Dip coating of charged colloidal suspensions onto substrates with patterned wettability: Coating regime maps

doi:10.1016/j.jcis.2010.08.028

Journal of Colloid and Interface Science

Volume 352, Issue 1, 1 December 2010, Pages 202-210

https://doi.org/10.1016/j.jcis.2010.08.028 Get rights and content

Abstract

Aqueous dispersions of silica nanoparticles were dip coated onto Si substrates that contained patterned wettability. The patterns were prepared by photolithography and consisted of groups of hydrophilic lines (5–100 μm wide) separated by hydrophobic areas (5–100 μm wide). Coating were made from two aqueous silica dispersions: a cationic dispersion in which particles have positive surface charge, and an anionic dispersion in which particles have negative surface charge. Coating morphology, thickness, and pattern quality were characterized. For a pattern containing 25 μm wide hydrophilic stripes separated by equally wide hydrophobic spaces, coating regime maps were created to show the effect of process variables on pattern features and morphology. Within the map there is a critical concentration for both dispersions, above which uniform stripes are formed and below which a segregated non-uniform structure results. Coatings prepared at withdrawal rates of 0.1 mm/s or lower resulted in a monolayer of coatings in the case of cationic silica and no deposition in the case of anionic silica. A maximum withdrawal rate was also found; above a critical speed, excess liquid is entrained and results in nonuniformity in the bottom of the pattern. The physical origin of the regimes and differences between the two types of particles are discussed.

Graphical abstract

Coating regime map for silica coating on a chemically patterned substrate.

Research highlights

► Dip coating of silica nanoparticle dispersions onto chemically patterned substrates. ► A ‘coating regime map’ shows effect of variables on morphology. ► Uniform particle patterns created with more concentrated suspensions (>10 wt.%). ► Silica with positive surface charge, cationic silica, forms monolayer at low rates.

Introduction

Research on using wettability differences to pattern devices has emerged recently due to interest in microelectronic devices [1], [2] and microfluidic applications [3], [4], [5]. While still a budding technique, patterning through wettability differences has been studied both experimentally [6], [7], [8], [9], [10] and theoretically [11], [12], [13], [14], [15]. However, routes that combine a substrate with patterned wettability and a conventional coating process for distributing materials onto patterns have not received much attention [16], [17], [18]. In this combination, a coating solution or dispersion is deposited onto a substrate patterned with a wettability difference, the coating liquid dewets from the hydrophobic (or hydrophilic) area, and material is deposited on the wetted hydrophilic (or hydrophobic), areas. The result is a patterned coating. Among the conventional coating processes, dip coating is a simple and inexpensive technique, and its physics are relevant to several industrial processes [19]. This work, therefore, explores dip coating of a substrate with patterned wettability.

The experimental research so far has focused on the preferential wetting of aqueous particulate dispersions onto substrates with patterned wettability [6], [7], [8], [20], [21], [22]. The intricacies of the coating process itself are not studied in these works. For example, Heule et al. [8] investigated the deposition of ceramic particulates through the use of both patterned wettability and coating processes. They prepared tin oxide and aluminum oxide patterns using dip coating or dropping the suspension onto a tilted patterned substrate. Unfortunately, information about dip coating rates was not included. Fustin et al. [21] describe the dip coating of silica particulates onto a substrate patterned with wettability. Colloidally crystalline patterns were made and parameters that influence these colloidal crystals were investigated. The typical coating rate to make a colloidally crystalline pattern was in the range of microns per minute, orders of magnitude lower than a practical coating rate. Interestingly, stripe patterns have also been demonstrated to form spontaneously from colloidal suspensions by dip coating on homogeneous surfaces [23]. Such stripe patterns form at low withdrawal rates on the order of 100 μm/min or lower, with stripes being oriented perpendicular to the direction of withdrawal.

Theoretical research in the area of patterned wettability has focused on dewetting of a model liquid coating on a patterned substrate [11], [12], [24]. While the results are useful in understanding how dewetting happens and how to create optimal conditions for dewetting, [3] these results do not account for how coating methods might affect dewetting on a patterned substrate.

