Anomalous Time Effect on Particle− Bubble Interactions Studied by

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Anomalous Time Effect on Particle-Bubble Interactions Studied by Atomic Force Microscopy Elena Taran,† Marc A. Hampton,† Anh V. Nguyen,*,† and Phil Attard‡ DiVision of Chemical Engineering, School of Engineering, The UniVersity of Queensland, Brisbane, Queensland 4072, Australia, and School of Chemistry, The UniVersity of Sydney, Sydney, New South Wales 2006, Australia ReceiVed August 12, 2008. ReVised Manuscript ReceiVed NoVember 25, 2008 The atomic force microscope was employed to investigate the time effect on normal interactions between a hydrophilic silica particle and an air bubble deposited onto a hydrophobic Teflon surface in pure water and 10 mM methyl isobutyl carbinol solutions. The force versus separation distance curves taken at different times after bubble generation were qualitatively compared. It has been found that the penetration distance, jump-in force, contact angle, rupture distance, force required for the film to rupture, interfacial spring constant, and bubble shape were time-dependent. The results were explained by the change of the air-water interface shape with time due to water droplet growth on the Teflon surface inside the air bubbles.

Introduction Interaction forces exerted between air bubbles and colloidal particles have been extensively studied in the past decade due to their significance in industrial applications of flotation separation in minerals processing, oil separation, effluent processing, deinking of wastepaper for recycling, and wastewater treatment. Air bubbles of a few hundred micrometers interacting with hydrophilic and hydrophobic particles in pure water and electrolyte solutions were first studied by direct measurements of colloidal forces using atomic force microscopy (AFM).1-4 The results have been explained by electrostatic double layer forces and van der Waals repulsion. In addition, the rupture thickness was estimated to be in the range of 10-20 nm.5 Other studies demonstrated the significance of the hydrodynamic interaction.6 Soon after, oil droplets in water were examined using silica or polymer particles.7-10 Additionally, interactions between a polystyrene particle and a water droplet in hexadecane were measured.11 The authors found that the interaction force was qualitatively similar to the force obtained with a water droplet in air, namely no long-range interactions were observed. As to the data interpretation, in the case of rigid bodies, the zero separation can be chosen as the point where the constant compliance region starts, that is, where the vertical deflection of the cantilever equals the vertical displacement of the piezo. However, in the case of deformable surfaces, it is difficult to determine the separation between the probe and bubble/droplet, * Corresponding author. Telephone: +61-7-33653665. Fax: +61-733655149. E-mail: [email protected]. † The University of Queensland. ‡ The University of Sydney.

(1) Butt, H.-J. J. Colloid Interface Sci. 1994, 166, 109. (2) Ducker, W. A.; Xu, Z.; Israelachvili, J. N. Langmuir 1994, 10, 3279. (3) Fielden, M. L.; Hayes, R. A.; Ralston, J. Langmuir 1996, 12, 3721. (4) Attard, P.; Miklavcic, S. J. J. Colloid Interface Sci. 2002, 247, 255. (5) Ishida, N. Colloids Surf. A 2007, 300, 293. (6) Nguyen, A. V.; Nalaskowski, J.; Miller, J. D. Miner. Eng. 2003, 16, 1173. (7) Mulvaney, P.; Perera, J. M.; Biggs, S.; Grieser, F.; Stevens, G. W. J. Colloid Interface Sci. 1996, 183, 614. (8) Snyder, B. A.; Aston, D. E.; Berg, J. C. Langmuir 1997, 13, 590. (9) Hartley, P. G.; Grieser, F.; Mulvaney, P.; Stevens, G. W. Langmuir 1999, 15, 7282. (10) Nguyen, A. V.; Nalaskowski, J.; Miller, J. D. J. Colloid Interface Sci. 2003, 262, 303. (11) Yakubov, G. E.; Vinogradova, O. I.; Butt, H.-J. J. Adhes. Sci. Technol. 2000, 14, 1783.

