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Langmuir 1999, 15, 2466-2476
Effect of Surface Hydrophobicity on the Hydrodynamic Detachment of Particles from Surfaces Alexandre M. Freitas and Mukul M. Sharma* Department of Petroleum and Geosystems Engineering, University of Texas at Austin, Austin, Texas 78712 Received June 16, 1998. In Final Form: December 17, 1998 Experiments were performed to evaluate the hydrodynamic force required to detach colloidal particles from substrates by fluids in simple shear flow. The role of DLVO and acid-base interactions on the process of adhesion and removal of colloidal particles, from model surfaces, in various fluid environments was investigated. Particles and substrates with varying degrees of hydrophobicity were used. The free energy of interaction between these surfaces in a number of fluid media was calculated using the acid-base approach. Good qualitative agreement was obtained between calculated values of free energy of interaction using the acid-base approach and the critical hydrodynamic force required to detach the particles if long range electrostatic repulsion is properly accounted for. Results for a range of particles and substrates of varying hydrophobicity and for a range of solvents can be consistently explained using this approach.
Introduction Dussan and Lee1 studied the behavior of a small spherical particle initially at rest on the surface of a flat plate in a laminar boundary layer. The adhesion of carbon black particles to a glass substrate in an aqueous media and their subsequent removal by ionic surfactant solutions were studied by Clayfield and Smith2 using a powder-bed technique. It was shown that, at certain surfactant and electrolyte concentrations, about 60% of the adherent particles could be removed with no appreciable hydrodynamic force. The adhesion force between carbon-black particles and a cellulose film in aqueous solution was studied by Visser.34 He also studied the adhesion of (1) Dussan, E. B.; Lee, S. L. Appl. Sci. Res. 1969, 20, 465. (2) Clayfield, E. J.; Smith, A. L. Environ. Sci. Technol. 1970, 4, 413. (3) Pokusayev, V. G.; Yermakov, V. A.; Bondarik, V. V. Fluid Mech. Sov. Res. 1982, 11, 50. (4) Grishin, N. N. Sov. Meteorol. Hydrol. 1981, 5, 60. (5) Hubbe, M. A. Colloids Surf. 1984, 12, 151. (6) Khilar, K. C.; Fogler, H. S. J. Colloid Interface Sci. 1984, 101, 214. (7) Kia, S. F.; Fogler, H. S.; Reed, M. G.; Soc. Pet. Eng. AIME Pap. SPE 1986, 15318, 50. (8) Kallay, N.; Biskup, B.; Tomic, M.; Matijevic, E. J. Colloid Interface Sci. 1986, 114, 357. (9) Ranade, M. B. Aerosol Sci. Technol. 1987, 7, 161. (10) Bardina, J. Part. Sci. Technol. 1988, 6, 121. (11) Kurtz, M. R., Busnaima, A., Kern, F. W., Proc. Annu. Technol. Meet. Inst. Environ. Sci. 1989 Proc.s35th Annu. Tech. Meet. May 1-5 1989 Anahein, CA 1989, 340. (12) Kaiser, R. Proc. Annu. Technol. Meet. Inst. Environ. Sci. 1989 Proc.s35th Annu. Techn. Meet., May 1-5 1989 Anahein, CA 1989, 306. (13) Busnaina, A.; Edler, J. E.; Gale, G.; Kern, F. W. J. Environ. Sci. 1989, 32, 46. (14) Sharma, M. M.; Chamoun, H.; Sita Rama Sarma, D. S. H.; Schechter, R. S. J. Colloid Interface Sci. 1992, 149, 121. (15) Amirtharajah, A.; Raveendran, P. Colloids Surf. A: Physicochem. Eng. Aspects 1993, 73, 211. (16) Yamamoto, T.; Periasamy, R.; Donovan, R. P.; Ensor, D. S. J. Adhes. Sci. Technol. 1994, 8, 543. (17) Das, S. K., Schechter, R. S., Sharma, M. M. J. Colloid Interface Sci. 1994, 164, 63. (18) Matijevic, E.; Ryde, N. P. J. Adhes. 1995, 51, 1-4. (19) Das, S. K.; Sharma, M. M.; Schechter, R. S. Part. Sci. Technol. 1995, 13, 227. (20) Mendoza, H. D.; Sasaki, H.; Matsuoka, I.; Sugimoto, T. J. Dispersion Sci. Technol. 1996, 17, 767. (21) Kuo, S. C.; Hammer, D. A.; Lauffenburger, D. A. Biophys. J. 1997, 73, 517.
colloidal polystyrene particles to the same substrate35 as a function of pH and ion concentration and concluded that the adhesion of the particles could be described in terms of the DLVO theory. Pokusayev et al.3 investigated models for particle detachment from flat substrates by laminar flow and Grishin4 studied the mechanisms of detachment of particles in turbulent flow. Hubbe5 presented models for the detachment of colloidal particles from solid surfaces exposed to shear flow. The models are most relevant for hard, spherical particles. It was concluded that the component of hydrodynamic force acting parallel to a sheared wall is usually much larger than the lifting force. He argued that limiting modes of incipient motion (e.g., rolling, sliding, and lifting) can be distinguished based on the dependency of the shear stress required for detachment on the size of particles. Khilar and Fogler6 and Kia et al.7 investigated the existence of a critical ionic concentration for particle release from a Berea sandstone. They concluded that a critical salt concentration exists below which colloids are mobilized from pore surfaces in sandstones. A fundamental study on particle adhesion and removal in model systems was conducted by Kallay et al.8 It was shown that the removal of spherical colloidal hematite (22) Ziskind, G.; Fichman, M.; Gutfinger, C. J. Aerosol Science 1997, 28, 623. (23) Das, S. K. Ph. D. Dissertation, The University of Texas at Austin, 1996. (24) Goldman, A. J.; Cox, R. G.; Bremmer, H. Chem. Eng. Sci. 1967, 22, 65. (25) Basu, S.; Sharma, M. M. Langmuir 1996, 12, 6506. (26) Israelachvili, J., Intermolecular and Surface Forces, 2nd. ed.; Academic Press: San Diego, CA, 1992. (27) van Oss, C. J. Interfacial Forces in Aqueous Media; Marcel Dekker: New York, 1994. (28) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Rev. 1988, 88, 927. (29) Yoon, R.-H.; Flinn, D. H.; Rabinovich, Y. I. J. Colloid Interface Sci. 1997, 185, 363. (30) Pashley, R. M. J. Colloid Interface Sci. 1981, 80, 153. (31) Parker, J. L.; Claesson, P. M. Langmuir 1994, 10, 635. (32) Freitas, A. M.; Sharma, M. M., Manuscript in preparation. (33) Razatos, A.; Ong, Y. L.; Georgiou, G.; Sharma, M. M., Proc. Natl. Acad. Sci. U.S.A., Appl. Biol. Sci. 1998, 95, 11059. (34) Visser, J. J. Colloid Interface Sci. 1970, 34, 26. (35) Visser, J. J. Colloid Interface Sci. 1976, 55, 664.
