inks used in the inkjet printing industry are usually polymer solutions which have different wetting behavior. A polymer based droplet is expected to form a coffee-ring pattern on a substrate under forces such as capillary force, counter force from compressed air trapped within the pore and the Marangoni effect

[29, 30]

. Changing the substrate structure affects Marangoni flow, which in turn impacts

coffee ring formation such as printing

[34]

[31-33]

. Coffee-ring formation will influence many practical applications

; biology

[35, 36]

and complex assembly

[37]

require uniform coatings. Despite

other methods as light or temperature to control the formation of a coffee ring [38-47], the focus of this study is to explore how surface structure and substrate modification can impact coffee ring formation on porous alumina. In addition to experimental observations of inkjet droplet behavior,


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we used a model with broad applicability to elucidate the underlying physical mechanisms. The kinetic model of wetting on porous media has been extensively discussed in previous literature [48-52]

. With small pores where the effect of gravity on the penetration process can be neglected, a

widely-accepted description is the Lucas-Washburn equation for penetration into a straight, cylindrical capillary [53, 54]:

ℎ12 =

𝑅𝛾𝑐𝑜𝑠𝜃 𝑡 2𝜇

(1)

Here hl is the distance of liquid penetrated into the capillary, R is the radius of the capillary, γ, θ and μ are the surface tension, contact angle, and viscosity of the liquid, respectively, and t is the time. The dynamics and equilibrium of capillary flow was described by Washburn assuming Poiseuille's law

[47]

. The driving pressure is expressed in three parts: the atmospheric pressure,

the hydrostatic pressure due to the liquid penetration, and the capillary pressure due to the surface tension. Later this equation was further developed by Bosanquet with the consideration of inertial forces and applied in porous substrates [55-57]. These theoretical studies provide the physical foundation for the phenomenological analysis of our work. We applied the pressure balance on porous alumina (Anodic Aluminum Oxide, AAO) with one-end-closed to calculate the penetration depth and volume of polymer solution. The physical insight will help the design and development of inkjet systems with improved resolution capability.

To this end, two approaches were explored in this investigation, namely i) the synergistic effects of pore structure and surface modification, and ii) the effect of the ink polymer structure. We found that the coffee ring and spreading of the polymer droplets can be controlled with the synergistic effects of the hierarchical structure and substrate surface modification. With the


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increasing of pore diameter and depth of the microstructure, the coffee ring disappeared. The results obtained in this work provide a method to improve the resolution of inkjet printing by decreasing the spreading.

EXPERIMENTAL SECTION Materials The porous alumina substrates with one-end-closed nano-pores were obtained from Hefei PuYuan Nano Technology Ltd., Hefei, China. 1H, 1H, 2H, 2H-perfluorodecyltrimethoxysilane (C13H13F17O3Si, FAS) was purchased from Aladdin Industrial Co., CA, USA. The polymers used to formulate the inks were obtained from the following sources: polyurethane (Mn=8806, Mw/Mn=1.86, Tianjin Atoz Fine Chemicals Co., Ltd, Tianjin, China), polyacrylic acid (Mn=4600, Mw/Mn=1.09, Aladdin Industrial Co., CA, USA), polyethylene glycol (Mn=13388 and Mw/Mn=1.06, Sinopharm Chemical Reagent Co., Ltd). The molecular weight and polydispersity of the polymers was determined by gel permeation chromatography (GPC, Waters HPLC, USA. Solvent: tetrahydrofuran (THF), Columns: HR1-100 and HR2-500, using polystyrene standards). The solvent, 2-ethoxyethanol was purchased from Sinopharm Chemical Reagent Beijing Co., Ltd, Beijing, China. The dye (acid blue) used in the ink formulations was water-based blue powder from Spectraflying Technology Ltd, Zhongshan, China. Substrate Preparation Porous alumina with pore diameters of 40~70 nm and 80~100 nm were selected, and the pore depth was either 50µm or 100µm (As shown in Table S1 in the Support Information). The anodized alumina substrates were cleaned consecutively with acetone, ethanol and deionized water under ultrasonic irradiation for 10 min to remove the oil and pollutant on the AAO. Then,


