Film Formation in Waterborne Coatings - American Chemical Society

Chapter 28

Fabrication and Application of Particle-Crystalline Films 1

2

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 19, 2015 | http://pubs.acs.org Publication Date: October 15, 1996 | doi: 10.1021/bk-1996-0648.ch028

Kuniaki Nagayama and Antony S. Dimitrov 1

Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Komaba, Meguro-ku, Tokyo 153, Japan L'Oreal Tsukuba Center, 5-5 Tokodai, Tsukuba 300-26, Japan 2

The formation of thin film organizations of small particles on a substrate has been studied since the beginning of this century. The main efforts were directed to studying the interparticle forces, which are responsible for the structure organization, but not to controlling these forces during the film formation process. Here, we present a novel approach to growing particle arrays, which allows the formation of centimeter-sized particle-crystalline monolayer films. We have specified three factors controlling the rate of an array growth, namely, the diameter of the particles, the particle volume fraction in the suspension, and the water evaporation rate, which depends on the temperature and the relative atmosphere humidity. We have also discriminated the forces arranging the particles into two-dimensional array, namely, the lateral capillary forces between the particles and the hydrodynamic pressure forces pressing each particle to the array's main body. Then, we proposed a simple way to adjust the rate of monolayer production to the growth rate of the densely-packed monolayer or bilayer two-dimensional (2D) array. The obtained large-sized monolayers consist of closely packed domains and, interestingly, in reflected light, mimic the morpho-coloring of the wings of rare types butterflies. In some scale, we are able to regulate the size of the obtained domains by controlling the thickness of the spreading suspension film. Enlarging the domain size can serve future technologies to producing new types of optical gratings, interferometers, antireflection coatings, selective solar absorbers, and especially mass-storage media and microelectronical units.

Ordered arrays of colloidal particles coated on surfaces can be used either as diffraction gratings, optical storage media, or interference layers. Monolayer or thicker layers of random or ordered colloidal particles has shown usage as lithographic masks for preparation of precisely controlled surface textures (/). Textured surfaces of controlled

0097-6156/96/0648-0468$15.50/0 © 1996 American Chemical Society In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


28. NAGAYAMA & DIMITROV

Fabrication of Particle-Crystalline Films

469

periodicity are of growing importance for several fields of science and technology. Surface roughness with a periodicity of 10 to 100 nm are responsible for enhanced Raman scattering (2). Random and periodic roughness on a submicron scale are parts of optical elements as interferometers (3), antireflection coatings (4,5), and optical gratings. Selective solar absorbers (6) utilize surfaces textured on a micron scale periodicity. Textured surfaces can play an important role in photovoltaics (7), and those with a perfect periodicity promise novel technologies for data storage, optics (8), and microelectronics. Especially promising are surfaces textured by layers of two-dimensional (2D) arrays of small particles. The simplest but uncontrolled way to form particle 2D arrays is to spread the particle suspension in a thin layer onto the substrate and leave the solution to evaporate (9). The spin-coating technique to form thin particle layers of 2D array, in which the suspension rapidly spreads on rotating substrates, has been developed by Deckman and his group (1,10-12). These approaches and the recently developed ones for formation of 2D particle array in wetting films in small cylindrical cells (13,14) do not allow to control the growth of the 2D array, which commonly results in a variety of defects, size restriction, and instabilities as, for example, sequences of uncontrollable voids and multilayers. The continuous growth of particle array on large surface areas has not been well documented In this article, we propose a novel approach to control the formation of large-sized monolayer 2D arrays from fine particles. The controlling mechanism is based on a quantitative analysis of the rate of the array growth. The obtained centimeter-sized monolayer 2D arrays illuminated by white light exert brilliant morpho-coloring as those of morpho butterfly wings. This report can be formally cuscriminated into three major focuses: 1) the idea for the continuous array growth and its analysis; 2) experimental realization for the formation of large-sized monolayer arrays; and 3) comparison of the structure and optical properties of monolayer arrays with those of Mo/pAo-butterfly wing surface. We would like to note that the experiments were carried out using negatively charged polystyrene particles. Our further experiments with silica particles confirmed that the phenomenon of a thin solid layer deposition on the substrate is quite general and does not depend on the particle internal properties. It is mainly determined by the physical properties of the suspension, particle-particle and particle-substrate interactions, and the particle geometry. The shape and uniformity of the particles are of primary importance for the size of the obtained ordered domains and, hence, for the color properties of the monolayer array. The essence of our fabrication of two-dimensional assembly of particles stands in the use of a stable wetting film that is made on the substrate as shown in Figure 1 (75). The wetting film plays two important roles: 1) it is a 2D liquid medium, where particles can be carried by water flow toward the array's boundary for growth (convective assembly shown in Figure 1C) (13-18), and 2) the deformation of its free liquid surface induces attractive force between the particles, when they are pressed between the film surfaces (lateral capillary force, Figure ID) (19-21). In parallel to the usual 3D crystallization, these processes can also proceed according to the free energy difference before and after the reaction in the non-equilibrium state. First, particles undergo the Brownian motion in the thick film (Figure 1A). When the wetting film becomes as thin as the particle size by removing water, ordered 2D domains start to