Darhuber et al. [25] and Davis [26] studied dip coating onto patterned substrates. Darhuber et al. [25] developed a scaling relation that describes the wet coating thickness as a function of pattern width and withdrawal speed. For dip coating of a substrate with a vertical hydrophilic strip of width W surrounded by a planar, hydrophobic surface, the entrained liquid film thickness at the center of the stripe is given by $h_{\infty} \sim W (Ca)^{1 / 3}$ where Ca denotes the capillary number, $μ U / σ$ . Here μ refers to the viscosity of the liquid, σ is the surface tension and U is the withdrawal rate. Davis [26] later showed that the proportionality constant in Eq. (1) is ∼0.356. It is worth noting that Eq. (1) was derived in the regime Ca ≪ 1. This equation has also been shown to agree with experimental data for silicon substrates patterned with octadecyl trichlorosilane and then dip coated with a model liquid [25]. However, rate limits, drying, and other practical concerns in implementing the method were not investigated.

This paper presents a systematic study of the effects of coating parameters on coating quality, morphology, and limits to create patterned coatings. A range of coating conditions are explored for aqueous silica dispersions deposited onto patterned silicon substrates. Photolithography is used to create patterns and chemical functionality is introduced by self assembled monolayers [27] (SAMs). Coatings of particulate dispersions are created using dip coating. Coating regime maps are used to show the effect of process conditions on the coating morphology and in particular to define the process space for uniform patterned coatings. The effects of particle charge, dispersion concentration, dip coating withdrawal rate, and pattern feature sizes are considered. Combining knowledge of the effects of processing variables along with interfacial science and liquid transport, we also identify possible mechanisms that govern coating morphologies, rate limits and quality.

Section snippets

Experimental methods

The experimental procedure involved three stages: patterning the substrate, dip coating and subsequent characterization.

Dispersion properties

Fig. 2 shows viscosity of the dispersions as a function of shear rate. The stock dispersions (20 wt.% silica) shear thin slightly; this behavior is more noticeable with cationic silica. As the dispersion concentration is decreased, the behavior becomes Newtonian and the viscosity approaches that of pure water (1 mPa s). The viscosities of the cationic and anionic dispersions are similar at low-concentration. At 20 wt.%, however, the cationic silica dispersion has a significantly higher viscosity

Discussion

In this section, the possible mechanisms behind formation of different regimes in the coating regime maps in Fig. 7 are explained.

Summary

Aqueous dispersions of colloidal silica, having positive and negative surface charges, were dip coated onto Si substrates patterned by wettability. The patterns on the substrate consisted of groups of alternating hydrophilic and hydrophobic lines of widths on the length scale of 5–100 μm. Coating regime maps were created for a group of 25 μm wide hydrophilic lines separated by equal hydrophobic spacing. The coating conditions required to produce uniform and segregated patterned coatings were

Acknowledgments

This work was supported through the Industrial Partnership for Research in Interfacial and Materials Engineering at the University of Minnesota. The research also made use of the Nanofabrication Center and Characterization Facility at the University of Minnesota.

References (45)

M. Heule et al.
J. Eur. Ceram. Soc
(2004)
A.L. Yarin et al.
J. Colloid Interface Sci.
(2006)
G. Mason et al.
J. Colloid Interface Sci.
(1986)
M.L. Chabinyc et al.
Appl. Phys. Lett.
(2002)
C.R. Kagan et al.
Appl. Phys. Lett.
(2001)
H. Gau et al.
Science
(1999)
D.E. Kataoka et al.
Nature
(1999)
H. Fan et al.
Nature
(2000)
F. Fan et al.
Langmuir
(2004)
C. Bae et al.
Ceram. Trans.
(2005)

M.J. Lee et al.

Nanotechnology

(2008)

S. Lee et al.

Mol. Cryst. Liq. Cryst.

(2007)

K. Kargupta et al.

Langmuir

(2002)

M. Brinkmann et al.

J. Appl. Phys.

(2002)

M. Schneemilch et al.

J. Chem. Phys.

(2003)

K. Kargupta et al.

Langmuir

(2000)

P. Lenz et al.

Phys. Rev. Lett.