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they were immersed in 1wt% sodium hydroxide aqueous solution for 30 seconds before use. Thus there were some hydroxyl groups on the surface of the AAO which was used to combine with FAS. The cleaned porous alumina substrates were modified with FAS in the following manner. The substrates and 3-5 drops of FAS in a small container were placed inside a vacuum desiccator, where they remained under vacuum conditions for 24 hours. Then, the substrates were removed and dried at 120 ℃ for 2 hours in a vacuum oven to make FAS react with the hydroxyl group on the surface. The components and distribution of elements on the modified substrates were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, USA). Ink Preparation The inks were formulated by mixing polymer 0.5g, acid blue 0.02g and ethylene glycol monomethyl ether 9.48g. The dye of acid blue was used to facilitate observation of the as deposited droplets with a microscope or scanning electron microscope (SEM). The acid blue had no effect on the experimental results. After all ink components were fully dissolved, the solutions were filtered through a 0.22 μm Teflon microfiltration membrane to remove any solid impurities. Characterization of the Ink The surface tension of the ink was measured by a full automatic force tensiometer (K100SF, KRUSS GmbH, Germany). The viscosity of polymer solution was measured by digital viscometer ((SNB-1, Shanghai, China). Contact angle of the polymer droplet (volume 2 μl) on modified AAO substrate was measured by optical contact angle measurement (KRUSS DSA100, Germany). Inkjet Process The inkjet droplet was fabricated using a Dimatix Fujifilm DMP-2831 printer. The droplet volume size was controlled to 10 pL by Dimatix Drop Manager software. The printer frequency


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was set at 5.0 kHz and a waveform with a maximum voltage of 22 V and a pulse width of 8.5 µs were used. The substrate temperature was 40 ℃ and the relative humidity within the printing chamber was 30~40%. Dot chart was 20×30 and the drop space was 50 µm. The droplets jetted onto the surface of the substrate were observed by optical microscope (Nikon Eclipse LV 100ND) with a CCD camera (Nikon DS-Ri2, Japan). The drops were further investigated by SEM (Hitachi S-4800, Japan) with an accelerating voltage of 5 kV and 15.0 kV. Upon contact with the substrate, each polymer ink exhibited different sorption and spreading behavior. After solvent evaporation, the residual polymer ink remaining on the modified porous alumina depends on both polymer molecular structure and substrate microstructure (Figure 1). When the droplets impinged the substrate, a portion of the ink solution infiltrated the substrate pores. After wetting and solvent evaporation, the patterns of the droplets’ dots on the porous substrate were either as coffee rings or uniform circles.

Inkjet printing

Polymer droplet

Modification FAS-AAO

AAO

Wetting

Evaporation

Figure 1. Illustration of the process of the behavior of polymer inkjet droplets on modified porous alumina (Anodic Aluminum Oxide, AAO), AAO modified with FAS.

RESULTS AND DISCUSSION


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Surface modification was performed to decrease the effect of surface free energy and the spreading behavior. The substrate surface modified by FAS became more hydrophobic. Wetting behavior of 5wt% PU dissolved in 2-methoxyethanol on unmodified and modified porous alumina substrates is shown in Figures 2a and 2b, respectively. Since PU solution is hydrophilic, the droplet spreads over a wider area on the unmodified hydrophilic alumina. On the modified hydrophobic surface, the droplet fails to wet easily. Hence, surface modification is an effective technique to restrict the spreading of an inkjet droplet.

Figure 2. The effect of surface modification on droplet wetting behavior on porous alumina, Pore diameter: 80~100 nm, pore depth: 100 µm. 1a: no modification; 1b: modified with FAS. Concomitantly, surface modification changed the elemental composition on the substrate surface as revealed by XPS analysis in Figure S1a and Table S2. After surface modification, the elemental fraction of Al, C, P and O decreased while the fraction of F and Si increased. The modification depth extended about 200 nm into the pores, as deduced from the F and Si distribution profile. Since F leads to the hydrophobicity and Al is the relative constant element on the substrate, the F/Al ratio can be used to quantify the degree of surface modification. As shown in Table S3 in the Support Information, the F/Al ratio increased with the increase in pore diameter and depth, indicating the increase in the degree of modification.


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From XPS analysis, it is apparent that some hydroxyl groups still remained on the AAO surface. For polymer inks with different functional groups, the interactions between the hydroxyl groups on the substrate surface and polymer functionalities are expected to be different. The polarity of NH2, OH and COOH in PU, PEG and PAA increases in sequence, so the interactions between the polymers and substrate surface are also expected to increase. Thus, the wetting behavior will also be influenced. The FAS modified porous alumina microstructure was characterized by top view (Figure 3a3d) and cross sectional (Figure 3e and 3f) SEM imaging. Figure 3a-d presents images of the substrates having different diameters and depths. Figures 3e and 3f illustrate that the pores run through the thickness of the substrate and appear capillary like. They can be assumed as vertical cylindrical tubes.


