Removal of Colloidal Particles from Quartz Collector Surfaces As

Surfaces As Stimulated by the Passage of Liquid-Air. Interfaces ... Particle removal could be increased by the passage of multiple air bubbles. In sum...
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Langmuir 1999, 15, 5123-5127

5123

Removal of Colloidal Particles from Quartz Collector Surfaces As Stimulated by the Passage of Liquid-Air Interfaces C. Go´mez Sua´rez,* J. Noordmans, H. C. van der Mei, and H. J. Busscher Department of Biomedical Engineering, University of Groningen, Bloemsingel 10, 9712 KZ Groningen, The Netherlands Received November 17, 1998. In Final Form: April 21, 1999 Micron-sized particles adhering to collector surfaces can be detached by passing a liquid-air interface over the adhering particles. Theoretically, the efficiency of particle detachment depends on the interface velocity, the liquid surface tension, the viscosity, and the particle-substratum interaction forces. In this study we perfuse an air bubble through a parallel-plate flow chamber to study detachment of polystyrene particles from a quartz collector surface, at different interface velocities and liquid surface tensions and upon multiple air bubble passages. A linear relation was found between particle removal and the liquidair interface velocity, with negligible removal at elevated velocities. Linear relations were also found between particle removal and the liquid-air surface tensions, with different slopes for different air bubble velocities. Particle removal could be increased by the passage of multiple air bubbles. In summary, this study shows that liquid-air interfaces can be employed to detach micron-sized particles from collector surfaces. The detachment process can be optimized by adjusting the air bubble velocities, the surface tensions, and the number of bubbles applied.

Introduction The adhesion of submicron particles to surfaces is generally nonspecific and can be very tenacious. Accordingly, their removal by simple mechanical procedures is difficult to achieve.1 Secondary minimum DerjaguinLandau-Verwey-Overbeek DLVO forces are believed to be responsible for the adhesion of submicron particles to surfaces rather than primary interaction forces. For particle diameters between 1 and 0.1 µm, the magnitude of the forces in the secondary minimum forces are up to 5 orders of magnitude greater than the weight of the particle.1 Several mechanical methods for the removal of small particles from surfaces are known, including ultrasonics,2,3 megasonics, wiping, brush scrubbing, low-pressure spraying, high-pressure jet spraying, etching, and centrifugal spraying.4,5 Frequently, however, these methods are ineffective when applied to particles with diameters of less than about 1 µm.1,4,6 The use of surfactants may be efficient for the removal of micron-sized particles but bears the risk of surfactant adsorption to the surface,7 which can be troublesome for future application. The removal of adhering submicron-sized particles from surfaces has been extensively studied in the semiconductor * Corresponding author. Phone: 31-50-3633121. Fax: 31-503633159. E-mail: [email protected]. (1) Kaiser, R. In Particle on Surfaces 2: Detection, Adhesion and Removal; Mittal, K. L., Ed.; Plenum Press: New York, 1988; p 269. (2) Menon, V. B.; Michaels, I. D.; Donovan, R. P.; Ensor, D. S. In Particle on Surfaces 2: Detection, Adhesion and Removal; Mittal, K. L., Ed.; Plenum Press: New York, 1988; p 297. (3) Berg, D. M.; Grimsley, T.; Hammond, P.; Soreson, C. T. In Particle on Surfaces 2: Detection, Adhesion and Removal; Mittal, K. L., Ed.; Plenum Press: New York, 1988; p 307. (4) Bardina, J. In Particle on Surfaces 1: Detection, Adhesion and Removal; Mittal, K. L., Ed.; Plenum Press: New York, 1988; p 329. (5) Khilnani, A. In Particle on Surfaces 1: Detection, Adhesion and Removal; Mittal, K. L., Ed.; Plenum Press: New York, 1988; p 17. (6) Leenaars, A. F. M. In Particle on Surfaces 1: Detection, Adhesion and Removal; Mittal, K. L., Ed.; Plenum Press: New York, 1988; p 361. (7) Velraeds, M. M. C.; Van der Mei, H. C.; Reid, G.; Busscher, H. J. Appl. Environ. Microbiol. 1996, 62, 1958.

