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Deposition of Poly(styrene/r-tert-butoxy-ωvinyl-benzyl-polyglycidol) Microspheres on Mica Plates Crossing the Liquid-Air Interface: Formation of Stripe Pattern Ewelina Przerwa, Stanislaw Sosnowski, and Stanislaw Slomkowski* Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland Received January 9, 2004. In Final Form: March 15, 2004 Formation of stripelike assemblies of poly(styrene/R-tert-butoxy-ω-vinyl-benzyl-polyglycidol) microspheres adsorbed on nonpatterned mica plates moving perpendicularly from suspension of particles through the water-air interface has been observed. It was found that ordered assemblies were formed by capillary forces acting on particles crossing the water-air boundary. At sufficiently high rates of plate movement (i.e., at appropriate dynamic loading conditions) the adsorbed microspheres approaching the water surface begin sliding on the plate, due to capillary forces, in the direction opposite to the plate movement and are kept below the water surface. Plate movement brings new adsorbed particles to the water-air interface, where particles are assembled into aggregates. When particle aggregates are large, the capillary forces cannot overcome shearing forces and the particle assemblies are withdrawn on the plate above the water surface. This process repeated during continuous movement of the plate results in the formation of the quite regularly distributed stripes of adsorbed microspheres. Formation of the regularly distributed particle assemblies depends on concentration of microspheres in suspension.
1. Introduction Adsorption of well-defined polymer microspheres from liquid suspensions onto solid supports has been extensively analyzed by many researchers.1-6 This process can be considered as a model of events occurring during many practically important processes such as transportation of colloidal suspensions in industrial installations, painting, deposition of particulate materials on ship’s hulls, surface modification of elements of biomedical equipment, fabrication of biosensor detectors, deposition of proteins on surfaces of implants, and many others. Typically, the adsorption of microspheres onto substrates with isotropic surfaces (with respect to the chemical structure and morphology), fully immersed in a suspension of polymer microspheres, results in the formation of particle monolayers. Obviously, the random spatial distribution of microspheres in monolayers is often affected by short distance particle-particle interactions. However, for some applications (e.g., for fabrication of elements of detectors of biosensors) there are needed methods allowing the formation of objects with surfaces covered with monolayers of microspheres assembled in a controlled, often regular fashion. Adsorption or covalent immobilization of microspheres in selected surface areas was accomplished by surface * To whom correspondence should be addressed. Phone: +4842-6826537. Fax: +48-42-6847126. E-mail: staslomk@ bhilbo.cbmm.lodz.pl. (1) Adamczyk, Z.; Siwek, B.; Zembala. M.; Belousek, P. Adv. Colloid Interface Sci. 1994, 48, 151-280. (2) Polverari. M.; Vandeven, T. G. M. J. Colloid Interface Sci. 1995, 173, 343-353. (3) Bar, G.; Rubin, S.; Cutts, R. W.; Taylor, T. N.; Zawodzinski T. A., Jr. Langmuir 1996, 12, 1172-1179. (4) Miksa, B.; Slomkowski, S.; Chehimi, M. M.; Delamar, M.; Majoral, J.-P.; Caminade, A.-M. Colloid Polym. Sci. 1999, 277, 58-65. (5) Adamczyk, Z.; Weronski, P.; Musial, E. J. Colloid Interface Sci. 2001, 241, 63-70. (6) Adamczyk, Z.; Weronski, P.; Musial, E. J. Chem. Phys. 2002, 116, 4665-4672.