as the constant compliance does not occur. Currently, there are several methods to measure the separation between deformable surfaces.2,7-9,12-14 Numerical analyses of the bubble/droplet deformation upon interaction with spherical colloids were performed by a number of authors,14-18 particularly Attard and Miklavcic,4,15 who in a theoretical treatment of bubble deformation showed that a gas bubble behaves similar to a simple spring, following Hooke’s law. The same behavior was found for the case of liquid droplets interacting with a hard surface under a loading force. On the other hand, Aston and Berg13 and Bhatt et al.19 approached the problem from a different perspective; by solving the Young-Laplace equation, they reached the conclusion that the assumption of linear elasticity for bubbles or droplets is not justified. The quest for a fundamental understanding of soft interface deformation was continued by extending the well-known colloidal probe technique20 with an innovative approach in which droplets and air bubbles were attached to an AFM cantilever.21,22 By using this method, the authors were able to quantify the interactions between two droplets in water and also between two bubbles in different aqueous solutions of electrolytes or ionic surfactants. The effects of surface-active agents on particlebubble/droplet interactions were systematically studied by many authors using both hydrophilic and hydrophobic particles,2,12 in sodium dodecyl sulfate (SDS)7,23,24 and dodecyl trimethylammonium bromide (DTAB).12 (12) Preuss, M.; Butt, H.-J. Langmuir 1998, 14, 3164. (13) Aston, D. E.; Berg, J. C. J. Colloid Interface Sci. 2001, 235, 162. (14) Gillies, G.; Prestidge, C. A.; Attard, P. Langmuir 2001, 17, 7955. (15) Attard, P.; Miklavcic, S. J. Langmuir 2001, 17, 8217. (16) Chan, D. Y. C.; Dagastine, R. R.; White, L. R. J. Colloid Interface Sci. 2001, 236, 141. (17) Webber, G. B.; Manica, R.; Edwards, S. A.; Carnie, S. L.; Stevens, G. W.; Grieser, F.; Dagastine, R. R.; Chan, D. Y. C. J. Phys. Chem. C 2008, 112, 567. (18) Nguyen, A. V.; Evans, G. M.; Nalaskowski, J.; Miller, J. D. Exp. Therm. Fluid Sci. 2004, 28, 387. (19) Bhatt, D.; Newman, J.; Radke, C. J. Langmuir 2001, 17, 116. (20) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature (London) 1991, 353, 239. (21) Dagastine, R. R.; Manica, R.; Carnie, S. L.; Chan, D. Y. C.; Stevens, G. W.; Grieser, F. Science 2006, 313, 210. (22) Vakarelski, I. U.; Lee, J.; Dagastine, R. R.; Chan, D. Y. C.; Stevens, G. W.; Grieser, F. Langmuir 2008, 24, 603. (23) Nespolo, S. A.; Chan, D. Y. C.; Grieser, F.; Hartley, P. G.; Stevens, G. W. Langmuir 2003, 19, 2124. (24) Aston, D. E.; Berg, J. C. Ind. Eng. Chem. Res. 2002, 41, 389.

10.1021/la802638s CCC: $40.75  2009 American Chemical Society Published on Web 02/03/2009

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Figure 1. Typical approaching force curves obtained for the interactions between a hydrophilic silica particle and an air bubble in pure water after 30, 60, and 120 min from the moment of bubble generation.

In this study, we have used the AFM technique to investigate the interaction forces between a hydrophilic silica particle and an air bubble in methyl isobutyl carbinol (MIBC) solutions. MIBC was used, as it is one of the most common surfactants used to control the air-water interfacial properties in the flotation separation of hydrophobic particles such as coal. The adsorption of nonionic MIBC surfactant at the water-silica interface was not expected. Interaction forces between a silica particle and an air bubble in pure DI water were measured as a reference. The force curves taken at different time intervals after bubble generation were compared. During these investigations, an anomalous time effect was revealed for both investigated systems with DI water and MIBC solutions. The use of MIBC excluded the influence of surface contamination on the anomalous time effect. This systematic study into the time effect on particle-bubble interactions is important for interpreting the bubble-particle interaction force data obtained by AFM.

Experimental Section Silicon wafers of (100) crystal orientation with a deposited thermal oxide layer of 100 nm thickness were obtained from Silicon Valley Microelectronics. Nonporous silica particles (Fuso Chemical Co., Japan) of 20 µm in diameter were cleaned by RCA SC-1 solution developed by Kern and Puotinen.25,26 The particles were attached to the end of a triangular cantilever using a very small amount of thermoplastic epoxy resin Epikote 1004.12,27 A silicon wafer cleaned by the same procedure was completely wetted by water, which indicates that the cleaning method produces a hydrophilic surface. A MFP-3D (Asylum Research) atomic force microscope was used for all the measurements. The AFM probes were calibrated by employing the thermal vibration method28 embedded in the Asylum Research AFM software. The cantilever used in the present set of data was found to have a spring constant of 0.377 N/m. Immediately priortheparticle-bubblemeasurements,thesystemcantilever-particle was UV treated for 20-30 min. A flat substrate of poly(tetrafluoroethylene) (PTFE or Teflon) was cleaned thoroughly by sonication in acetone, ethanol, and deionized water followed by drying with a stream of high purity nitrogen. A DI water droplet was placed on the PTFE plate with a Pasteur pipet, and then an air bubble of 800-900 µm in diameter was attached on the PTFE surface within the droplet by injecting air through a Hamilton 5 µL syringe.6 Pure DI water was produced by a system consisting of a Reverse Osmosis Unit in combination with a Milli-Q Academic Unit (Millipore). MIBC (99%, Acros Organics) was used as received. All measurements were performed at a room temperature of 22 ( 0.5 °C. (25) Kern, W.; Puotinen, D. A. RCA ReV. 1970, 31, 187. (26) Donose, B. C.; Taran, E.; Vakarelski, I. U.; Shinto, H.; Higashitani, K. J. Colloid Interface Sci. 2006, 299, 233. (27) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. (28) Sader, J. E.; Larson, I.; Mulvaney, P.; White, L. R. ReV. Sci. Instrum. 1995, 66, 3789.