In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


470

FILM FORMATION IN WATERBORNE COATINGS

Figure 1. Two-dimensional assembly of particles in a wetting film. (A) Particles undergo the Brownian motion in the liquid layer, which has thickness much larger than the particle size. (B) Particles start to assemble in the wetting film as the thickness of the film becomes comparable to or slightly smaller than the particle diameter. (C) Convective flow responsible for the assembling of particles stimulated by the liquid evaporation. (D) Lateral capillary forces responsible for the hexagonally closed packing of the particles. (Adapted from ref. 15.)



28.

NAGAYAMA & DIMITROV


471

grow (Figure IB). The removal of water by evaporation is the only one explicit nonequilibrium condition in our system. The most difficult task in this fabrication is not to control the evaporation but to create stable wetting films that are appropriate for the submicron or nanometric particles. At common conditions, the thinning process on solid substrates usually leads to rupturing the film at nanometer-scaled thickness (22). Cleaved mica or acid-rinsed glass provides a very wettable surface and, consequently, is suitable for the array formation of colloidal particles when they are larger than 50 nm in diameter. The main our efforts were spent to find experimental conditions, at which the thickness of the spreading suspension films can be gradually changed. For a wettable solid plate dipped vertically in a suspension of fine particles, we found that monolayers and successive multilayers of 2D particle array spontaneously start to form on the plate surface from the plate-suspension-air contact line downward toward the bulk suspension. The successive formation of monolayers, bilayers, trilayers, etc. (23,24) was due to the increase in the wetting film thickness from the plate-suspension-air contact line toward the bulk suspension and to the continuous flux from the bulk suspension toward the film filling it up with densely packed particles. We were able to control the formation of a monolayer or multilayer by carefully withdrawing the substrate (the solid plate) together with the already formed particle arrays from the suspension. Theoretically, when the withdraw rate equals the rate of 2D array formation the array can be continuously formed to any size. The mechanism for an array formation on substrate plates dipped in water suspensions or solutions is the same as those behind scum formation on the walls of swimming pools, kitchen sinks, etc. Everywhere water contacts wettable surfaces and evaporates, the substances dissolved in the water accumulate in the spreading films at the vicinity of the three-phase contact line. The phenomenon is quite general and depends on the wettability of the surfaces and the properties of the wetting films, rather than the individual chemical properties of the substances dissolved or dispersed in the water. Materials, Apparatuses, and Procedures Materials. We used Matsunami microslides (76x26x1 mm) as the solid substrates. The water used in the experiments was purified by using MILLI-Q SP.TOC reagent water system. To control the properties of the spreading suspension film on the glass plates, we added to the suspensions no more than 0.001 mol/1 sodium dodecyl sulfate (SDS), 10^ mol/1 sodium chloride (NaCl), 0.001 mol/1 octanol (Wako Pure Chemical Industries, LTD), and 0.01 mg/ml protein (milk casein, Chameleon Reagent, Japan, or ferritin, Boehringer Mannheim GmbH, Germany). The total amount of the additives was kept to not exceed 5 vol.% of the particle volume fraction in the suspension. This restriction appeared to reserve the brilliant coloration of the obtained particlecrystalline films. The additives were mixed together in a closed 20-ml flask. Their concentrations were as follows: 0.01 mol/1 SDS, 0.01 mol/1 octanol, 0.1 mg/ml protein, and 0.001 mol/1 NaCl. This solution was being mixed and slightly warmed up to 40 °C until a transparently clear solution is formed. After cooling to the room temperature, portions of this solution were added to the particle suspension having in mind the restrictions above.