patterning preceding the adsorption or covalent immobilization steps.7 The patterning was performed most often by various microlithography-related procedures. The processes introduced chemical groups in chosen places, enhancing (or reducing) physical interactions or chemical reactions with groups in surface layers of microspheres. Extensive studies have been carried on self-organization of polymer microspheres during drying of droplets of suspensions microspheres on flat and smooth supports (so-called convective assembly).8-10 Ordering of microspheres in these experiments was induced by capillary forces emerging when a thickness of the layer of suspending medium was equal or lower than diameters of adsorbed particles. Lateral components of these forces moved the particles together, packing them into two-dimensional crystals. Moreover, evaporation of liquid from the already formed array causes a flux of liquid, bringing new particles to this area.10 In principle, any self-organization requires action of properly directed forces onto particles. A field of welldirected forces is usually present at interfaces. In this paper we present results of our studies on morphology of particle assemblies formed on surface mica plates (modified with γ-triethoxyaminopropylsilane) crossing with a controlled rate the interface between the water suspension of poly(styrene/R-tert-butoxy-ω-vinyl-benzyl-polyglycidol) microspheres and air. Mica has been chosen for our studies since mica plates with clean and well-defined surfaces could be easily obtained by cleaving them from thick mineral specimens. Modification of mica with γ-triethoxyaminopropylsilane yields a surface with amino (7) Slomkowski, S.; Kowalczyk, D.; Trznadel, M. Trends Polym. Sci. 1995, 3, 297-304. (8) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. (9) Kralchevsky, P.; Nagayama, K. Adv. Colloid Interface Sci. 2000, 85, 145-192. (10) Kralchevsky, P. A.; Denkov, N. D. Curr. Opin. Colloid Interface Sci. 2001, 6, 383-401.
10.1021/la0499259 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/22/2004
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Table 1. Recipes for Syntheses and Parameters Characterizing Microspheres microspheres
styrene, g
BVBpolyGL,a g
K2S2O8, g
H2O, mL
Dn, nm
Dw/Dn
SCAG,b mol/m2
P(S/PGL)1 P(S/PGL)2 P(S/PGL)3
8.0 12.5 8.0
0.40 0.11 0.08
0.16 0.25 0.16
100 100 100
350 650 1000
1.016 1.008 1.054
8.77 × 10-7 1.57 × 10-7 1.65 × 10-6
a BVBpolyGL, an abbreviation for R-tert-butoxy-ω-vinyl-benzyl-polyglycidol. b SCAG denotes the concentration of acidic groups on the surface of microspheres.
Scheme 1. Structure of ω-Vinyl-benzyl-polyglycidol
groups which protonated form cations suitable for ionic interactions. Synthesis of poly(styrene/R-tert-butoxy-ω-vinyl-benzylpolyglycidol) microspheres, which have been selected for experiments, has been elaborated recently in our laboratory.11 These particles have the core-shell structure with a core rich in polystyrene and shell enriched in hydrophilic polyglycidol. In water media the hydrophilic polyglycidol shell provides a protective layer eliminating irreversible adsorption of microspheres from water suspensions. However, physical reversible adsorption of poly(styrene/ R-tert-butoxy-ω-vinyl-benzyl-polyglycidol) microspheres on positively charged surfaces should be facilitated by negative charge of these particles introduced during initiation with K2S2O8.11,12 2. Experimental Part Microspheres. Poly(styrene/R-tert-butoxy-ω-vinyl-benzylpolyglycidol) microspheres have been synthesized by a free radical emulsion/dispersion copolymerization of styrene and R-tertbutoxy-ω-vinyl-benzyl-polyglycidol (BVBpolyGL) (cf. Scheme 1) according to the recipe published in our earlier paper.11 Molecular weight of BVBpolyGL used for the synthesis was 2700. Briefly, the known amounts of BVBpolyGL solution in water (distilled three times, pH adjusted to 6.0 by addition of K2CO3) and styrene free from a stabilizer (4-tert-butylcatechol stabilizer was removed by distillation of styrene at 30 °C under reduced pressure) were charged into a reactor. An initiator, K2S2O8, was added to the mixture. Oxygen was removed from the mixture with argon. Polymerizations were carried out under argon, at 65 °C with stirring at 60 revolutions per minute. Polymerization time was 28 h. Traces of unreacted styrene were removed from the mixture by steam stripping. This procedure consists of bubbling steam through the mixture kept under slightly reduced pressure with the purpose of avoiding excessive water condensation and dilution of the suspension of microspheres. Thereafter, the microspheres were isolated by centrifugation and washed with 10-3 mol/L HCl and then with new portions of water. The cenrtifugation/washing steps were repeated four times. Diameters of microspheres were determined from scanning electron microscopy (SEM) microphotographs registered using a JEOL 5500LV apparatus. The averages were calculated on the basis of the measurements of diameters of at least 600 randomly chosen particles in various SEM pictures. The concentration of acidic groups on the surface of microspheres was determined by conductometric titration with KOH. Prior to titration, a sample of suspension of microspheres was passed through a Dowex 50WX4 ion-exchange resin. Three types of microspheres were synthesized. They were denoted as P(S/PGL)1, P(S/PGL)2, and P(S/PGL)3. Recipes for the syntheses, the number-average (11) Basinska, T.; Slomkowski, S.; Dworak, A.; Panchev, I.; Chehimi, M. M. Colloid. Polym. Sci. 2001, 279, 916-924. (12) Radomska-Galant, I.; Basinska, T. Biomacromolecules 2003, 4, 1848-1855.