(1998)

C.L. Bower et al.

AIChE J.

(2007)

T.D. Reynolds, M.S. Thesis, Investigations in Patterned Particulate Coatings, University of Minnesota,...

B.J. Brasjen et al.

European Coating Symposium

(2009)

S.F. Kistler et al.

Liquid Film Coating: Scientific Principles and Their Technological Implications

(1994)

Z. Huang et al.

Langmuir

(1997)

Cited by (17)

Features of colloidal silica deposits dip coated onto porous alumina membranes from aqueous suspensions
2021, Colloids and Interface Science Communications
Citation Excerpt :
Therefore, the efficiency of momentum transfer from the moving plate to the liquid in the bath will be lesser, thereby leading to the formation of thinner films than predicted by the Landau-Levich result [50]. In case of colloidal suspensions, shear thinning is also inherently connected to inter-particle interactions, which can affect the coating microstructure especially when the particles are charged or electrostatically stabilized [54,55]. Wang et al. [56] have discussed the mismatch between theoretical and experimental thickness of polymer-derived ceramic composite films dip coated from a shear thinning complex fluid.
The present work is directed towards understanding the development of coating morphologies and the accompanying hydrodynamics of suspension flows during dip coating on porous substrates. Colloidal silica particles are deposited from aqueous suspension onto nanoporous alumina membranes. The choice of colloidal silica as the model system is motivated by its popularity in fabrication of antireflection coatings, and as nanostructured material templates for electrochemical and catalytic applications. Nanoporous alumina as substrate is notable for its chemical and thermal stability, and its ability to form self-organized pore structures. The objective here is to characterize the dependence of coating morphologies on membrane pore size, wettability, dip coating speed, and size of colloidal particles in suspension. A coating regime map is formulated based on different morphologies obtained for coatings deposited under different operating parameter spaces. The map indicates existence of two perceptibly distinct regimes which emanate from particle-induced interfacial deformation of the meniscus: Scarcely populated particulate deposition regime and densely packed coating regime having significant amount of particle deposition. The experimental results are analysed and explained using numerical results from earlier mathematical models in literature which cater to modified Landau-Levich formulation for porous substrates, as well as the hydrodynamics of colloidal assembly on moving substrates.
Dip- and die-coating of hydrophilic squares on flat, hydrophobic substrates
2017, Chemical Engineering Science
Citation Excerpt :
For the purpose of delineating individual devices, chemical patterns can be defined on the substrate. These typically consist of isolated hydrophilic regions, where the deposition of functional material is desired, on an otherwise hydrophobic substrate (Biebuyck and Whitesides, 1994; Qin et al., 1999; Braun and Meyer, 1999; Bechinger et al., 2000; Darhuber et al., 2000; Hartmann et al., 2001; Oh et al., 2003; Ko et al., 2004; Fan and Stebe, 2005; Darhuber and Troian, 2005; Choi and Park, 2006; Benor et al., 2007; Makaram et al., 2007; Woodson and Liu, 2007; Chowdhury et al., 2007; Zeira et al., 2008; Bardecker et al., 2008; Lin et al., 2008; Li et al., 2008; Zeira et al., 2009; Lee et al., 2009; Lin et al., 2009; Park et al., 2009; Mastrangeli et al., 2009; Meyer et al., 2009; Na et al., 2009; Cai, 2009; Lee et al., 2010; Miller et al., 2010; Reynolds et al., 2010; Svarnas et al., 2010; Zhu et al., 2010; Furuta et al., 2011; Kim et al., 2011; Mastrangeli et al., 2011; Brasjen et al., 2011; Ke and Tang, 2013; Nakajima et al., 2013; Wang and McCarthy, 2014; Wang et al., 2014; Watanabe et al., 2014; Kim et al., 2014; Hou et al., 2014; Wu et al., 2015; Hou et al., 2015; Kobaku et al., 2015; Chang et al., 2016; Bao et al., 2016). In this manuscript, we consider the selective deposition of liquids on isolated, geometrically convex, hydrophilic regions on otherwise hydrophobic substrates.