10.1021/la9807107 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/05/1999
Hydrodynamic Detechment of Particles
particles from glass surfaces in basic media is enhanced with increasing concentration of NaNO3. Their analysis indicated that double layer expansion is responsible for the observed phenomenon. Ranade9 and Bardina10 conducted experiments to evaluate the effectiveness of different methods used to remove particles in the size range of 0.1-10 µm from a wide range of surfaces. Kurtz et al.11 investigated the hydrodynamic force required to remove different spherical microparticles from silicon substrates in order to evaluate the effectiveness of various particle removal techniques. An enhanced method of removal of submicrometer particles from surfaces by high molecular weight fluorocarbon surfactant solutions was studied by Kaiser.12 In a preliminary result it was shown that submicrometer particles were more effectively removed from a solid surface by washing the surface with a one percent solution of a high molecular weight highly fluorinated surfactant in a highly fluorinated carrier fluid than washing it in the carrier liquid without surfactant. Busnaina et al.13 studied the fluid dynamics of microcontaminant particle removal from silicon wafers using deionized water in a centrifugal field. Flow visualization was used to observe the jet trajectory and the flow pattern for different flow rates at different rotational speeds. A systematic study on detachment of colloidal particles was conducted by Sharma et al.14 It was demonstrated, by conducting centrifuge experiments, that the mechanism of detachment is rolling rather than sliding or lifting. The influence of particle size and elasticity as well as the surface chemical interactions between the particle and the substrate was included in a model. Computed and experimentally measured forces showed good agreement. It is important to note that for spherical particles on a flat substrate there is no lift force (at low Reynolds number). This implies that particle removal must be induced by the drag force on the particle. This is the primary reason that in all experimental data plots presented in this work we use the critical hydrodynamic drag force as a measure of the fluid’s ability to remove particles. Amirtharajah and Raveendran15 conducted experimental and theoretical studies on the detachment of colloids from sediments and sand grains. Latex particles (2 and 5 µm) were attached to a packed column containing glass beads as collector grains. Experimental results indicated that the detachment of latex particles was independent of the ionic strength of the solution during attachment, but was dependent on the ionic strength during detachment. It was found that the detachment efficiency was higher with lower ionic strength solutions. Yamamoto et al.16 investigated the removal of polystyrene particles (10 µm) from various model surfaces in a flow cell. They observed that, as the flow rate increased, single spheres detached first, then doublets, then triplets and finally larger agglomerates. The effects of elastic deformation and surface roughness on the hydrodynamic detachment of colloidal particles from surfaces was studied by Das et al.17 It was shown that to release a smooth, rigid spherical particle from a deformable substrate, the hydrodynamic force required is infinitesimally small. In the case of a deformable particle interacting with a rigid substrate the deformation of the contact region caused by the frictional shear is negligibly small and hence does not provide a restraining torque large enough to balance the imposed hydrodynamic torque. They concluded that surface roughness is necessary to account for the observed critical hydrodynamic force. The magnitude of this critical force for a given fluid-particle-
Langmuir, Vol. 15, No. 7, 1999 2467
Figure 1. Schematic presentation of the experimental setup used for hydrodynamic detachment experiments (adapted from ref 23).
substrate system was found to depend on three factors: particle and substrate roughness, force of adhesion, and the extent of the contact deformation. Matijevic and Ryde18 conducted studies on kinetics of particle deposition and detachment. Das et al.19 in a review paper have summarized earlier experimental and theoretical studies on the mechanisms of colloid detachment. Kuo et al.20 conducted experiments on detachment of specifically bound particles from surfaces by shear flow. Mendoza et al.21 studied the detachment of fine particles of silica (0.12 µm) and hematite (0.10 µm) co-attached on glass beads (80 µm) in aqueous solution as a function of pH and electrolyte concentration. It was observed that the selective separation of silica particles was governed by the balance of short-range forces. Ziskind et al.22 developed an adhesion model for estimating particle detachment from a surface. They showed that the hydrodynamic moment could cause particle detachment, while the hydrodynamic lift force is smaller than the adhesion force by several orders of magnitude. Razatos et al.33 studied bacterial adhesion to surfaces and have developed a protocol for measuring the adhesion force between bacteria and substrates using an AFM. Materials and Methods The equipment used in the flow cell experiments has been described elsewhere.23 The apparatus consists of a liquid reservoir, a pressure control valve, a flow cell, and a optical microscope. Figure 1 shows the apparatus schematics. The liquid reservoir can be pressurized to 15 psig using a pressure control valve. The liquid flow rate can be controlled through either the pressure control valve or the needle valve or both. Flow rate can be varied from 0.01 to 9 mL/s (approximately). The flow cell is made of Plexiglas and consists of two parts. The lower part is designed to hold a substrate of dimensions 3 in. × 1.5 in. The upper part of the cell has three ports; two of these are the inlet and outlet for the fluid while the third is used to inject particles in the cell. The substrate and the upper part of the cell are separated by an O-ring with a thickness of approximately 1.5 mm. Particle movement was monitored visually through an optical microscope. The hydrodynamic drag force acting on a sphere on a flat surface, due to a fluid in simple shear flow is given by24
Fh ) 1.7 × 6πηRVR
(1)
2468 Langmuir, Vol. 15, No. 7, 1999
Freitas and Sharma
Table 1. Contact Angles (deg) of Probe Liquids on SiO2 Wafer Surfacesa probe liquids
wafer as received
wafer conditioned
DI-water glycerol DIM
28 15 42
25 15 45
a
silicon wafer glass slide I glass slide II glass slide III
Note that the values are virtually unaltered.