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Figure 3. SEM images of different porous alumina substrates, 3a: pore diameter 40~70 nm, depth 50µm; 3b: pore diameter 40~70 nm, depth 100µm; 3c: pore diameter 80~100 nm, depth 50µm and 3d: pore diameter 80~100 nm, depth 100µm; 3e: pore diameter 40~70 nm; 3f: pore diameter 80~100 nm. The wetting of PU solution droplets (5wt% PU dissolved in 2-methoxyethanol) are shown in Figures 4a-d. Notably, pore diameter has more effect on the coffee ring formation than pore depth. By comparing Figures 4a and 4b (the same diameter, different pore depth, 50 μm and 100 μm, respectively), pore depth affects the size of the circle and the width of the coffee ring edge. The size of the circle is smaller and the edge of the coffee ring becomes narrower as pore depth increases. Figures 4c and 4d (the diameter is 80~100 nm which is larger than that of Figure 4a and 4b: 40-70 nm) exhibit no coffee ring pattern upon drying, suggesting that almost all the polymer had penetrated into the pores. With the increase of pore size and depth, the coffee ring became narrower and eventually disappeared.


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Figure 4. Wetting phenomena of 5wt% PU solutions on modified porous alumina substrates, 4a: diameter 40~70 nm, depth 50µm; 4b: diameter 40~70 nm, depth 100µm; 4c: diameter 80~100 nm, depth 50µm and 4d: diameter 80~100 nm, depth 100µm. Coffee ring forms when solvent evaporating from the edge of a droplet is replenished by the flow from the interior. The circular Marangoni flow carries entrained solutes to the drop periphery, affording a dense ring-like deposit. For PU droplets on porous substrates, the penetration process of droplets into the pores occurs simultaneously with the evaporation of solvent. The coffee ring forms only when the amount of solution remaining on the surface is enough to form outward capillary flow and inward Marangoni flow. The wetting behavior of PEG solution (5wt% dissolved in 2-methoxyethanol) was also investigated. PEG is another polymer commonly used in ink formulations and has hydrophillic hydroxyl group functionalities with polarity higher than PU. As shown in Figure 5a-d, PEG droplets exhibited similar behavior as PU on substrates with different pore diameters and depths. The droplets formed coffee rings on substrates with small pore diameter (40~70 nm) shown in Figure 5a and 5b. However, the ring diameter and amount of polymer remaining in the core of the ring differed from PU. These differences include the morphology and the diameter of the dots formed from the droplets. These may derive from polymer functional group interactions


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with the substrate. For PU, the surface interaction is associated with hydrogen bonding between the NH2 functional group in the polymer and OH on the AAO surface. While for PEG the OH functional group will interact with the OH on the AAO surface by forming hydrogen bond. The higher polarity of the PEG OH group will impact the wetting behavior.

Figure 5. Wetting behavior of 5wt% polyethylene glycol solutions on modified porous alumina substrates, 5a: diameter 40~70 nm, depth 50µm; 5b: diameter 40~70 nm, depth 100µm; 5c: diameter 80~100 nm, depth 50µm and 5d: diameter 80~100 nm, depth 100µm. Scale bar: 10µm. Then PAA was selected as an inkjet ink which has a higher polarity functional group, a carboxyl group. Similar experimental results were achieved as shown in Figure 6a-6d: the initially visible coffee ring disappeared with increasing of pore diameter. With the increasing pore diameter and pore depth, the F/Al ratio increased which means that the substrate became more hydrophobic (as shown in Table S3 in support information). Generally a higher degree of modification implies a lower surface free energy, which will in turn limit the ability of polymer


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droplets which contain hydrophilic functional groups to infiltrate into the pore. It is known that the hydrophilicity of a substance relates to the polarity of functional groups present. Despite the degree of modification and difference in functional group, all the three polymer droplets penetrated into the pores upon increased pore diameter and depth. Importantly, changing of structure has more effect than surface modification on the wetting behavior. In contrast to PU and PEG, the coffee ring inner edge formed by PAA droplets was non uniform (Figure 6a and 6b). Conceivably, the COOH group of PAA reacted with Al2O3 on the AAO surface. From XPS analysis, the surface modifier, FAS, did not completely cover the surface, allowing the regions not modified by FAS to react with PAA. Thus, the resulting coffee ring will be irregular in shape and appear deformed.