industry because of their detrimental effect in the production of integrated circuits. A cleansing method involving the detachment of particles stimulated by the passage of a liquid-air interface over the surface was proposed, therewith avoiding the use of surfactants.6,8 Rutile (TiO2), amorphous silica (SiO2), and haematite (RFe2O3) particles adhering to silicon wafers could be removed by the passage of a liquid-air interface with efficiencies up to 97%. The detachment of these particles was studied by moving the substrates vertically through a liquid-air interface at a controlled speed, and the efficiency of cleaning depended on the wettability of both particle and substrate surfaces and not on the particle size.8 The interaction between colloidal particles and air bubbles in aqueous electrolytes9-11 is also of great interest in froth flotation,12,13 water purification, and deinking of paper.14 The passage of a liquid-air interface over bacteria adhering to substrates in vitro has been applied to estimate the ability of microorganisms adhering to dental enamel surfaces15,16 to withstand detachment forces in the oral cavity such as during eating, speaking, drinking, or swallowing or to contact lenses17 during blinking of the eye. Other studies report displacement of bacteria adhering to biomaterial implants such as silicone rubber catheters, larynx prostheses,18-22 and glass surfaces23 after the passage of a liquid-air interface. (8) Leenaars, A. F. M.; O’Brien, S. B. G. Philips J. Res. 1989, 44, 183. (9) Preuss, M.; Butt, H. J. Langmuir 1998, 14, 3164. (10) Ducker, W. A.; Xu, Z.; Israelachvili, J. N. Langmuir 1994, 10, 3279. (11) Fielden, M. L.; Hayes, R. A.; Ralston, J. Langmuir 1996, 12, 166. (12) Ahmed, N.; Jameson, G. J. In Frothing in Flotation; Laskowski, J., Ed.; Gordon & Breach: New York, 1989; p 77. (13) Schulze, H. J. In Physicochemical Elementary Processes in Flotation; Elsevier: Amsterdam, The Netherlands, 1984. (14) Bloom, F.; Heindel, T. J. J. Colloid Interface Sci. 1997, 190, 182. (15) Bos, R.; Van der Mei, H. C.; Busscher, H. J. J. Dent. Res. 1996, 75 (2), 809. (16) Landa, A. S.; Van der Mei, H. C.; Busscher, H. J. Adv. Dent. Res. 1997, 11 (4), 528. (17) Landa, A. S.; Van der Mei, H. C.; Van Rij, G.; Busscher, H. J. Cornea 1998, 17 (3), 293.

10.1021/la981608c CCC: $18.00 © 1999 American Chemical Society Published on Web 06/09/1999

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Go´ mez Sua´ rez et al.

approach the three-phase contact line moves, and its position changes continuously. Under flow, a viscous drag force acts on the adhering particle in addition to the abovementioned forces under static conditions.28 The surface tension force can oppose the adhesion force between a particle and a collector surface, depending on the position of the three-phase boundary line as expressed by

Fγ ) 2πRpγlv sin[φ(t)] sin[θ - φ(t)] Figure 1. Schematic presentation of the three-phase contact between an air bubble, micron-sized particle, and fluid regions. Rp represents the radius of the particle, φ the angle giving the position of the particle with respect to the liquid-air interface, θ the contact angle between the surface of the particle and the liquid-air interface, h0 the height of the undisturbed meniscus, H the distance between the particle and collector surface, and γlv the liquid-air interfacial tension.