where η is the fluid viscosity, R is the particle radius. VR is the fluid velocity at the center of the spherical particle and is given by
VR ) 6(Q/A)(R/l)(1 - R/l)
Table 2. Contact Angles (deg) of Probe Liquids on Various Substrate Surfaces
Substrate Preparation All flow cell experiments were conducted using glass slides, SiO2 wafers (as model silica substrates), and muscovite mica. The conditioning sequence for each substrate (except mica) is described below. The glass slides received different types of surface treatment in order to have different surface energies and are referred to as I, II, and III. After conditioning, slides I and II can be considered hydrophilic, slide II being more hydrophilic (smaller water contact angle) than slide I. Slide III had a strong hydrophobic character. A thin film of silane was covalently bonded on to the silica on slide III, and its acid-base parameter γLW was changed from 42.03 to 25.7 mN/m (refer to Table 3). For glass slide I the following conditioning sequence was adopted. The surface was washed and rubbed with detergent (Alconox) and rinsed with DI-water; then it was washed with methanol, acetone, and deionized water and blow-dried with filtered air. Slide II was washed and rubbed with detergent (Alconox), rinsed with DI-water; soaked for 3 h in H2SO4/HNO3, rinsed in deionized water, and blow-dried with filtered air.
DIM
glycerol
28 25 7 105
42 35 35 65
15 25 7 97
Table 3. Acid-base Parameters for Various Solid Substrates and Liquids with Values of Surface Tension Components Given in mN/m silicon wafer glass slide I glass slide II glass slide III polystyrenea watera ethanola CCl4a chloroforma benzenea toluenea diiodomethanea glycerola
(2)
where Q is the volumetric flow rate, A is the cross sectional area, and l is the spacing between the substrate and the upper part of the cell holder (the thickness of the cell channel). Before starting an experiment, the liquid reservoir was filled with the fluid to be used in the experiment and the system was opened to flow at a low flow rate until half the volume of the reservoir has been drained to guarantee complete washing of the system. In the next steps the upstream and needle valves were closed, in that order, and the liquid reservoir was refilled and pressurized to the desired pressure. The fluid containing the particles in suspension was then injected, by means of a syringe, very slowly in order to obtain a uniform deposit of particles on the substrate. The model particles studied were glass and polystyrene microspheres (Duke Scientific Corporation) of known size distribution. All experiments were conducted with microspheres of diameter 10 µm ( 2% according to the manufacturer. However, visual inspection using an optical microscope showed that some of the particles had variations in diameter on the order of 1020%. The settling time allowed for the particles to reach equilibrium was 1 h for glass beads and 20 h for polystyrene beads. This is because polystyrene beads settle much slower than glass because their density (F ) 1.06) is very close to that of water (F ) 1.0). At the end of the deposition time the number of particles on the surface was estimated using a grid on the microscope eyepiece. This count includes a large number of particles, and therefore, errors induced by the procedure were found to have a margin of error on the order of 10%. If a large number of particles are taken into account, errors induced by surface heterogeneity are minimized. Flow was initiated and the flow rate increased gradually with the help of the needle valve. The flow rate was measured volumetrically. At each flow rate the number of particles remaining on the surface was estimated. This procedure continued until either all particles were detached or the maximum attainable flow rate was reached.
DI-water
a
γLW
γ+
γ-
38.59 42.03 42.03 25.70 42.00 21.80 18.80 27.00 27.15 28.85 28.5 50.8 34.0
4.00 1.97 2.82 0.24 0.00 25.5 0.02 0.0 3.8 0.0 0.0 0.0 3.92
33.98 40.22 44.76 1.32 1.10 25.5 68.0 0.0 0.0 2.7 2.3 0.0 57.4
Values of surface tensions from ref 27.
Slide III was washed and rubbed with detergent (Alconox) and rinsed with water, soaked in concentrated HNO3 for 15 min to activate silanol groups, rinsed in distilled water, and blowdried with filtered air. The glass slide was then dipped in a 2% solution of n-octadecyltrichlorosilane (OTS) in chloroform for 10 min. The slide was removed from the chloroform solution and dried by solvent evaporation. It was washed again in detergent and hot water to remove any excess of nonreacted OTS from the surface. This process was followed by a rinse in deionized water and blow-drying with filtered air. Contact angles of probe liquids and acid-base parameters for glass slides I, II, and III are shown in Tables 2 and 3. Mica sheets received no conditioning. They were first glued on a glass slide by means of a epoxy-hardener mix and then cleaved just before the experiment. The freshly exposed surface was assumed to be free of any contaminants. A polished silicon wafer surface was cleaned using detergent (Alconox) and rinsed with hot tap water and deionized water. This cleaning procedure does not alter the surface energy of the wafer. Contact angles measurements were conducted on the silicon wafer after the cleaning procedure and compared with those measured on the wafer as received from the manufacturer. Table 1 shows contact angles of probe liquids on conditioned and nonconditioned wafer surfaces. Contact angles of probe liquids on various substrates and acid-base parameters of various substrates and liquids are shown in Tables 2 and 3.