Figure 1. Setup used for deposition of microspheres: (a) mica plate, (b) supporting glass plate, (c) container with suspension, (d) environmental chamber, and (e) step motor. diameters of microspheres (Dn), diameter polydispersity parameters (Dw/Dn, where Dw denotes the weight-average diameter), and surface charge of obtained microspheres are given in Table 1. Mica Plates. The freshly cleaved mica plates (cut to a size ca. 2.5 × 5 mm) were washed with a 1:1 (by volume) 2-propanolwater mixture and then with toluene, ethanol, and water (three times distilled). Afterward, the plates were dried at room temperature. In the next step, the plates were transferred to the toluene solution of γ-triethoxyaminopropylsilane (concentration 12.5 vol %) and kept in this solution for 24 h at room temperature. Thereafter, the plates were washed three times with toluene, six times with ethanol, and three times with water. Finally, the plates were heated in an oven at 50 °C for 2 h. The plates, prepared in the described way, were used as substrates for deposition of microspheres. Mica plates not modified with γ-triethoxyaminopropylsilane were also used in some experiments. Setup Used for Deposition of Microspheres. Microspheres were deposited on mica plates using a setup illustrated in Figure 1. Mica plates attached to a glass holder and a glass beaker, containing a suspension of microspheres with known concentration of particles, were placed into a metal chamber allowing for control of internal temperature (environmental chamber). The environmental chamber was equipped with glass windows and with a small aperture for a thread. With use of the thread the glass holder was mounted to an axle of a step motor. The whole setup had been placed on a table providing isolation from accidental vibrations. In a typical experiment three mica plates modified with γ-triethoxyaminopropylsilane were mounted on the glass holder and immersed in a suspension of microspheres (pH ) 6.0, ionic strength adjusted to 10-3 mol/L by addition of NaCl). Immediately afterward, the step motor was switched on and the plates were withdrawn from suspension with rates controlled in the range from 4.7 to 18.3 µm/s. The plates withdrawn from suspension were detached from the glass holder and microphotographs of their surfaces were registered by scanning electron microscopy (SEM).
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Figure 2. Photo of mica plate crossing the interface between the suspension of microspheres and air. Registration of SEM Microphotographs of Microspheres Adsorbed to Mica Plates. Microphotographs of mica plates with adsorbed microspheres were recorded using a JEOL 5500LV scanning electron microscope. Since the initial plates were too large for placing them in the designated SEM compartment, they were placed on the SEM holder and cut to the needed dimensions. The plates were gold-sputtered using the standard JEOL provided equipment. In this way the static charge, developing during registration of SEM pictures, was eliminated. All operations were done with special care to avoid any adventitious touching of the surfaces of plates prepared for analysis. Microphotograps were recorded in electronic form and stored in the tif format. Positions of the microspheres on mica plates (their X,Y coordinates) were determined by using the SigmaScan Pro Version 5.0.0. Contact angle of water on the plates was determined using a Rame-Hart goniometer.