We studied the selective deposition of a Newtonian liquid on the hydrophilic domains of chemically patterned substrates. We present experiments and numerical simulations of the dip-coating and self-metered die-coating of compact, geometrically convex patterns such as squares and diamonds. The coating of such patterns is intrinsically an instationary process. Nevertheless, the maximum film thickness entrained on diamonds that are either much smaller or larger than the lengthscale governing the coating process follows the theoretical predictions derived for steady-state coating of either narrow hydrophilic stripes or chemically homogeneous surfaces. The transition between these two regimes is determined by the ratio of the pattern dimensions and the characteristic length of the reservoir meniscus, i.e. the capillary length for dip-coating and the die-gap for die-coating processes. While the film thickness entrained on diamonds scales with the coating speed as a powerlaw, the entrained thickness on squares with sides oriented parallel and perpendicular to the coating direction appears to saturate for small coating speeds. This influence of the azimuthal pattern orientation is related to the capillary break-up of the coating meniscus. Residual satellite droplets observed underneath the hydrophilic patterns after coating are a consequence either of the break-up or the subsequent retraction of the coating meniscus.
Soft Substrates
2015, Droplet Wetting and Evaporation: From Pure to Complex Fluids
Performed experimental investigations show that soft substrates (Young’s modulus below 10 MPa) are able to directly influence the dynamics of the triple-phase-contact-line (TPCL) of a sessile drop due to deformation caused by interfacial forces. By fine-tuning the Young’s modulus of the substrate underneath an evaporating droplet, it is possible to control the evaporation mode, and consequently, evaporation time of the droplet. Further investigations on particle deposition during the evaporation of water-silica suspension drops showed that the TPCL velocity increases with decreasing Young’s modulus of the substrate. Fine-tuning Young’s modulus of soft polymer substrates also allowed the control of particle deposition without modifying the used suspension.
Micropatterning of functional dye films on wettability-patterned surfaces using soft liquid-phase adsorption
2014, Colloids and Surfaces A: Physicochemical and Engineering Aspects
Citation Excerpt :
Phase-separated Langmuir–Blodgett films containing silane-coupling agents can represent micro- or nanoscale phase-separated structures in the films which can enable us to fabricate wettability-patterned surface with low cost and small material consumptions [10–14]. These wettability-patterned surfaces are utilized as templates for the patterning of graphene [15,16], carbon nanotubes [17], organic semiconductors [18,19], functional material nanoparticles [20–24], and organic dyes [25,26] to fabricate organic field effect transistors, organic thin film solar cells, and emitting devices. Liquid-phase adsorption techniques have attracted much attention for the film-fabrication with low cost, energy, and facility investment in comparison with dry processes and some of the conventional wet process techniques such as spin-coating and dip-coating [25–30] because of large material consumption and difficulty in forming homogeneous films on wettability-patterned surfaces.
We discuss a “soft” liquid-phase adsorption (SLPA) technique to control formation of organic dye films on wettability-patterned templates by using methanol/hexane emulsions. Wettability-patterned templates were fabricated by the formation of self-assembled monolayers of silane-coupling agents on glass plates followed by UV-ozone treatment through a metal mask. Rhodamine B and thymol blue films were formed selectively on the hydrophilic regions of the wettability-patterned templates. The thickness of the rhodamine B films increased with a decrease in immersion temperature and with an increase in immersion angle of the wettability-patterned templates in the emulsions. The thickness of the thymol blue films was significantly larger than that of the films fabricated by conventional film-forming techniques such as spin-coating and dip-coating. The thymol blue films served as pH-responsive devices under exposure to HCl and NH₃ gasses. The SLPA technique allows for the fabrication of organic thin films on wettability-patterned templates with little material consumption and a few film-forming processes, contributing to application to the fabrication of organic photonics and electronics devices with reduction of environmental burdens.
Thin-film models of liquid displacement on chemically patterned surfaces for lithographic printing processes
2012, Journal of Colloid and Interface Science
Citation Excerpt :