Theoretical Background The acid-base approach developed by van Oss, Chaudhury, and Good28 suggests that if a surface involves both Lifshitz-van der Waals (LW) and polar or acid-base (AB) interactions, the total surface tension should be the sum of two components, γLW and γAB. Their methodology uses surface tensions, interfacial tensions and contact angles to determine the LW (γLW) and AB (γ+, γ-) parameters of materials. These parameters can be used to calculate the free energy of a solid surface and free energy of interaction between two surfaces in vacuo or immersed in liquids. The methodology is briefly described below. The components of the free energy of adhesion are AB ∆Gij ) ∆GLW ij + ∆Gij
(3)
we can write the combining rule for ∆GLW ij in terms of surface tensions as LW LW 1/2 ∆GLW ij ) -2(γi γj )
(4)
Hydrodynamic Detechment of Particles
Langmuir, Vol. 15, No. 7, 1999 2469
Table 4. Gibbs Free Energy of Interaction at Contact, As Calculated from Acid-Base Theory, and Experimentally Measured Critical Hydrodynamic Force for Various Systemsa × ∆G132 Fh (mJ/m2) (mN/m) /Rcrit
substrate
liquid
particle
silicon wafer silicon wafer silicon wafer silicon wafer hydrophobic glass III hydrophobic glass III hydrophilic glass II hydrophilic glass II hydrophobic glass III hydrophilic glass II mica hydrophobic glass III hydrophobic glass III hydrophilic glass I hydrophilic glass I
water water ethanol ethanol water ethanol water ethanol water water water CCl4 chloroform benzene toluene
glass polystyrene glass polystyrene polystyrene polystyrene polystyrene polystyrene glass glass glass glass glass glass glass
a
8.84 -22.11 6.60 18.20 -77.34 -0.04 -22.78 8.74 -18.11 14.11
104
6 ND 30 7 ND 140 ND 31 ND 170 500 153 3 ND 290
-9.2 19.9 -5.05 -4.80
van Oss et al.28 also suggested splitting the asymmetric acidbase component of a bipolar system into two separate parameters: a (Lewis) acid parameter of surface tension γ+ and a (Lewis) base parameter of surface tension γ-. The postulated combining rule for acid-base interactions across an interface is
(5)
The complete combining rule for apolar (LW) and acid-base (AB) components together is obtained by substituting eqs 4 and 5 into eq 3 LW - + ∆Gij ) -2(xγLW + xγ+ i γj i γj + xγi γj )
(6)
The expression for the total interfacial tension between condensed materials i and j is given by + + + 2 γij ) (xγLW - xγLW i j ) + 2(xγi γi + xγj γj - xγi γj +
xγi-γ+j )
(7)
The free energy of interaction between two molecules or particles of material 1 immersed in liquid 2 may be expressed, following the Dupre´ equation, as
∆G121 ) -2γ12
(8)
and the equation for the free energy of interaction between two dissimilar materials 1 and 2 immersed in liquid 3, according to Dupre´ is
∆G132 ) γ12 - γ13 - γ23
(9)
where the γij’s in eq 8 and 9 are calculated from eq 7. Once the γLW, γ+, and γ- values of all materials in a given system have been determined (see next section and Table 3), the values of the free energies ∆G121 and ∆G132 can be calculated (see Table 4). The total surface energy of the solid surfaces considered in this work are calculated using γLW, γ+, and γ- values from Table 3 and using eq 3 and 4. Determination of the Acid-Base Parameters (γLW, γ+, γ-) for Low-Energy Solid Surfaces. When a liquid and a solid phase are placed in contact, the free energy is related to the contact angle of the liquid on the solid surface by the YoungDupre´ equation
-∆Gsl ) γl(1 + cos θ)
(11)
+ The three unknown terms for the solid surface, γLW s , γs , and γ, can be found by using probe liquids with known LW and AB s properties. A general approach is to measure the contact angles of three probe liquids with known acid-base parameters (refer to Table 3) and simultaneously, solve the three linear equations given by eq 11. Another approach, which was adopted in this and two other study, is to use an apolar liquid for finding γLW s polar liquids for solving eq 11. The liquids we used for most of the contact angle measurements were diiodomethane (apolar), glycerol (polar), and water (polar).
Results
ND stands for no detachment.
+ + ∆GAB ij ) -2(xγi γj + xγi γj )
LW - + γl(1 + cos θ) ) 2(xγLW + xγ+ s γl s γl + xγs γl )
(10)
by combining eqs 7 and 10 and letting i ) s and j ) l, for a polar system, the Young-Dupre´ equation can be written as
Particle detachment experiments were performed in the flow cell. The interaction between substrate and particle are classified as those between hydrophilic (phiphi), hydrophilic-hydrophobic (phi-pho), and hydrophobic (pho-pho) surfaces. The effect of aqueous solutions at various electrolyte concentrations as well as organic solvents were investigated for each interacting system. The effect of pH was systematically studied earlier in our laboratory17 and was not repeated here. AFM experiments32 have shown that pH variation affects the longrange electrostatic repulsion but have very little effect on the work of adhesion or free energy at contact. Our results are presented graphically as the fraction of particles detached (%) as a function of the hydrodynamic force normalized by the particle radius (Fh/R), calculated from eq 1. A measure of the onset of particle release is the critical hydrodynamic force, which is defined in this work as the force required to release 10% of the particles deposited initially. Interaction between Hydrophilic Surfaces. The interaction between hydrophilic surfaces was studied by conducting experiments with hydrophilic glass microspheres and hydrophilic substrates in various liquid media. Glass particles of 10 µm diameter were deposited on four types of hydrophilic substrates: glass slides I and II, silicon wafer, and freshly cleaved mica. Deionized water was used as the liquid medium. The results are presented in Figure 2. It can be seen that the critical hydrodynamic force needed to release the glass particles from all substrates shows about the same value. It is noted that for all substrates about 50% of particles are readily released at a very small flow rate (or hydrodynamic force). This clearly indicates the presence of a repulsive electrostatic barrier to deposition. As expected, the behaviors of glass and the silicon wafer were very similar; they presented similar curves and complete particle detachment at about the same hydrodynamic force. Surprisingly glass particles were detached more easily from silica surfaces than from mica, despite mica being molecularly smooth, indicating that roughness plays a less important role in this case. The effect of electrolyte was studied by conducting flow cell experiments with a ionic solution of 0.1M potassium chloride (KCl). The particles used were 10 µm glass microspheres deposited on glass slide II. From Figure 3a it can be seen that the critical hydrodynamic force required to release the glass particles from a silica substrate in salt solution is greater than that to release particles in deionized water. Electrolytes have the effect of reducing the Debye length or thickness of the diffuse double layer, reducing the electrostatic repulsive barrier present in the deionized water system.26 The particles can, therefore, come closer to the surface where AB and VDW forces are larger. Acid-base theory predicts a repulsive (or nonad-
2470 Langmuir, Vol. 15, No. 7, 1999
Figure 2. Detachment of 10 µm hydrophilic glass particles from hydrophilic substrates in deionized water.