3. Results 3.1. Wettability of Mica Plates. Measurements of contact angle of water drops on the surface of mica revealed that modification with γ-triethoxyaminopropylsilane significantly increased hydrophobicity of the plates. Before modification with γ-triethoxyaminopropylsilane the contact angle of water on a mica plate was equal to 10.6° ( 0.4°. However, after modification, the contact angle became much higher (79.4° ( 0.8°). It is worth noting that there was a significant difference between the advancing and receding contact angle of water on mica plates modified with γ-triethoxyaminopropylsilane. Whereas the advancing contact angle equal to 78° ( 2° did not differ significantly from the initial one (79.4° ( 0.8°), the receding contact angle was much lower (58° ( 2°). Thus, it is obvious that mica plates modified with γ-triethoxyaminopropylsilane become more hydrophilic after contact with water. Probably, this effect is due to binding water molecules (via hydrogen bonds) to amino groups of γ-triethoxyaminopropylsilane derivatives attached to the plates. Conversion of hydrophobic γ-triethoxyaminopropylsilane-modified mica plates to hydrophilic ones after contact with water is clearly visible when such a plate is withdrawn from suspension of microspheres to air (cf. Figure 2). The meniscus on the plate is concave, indicating that after contact with water the initially hydrophophobic γ-triethoxyaminopropylsilane-modified mica plate became hydrophilic. The contact angle on the moving plate was ca. 30°. 3.2. Morphology of Particle Assemblies on Mica Plates Not Modified and Modified with γ-Triethoxyaminopropylsilane. Mica plates modified and not modified with γ-triethoxyaminopropylsilane were used in this set of experiments. Microspheres (P(S/PGL)2; with Dn ) 650 nm) were deposited from the suspension in which the concentration of particles was equal to 10 g/L. Plates were withdrawn from the suspension with a constant rate
Figure 3. SEM microphotographs of (P(S/PGL)2 deposited on mica plates: not modified, (a) and (b), and modified with γ-triethoxyaminopropylsilane, (c) and (d). Arrows indicate direction of the plate movement during withdrawal from the suspension of microspheres.
equal to 4.7 µm/s. The temperature of the suspension was maintained at 20 °C. In Figure 3a-d are shown typical microphotographs of particle assemblies. There is a significant difference between shape of particle assemblies adsorbed on not modified and on modified (with γ-triethoxyaminopropylsilane) mica plates. In the first case the particle assemblies either tend to form two-dimensional colloidal crystals with hexagonal packing of microspheres or elements of three-dimensional colloidal crystals with various defects. It has to be mentioned that such assemblies were already observed and their formation was attributed to capillary forces between microspheres in thin liquid films.8-10 On the other hand, the particle assemblies adsorbed on mica plates modified with γ-triethoxyaminopropylsilane displayed a different morphology. This morphology was characterized by an arrangement of particles into stripes that were perpendicular to the plate movement from suspension. 3.3. Morphology of Particle Assemblies on Mica Plates Modified with γ-Triethoxyaminopropylsilane: The Role of Rate of Plate Movement. P(S/PGL)3 microspheres (Dn ) 1.00 µm) were adsorbed from suspension (10 g/L) onto mica plates modified with γ-triethoxyaminopropylsilane. The plates were moved across a liquid-air interface with constant rates ranging from 4.7 to 18.5 µm/s. Typical SEM microphotographs of obtained particle assemblies deposited on the plates are collected in Figure 4. From Figure 4 it is evident that in experiments with the rates of plate movement below 10 µm/s particles were adsorbed in an irregular fashion. When the plates were moved with the rate 11.1 µm/s, some orientation of particle assemblies became visible. At still a higher rate (13.8 µm/ s) adsorbed particles were assembled into quite regularly distributed stripes. The distance between these stripes was equal to 70 ( 20 µm/s and the angle they formed with the direction of the plate movement was 90° ( 15°. When the rate of the plate movement was equal to 18.5 µm/s, the large areas of plates were free from the particles. Only in the lower parts of these plates were the thick multilayered particle assemblies deposited. A similar set of experiments was performed for P(S/PGL)1 microspheres (Dn ) 0.35 µm). The particles were adsorbed also from suspension with a concentration of microspheres of 10 g/L. The plates were moved across the liquid-air interface
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Figure 5. SEM microphotographs of (P(S/PGL)3 microspheres deposited on mica plates modified with γ-triethoxyaminopropylsilane. For all plates the rate of crossing the liquid-air interface was the same and equal to 13.8 µm/s. Concentrations of microspheres were equal: 2.5 g/L (a), 10 g/L (b), 15 g/L (c), and 20 g/L (d).