Figure 6. Wetting of 5wt% PAA solution on different modified porous alumina substrates, 6a: diameter 40~70 nm, depth 50µm; 6b: diameter 40~70 nm, depth 100µm; 6c: diameter 80~100 nm, depth 50µm and 6d: diameter 80~100 nm, depth 100µm. Scale bar: 10µm.


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From analysis of the distribution of the diameters of the above three polymer droplets (as shown in Table S4), it becomes possible to summarize the relationship between pore diameter and wetting behavior. For the three types of polymer droplets investigated here, increased pore diameter and depth resulted in decreased diameter of the remaining dot formed from the dried droplet. It is likely that changes in substrate microstructure facilitated more rapid infiltration of polymer droplets into the porous substrates. For comparison, the oligomers of PAA and PEG were also investigated in the same manner as above (Figure S2 and Figure S3 in the Support Information). In the case of the oligomers, no coffee ring patterns were observed, irrespective of the substrate porosity. Thus, polymer molecular weight is an additional factor that can impact interactions between ink droplets and substrates. Analysis of the diameter of polymer droplets in Figures 4-6 and Figures S2-S3 demonstrated that for PU and PAA, increasing pore diameter and depth led to decreased spreading. For PEG droplets, however, the spreading behavior was affected primarily by changes in pore depth. From a mechanistic perspective, these results suggest that in addition to differences in functional group polarity, chemical reactivity of anodized alumina with functional groups such as COOH should also be considered. Comparison of the size of droplet diameter achieved from PAA and PEG solutions with higher molecular weight to their oligomers, the spreading was very similar. But polymers with high molecular weight formed the coffee ring, while the oligomer did not. From Table S5 in supporting information, the surface tensions of either polymer with high molecular weight or the oligomer are almost the same. However, polymer viscosity may influence the penetration speed into the pore on porous AAO. The contact angles of polymer droplets on the modified AAO are in accordance with the polarity of functional groups in polymer.


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To help elucidate the experimental results, a model based on the force balance of a droplet on a porous substrate was used to reveal the underlying physical mechanisms. First, we validated that the solution penetration reached equilibrium in the experimental time scale. The time required to reach equilibrium was estimated using the Lucas-Washburn equation (1) with R = 50 nm and hl = 50 μm. The viscosity (μ), contact angle (θ), and surface tension (γ) of the polymer solutions on the modified substrate were measured and are listed in supporting information Table S1. The time required to reach equilibrium was estimated as 0.12s, much shorter than the experimental time scale (more than 1 hour). As a result, the equilibrium state considered in the following model is balanced with respect to all the driving pressures. Besides the atmospheric pressure (P0), the hydrostatic pressure (Ph) and the capillary pressure (Pr =

2𝛾𝑐𝑜𝑠𝜃 𝑅

) mentioned by

Washburn[6], the pressure of air trapped in the pores (Pa) is also included in the force balance for the one-end-closed porous substrate used here. As illustrated in Figure 7, at equilibrium, these four pressures balance at the interface of air and solution penetrating into the pores.

P0

Ph H

h

γ

0

h1

2R Pa

Figure 7. The force balance during the wetting process. The force balance is described by the following equation:

( P0  Ph  Pa ) R2  2 R cos   0

(2)


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Where R is the pore radius, and γ is the surface tension of the solution permeating into the pore. The surface tension, viscosity and contact angle of polymer droplet (volume: 2 μl) on the modified AAO substrate were measured (Table S5 in support information). The pressure of the polymer solution on the interface can be calculated by equation (3): Ph  l g (h0  h1 )

(3)

Where l is the density of polymer solution, which is estimated as the density of the solvent, ethylene glycol monomethyl ether; h1 is the height of the droplet penetrating into the pore; and h0 is the height of the droplet on the surface of the substrate, which is assumed to be 30 μm based on SEM images. The pressure of the air trapped inside the pore was estimated using the ideal gas equation. Since the ideal gas assumption may not be accurate when the air is under significant compression, the Van der Waals equation of state was also used for the calculation. Similar results were obtained. Hence for simplicity, air was assumed to be ideal gas in the model presented here. The pressure of the trapped air was calculated by equation (4):