472



Table 1. Specifications of the latex particles given by the manufacturer Latex code

Diameter [nm]

Polydispersity [nm]

SS-021-P SC-171-S SC-108-S SC-953-S SC-081-S SC-051-S SC-048-S SC-032-S SC-015-S SC-008-S

2106 1696 1083 953 814 506 479 309 144 79

±17 ±47 ±10 ± 9 ±23 ±10 ± 5 ± 4 ± 2 ± 2

Source: Reprinted with permission from ref. 27. Copyright 1996.

The specifications of the polystyrene particles (Stadex, Japan Synthetic Rubber) that were used are given in Table 1. Originally, the suspensions contained 1 vol.% of particles. To exclude the eventual aggregates, the suspensions were filtered through cellulose acetate membrane filters, which had a pore size ranging from 2 to 4 times greater than the particle diameter. We assumed that the particle concentration did not change significantly during the filtration process. To concentrate the particles up to 6 vol.% in some of the experiments we used Millipore molecular filtration units with a cut-off pore size of 300 KDa. Experimental Setups. The simplest experimental setup designed for the preliminary experiments was, in fact, an inclined precleaned microslide plate (25). We placed a 100-μ1 drop of suspension onto the microslide plate and spread it uniformly along the plate surface. Then, we placed the plate on the goniometer stage, which was inclined at a desired angle. Due to gravity the suspension started to flow downward along the microslide plate and the layered array remained behind the receding drop-substrateair three-phase contact line. We used the above method to figure out the approximate array growth rate when changed the experimental conditions. Quantitatively detailing the array growth, we have designed and prepared a laboratory setup for withdrawing substrates from particle suspensions. The schematic of this setup and a photograph of the prototype are shown in Figure 2. The working cell was made in-house from pieces of microslides connected to each other by a polymeric glue. It has a narrow gap at the top for withdrawing the substrate plate. A 1mm gap was chosen to minimize solvent evaporation from the meniscus and confine the evaporation to the particle array film. The substrate plate hangs on a nonelastic string, which is attached to the periphery of a 2-cm in diameter wheel. A stepper motor driver rotates the wheel using a gear box. Thus, the substrate can be withdrawn from the suspension at a rate from 0.1 to 30 μηι/s. The array growth process is observed and



473



Figure 2. The setup for fabrication of 2D particle array on microslide substrate plates: (A) Schematics and (B) Photograph.