Figure 4. SEM microphotographs of (P(S/PGL)3 microspheres deposited on mica plates modified with γ-triethoxyaminopropylsilane. Rates with which the plate crossed the liquid-air interface were equal: 4.7 µm/s (a), 6.6 µm/s (b), 8.3 µm/s (c), 11.1 µm/s (d), and 13.8 µm/s (e) and (f).
with rates from 4.7 to 8.3 µm/s. For P(S/PGL)1 microspheres the particle assemblies arranged into stripes perpendicular to the plate movement were observed for the rate of the plate movement equal to 6.6 µm/s. The distance between stripes of adsorbed P(S/PGL)1 microspheres was 35 ( 5 µm/s and the angle the stripes formed with the direction of the plate movement was 92° ( 11°. 3.4. Morphology of Particle Assemblies on Mica Plates Modified with γ-Triethoxyaminopropylsilane: Role of Particle Concentration. P(S/PGL)3 microspheres (Dn ) 1.00 µm) were adsorbed from suspensions in which particle concentrations were varied from 2.5 to 20 g/L. All plates were moved through the liquidair interface with the same rate equal to 13.8 µm/s. It was noticed (examples of microphotographs are shown in Figure 5) that at low concentration of microspheres (2.5 g/L) only isolated particles were adsorbed onto the surface of mica. For a concentration of microspheres equal to 10 g/L adsorbed particles were arranged into the regularly distributed stripes (Figure 5c). At concentrations of microspheres equal to 15 and 20 g/L the surface concentrations of adsorbed particles were high but any orientation of the assemblies of adsorbed particles was rather weak. 4. Discussion Previous studies revealed that adsorption of microspheres onto nonpatterned plates immersed in suspension proceeds in such a way that adsorbed particles and oneparticle thick microsphere assemblies are distributed randomly on the surface of the plates without any specific orientation.4,7,13 Thus, it was obvious that adsorption of microspheres in the form of the stripelike assemblies obtained in experiments described in this paper resulted from interactions close to the solid-liquid-gas (platewater-air) boundary.
It has been shown already (cf. Figure 2) that liquid suspension of microspheres forms a concave meniscus on the mica plate moving upward from the liquid. The form of menisci was investigated in detail for various static systems. However, in principle, the exact description of the shape of the meniscus formed by water on the plate moving across the liquid-air interface poses some difficulties. Generally, the shape of the meniscus close to the plate immersed in liquid could be described by the Laplace equation of capillarity with boundary conditions taking into account hysteresis of the contact angle. However, in the case of the moving plate in the Laplace equation one should replace pressure by a tension tensor introduced in equations of motion in viscous fluids.14 Thus, the modified Laplace equation should contain an additional term (third term in eq 1) corresponding to the viscous drag of the liquid by the plate moving upward:
ηVχ 1 Fg + zexp(-χx) ) 0 R σ σ
(1)
In eq 1 R denotes radius of curvature of the line determined by the cross section of the surface of liquid and the vertical plane perpendicular to the plate (cf. Figure 6), F, η, and σ denote density, viscosity, and surface tension of liquid, respectively. Acceleration due to gravity is denoted as g. Elevation of the surface of liquid above the level far from the plate is denoted as z and the distance from the plate is denoted as x. V denotes rate of the vertical plate movement and χ reciprocal of the characteristic distance at which the drag forces are transmitted. Remembering the following fundamental relation
dz 1+( ) ) ( dx R)-
2 3/2
d2z dx2
(2)
(13) Slomkowski, S.; Miksa, B.; Chehimi, M. M.; Delamar, M.; CabetDeliry, E.; Majoral, J.-P.; Caminade, A.-M. React. Funct. Polym. 1999, 41, 45-57. (14) Landau, L. D.; Lifszyc, E. M. Fluid Mechanics, 2nd ed.; Reed Educational and Professional Publ.: Oxford, 2000; Chapters II and VII, pp 44-92, 238-248.
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Figure 6. Meniscus of suspension of microspheres at the plate crossing the liquid-air interface.