ρa πR 2 H Pa πR (H  h1 )  R0T Ma 2

(4)

Where Ma is the molar mass of air, estimated to be 28.97 g/mol. R0 is the ideal gas constant (8.3145 m3Pa·mol-1·K-1); T is the temperature which is taken as 273K;  a is the density of trapped air in the pore, estimated to be 1.225 kg/m3; and H is the depth of the pore. Equations (2)-(4) enable the calculation of the depth the polymer solution infiltrating the pore h1. The properties of PU were used in the calculation. Since this model is based on the universal


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force balance, the calculated tendency shown in Figure 8 is generally applicable to the other two polymers, PAA and PEG. The calculated infiltration depth of the polymer solution is depicted by the color line in Figure 8a and is presented for substrates with different pore radii and depths. As pore depth increases and pore radius decreases, the polymer solution infiltrates deeper into the pore. The calculated dependence on pore radius is consistent with the experimental results for water (see Ref. 15). The capillary force is larger for smaller pore radius, which enables deeper penetration. Notably, here, the calculated dependence on pore depth exhibits a trend that is opposite to that found in reference 15. This difference is attributed to the impact of surface modification of the alumina substrates, coupled with the use of polymer solutions rather than water in the current investigation. Further, the substrates used here have a larger pore depth (50100 μm) than those of reference 15 (0.8-10 μm). Clearly, surface interactions and pore size affect droplet contact angle and hence the wetting behavior. The volume of solution inside the pore is perhaps, however, more importantly, because it determines the spreading behavior of the ink on the surface. With knowledge of the penetration depth, the volume of solution inside the pore can be easily calculated by V = πR2h1. As shown in Figure 8b, the volume of polymer solution infiltrating into the substrate increased with an increase in both pore depth and radius, which suggests that less polymer solution remains on the substrate surface when pore size increases. The results from the simple model presented above readily explain the experimental results. The penetration depth increased with a decrease in pore radius or increase in pore depth, while the penetration volume increased with an increase in both pore radius and depth. The calculated results effectively explain the phenomena observed in the experiments, and reveal important physical insights surrounding wetting behavior.


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As pore radius and depth increase, the amount of solution remaining on the substrate surface is insufficient to form the coffee ring under the capillary flow and Marangoni flow effects during solvent evaporation. Consequently, the coffee ring was observed only for substrates with small pore diameter and depth.

Figure 8. The penetration depth (a) and volume (b) of polyurethane solution infiltrating alumina substrates with different pore radius and pore depth. The experimental results for polymer solution droplets differ from those observed for water wettability upon structural and chemical modification of AAO. For instance, after modification with perfluorooctyltrichlorosilane very little water went into the pores

[21]

. However, from the

studies conducted here, the volume of polymer solution remaining on the AAO surface is quite important for the resolution of inkjet printing, because it determines the type of spreading or infiltration behavior of the ink. CONCLUSION Polymers with amino (PU), hydroxyl (PEG) or carboxyl (PAA) groups are used as representative solutes in this paper. Experimental results demonstrated that with the increasing


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pore diameter and depth of a porous structure, the anticipated coffee ring disappeared. Thus, the wetting behavior of polymer droplets can be controlled by synergistic effects of porous structure and surface modification. During inkjet printing, residual ink on the substrate surface is vital to achieve color saturation, which in turn correlates to print quality. From this study it was found that intrinsic polymer properties and solution surface tension effects should also be considered, especially when high resolution is desired. The results of this investigation provide significant insight into the relationships between intrinsic polymer properties, the spreading behavior of polymer based inks, and porous substrate microstructure. To continue to advance the state of the art in ink jet printing technology, the impact of ink characteristics with a range of substrates common to the industry should be investigated. The insights generated by this investigation will facilitate the design and selection of materials for future fabrication technologies based upon inkjet printing. ASSOCIATED CONTENT Supporting Information The distribution of the modification elements on the alumina surface and in the alumina pores were measured by XPS. The SEM images of the oligomer of PAA and PEG are presented in the supporting information. The diameters of all the droplets are measured and showed in Table S4. The viscosity, surface tension and contact angle between polymer droplets and the surface of porous AAO of polymer or oligomer solution was represented in Table S5. AUTHOR INFORMATION Corresponding Author *Phone: +86 10 62529284. Fax: +86 10 62553912. E-mail: [email protected],