Figure 3. (a) Effect of electrolyte on the detachment of 10 µm hydrophilic glass particles from a hydrophilic glass substrate (glass slide II). (b) AFM force curve for interaction between a hydrophilic 30 µm glass bead and a hydrophilic silica substrate in DI-water and 0.1 M KCl. Arrows denote jump into contact. In DI-water, the strong double layer keeps particles away from surface. A pull-off force of 1.2 mN/m was measured for both aqueous media. Solid lines are fits to DLVO theory: DI-water, ι0 ) 50 mV, κ-1 ) 68 nm, A ) 1 × 10-20 J; 0.1 M KCl, ι0 ) 10 mV, κ-1 ) 1 nm, A ) 1 × 10-20 J (data from ref 32).
hesive) interaction (∆G132 ) 14.11 mJ/m2 in Table 4) at contact, which agrees with the experimental results shown in Figure 3a (squares), where all particles were released
Freitas and Sharma
Figure 4. Effect of electrolyte on the detachment of 10 µm hydrophilic glass particle from a hydrophilic mica substrate.
at relatively low hydrodynamic force (although detachment was easier in DI-water due to a double layer repulsive barrier). Figure 3b shows AFM force curves for a similar system using a 30 µm glass microsphere attached to the apex of the cantilever.32 This experiment confirms the existence of a strong repulsive double layer in DIwater (diamonds) and virtually no electrostatic repulsion in 0.1 M KCl (squares). The measured pull-off force for both aqueous media was 1.2 mN/m. Note that this pull-off force is 2 orders of magnitude higher than the critical hydrodynamic force (∼ 0.017 mN/m) measured in 0.1 M KCl. This confirms that the release mechanism is rolling, as suggested by Sharma et al.14 Hydrodynamic detachment experiments were performed on a mica substrate using 10 µm glass microspheres in a aqueous solution of 0.1 M potassium chloride. Results can be seen in Figure 4, which depicts the dramatic effect of ionic strength on the number of particles released in the flow cell experiment. The critical hydrodynamic force needed to release the particles in the ionic solution is much higher than that needed to release the particles in deionized water. From Figures 3 and 4 it is noted that for both glass and mica the repulsive double layer was screened by the salt. However all particles were detached from the silica substrate while only 30% of them were released from the mica substrate, despite mica being molecularly smooth. The effect of ethanol was studied with 10 µm glass microspheres deposited on a silicon wafer substrate in ethanol. The results are shown in Figure 5 from which it can be seen that the critical hydrodynamic force required to release glass particles in ethanol is greater than that required to release glass particles in deionized water. Acid-base theory shows approximately the same repulsive energy for both water and ethanol (∆G132 ) 8.84 and 6.60 mJ/m2 in Table 4), but in this case, particles are more easily detached in water due to a long-range repulsive barrier probably of electrostatic origin. It is noted that although acid-base theory predicts a repulsive interaction at contact in ethanol, not all particles were released in the experiment. This effect may be attributed to surface heterogeneities on the particle and substrate. It is useful to compare the ethanol curve with an aqueous solution of 0.1 M potassium chloride (see Figure 5). In this case, it can be seen that the critical hydrodynamic force required to release the particles in the ionic solution is greater than that in ethanol. All particles were detached in the aqueous solution during the experiment.
Hydrodynamic Detechment of Particles
Figure 5. Effect of ethanol on the detachment of 10 µm hydrophilic glass particles from a hydrophilic oxidized silicon wafer substrate.
Figure 6. Effect of aromatics on the detachment of 10 µm hydrophilic glass particles from a hydrophilic glass substrate (glass slide I).
Hydrodynamic detachment experiments were conducted on a glass substrate (glass slide I) using 10 µm glass spheres in benzene and toluene. The results are presented in Figure 6. It was found that the critical hydrodynamic force for toluene is much larger than that for deionized water, and almost no release of particles was noted in the case of benzene. This dramatically different behavior between benzene and toluene cannot be explained on the basis of existing theories such as DLVO and acid-base since both have similar Hamaker constants, dielectric constants, and acid-base parameters. A possible explanation may lie in the liquid structure. Since these fluids are confined between two solid surfaces, solvation or structural forces could arise from this interaction. However very little is known about such forces, and there is no adequate theory at present. The effect of adsorbed surfactants was studied with 10 µm glass microspheres deposited on a silicon wafer substrate in an aqueous solution of the cationic surfactant cetyltrimethylammonium bromide (CTAB). The results are shown in Figure 7. The critical hydrodynamic force required to release the glass particles in a solution of 10-4M CTAB is greater than that required to release glass particles in deionized water and in a solution of 10-3 M CTAB. These experiments using cationic surfactant CTAB can be explained by the fact that CTAB forms a monolayer or
Langmuir, Vol. 15, No. 7, 1999 2471
Figure 7. Effect of concentration of cationic surfactant CTAB on the detachment of 10 µm hydrophilic glass particles from a hydrophilic oxidized silicon wafer substrate.