Figure 7. Capillary forces acting on the particle adsorbed onto a plate at the water-air interface.
and substituting formula (2) into eq 1, one obtains the equation relating z, dz/dx, d2z/dx2, and x. This equation could be solved numerically. Fortunately, for experiments described in this report rates of plate movement are very slow and do not exceed 10-4 m/s. For such system influence of the drag of liquid by the moving plate could be neglected. Indeed, for water at 20 °C F ) 1 g/cm3 and σ ) 73 g/s2. Close to the mica plate z does not exceed 5 mm. Thus, next to the wall of the plate the second term in eq 1 Fgz/σ is close to 7 × 10-3 cm-1. For a plate moving with rate V ) 10-3 cm/s in water the maximal value of the last term in eq 1 (ηVχ/σ) equals 1.4 × 10-7 cm-1 and is 4 orders of magnitude lower than the second term (for water η ) 10-2 g/(cms) and reciprocal of the thickness of the boundary layer on the plate χ ≈ x(zη)/(FV) equals 1 cm-1 at the distance z ) 1 mm from the surface).15 Thus, it is evident that the shape of the meniscus can be described by the Laplace equation for the static system, that is, by eq 1 in which V ) 0 and R is determined by formula (2). Integration of this equation gives a known solution relating x and z,
radius of the contact line between water and air on the surface of the particle, and ψ an angle between the plane of the three-phase contact line and tangential to the surface of water. For small particles adsorbed on the plate R is close to the contact angle of water on mica. One component of the capillary force (Fσ|) acts on the particle toward the plate surface. The second one (Fσ⊥) is directed parallel to the plate, in the direction opposite to the plate movement (cf. Figure 7).
Fσ⊥ ) 2πσr sin ψ cos R Fσ| ) 2πσr sin ψ sin R
(5)
(4)
It is worth noting that, for the highly hydrophilic plates, for which the contact angle of water is very low, Fσ| is very low too. On the contrary, for particles adsorbed onto plates for which the contact angle equals 90° the whole capillary force is directed parallel to the plate. Capillary forces could be relatively very strong. For example, for a particle with diameter equal to 1 µm, made from a material with density 1 g/cm3, for which the contact angle of water equals 45° and adsorbed on a plate for which the contact angle also equals 45°, the maximal value of Fσ| is 0.115 µN whereas the gravitational force acting on this particle in air is 230 times weaker. For a microsphere adsorbed onto the mica plate taken out from suspension with a rate ranging from 4.7 to 18.3 µm/s, the process of crossing the water-air interface is a rapid event. In the case of particles with diameter 1 µm, adsorbed onto the mica plate modified with γ-triethoxyaminopropylsilane (for which sin R is ca. 0.5), the time between the moment the particle is just below the surface of water and the moment the whole particle is above water takes only from ca. 50 to 200 ms. During this time the capillary force acting on the particle increases from 0 to its maximal
is directed at an angle R to normal to the plate (cf. Figure 7). In formula (4) σ denotes surface tension of water, r
(15) Landau, L. D.; Lifszyc, E. M. Fluid Mechanics, 2nd ed.; Reed Educational and Professional Publ.: Oxford, 2000; Chapter IV, pp 157189.
[ x
2 1 + x ) ln q zq
]
4 2 - 1 + x1 - (qz)2 + A (3) 2 q (zq)
where q2 ) Fg/σ and A is chosen in such a way that at the wall of the plate the needed relation for contact angle is fulfilled
(dxdz|
) -ctgθ x)0
)
Adsorbed particle experiences capillary forces when transported on the plate across the surface of water. The force equal to
Fσ ) 2πσr sin ψ
Deposition of Microspheres on Mica Plates
Figure 8. Scheme illustrating the formation of microsphere assemblies on plates crossing the water-air interface. “A” denotes the line of the water-air-plate contact, Fσ| denotes the capillary force component parallel to the plate and directed against the plate movement, Fp denotes reaction force transmitted by the adsorbed particle moving upward with the plate, and “a”, “b”, “c”, and “d” are snapshots of microsphere assemblies at various states of plate movement.