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This research is supported by National Natural Science Foundation of China (Grant No. 21303218), the 973 Program (2013CB933004) and the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No.XDA09020000). ABBREVIATIONS AAO

Anodic Aluminum Oxide

FAS

1H, 1H, 2H, 2H-perfluorodecyltrimethoxysilane (C13H13F17O3Si)

PU

Polyurethane

PAA

Polyacrylic acid

PEG

Polyethylene glycol

REFERENCES (1) Sweet, R. G. High frequency recording with electrostatically deflected ink jets. Rev. Sci. Instr. 1965, 36, 131-136. (2) Kettle, J.; Lamminmäki, T.; Gane, P. A review of modified surfaces for high speed inkjet coating. Surface & Coatings Technology 2010, 204 , 2103-2109. (3) J de Gans, B.; Duineveld, P. C.; Schubert, U. S. Inkjet Printing of Polymers: State of the Art and Future Developments. Adv. Mater. 2004, 16, 203-213. (4) Calvert, P. Inkjet Printing for Materials and Devices. Chem. Mater. 2001,13, 3299-3305. (5) Kuang, M. X.; Wang, L. B.; Song, Y. L. Controllable Printing Droplets for High-Resolution Patterns with Uniform Morphology. Adv. Mater. 2014, 26, 6950-6958. (6) Kuang, M. X.; Wang, J. X.; Wang, L. B.; Song, Y. L. Research Progress of High-quality Patterns by


20

Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Directly Inkjet Printing. Acta Chim. Sinica. 2012, 70,1889-1896. (7) Danzebrink, R.; Aegerter, M. A. Deposition of micropatterned coating using an ink-jet technique. Thin Solid Films 1999, 351, 115-118. (8) Boreyko, J. B.; Baker, C. H.; Poley, C. R.; Chen, C. H. Wetting and dewetting transitions on hierarchical superhydrophobic surfaces. Langmuir 2011, 27, 7502-7509. (9) Wu, J.; Liu, L.; Jiang, B.; Hu, Z.; Wang, X. Q.; Huang, Y. D.; Lin, D. R.; Zhang, Q. H. A coating of silane modified silica nanoparticles on PET substrate film for inkjet printing. Appl. Surf. Sci. 2012, 258, 5131-5134. (10) Kim, H.; Lee, C.; Kim, M. H.; Kim, J. Drop impact characteristic and structure effects of hydrophobic surfaces with micro- and/or nanoscaled structures. Langmuir 2012, 28, 11250-11257. (11) Chen, L.; Wang, C.; Tian, A.; Wu, M. An attempt of improving polyester inkjet printing performance by surface modification using β-cyclodextrin. Surf. Interface Anal. 2012, 44, 1324-1330. (12) Tsuchiya, Y.; Haraguchi, S.; Ogawa, M.; Shiraki, T.; Kakimoto, H.; Gotou, O.; Yamada, T.; Okumoto, K.; Nakatani, S.; Sakanoue, K.; Shinkai, S. Fine wettability control created by a photochemical combination method for inkjet printing on self-assembled monolayers. Adv. Mater. 2012, 24, 968-972. (13) Somasundaram, R.; Kanagaraj R.; and Kalakkath, P. Dynamic characteristics of drop-substrate interactions in direct ceramic ink-jet printing using high speed imaging system. Defence Science Journal 2009, 59, 675-682. (14) Mielonen, K., Geydt, P.;

österberg, M.; Johansson, L.-S.; Backfolk K. Inkjet ink spreading on

polyelectrolyte multilayers deposited on pigment coated paper. J. Colloid Interf. Sci. 2015, 438, 179-190. (15)

Öhlund, T.; örtegren, J.; Forsberg S.; Nilsson, H.-E. Paper surfaces for metal nanoparticle inkjet printing. Appl. Surf. Sci. 2012, 259, 731-739.

(16) Kettle J.; Lamminmäki, T.; Gane, P. Areview of modified surfaces for high speed inkjet coating. Surf. Coat. Tech. 2010, 204, 2103-2109. (17) Lamminmäki, T.; Kettle J. Gane, P. Absorption and adsorption of dye-based inkjet inks by coating layer components and the implications for print quality. Colloids and Surface A: Physicochem. Eng. Aspects 2011, 380, 79-88.