bilayer on the substrate as a function of surfactant concentration. At a concentration of 10-4 M (in DI-water) a hydrophobic monolayer is formed (as indicated by contact angle measurements25) on hydrophilic silica and glass surfaces and it is expected that an attractive hydrophobic interaction will occur between the glass substrate and the glass beads. At a concentration of 10-3 M (in deionized water) a hydrophilic bilayer is formed,25 and a repulsive interaction between particles and substrate due to acidbase and electrostatic interactions is expected. The experiments show good agreement with this prediction. The particles are easily removed at a relatively low flow rate, indicating a net repulsive interaction between particles and substrate at 10-3 M CTAB. This behavior is similar to that of the interaction between hydrophilic glass surfaces in deionized water (open squares in Figure 7). Interaction between Hydrophilic and Hydrophobic Surfaces. The interaction between hydrophilic and hydrophobic surfaces was studied by performing experiments with 10 µm polystyrene microspheres (hydrophobic) interacting with glass slide II (hydrophilic) and also conducting experiments with 10 µm glass microspheres (hydrophilic) interacting with silanated glass slide III (hydrophobic) in various liquid media. The important observations are discussed below. For 10 µm polystyrene microspheres deposited on a silicon wafer substrate and a glass substrate in deionized water, acid-base theory predicts a negative free energy at contact (∆G132 ) -22.11 and -22.78 mJ/m2 in Table 4); therefore, a strong adhesion force between particles and substrate is expected. In Figure 8 an electrostatic repulsive barrier (denoted by the steep initial slope of the curve) is clearly observed. This is responsible for the easy detachment of particles from both substrates even though acidbase theory predicts a strong adhesive minimum at contact. This clearly illustrates the importance of longrange electrostatic interactions. These interactions prevent the particle from coming into “contact” with the substrate invalidating the use of the adhesion energy at “contact” for interpreting the results. Results of flow cell experiments conducted with polystyrene microspheres on glass and silicon wafers in ethanol are presented in Figure 8. This experiment is particularly important because in ethanol electric double layer forces are expected to be small with only AB and VDW forces playing a major role. Acid-base theory27,28 predicts repulsive interaction at contact (see Table 4) for both substrates with a stronger repulsion for silicon (∆Gglass )
2472 Langmuir, Vol. 15, No. 7, 1999
Figure 8. Effect of glass (diamonds) and SiO2 wafer (squares) on the detachment of 10 µm hydrophobic polystyrene particles in deionized water (filled markers) and ethanol (open markers).
Figure 9. Effect of KCl concentration on the detachment of 10 µm hydrophobic polystyrene microspheres from hydrophilic glass substrate (glass slide II).
8.74 mJ/m2 and ∆Gsilicon ) 18.20 mJ/m2). This is in good agreement with the particle detachment versus hydrodynamic force curves in Figure 8 which shows particles being released more easily from a silicon wafer (open squares) than from glass (open diamonds). It can be noted from the same figure that the silicon wafer curve (open squares) shows a long-range repulsive barrier similar to that of aqueous systems. This long-range repulsion was also verified in the AFM experiments conducted by Freitas and Sharma.32 The effect of electrolyte concentration was studied with 10 µm polystyrene microspheres deposited on the glass substrate in aqueous solutions of varying ionic concentrations. The results are presented in Figure 9. Increasing salt concentration increases the critical hydrodynamic force. As expected, as electrostatic repulsion is screened by increasing electrolyte concentration, particles are more strongly attached to the substrate. Acid-base theory predicts a strong attractive interaction (∆G ) -22.78 mJ/ m2) for this system. The effect of depositing fluid was studied with 10 µm polystyrene microspheres deposited on a glass substrate in an ionic solution of 0.1 M potassium chloride and ethanol. After particles have settled in each depositing fluid, ethanol was used as the detachment fluid. As shown in Figure 10 virtually all particles deposited in 0.1 M KCl remained attached to the substrate while particles deposited in ethanol were easily detached. The experiment
Freitas and Sharma
Figure 10. Effect of depositing fluid on the detachment of 10 µm hydrophobic polystyrene particles from a hydrophilic glass substrate (slide II). Removing fluid is ethanol.
Figure 11. Effect of chlorocarbons on the detachment of 10 µm hydrophilic glass particles from a hydrophobic silanated glass substrate (glass slide III).
clearly shows that despite the positive (repulsive) energy of interaction, ethanol was unable to release particles previously deposited in a saline solution of KCl. The effect of chlorocarbons was studied with 10 µm glass microspheres deposited on a silanated glass substrate (slide III) in chloroform and carbon tetrachloride. From Figure 11 it can be seen that the critical hydrodynamic force required to release glass particles from the substrate in carbon tetrachloride is much greater than that required to release particles in chloroform. This is a particularly interesting system since acid-base theory predicts (see Table 4) attraction for carbon tetrachloride (∆G132 ) -9.2 mJ/m2) and repulsion for chloroform (∆G132 ) 19.9 mJ/ m2). The agreement of this prediction with the curves in Figure 11 is very good. Particles were detached much more easily in chloroform than in carbon tetrachloride. It can also be noted from Figure 11 that the chloroform curve suggests a long-range repulsive barrier similar to that of water (triangles in Figure 11) and ethanol (open squares in Figure 8). This experiment demonstrates that acidbase theory can be used as a tool to evaluate the efficiency of organic solvents, such as hydrochlorocarbons, as “adhesion inhibitors” in surface chemical cleaning for semiconductor processing, since particle removal is promoted by the reduction of adhesion forces between particle and substrate.
Hydrodynamic Detechment of Particles
Figure 12. Effect of KCl concentration on the detachment of 10 µm hydrophobic polystyrene particles from a hydrophobic silanated glass substrate (glass slide III).
Interaction between Hydrophobic Surfaces These interactions were studied by conducting experiments with 10 µm polystyrene microspheres (hydrophobic) deposited on silanated glass (hydrophobic) in ethanol and aqueous solutions of varying electrolyte concentrations. The behavior of these systems was found to be very similar to that of hydrophilic-hydrophobic systems studied earlier. From Figure 12 the dramatic effect of salt concentration on the critical hydrodynamic force can be seen. As was the case for the hydrophilic-hydrophobic interaction, as electrostatic repulsion is screened by increasing salt concentration, the particle approaches the substrate to “contact” where short-range acid-base or hydrophobic interactions dominate. Acid-base theory predicts very strong attraction at contact for interaction between hydrophobic surfaces (∆G132 ) -77.34 mJ/m2 in Table 4). Comparing Figure 12 and Figure 9, it can be seen that for 0.001 M potassium chloride more particles were released (∼30%) in the hydrophilic-hydrophobic system than in the hydrophobic-hydrophobic (∼3%) system. The larger negative value of ∆G at contact (Table 4) obtained from acid-base theory is consistent with the experiments. In some systems the hydrophobic force is so long range that the repulsive double layer forces are overcome and a purely attractive interaction is obtained.29,32 Discussion Effect of Surface Hydrophobicity on Particle Detachment. The interactions among the three main groups of substrate-particle pairs chosen (hydrophilichydrophilic, hydrophilic-hydrophobic and hydrophobichydrophobic) in aqueous media and ethanol were compared and the results are presented graphically in Figures 13-15. From Figure 13 it can be seen that for all three pairs the critical hydrodynamic force needed to release particles from a substrate is very small for deionized water. It is observed that about 60% of the particles were released at a very small flow rate, which confirms the presence of a repulsive electrostatic double layer barrier for particle deposition. This electrostatic barrier is the reason particles are easily released from all substrates despite the strong short-range attraction predicted by acid-base theory for hydrophobic surfaces. Figure 14 shows the dramatic effect of reducing the electrostatic repulsion by adding 0.1 M KCl. For the hydrophilic-hydrophilic interaction, there is no electro-
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Figure 13. Comparison among various substrate-particle interactions in deionized water.