Recently, the detachment of polystyrene particles from glass collector surfaces by surface tension forces induced by air bubble passage was studied in a parallel-plate flow chamber, demonstrating that surface tension detachment forces were in excess of DLVO interaction forces and could consequently remove the majority of colloidal particles adhering to different collector surfaces.24 The aim of the present work is to study the factors affecting the detachment of colloidal particles from collector surfaces stimulated by the passage of air-liquid interfaces in a parallel flow chamber. The efficiency of particle removal as a function of the liquid-air interfacial tension, the velocity of the air bubble, and the effect of multiple passages of air bubbles over a quartz collector surface is presented, together with an analysis of the factors influencing the detachment process. Theory A sequence of microprocesses controls the detachment of colloidal particles from collector surfaces by the passage of a liquid-air interface, including the approach of the liquid-air interface to the particle, interception of the particle by the interface, sliding of the particle along the surface of the liquid film separating particle and interface, film rupture, the subsequent formation of a three-phase contact line, and stabilization of the particle/air bubble aggregate and its subsequent removal away from the collector surface. In a static approach, after the formation of a threephase contact line during the passage of a liquid-air interface over an adhering colloidal particle, gravitational, hydrostatic, buoyancy, and surface tension forces act on the particle,14 as shown in Figure 1. However, for particles with a radius smaller than 10 µm, the surface tension force dominates and the value of the static contact angle can be considered constant.6,8 Note that in a dynamic

(1)

where Rp is the radius of the particle, γlv is the liquid-air interfacial tension, φ represents the angle determining the position of the particle with respect to the liquid-air interface, and θ is the contact angle between the surface of the particle and the liquid-air interface. Depending on the degree of immersion of the particle (see Figure 1), three situations can be distinguished: φ < θ, where the surface tension forces are directed upward tending to remove the adhering particle; φ ) θ, where the surface tension forces act parallel to the liquid-air interface; and φ > θ, where the surface tension forces are directed down, tending to draw the particle into the liquid and thus opposing its removal. For the dimensions considered here (i.e., H/Rp , 1), the viscous force (Fη) opposing the separation of the particle from the surface is given by

Fη ) -6πR2pηW/H

(2)

where W is the velocity of the displaced particle (note that as the momentum is conserved W will also be the speed of the air-liquid interface), H is the distance between the particle and the substrate, and η is the kinematic viscosity of the fluid. Under flow and when the particle begins to move away from the collector surface by attachment to the liquid-air interface, conservation of the momentum dictates

mW )

∫0t(Fγ - FA - Fη) dt′

(3)

where Fγ, FA, and Fη are the surface tension, the particle adhesion, and the viscous forces, respectively, and m is the mass from the particle. The particle adhesion force can be described according to the classical DLVO approach as a combination of Lifshitz-van der Waals and electrostatic forces26

FA (H) ) FLW (H) + FEL (H)

(4)

in which FLW (H) and FEL (H) denote the Lifshitz-van der Waals and electrostratic interaction forces, respectively. According to the DLVO approach, estimates for the adhesion force between polystyrene particles and silicate surfaces24 range from 10-13 to 10-12 N. Materials and Methods

(18) Millsap, K. W.; Reid, G.; Van der Mei, H. C.; Busscher, H. J. Biomaterials 1997, 18 (1), 87. (19) Van der Mei, H. C.; Van de Belt-Gritter, B.; Reid, G.; BialkowskaHobrzanska, H.; Busscher, H. J. Microbiology 1997, 143, 3861. (20) Everaert, E. P. J. M.; Van der Mei, H. C.; Busscher, H. J. Colloids Surf. B 1998, 10, 179. (21) Everaert, E. P. J. M.; Van de Belt-Gitter, B.; Van der mei, H. C.; Busscher, H. J.; Verkerke, G. J.; Dijk, F. J. Mater. Sci. 1998, 9, 147. (22) Busscher, H. J.; Geertsema-Doornbusch, G. I.; Van der Mei, H. C. J. Biomed. Mater. Res. 1997, 34, 201. (23) Pitt, W. G.; McBride, M. O.; Barton, A. J.; Sagers, R. D. Biomaterials 1993, 14 (8), 605. (24) Noordmans, J.; Wit, P. J.; Van der Mei, H. C.; Busscher, H. J. J. Adhes. Sci. Technol. 1997, 11 (7), 957.