474


recorded using a horizontally placed video-microscope having a resolution power of about 350 nm. In fact, simultaneously keeping constant values for the humidity, temperature, and the particle concentration at the array's leading edge is quite difficult task. Furthermore, our controller of the stepper motor driver allowed only a manual stepwise change of the velocity in 300 steps. Both these inconveniences forced as to select a slightly higher than the calculated substrate withdrawal rate. Keeping this high withdrawal rate for about 1-2 min causes a rupture in the regular array growth and formation of striped particle films - see ref. 26 for the mechanism of the striped pattern formation. The state before rupturing the regular array formation is characterized with a rough leading edge of the array and an accelerated particles' speed with about 30%. We were continuously observing the array growth process and held on the substrate withdrawal (switch off the motor driver) just before the array to rupture. When the regular growth was reestablished we continued the substrate withdrawal (switch on the motor driver). We were able to apply this ofïïon switch procedure when the array's growth rate did not exceed 3 to 5 um/s. Array Formation Procedure. When a protein was not added to the suspension, the glass plates were kept in a chromate cleaning solution at least overnight, then rinsed with water, soaked for more than one hour in 0.1 M SDS solution (when particles larger than 506 nm particles were used) or in pure ethanol (when particles smaller than 479 nm were used). The plates soaked in the SDS solution were rinsed again with water and placed under a beaker to dry. The plates soaked in ethanol were placed directly under a beaker. The difference in the washing procedure was necessary to manage the thickness of the spreading suspension film. The SDS molecules adsorbed on the glass substrate kept the film thicker than the ethanol molecules did. The desorption process was slow enough and the surface properties were generally preserved for the time of the experiment - 2 to 3 hours. Because in most of the experiments the laboratory setup and the cell were opened to the room atmosphere, the existence of organic traces in the air often led to local dewettings on the substrate plates. Adding protein molecules significantly improved the plates' wettability. When a protein, ferritin or casein, was added to the suspension, the glass plates were used directly from the original packaging, without additional treatment. However, these additives hampered in some aspects the brilliant coloring of the obtained monolayer arrays. The experimental cell was sonicated for about 10 sec in a Ney 300 Ultrasonic water bath before each experiment. Then, the cell was washed using a soap solution, rinsed with plenty of water, and dried. After drying, the experimental cell and a glass substrate were mounted in the apparatus. The stepper motor driver for withdrawing the substrate was set at a rate 3 to 5 times higher than that estimated for a monolayer formation. Then, we filled the cell with a suspension and started to withdraw the substrate plate. To obtain an uniform thickness at the leading edge of the growing particle arrays, we monitored the array growth and gradually decreased the substrate withdrawal rate accordingly. We further proceeded the continuous formation of a monolayer 2D array as described above in the setup description.


28. NAGAYAMA & DIMTTROV

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To initiate a bilayer formation, we reduced the withdrawal rate by half. When the bilayer started to grow we readjusted the withdrawal rate to obtain uniform thickness at the leading edge of the growing arrays. A l l the experiments were done in a room at 25 °C and relative humidity of 48 % R H , measured using a thermo-hygrometer (TRH-10A, Shinyei, Japan). Color enhancement. The array from micrometer-sized particles exhibit brilliant coloring when illuminated by white light in their native state, after forming and drying. We enhanced this brilliance by coating the particles with silver or gold (up to 10 nm thick) using a vacuum coating technique. A n added benefit of this coating is the stabilization of the arrays on the substrate, which, in fact, are fragile before the metal coating. Characterization of the Obtained 2D Particle Array. The obtained centimeter-sized monolayer particle arrays show interesting color properties resembling natural beauties. We have studied the structure of the 2D particle array in comparison to the structure of butterfly wings, which in some aspects exhibit similar coloring due to the scattering of white light from a periodic surface texture. We used different metric-scale approaches in this investigation. The macroscopic outside view and scattering of white light were observed by naked eyes. The surface structure was figured out by using an optical microscope (Olympus B H , Japan) in reflected light with a dark-field option. To get details in the structures of both the obtained particle arrays and the wings of a butterfly we performed electron microscopy by using a field emission scanning electron microscope (S-5000H, Hitachi, Japan). We measured the thickness and the refractive index of densely packed monolayers of 144-nm particles by using an ellipsometer (Gaertner Scientific Corporation, Chicago). The incidence angle of the helium-neon laser beam was set at 70°. Using the same apparatus, we also measured the thickness of horizontal wetting water films formed on the microslides by spreading of large water drops. Schematics of the Array Growth Process The profile of growing 2D particle array from bulk suspension onto a withdrawing substrate plate, water evaporation flux,./*, and water and particle fluxes,^ and j , in the vicinity of the array's leading edge are schematically shown in Figure 3 - see also refs. 27 and 28. The width of the plate is large enough and the growth disturbances at the edges can be neglected, namely, in our model the array's leading edge is a straight line parallel to the plane of the horizontal suspension surface. The formation of layered 2D array can be conveniently split in two main stages: 1) convective transfer of particles from the bulk of the suspension to the thin spreading film (upward in our experiments) due to water evaporation from the film surface and 2) interaction between the particles that lead to specific textures. p