Figure 14. Comparison among various substrate-particle interactions in aqueous solution of 0.1M KCl.
static barrier keeping particles away from the substrate (no steep slope at low flow rates); however, all particles were released, which agrees with the acid-base prediction of positive (repulsive) interaction at contact. For hydrophobic interactions (the other two curves), with the double layer compressed, particles can approach the substrate to contact where strong short-range attractive forces prevail. As a result, virtually no particles are released during the flow experiment. For interactions in ethanol, it is observed from Figure 15 that the critical hydrodynamic force needed to detach the particles from a substrate are very similar irrespective of surface hydrophobicity. Acid-base theory predicts no adhesive interactions at contact for all three systems in ethanol, which agrees with the general trend of the curves where virtually all particles were easily released from all substrates. Equilibrium Distance between Particle and Substrate. The effects of gravity as well as size and density of particles cannot be neglected in the flow cell experiments since they play an important role, together with electric double layer forces, in determining the separation distance between particle and substrate. The equilibrium separation distance is determined by a balance between gravity and repulsive double layer forces acting on the particle during the deposition process. When the gravitational force is greater than the repulsive maximum, the particles come into contact with the substrate and the resulting interaction is dictated by short-range forces (such as acid-base). When gravity is
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Freitas and Sharma
Figure 15. Comparison among various substrate-particle interactions in ethanol.
not large enough to overcome the repulsive electrostatic barrier, particles are kept at some distance away from the substrate and short-range interactions are not important. The net body force pulling down the particle is simply
Figure 16. Force between a hydrophobic sphere and a untreated (hydrophilic) glass surface in pure water (solid line) and force between hydrophobic surfaces (dotted line). Note the normalized body force acting on glass particles (1.64 × 10-3 mN/m) and on polystyrene particles (6.2 × 10-5 mN/m) equilibrates the repulsive double layer at a distance about 190 and 420 nm respectively.
FB ) (Fp - FL)Vpg where Fp and FL are particle and liquid densities, Vp is the particle volume, and g is the acceleration due to gravity. For this estimate we assume
Fwater ) 1000 kg/m3; Fpolystyrene ) 1060 kg/m3; Fglass ) 2600 kg/m3 Substituting these values in the equation above and normalizing the force with the particle radius (5 µm) we obtain
FB/R(glass) ) 1.64 × 10-3 mN/m FB/R(polystyrene) ) 6.16 × 10-5 mN/m Force measurements between two hydrophobic silanated glass microspheres and between a silanated glass and a hydrophilic (untreated) glass using a surface force apparatus performed by Parker and Claesson31 are used here for comparison purposes. It can be seen from Figure 16 that the equilibrium “contact” distance for polystyrene particles is on the order of 420 nm, while the equilibrium distance for glass particles is 190 nm. At these distances the body forces acting on the particle balance the repulsive double layer forces. At these large separation distances short-range interactions, such as acid-base, will not play any role and the particles will be easily detached. Since the gravity force scales as the density and the cube of particle radius, the smaller and less dense the particles the greater the separation distance. For submicrometer particles the thermal energy associated with Brownian motion (kT) plays a important role since the gravitational potential is negligible. Small particles will, therefore, be extremely sensitive to changes in the longrange DLVO interactions because these will determine if the particle comes into “contact” with the surface. Comparisons with Acid-Base Theory. In general, acid-base interactions are short range and when double layer forces dominate the interaction, inconsistencies with acid-base predictions arise because the surfaces are not in contact. For example, for hydrophobic systems in
Figure 17. Critical hydrodynamic force as a function of the free energy of interaction at contact between particle and surface for some selected systems. ND stands for nondetachment. By converting the free energy at contact to force/R (using the Derjaguin approximation for the case of interaction sphereflat plate, F/R ) -2π∆G) it can be seen that the attractive surface force is 2 orders of magnitude larger than the measured critical hydrodynamic force. R is the particle radius.
aqueous media where the interaction energy calculated by acid-base theory is negative and strong attractive interaction is expected, easy detachment of particles was observed in deionized water, suggesting noncontact between particle and substrate. Experiments at higher salinity have shown that the particles are kept away from the surface by a long-range repulsive energy barrier sensitive to ionic concentrations, suggesting an electrostatic origin to this barrier. For systems in which particles and substrate do come into contact, good qualitative agreement between the acid-base predictions and the hydrodynamic detachment experiments was obtained. Figure 17 and Table 4 show a comparison between the critical hydrodynamic force (measured experimentally) and ∆G132 obtained from acidbase theory. It is clear that large negative values of ∆G132 always correspond to no detachment in the flow experiments. Large positive ∆G132 values correspond to small critical hydrodynamic forces (easy detachment) as might
Hydrodynamic Detechment of Particles
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Figure 18. Free energies at contact for a number of interacting systems in various liquids according to acid-base theory. Positive values denote repulsion and negative values denote attraction.