Polystyrene Latices and Suspensions. Monodisperse polystyrene latices were synthesized as described (25) Shaw, D. J. In Introduction to Colloid and Surface Chemistry; Shaw, D. J., Ed.; Butterworth: London, 1985; p 19. (26) Tadros, Th. F. In Microbial Adhesion to Surfaces; Lynch, J. M., Melling, J., Rutter, P. R., Vicent, B., Eds.; Ellis Horwood Limited: London, 1980; p 93. (27) Goodwin, J. W.; Hearn, J.; Ho, C. C.; Ottewill, R. H. Colloid Polym. Sci. 1974, 252, 464. (28) Hiemenz, P. C. In Electrophoresis and Other Electrokinetic Phenomena; Lagowski, J. J., Ed.; Marcel Dekker: New York, 1977; p 452.

Removal of Colloidal Particles from Quartz Collector Surfaces

by Goodwin et al.27 Styrene (Baker grade, J. T. Baker B.V., Deventer, Holland) was purified before use by vacuum distillation at 40-50 °C. Potassium persulfate (Merck, Darmstadt, Germany) was employed as the initiator of the reaction. Potassium bicarbonate (Merck, Darmstadt, Germany) was used as the ionic medium. The polymerization reaction was carried out in the absence of emulsifiers under a nitrogen atmosphere at a temperature of 60 °C. The concentrations used for the reaction were 2.22 × 10-3 M initiator, 43.3 × 10-3 M potassium bicarbonate, and 0.87 M monomer. After the reaction time (i.e., 24 h), unreacted monomer forming a separate phase on the surface of the vessel was decanted and removed. The latex solution was filtered, washed with distilled water, and rotaevaporated. The cleaning process was repeated by adding new distilled water to the latex suspension and reevaporating and was considered to be complete when no monomer was detected by UV in the evaporated phase. After cleaning, the latex was treated with cation- and anion-exchange resins (analytical grade, AG 50W-X4 resin and AG 1-X4 resin, respectively, BioRad Laboratories, Richmond, CA) to remove potassium bicarbonate, unreacted monomer, potassium sulfate, and sulfuric acid formed in the reaction. Polystyrene particles were suspended to a concentration of about 3 × 108 cm-3 in 10 mM potassium nitrate for the deposition assays in the parallel-plate flow chamber. Samples of the cleaned latices were diluted with distilled water, and scanning electron microscopy was used to determine the radius of the polystyrene particles, which was found to be 806 ( 16 nm. The ζ potential of the polystyrene particles was measured by particulate microelectrophoresis28 at room temperature in 10 mM potassium nitrate as used for the flow experiments (pH ) 5.8) and amounted to -68 ( 1 mV. Quartz Collector Surfaces. Quartz surfaces of 76 × 25 × 1.5 mm (Lowers Hapert Glastechniek, Hapert, The Netherlands) were used as collector surfaces. The quartz plates were cleaned thoroughly with 2% RBS 35 detergent (Omnilabo International B.V., Breda, The Netherlands), extensively rinsed in water and Milli-Q water, washed in methanol, and again rinsed in water and finally with Milli-Q water. The ζ potential of the quartz surfaces was determined using streaming potential measurements29 and found to be -41 ( 1 mV at room temperature in the potassium nitrate solution used. Equilibrium, advancing, and receding contact angles on the quartz surfaces were measured at room temperature with a standard deviation of (2° employing sessile potassium nitrate droplets30 and amounted to 32, 52 and 24°, respectively. Interfacial Tension Measurements. Axisymmetric drop shape analysis by profile (ADSA-P)31 was used for the measurement of the liquid surface tensions (γlv) of the perfusion fluids. Potassium nitrate was employed as an aqueous perfusion fluid, whereas in order to change the liquid-air interfacial tension, also 1-propanol was added to the potassium nitrate in varying amounts. ADSA-P calculates the liquid surface tension of an axisymmetric droplet from its shape, on the basis of the classical Laplace equation of capillarity32 that describes the shape of a droplet. In these experiments, ADSA-P was applied as (29) Van Wagenen, R. J.; Andrade, J. D. J. Colloid Interface Sci. 1980, 76, 305. (30) Spelt, J. K.; Vargha-Butler, E. I. In Surface Thermodynamics; Neumann, A. W., Spelt, J. K., Eds.; Marcel Dekker: New York, 1996; p 389. (31) Rotenberg, Y.; Boruvka, L.; Neumann, A. W. J. Colloid Interface Sci. 1983, 93, 169.