value and then drops to 0 again. Under action of the capillary force component directed parallel to the plate the particle may begin to slide or roll on the plate surface. Fundamental laws of mechanics indicate that when force acting on an object increases very slowly (quasistatic conditions), movement begins when the applied force exceeds the static shearing (or rolling) force. This force is proportional to the force acting on an object toward the surface. However, at dynamic conditions (force increases rapidly) detachment of an object and its movement could be induced even under subcritical loading.16 Thus, whether the particle crossing the water-air interface begins to slide (or roll) on the plate or not may depend not only on the value of surface tension forces but also on the rate at which crossing of the water-air interface does occur. This explains the marked changes in morphology of the assemblies of adsorbed particles (cf. Figure 4) when the rate of the plate movement exceeds some critical value. The particle sliding on the plate is confined in a thin wedgelike film of water. Further movement of the plate may bring sequentially new particles to the surface (cf. Figure 8). It has to be noted that the capillary forces act only on the partially immersed particles whereas the shear forces are due to all microspheres in the cluster. Thus, (16) Seifert, U. Phys. Rev. Lett. 2000, 84, 2750-2753. (17) Slomkowski, S.; Kowalczyk, D.; Trznadel, M.; Kryszewski, M. In Hydrogels and Biodegradable Polymers for Bioapplications; Ottenbrite, R., Huang, S., Park, K., Eds.; ACS Symposium Series 627; American Chemical Society: Washington, DC, 1996; pp 172-186.
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when the size of the cluster is large enough, friction forces prevail over capillary forces and eventually the aggregate remains immobilized on the plate and is withdrawn above the water-air interface. Capillary forces and shear forces may not act along one line and thus may create a moment of force, turning the aggregate (e.g., as is shown in Figure 8). Rotation of particle aggregates will orient them in such a way that their longer axes are perpendicular to the plate movement. In effect the stripes of the particles are formed. Initial random adsorption of particles on the plates should lead to the formation of particle aggregates with similar size after the plate moves in a similar way. This may explain why the stripes of adsorbed particles are separated roughly by a similar distance. Concentration of particles in suspension also plays an important role. For high concentrations of particles in suspensions the surface density of particles adsorbed on immersed plates is also high and eventually adsorbed microspheres are not isolated but form assemblies in which particles are in contact. The capillary forces can move only isolated microspheres and small particle aggregates; thus, the large aggregates formed at high concentrations of microspheres survive intact the process of crossing the water-air interface. In effect, morphology of particle assemblies formed at such conditions (cf. Figure 5c,d) is similar to the morphology of assemblies of irreversibly adsorbed microspheres.4,17 It has to be noted also that, on hydrophilic mica, not modified with γ-triethoxyaminopropylsilane, we did not notice any formation of the stripelike assemblies of microspheres. This conforms to the model described above. For strongly hydrophilic plates the contact angle is low (e.g., for mica plates not modified with γ-triethoxyaminopropylsilane R ) 10.6° ( 0.4°) and the force Fσ| is apparently too low to move microspheres on the plate.8-10 One also has to remember that between particles on plates above the water-air interface still some residual water could remain. Presence of this very thin film of water will result in interparticle capillary forces that additionally could move particles together. 5. Concluding Remarks The main results of this study can be summarized as follows. At properly chosen conditions a simple process consisting of moving a smooth, non-patterned mica plate from a suspension of microspheres allows for controlled particle deposition. Particles are assembled into stripes oriented perpendicularly to the plate movement. The stripes are distributed quite regularly on the plate. Assembly of microspheres into the stripelike patterns is due to capillary forces acting on microspheres crossing the water-air interface. Formation and quality of particle assemblies and of particle assembly distribution are strongly affected by the particle-plate adhesion forces (microspheres should be weakly adsorbed to allow their sliding or rolling on the plate) and shape of the water surface in the vicinity of the plate and on microspheres crossing the water-air interface as well as on the rate of plate movement. Concentration of microspheres in suspension also affects the formation of adsorbed particle assemblies. Acknowledgment. This work was supported by the Polish Committee for Scientific Research Grant No. 7 T09A 05820. LA0499259