Regular Formation of Particle Array Films. The primary driving force for the convective transfer of particles is the water evaporation from the freshly formed particle array. In an atmosphere saturated with water vapor, and after establishing



476


Figure 3. Schematics of the particle and water fluxes at the leading edge of the monolayer particle array growing on a substrate plate that is being withdrawn from a particle suspension. Here, v is the substrate withdrawal rate, j is the water influx, j is the respective particle influx, and j is the water evaporation flux. (Adapted from ref. 27) w

p

w

e



477


mechanical equilibrium, the pressure balance in an infinitely small bulk volume inside the spread suspension film is: Π + p = p + p , where p = Apghc (1) where Π is the sum of van der Waals and electrostatic adjoining pressures for the suspension wetting films on the substrate plate, Pep is the capillary pressure due to the curvature of the liquid surface between neighboring particles in the particle film, P is a reference capillary pressure, Ph is the hydrostatic pressure in a vertical film, h is the relative height, Δρ is the density difference between the suspension and the surrounding gas atmosphere, and g is the gravity acceleration. When the evaporation of water starts, the right-side terms in eq 1 stay almost constant for a given h . If we assume that P is related to the horizontal suspension surface, i.e., P = 0, then due to the decrease in the total suspension volume as the water evaporates, h slowly increases. We minimized the change in h by adding an amount of suspension to compensate the suspension volume decrease. The left-side terms in eq 1, however, increase due to the increase in the curvature of the menisci between the particles, namely, Pep increases, and due to the thinning of the film Π also increases. In some cases, σΠ/dh > 0, where h is the film thickness, but these films are not stable and not suitable for our technique. Therefore, in an atmosphere, unsaturated with water vapor, a pressure gradient, AP, from the suspension toward the wetting film arises due to the water evaporation from the freshly formed particle array (the flux j in Figure 3). The pressure gradient AP = (Π + ) - ( + p) (2) produces then a suspension influx from the bulk suspension toward the wetting suspension film. This influx consists of a water component, j , and of a particle flux component, j . The water part of the suspension influx j compensates for the water evaporated from the film j , and the particle complementary part,yp, causes particles to accumulate in the film, thus forming dense structures. Naturally, the particle structures thus formed follow the film geometry (23,29). The thickness of vertical wetting films increases from the plate-suspension-air contact line downward toward the bulk suspension due to the hydrostatic pressure. Then, successive monolayers, bilayers, trilayers, etc. are expected to be formed as the continuous particle flux j fills up the space between the substrate and the film surface. In fact, we observed the formation of successive multilayers when a wettable plate dipped in a suspension of fine particles was kept stationary. To model the process of regular array growth, we applied the approach of the material flux balance at the array's leading edge (14,25). Thus, we calculated the rate of the substrate withdrawal, which must actually be equal to the rate of the array growth. In the developed model we assume two idealized conditions; 1) complete wetting: the suspension wets the substrate by forming a stable wetting film and 2) frictionless: the particles do not stick onto the substrate, if they are not strongly pressed by the film surface as, for example, the larger particle pointed in Figure 4. Both assumptions can be validated for many systems with practical applications. For a regular and continuous formation of 2D particle array onto a substrate plate, which is schematized in Figure 3, the total water evaporation flux from the particle arrays per unit length of the array's leading edge, Λνύρ, is the integral of water cp