Langmuir, Vol. 15, No. 15, 1999 5125 Table 1. Interfacial Tension (γlv) and Kinematic Viscosity (η), for Different Mixtures of 10 mM KNO3/1-Propanol % 1-propanol (w/w)

γlv (10-3 N‚m-1)

η (10-3 N‚s‚m-2)a

0 1 3 5 12 40 60

70.08 58.90 49.59 41.74 33.59 24.04 19.31

1.002 1.055 1.161 1.280 1.745 3.241 3.590

a

Values of kinematic viscosity at 20 °C from ref 34.

Figure 2. Overview of the flow chamber system, including the parallel-plate flow chamber system, image analysis components, and the syringe pump.

described by Noordmans and Busscher.33 An objective function, expressing the difference between a set of measured droplet profile coordinates and a theoretical Laplacian profile, is numerically minimized and makes use of the three-phase line (solid-liquid-air), the droplet profile coordinates, gravity, density, and x and y magnification factors as input parameters. Table 1 summarizes the interfacial tensions for the different perfusion liquids employed. Parallel-Plate Flow Chamber System and Image Analysis. The experiments were carried out in a parallelplate flow chamber that has previously been described in detail.35,36 Figure 2 shows a schematic presentation of the flow chamber system. It contains three basic modules: the flow loop, the syringe pump, and the image analysis components. The chamber is made of poly(methyl methacrylate) (PMMA), while the channel dimensions were reduced with respect to the original design to facilitate control of the passage and velocity of the air bubbles introduced. The effective dimensions of the modified channel are 76 × 5 mm (l × w), and the top and bottom collector plates are separated by two Teflon spacers of 0.6 mm thickness. The top plate is made out of glass. The bottom plate is made out of PMMA with a rectangular section cut out in the center with dimensions 76 × 25 × 1.5 mm (l × w × h) in which the quartz collector surfaces could be inserted. A pulse free fluid flow was achieved by hydrostatic pressure, and fluid was recirculated by a roller pump. For the enumeration of the number of particles adhering to the quartz bottom plate37 before and after the passage (32) Lahooti, S.; Neumann, A. W.; Del Rio, O. I.; Cheng, P. In Surface Thermodynamics; Neumann, A. W., Spelt, J. K., Eds.; Marcel Dekker: New York, 1996; p 441. (33) Noordmans, J.; Busscher, H. J. Colloids Surf. 1991, 58, 239. (34) Weast, R. C.; Astle, M. J. CRC Handbook of Chemistry and Physics, 59th ed.; CRC Press Inc.: Boca Raton, FL, 1978; p D-295. (35) Sjollema, J.; Busscher, H. J. J. Colloid Interface Sci. 1989, 132, 382. (36) Van Kooten, T. G.; Schakenraad, J. M.; Van der Mei, H. C.; Busscher, H. J. J. Biomed. Mater. Res. 1992, 26, 725.