be expected. For intermediate values of ∆G132 (small positive or negative values) clear trends with critical hydrodynamic force (CHF) are not obtained. This might be expected since the adhesion force is only one of the factors determining CHF, other factors such as surface roughness, surface heterogeneity, etc. begin to play a role. It should be pointed out that DLVO theory alone normally fails to predict adhesion energies between surfaces in contact because it does not take into account hydrogen bonding and other acid-base interactions which are always present when water is the intervening liquid. It was noted that hydrophilic glass beads were more easily removed from hydrophilic glass substrates than from molecularly smooth mica for both DI-water and a 0.1 M solution of potassium chloride. These observations can be explained as follows. In deionized water, particles are easily detached from both glass and mica indicating strong electrostatic repulsion. In 0.1 M KCl, particles are more easily detached from glass (compared to mica) due to two possible reasons: (i) a lower Hamaker constant (0.6 × 10-20 J) for glass compared to mica (1.2 × 10-20 J); (ii) the possibility of larger hydration forces on glass compared to mica.30 A similar conclusion can be drawn from experiments with hydrophobic polystyrene particles deposited on two hydrophilic substrates (SiO2 wafer and glass; see Figure 8). In these experiments all particles were detached from glass while not all particles (∼90%) were detached from the smoother silicon wafer. Glass substrate II is more hydrophilic than the silicon wafer surface (refer to Table 2). The hydrophilic nature of glass substrate II is attributed to a higher density of silanol groups which is a consequence of the acid pretreatment. Such highly charged hydrophilic surfaces manifest strong hydration orientation. This orientation decreases the permittivity of the aqueous layer near the solid-liquid interface, resulting in a higher surface potential. In this case the result is a stronger and longer ranged repulsive double layer for the more hydrophilic substrate (glass), which results in a greater separation distance between particles and substrate and hence in easier detachment of particles. The unexpected behavior of benzene and toluene in the flow cell (see Figure 6) cannot be explained on the basis
of existing theories. However, we speculate that this difference may be due to liquid structure. According to particle detachment experiments conducted by Das23 with hydrocarbon fluids, he found that no detachment at all occurred with n-decane, hexadecane, and tetradecane but easy detachment was verified with isooctane. He also found no detachment with cyclohexane and benzene but easy detachment with toluene. While this effect appears to be related to the hydrocarbon side chain, we do not have a good explanation for it. It should be mentioned that attempts to characterize mica in terms of AB parameters were not successful. Acidbase parameters may only be applied to estimate ∆G when low energy surfaces are involved, i.e., when the contact angle of water on these surfaces is greater than zero. Attempts to use probe liquids other than water lead to inconsistent values of acid-base parameters for mica. Particle Removal by Other Non-Aqueous Solvents. A reasonable correlation is obtained between the critical hydrodynamic force and the free energy at contact (or work of adhesion) for most of the systems studied (Figure 17). It can be seen that as the free energy goes from high negative values (attraction) toward high positive values (repulsion), the critical hydrodynamic force tends to decrease as expected. Note that for high negative free energy values no detachment of particles from the substrate was observed. On the basis of these results, some generalizations can be made. Figure 18 shows the calculated free energies using acid-base theory for surfaces interacting in various liquid media. Many of these results were verified experimentally in our flow cell experiments. However more experimental work is needed in order to validate and test this approach for a wide range of fluid media. Conclusions Experiments were performed using a flow cell to study the effects of DLVO and acid-base interactions on the process of adhesion and removal of particles from model surfaces in various fluid environments. The major conclusions from the experiments are summarized below. Experiments with SiO2 (wafers) were found to agree well with acid-base theory predictions modified to account
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for electrostatic interactions. For polystyrene particles being deposited on SiO2 in deionized water, the interaction energy at contact is negative, and strong adhesion of polystyrene particles on the silica substrate was expected. However, when double layer repulsion is accounted for during approach, the particle is found to be unable to overcome the repulsive energy barrier and come into contact with the substrate. Experiments, therefore, show easy detachment. To confirm that the reason for the easy detachment of polystyrene particles in pure water was the repulsive electrostatic double layer, the same experiment was run in an electrolyte solution of potassium chloride. Virtually all polystyrene particles remained attached to the silicon wafer surface, as expected from acid-base calculations. It was found that the adhesion between particle and substrate increases with the hydrophobicity of the surfaces in aqueous media as predicted by acid-base theory. Good agreement was also obtained for nonaqueous systems. Experiments with cationic surfactant CTAB showed good agreement with acid-base predictions and with the observation that the adhesion force increases with hydrophobicity. It was shown that the equilibrium distance between a polystyrene particle and a hydrophobic substrate in deionized water is greater than the equilibrium distance between a glass particle and the same substrate, thus explaining why polystyrene particles are detached more easily than glass particles from glass substrates in deionized water. The opposite was true in 0.1 M KCl, when the particles are in contact with the surface. This is consistent with acid-base predictions. Chlorocarbon experiments showed excellent agreement with calculated values of surface free energy ∆G. The particles were detached much more easily by chloroform (∆G > 0) than by carbon tetrachloride (∆G < 0). This experiment demonstrates that acid-base theory can be used as a tool to evaluate the particle removal efficiency of organic solvents, such as hydrochlorocarbons. Experiments with aromatics have shown that toluene detached particles much more easily than benzene.
Freitas and Sharma
Although the free energy of interaction in toluene (∆G ) -4.80 mJ/m2) is slightly smaller (less attraction) than of that of benzene (∆G ) -5.05 mJ/m2), this small difference is unlikely to be responsible for the far better toluene performance. We conjecture that there may be some surface molecular packing effects involving liquid structure. It was found that glass particles were more difficult to detach from mica than from the glass substrate despite mica being molecularly smooth. This behavior can be explained in terms of large electrostatic and hydration forces on glass surfaces and may also be due to a smaller surface area of contact between glass and the particle (due to surface asperities), resulting in smaller short-range attractive forces. It was found that polystyrene particles deposited in different liquids such as ethanol and electrolyte solution of potassium chloride showed different behavior when ethanol was used as the removing liquid. The experiment showed that despite the system having a positive (repulsive) energy of interaction, ethanol was unable to release the particles previously deposited in a saline solution of potassium chloride. This indicates specific short-range interactions, perhaps hydrogen bonding or polymer chain entanglement, at contact. It was concluded that when acid-base forces are considered in interfacial interactions, surface energies predominate over other aspects. However, other factors such as elastic properties and roughness of both particle and substrate play an important role in a comparative analysis of adhesion/detachment of particles among systems with similar interaction energies. In general the acid-base approach was found to provide reasonable agreement with experiments if long-range electrostatic repulsion is properly accounted for. Results for a range of particles and substrates of varying hydrophobicity and for a range of solvents can be consistently explained using this approach. Important exceptions remain. LA9807107