21

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

(18) Singh, S. K.; Khandekar, S.; Pratap, D.; Ramakrishna, S. A. Wetting dynamics and evaporation of sessile droplets on nano-porous alumina surfaces. Colloids and Surface A: Physicochem. Eng. Aspects 2013, 432, 71-81. (19) Ran, C. B.; Ding, G. Q.;Liu, W. C.; Deng, Y.; Hou, W. T. Wetting on nanoporous alumina surface: transition between Wenzel and Cassie states controlled by surface structure. Langmuir 2008, 24, 99529955. (20) Li, Z. R.; Wang, J. X.; Zhang, Y. Z.; Wang, J. J.; Jiang, L.; Song, Y. L. Closed-air induced composite wetting on hydrophilic ordered nanoporous anodic alumina. Appl. Phys. Lett. 2010, 97, 233107. (21) Norek, M.; Krasiński, A. Controlling of water wettability by structural and chemical modification of porous anodic alumina (PAA): towards super-hydrophobic surfaces. Surf. Coat. Tech. 2015, 276, 464470. (22) Wang, H.; Dai, D.; Wu, X. Fabrication of superhydrophobic surfaces on aluminum. Appl. Surf. Sci. 2008, 254, 5599-5601. (23) Park, B.G.; Lee, W.; Kim, J.S.; Lee, K.B. Superhydrophobic fabrication of anodic aluminum oxide with durable and pitch-controlled nanostructure. Colloids Surf. A 2010, 370, 15-19. (24) Buijnsters, J.G.; Zhong, R.; Tsyntsaru, N.; Celis, J.-P. Surface wettability of macroporous anodized aluminum oxide. ACS Appl. Mater. Interfaces 2013, 5, 3224-3233. (25) Mateo, J.N.; Kulkarni, S.S.; Das, L.; Bandyopadhyay, S.; Tepper, G.C.; Wynne, K.J.; Bandyopadhyay, S. Wetting behavior of polymer coated nanoporous anodic alumina. films: transition fromsuperhydrophilicity to super-hydrophobicity. Nanotechnology 2011, 22, 035703. (26) Lee, W.; Park, B.G.; Kim, D.H.; Ahn, D.J. ; Park, Y.; Lee, S. H.; Lee, K.B. Nanostructure dependent water-droplet adhesiveness change in superhydrophobic anodic aluminum oxide surfaces: from highly adhesive to self-cleanable. Langmuir 2010, 26, 1412-1415. (27) Leese, H.; Bhurtun, V.; Lee, K.P.; Mattia, D. Wetting behaviour of hydrophilic and hydrophobic nanostructured porous anodic alumina. Colloids Surf. A 2013, 420, 53-58. (28) Tasaltin, N.; Sanli, D.; Jonáš, A.; Kiraz, A.; Erke, C. Preparation and characterization of superhydrophobic surfaces based on hexamethyldisilazane-modified nanoporous alumina. Nanoscale Res. Lett. 2011, 6, 487.


22

Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(29) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel S. R. and Witten, T. A. Cappillary flow as the cause of ring stains from dried liquid drops. Nature 1997, 389, 827-829. (30) Marangoni, C. Nota circa il barometro del dott. Vecchi. Il Nuovo Cimento 1867, 28, 358-359. (31) Park, J.; Moon, J. Control of colloidal particle deposit patterns within picoliter droplets ejected by ink-jet printing. Langmuir 2006, 22, 3506-3513. (32) Dugas, V. Immobilization of single-stranded DNA fragments to solid surfaces and their repeatable specific hybridization: covalent binding or adsorption? Sens. Actuators B 2004, 101, 112-121. (33) Dugas, V.; Broutin, J.; Souteyrand, E. Droplet evaporation study applied to DNA chip manufacturing. Langmuir 2005, 21, 9130-9136. (34) De Gans, B. J.; Duineveld, P. C.; Schubert, U. S. Inkjet printing of polymers: state of the art and future developments. Adv. Mater. 2004, 16, 203-213. (35) Martin, C. P.; Blunt, M. O.; Pauliac-Vaujour, E.; Stannard, A.; Moriarty, P.; Vancea, I.; Thiele, U. Controlling pattern formation in nanoparticle assemblies via direct solvent dewetting. Phys. Rev. Lett. 2007, 99, 116103 (4). (36) Crivoi, A.; Duan, F. Evaporation-induced formation of fractal like structures from nanofluids. Phys. Chem. Chem. Phys. 2012, 14, 1449-1454. (37) Anyfantakis, M.; Baigl, D. Dynamic photocontrol of the coffee-ring effect with optically tunable particle stickiness. Angew. Chem. Int. Ed. 2014, 53, 14077-14081. (38) Soltman, D.; Subramanian, V. Inkjet-printed line morphologies and temperature control of the coffee ring effect. Langmuir 2008, 24, 2224-2231. (39) Trantum, J. R.; Wright, D. W.; Haselton, F. R. Biomarker-mediated disruption of coffee-ring formation as a low resource diagnostic indicator. Langmuir 2012, 28, 2187-2193. (40) Anyfantakis, M.; Baigl, D. Dynamic photocontrol of the coffee-ring effect with optically tunable particle stickiness. Angew. Chem. Int. Ed. 2014, 126, 14301-14305. (41) Dugyala, V. R.; Basavaraj, M. G. Control over coffee-ring formation in evaporating liquid drops containing ellipsoids. Langmuir 2014, 30, 8680-8686. (42) Mampallil, D.; Reboud, J.; Wilson, R.; Wylie, D.; Klug, D. R.; Cooper, J. M. Acoustic suppression of the coffee-ring effect. Soft Matter 2015, 11, 7207-7213.