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of an air bubble, the entire flow chamber was mounted on the stage of a phase contrast microscope (Olympus BH-2) equipped with a 40× ultralong working distance objective (Olympus ULWD-CD Plan 40 PL). A CCD camera (CCDMXR/5010, Adimec Advanced Image Systems B.V., Eindhoven, The Netherlands) was mounted on the phase contrast microscope and connected to a PCVision+ video frame grabber interface card (Imaging Technology Incorporated, Woburn, MA) to digitize the video signal into an image consisting of 512 × 512 pixels, with each pixel having 256 possible gray values. The frame grabber was installed in a Pentium II processor based personal computer, and the number of particles adhering to the collector surface was enumerated by homemade image processing software. Image analysis procedures applied here included the subtraction of an out-of-focus image, Laplace filtering, and black-and-white thresholding of the image.38 A cell-finder slide (76 × 26 mm) attached to the outside of the bottom plate was focused in order to obtain an accurate repositioning ((2 µm) of the flow chamber in the particle enumeration process. Experimental Protocol and Data Analysis. In all experiments, a 10 mM potassium nitrate solution was perfused through the chamber for 30 min. Thereafter, polystyrene particles from the suspensions were allowed to adhere to the collector surface until a final density of about 2 × 106 cm-2. The volumetric flow was 0.025 mL‚s-1 (shear rate 10 s-1), which yields a laminar flow (Reynolds number 0.6). Thereafter, flow was switched again for 30 min to potassium nitrate in order to remove all nonadhering particles from the flow chamber and the tubing system. For experiments involving 1-propanol, flow was switched at this stage to the appropriate nitrate/propanol mixture. Prior to air bubble introduction, the system was closed by a three-way valve to a syringe pump (Terumo, SC512) filled with the potassium nitrate or the nitrate/propanol solution and connected to the parallel flow chamber (see Figure 2). Herewith, the velocity of the air bubble could be adequately controlled. A 0.1 ( 0.02 mL air bubble was introduced in the flow system through injection in the tubing connected to the chamber by a 1 mL syringe provided with a needle. The approximate projected dimensions of the air bubbles are 25 × 5 mm (l × w), i.e., fully spanning the width of the flow chamber. The particles adhering to the collector surfaces were enumerated by choosing arbitrarily 10 areas of 0.017 mm2, distributed over the length of the collector surface. These positions could be retrieved after the passage of the liquidair interface with the aid of the cell finder. All experiments were performed at room temperature in triplicate with separate particle suspensions and collector surfaces. Results Particle Removal as a Function of the Velocity of the Liquid-Air Interface. Figure 3 shows the percentage of the adhering particles removed by the passing air bubble in the flow chamber, for different air bubble velocities in a 10 mM KNO3 solution. A maximal particle removal of 92% was found for the lowest liquid-air interface velocity of 2.37 × 10-3 m‚s-1, and the percentage of particles removed decreased linearly with increasing (37) Meinders, J. M.; Van der Mei, H. C.; Busscher, H. J. J. Microbiol. Methods 1992, 16, 119. (38) Meinders, J. M.; Noordmans, J.; Busscher, H. J. J. Colloid Interface Sci. 1992, 152, 265.

Go´ mez Sua´ rez et al.

Figure 3. Percentage of the adhering particles removed from the collector surface by the passage of a air-liquid interface in a parallel-plate flow chamber as a function of the liquid-air interface velocity. The dashed line corresponds to the linear regression (r ) 0.995).

Figure 4. Percentage of the adhering particles removed from the substrate as a function of the interfacial tension of different liquid-air interfaces for velocities of (b) 2.37 × 10-3 and (2) 7.75 × 10-3 m‚s-1. The dashed line corresponds to the linear regression for a velocity of 2.37 × 10-3 m‚s-1 (r ) 0.98) and the dotted line to that for a velocity of 7.75 × 10-3 m‚s-1 (r ) 0.93).

air bubble velocity (linear correlation coefficient 0.995). Most particles not detached by the liquid-air interface had initially formed aggregates on the collector surfaces prior to the passage of the air bubbles. Above air bubble velocities of 15 × 10-3 m‚s-1 virtually no particle removal was observed, and at higher velocities, a great number of adhering particles was transported to the sides of the flow chamber. Particle Detachment as a Function of the LiquidAir Interfacial Tension. Figure 4 summarizes the particle removal as a function of the liquid-air interfacial tension for air bubble velocities of 2.37 × 10-3 and 7.75 × 10-3 m‚s-1. Particle removal decreased linearly (linear correlation coefficient 0.98 and 0.93 and slopes 1.6 and 0.9 mJ-1‚m2, respectively). Note that this linearity is not affected by the variation in kinematic viscosity of the different potassium nitrate/propanol mixtures (see Table 1). Particle Removal as a Function of Multiple Air Bubble Passages. Figure 5 presents the percentage of particles detached upon multiple passages of air bubbles at a velocity of 13.48 × 10-3 m‚s-1 as a function of the number of air bubbles. The percentage of particles removed, calculated with respect to the initial number of