Details from the manufacturer reveal that the dispersion is composed of 20 nm individual particles in open clusters with a median aggregate diameter of 150 nm. The dispersion shear thins slightly, with an average viscosity of ∼5 mPa s as reported in experimental measurements elsewhere [41]. The solvent evaporates after coating and the dried films are imaged with an optical microscope.
We examine in this work a model problem relevant to the liquid displacement that occurs in lithographic printing processes. The model problem consists of two stratified thin liquid films confined between parallel plates, one of which is chemically heterogeneous. The films are assumed to be thin enough so that intermolecular forces are important and the lubrication approximation can be invoked. Both linear stability analysis and nonlinear simulations are applied to a partial differential equation governing the behavior of the liquid–liquid interface. The results provide physical insights into and numerical estimates of the smallest and largest feature sizes that can be printed, as well as the minimum spacing between feature sizes that can be tolerated. The results also provide insight into experimental observations on a closely related process, wire-wound rod coating on chemically patterned surfaces. The work presented here has important implications for the production of electronic devices and displays by lithographic printing, as well as for other processes that rely on coating and printing on chemically patterned surfaces.
Immobilization of active and stable goethite coated-films by a dip-coating process and its application for photo-Fenton systems
2012, Chemical Engineering Journal
Citation Excerpt :
Additionally goethite films obtained at 1.72 and 3.30 mm s−1 speeds exhibited less homogeneity of the layer with clear spots and agglomerates of goethite. The presence of a critical withdrawn speed is in agreement with the observed in the literature [32]. On the one hand, the withdrawal speed of the coating process promotes hydrodynamic properties on the interface glass-tube/goethite suspension that affects the thickness and the homogeneity of the goethite layer.
The immobilization of active photo-Fenton heterogeneous catalysts over different materials, mainly polymers, has been studied in the literature to avoid their separation from the reaction solution. However, those materials exhibited a high preparation cost and low stability in the process, being difficult their application in an industrial wastewater treatment plant. In this work, a dip-coating method for the immobilization of goethite film on the wall of a photo-Fenton reactor was studied. The influence of the concentration of the initial goethite suspension as well as the withdrawal speed on the coating process, were evaluated. Both variables have shown to be critical in the homogeneity and the optical properties of the layer and therefore in its activity in the photo-assisted reaction.
The immobilized photo-Fenton system was tested in the degradation of a mixture of six selected pharmaceutical compounds (nicotine, 4-acetamidoantipyrine, hydrochlorothiazide, ranitidine, diclofenac sodium and sulfamethoxazole) in aqueous solution. The influence of the goethite layer thickness in the photo-activity of the system was established and the results were compared with those obtained using goethite powder in a slurry photo-reactor. One coating cycle, corresponding to 0.05 g/L of catalyst was enough to obtain 100% degradation of the selected pharmaceutical compounds after 6 h of reaction using a low oxidant concentration, except for the nicotine, that seems to be refractory to the tested photo-Fenton system. The use of a commercial goethite as catalyst and the simplicity of the dip-coating procedure make this process an attractive alternative for the design of wastewater treatments based on the heterogeneous photo-Fenton reaction.

View all citing articles on Scopus

¹: Current address: Tru Vue, Inc., 2150 Airport Drive, Faribault, MN 55021, USA.

View full text

Dip coating of charged colloidal suspensions onto substrates with patterned wettability: Coating regime maps

Abstract

Graphical abstract

Research highlights

Introduction

Section snippets

Experimental methods

Dispersion properties

Discussion

Summary

Acknowledgments

J. Eur. Ceram. Soc

J. Colloid Interface Sci.

J. Colloid Interface Sci.

Appl. Phys. Lett.

Appl. Phys. Lett.

Science

Nature

Nature

Langmuir

Ceram. Trans.

Nanotechnology

Mol. Cryst. Liq. Cryst.

Langmuir

J. Appl. Phys.

J. Chem. Phys.

Langmuir

Phys. Rev. Lett.

AIChE J.

European Coating Symposium

Liquid Film Coating: Scientific Principles and Their Technological Implications

Langmuir