c

h

h

c


c

c

c

c

c

c

e

Pcp

Pc

h

w

p

w

e

p



478


Figure 4. A part of the leading edge of a growing monolayer particle array. In the upper-half of the photograph, a forming single domain of ordered 953-nm particles is shown. The lower-half shows the suspension meniscus, where the water influx presses the particles toward the fonning monolayer. Because of the high velocity in the microscale, v ~ 100 μιη/s, the particles are seen as short fuzzy lines. The few particles seen as bright spots (one is indicated by an arrow) have diameters larger than the average one. They are wedged into the wetting film. The hydrodynamic force is not sufficient to overcome the capillary resistance near the growing arrays boundary. (Adapted from ref. 27) p


28. NAGAYAMA & DIMTTROV

479


evaporation fhix,j (z), along the axis Z,

= J j (z) dz. For practical treatment,

e

e

ο

we introduce an evaporation length, / = I j . Here, the evaporation flux from a pure water surface, j , depends on the temperature and humidity of the surrounding atmosphere. Both parameters can be experimentally determined. For the steady state process of an array growth the water evaporation, is exactly compensated by the water flow from the bulk suspension into the arrays, J that is, J = J^p or e

e

W9

where J Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 19, 2015 | http://pubs.acs.org Publication Date: October 15, 1996 | doi: 10.1021/bk-1996-0648.ch028

w

= h/ j

and J

w

w

hfh = IJ., (?) = I j . Here, A/is the thickness of the wetting film at

evap

e

the height of the array's leading edge, and is usually slightly smaller than the thickness of the particle arrays, h. The compensation water influx, j , is defined as j Nw Vw Vwt, where N is the number of water molecules per unit volume, V is the water molecule volume, and v is the macroscopic mean velocity of the water molecules. The macroscopic mean velocity of the suspended particles, v , is proportional to v namely, v = βν**· The value of the coefficient of w

=

w

w

w

wt

p

wh

P

proportionality, /?, depends on the particle-particle and particle-substrate interactions and should vary from 0 to 1. The stronger the interactions, the smaller the value of β. For non-adsorbing particles and dilute suspensions β approaches 1. Then, the particle flux, j , which is defined as j = N V v (where N and V are the number of p

p

P

P

P

p

p

particles per unit volume and the volume of a single particle, respectively), is proportional to j , w

>. - 7 ^ >φ = NV is the particle volume fraction in the suspension and - φ = NwVw is the water volume fraction.

where 1

P

P

The particle flux, j , drives the growth of particle arrays because the particles attach to the array's leading edge and remain there. The stock of the particles at the array leading edge, h/j , is equal to the increase in the total particle volume in the array, namely, the product of the array growth rate, v , thickness of the array, h, and array density, 1 - ε, (ε is the porosity of the array) h(l - ε) = h j . (5) p

p

c

Vc

f

p

By substituting^,, from eq 3 into eq 4, and the resulting expression for j into eq 5, the rate of the array growth is: p

_

βΐ

Je

Ψ

( f

.

In the derivation of eq 6, the values of h and (1 - ε) are not important. What is important is their product, which shows the total volume of particles per unit area. To connect h and (1 - ε) with the real geometry of the particle array we assume that h is the distance from the substrate to the tops of the particles, namely, for a particle monolayer h = d (d is the diameter of the particles). Then (1 - ε) is geometrically calculated from the conditions for densely (hexagonally) packed spheres, namely, 1 - ε = 0.605. Taking into account that the dense particle multilayers are, in fact, monolayers displaced one


480


over another, we substitute h(\ - ε) with 0.605&/ for flayer particle array - see for example ref. 17 for the particle film thickness. Then, for the formation of dense twodimensional particle array (monolayers and multilayers as well), we estimate the withdrawal rate of the substrate plate, v , by rewriting eq 6 w

v w

= y v

=

-ÊL

.

(7)

0.605 kd(l-

Film Formation in Waterborne Coatings - American Chemical Society

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