Removal of Colloidal Particles from Quartz Collector Surfaces

Figure 5. Percentage of the adhering particles removed from the substrate by the passage of multiple air bubbles moving at a velocity of 13.48 × 10-3 m‚s-1. The dashed line corresponds to the linear regression (r ) 0.994). 10 mM KNO3 was used as a perfusion liquid.

adhering particles, increased linearly (linear correlation coefficient 0.994) with the number of air bubbles passed through the parallel-plate flow chamber. Discussion In this paper we studied submicron particle removal from collector surfaces by liquid-air interface passages in a parallel-plate flow chamber. In line with theoretical analyses, removal was sensitive to the liquid-air interfacial tension and velocity and no removal of adhering particles was observed at elevated interface velocities because of the lack of a sufficient transfer of momentum. Also, at elevated air bubble velocities, the thickness of the liquid film between the front and the rear menisci of the air bubble increases, therewith changing the degree of immersion and decreasing the resulting detachment force, as illustrated by eq 1. The increase in the liquid film thickness with the air bubble velocity, U, can be estimated as39

( )

b 3ηU ) 0.643 w γlv

2/3

(5)

in which b represents the liquid film thickness and w the width of the flow channel. From eq 5 it can be calculated (39) Bretherton, F. P. J. Fluid Mech. 1961, 10, 166.

Langmuir, Vol. 15, No. 15, 1999 5127

that for an aqueous solution the liquid film thickness varies from 1491 × 10-9 m for the highest (13.48 × 10-3 m‚s-1) to 468 × 10-9 m for the lowest (2.37 × 10-3 m‚s-1) air bubble velocity employed in this study. Viscous drag forces have little or no effect on the efficiency of submicron particle removal, as can be seen in Figure 4. However, on the basis of the theory (see eq 2), it may not be ruled out that a stronger influence of viscosity will be revealed for larger particles and higher air bubble velocities. It is a puzzling observation that one air bubble seems to have a maximal capacity with regard to particle removal, especially at elevated velocities (see Figure 5), for which particle removal increased linearly with the number of bubbles passed through the flow chamber. In contrast, at low bubble velocities, one air bubble seems to be able to carry nearly all adhering particles upon one passage. This may point to an additional effect, not accounted for in the theory outlined, involving diffusion of detached particles from the liquid-air interface into the bulk perfusion liquid. At slow velocities, there is ample time for detached particles to detach into the bulk liquid, whereas at elevated velocities a maximal capacity of the interface may be reached more readily and impede further detachment of adhering particles. This reasoning is contradicted, however, by the observation that particle removal is homogeneous over the entire length of the flow chamber, irrespective of bubble velocities. The results presented here not only constitute a warning for those involved in particle deposition studies to enumerate adhering particles solely after careful control of hydrodynamic and rinsing conditions (in fact, enumerations should preferably be done in situ, i.e., in the liquid phase) but also points to a variety of potential applications in cleansing, like in dishwashers, oil and water pipe lines, and several biomedical applications. Conclusions (1) Micron-sized particles adhering to a collector surface under flow can be removed with high efficiencies by the passage of a liquid-air interface. (2) This type of particle removal increases linearly with the air-liquid interfacial tension. (3) Above a certain air bubble velocity, particle removal is virtually absent, as the surface tension detachment force does not act sufficiently long upon the adhering particle. (4) Viscous drag forces have little or no effect on the removal of adhering submicron particles. LA981608C