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Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

(43) Anyfantakis, M.; Geng, Z.; Morel, M.; Rudiuk, S.; Baigl, D. Modulation of the coffee-ring effect in particle/surfactant mixtures: the importance of particle-interface interactions. Langmuir 2015, 31, 41134120. (44) Oko, A.; Swerin, A. and Claesson, P. M. Imbibition and evaporation of water droplets on paper and solid substrates. J. Imaging Sci. Technol. 2011, 55, 010201-1-010201-6. (45) Siebold, A.; et al. Capillary Rise for Thermodynamic Characterization of Solid Particle Surface. J. Colloid Interface Sci. 1997, 186, 60-70. (46) Ayala, R.E.; Casassa, E.Z. andParfitt, G.D. A Study of the Applicability of the Capillary Rise of Aqueous Solutions in the Measurement of Contact Angles in Powder Systems. Powder Technol. 1987, 51, 3-14. (47) Sun, J.; Bao, B.; He, M.; Zhou, H.; Song, Y. Recent advanced in controlling the depositing morphologies of inkjet droplets. ACS Appl. Mater. Interfaces 2015, 7, 28086-28099. (48) Gruener, S.; et al. Capillary rise of water in hydrophilic nanopores. Phys. Rev. E 2009, 79, 067301-1067301-4. (49) Marmur, A. Kinetics of Penetration into Uniform Porous Media: Testing the Equivalent-Capillary Concept. Langmuir 2003, 19, 5956-5959. (50) Lucas, R. Rate of capillary ascension of liquids. Kolloid. Zeit. 1918, 23, 15-22. (51) Washburn, E.W. The Dynamics of Capillary Flow. Phys. Rev. 1921, 17, 273-283. (52) Bosanquet M. A., C.H., LV. On the flow of liquids into capillary tubes. Lond.Edinb.Dubl.Phil.Mag. 1923, 45, 525-531. (53) Ridgway, C.J., Gane, P.A. and Schoelkopf, J. Effect of capillary element aspect ratio on the dynamic imbibition within porous networks. J Colloid Interface Sci. 2002, 252, 373-82. (54) Marmur, A. and Cohen, R.D. Characterization of Porous Media by the Kinetics of Liquid Penetration: The Vertical Capillaries Model. J Colloid Interface Sci. 1997, 189, 299-304. (55) Crivoi, A.; Duan, F. Three-dimensional monte carol model of the coffee-ring effect in evaporating colloidal droplets. Scientific Reports 2014, 4 , 4310. (56) Bormashenko, E.; Bormashenko, Y.; Pogreb, R.; Stanevsky, O.; Whyman, G. Droplet behavior on flat and texture surfaces: co-occurrence of Deegan outward flow with Marangoni solute instability. J. Colloid Interf. Sci. 2007, 306, 128-132.


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(57) Saleem, M.; Anwar Hossain, Md.; Mahmud, S.; Pop, I. Entropy generation in Marangoni convection flow of heated fluid in an open ended cavity. Int. J. Heat Mass Tran. 2011, 54, 4473-4484.


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Polymer droplet

Modification FAS-AAO

AAO

Wetting

Evaporation


Wetting of Inkjet Polymer Droplets on Porous Alumina Substrates

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