Cleanness of the system was routinely checked by carrying out the force measurements between the silica particles and the silica wafer in DI water. As shown in the Results and Discussion, the force curves for the silica-silica measurements are repulsive at all separations even for longer than 120 min. The retraction force curve followed the same path as the approach force curve, which indicates that clean silica particles and silica surfaces were used.

Results and Discussion The initial step in our experiments was to investigate how time influenced particle-bubble interactions in pure DI water. Figure 1 shows typical force versus nominal separation curves obtained during approach of a hydrophilic silica particle toward an air bubble in pure DI water taken after 30, 60, and 120 min from the moment of bubble generation. Such force curves were recorded at 15 min intervals over 120 min, but for clarity only a selection of data is presented. Prior to the particle-bubble measurements, the AFM photodetector was calibrated by pushing the cantilever against the hard surface. Here, we only show the approaching part of the force curve because in retraction the capillary forces overcame the spring constant of the cantilever and the laser was deflected away from the photodetector. The measurements were done at the apex of the air bubble, using a velocity of 1 µm/s and applying a maximum pushing force of 10 nN. At large separation distances, the force is constantly zero, which is equivalent to the absence of interaction. On approach to the bubble, the cantilever starts to deflect because of the cumulative effect of electrostatic and van der Waals repulsive forces.29 The repulsive regime ends with a jump-in corresponding to the rupture of the film, as highlighted in Figure 1. The zero of separation is established by shifting the measured data horizontally to coincide with the linear Poisson-Boltzmann law at large separations. Similar behavior has been found previously by a number of authors.3,10,12 The inset in Figure 1 shows the magnitude of the repulsive forces and the force required for the film to rupture. In the case of interactions between two hard surfaces, it is possible to plot the data as force versus separation, but for the particle-bubble interactions we have to consider the deformation of the air-water interface. Attard and Miklavcic15,30 developed a method to determine the zero separation for measurements on deformable surfaces. It was proposed that the nominal separation between the interacting surfaces was a more accurate method compared to actual separation. The nominal separation is defined as

h ) x - z + const

(1)

where x is the measured cantilever deflection, z is the measured piezo-distance, and the constant is chosen so that the measured data coincide with the calculated rigid body force curve at

Time Effect on Particle-Bubble Interactions

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Figure 2. Force versus nominal separation for the interaction between a silica particle and an air bubble in pure water, as measured after 30, 60, and 120 min from the moment of bubble generation. The solid line represents the linear rigid body Poisson-Boltzmann result using eq 2 for ψbubble ) -66 mV and ψparticle ) -75 mV. The dashed line represents the interface deformation modeled according to ref 4 with a contact angle of 93° for a water drop on Teflon.

Figure 3. Schematic representation of a force curve between a bubble and a colloidal particle. Dr represents the penetration distance of the particle into the bubble.

decay length which is not precisely known for DI water. The values of the decay length are presented in Table 1. For two rigid bodies, the electrical double layer force is given by the renormalized linear Poisson-Boltzmann equation.14,31

F(h) ) 64πε0εr Re(kBT ⁄ q)2 γ1γ2κD e-κDh

Figure 4. Typical approaching force curves obtained for the interactions between a hydrophilic silica particle and an air bubble in 10 mM MIBC solution after 30, 60, and 120 min from the moment of bubble generation.

large separation as presented in Figure 2. There are two fitting parameters, namely, the shift in the zero of separation and the

(2)

where Re ) 1/(R-1 + Rb-1) is the effective radius, R and Rb are the particle and bubble radii, respectively, ε0 is the permittivity of vacuum, εr is the relative permittivity of the solvent, κD -1 is the Debye length, T is the absolute temperature, and q ) ze is the valence of the ions multiplied by the electron charge. The potential terms are defined as:

γ ) tanh(qψ ⁄ 4kBT)

(3)

where ψ is the surface potential. The value of surface potential for the air-water and water-silica interfaces was taken from the literature.32-34

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Figure 5. Force versus nominal separation for the interaction between a silica particle and an air bubble in the MIBC solution, as measured after 30, 60, and 120 min from the moment of bubble generation. The solid line represents the linear rigid body Poisson-Boltzmann result using eq 2 for ψbubble ) -66 mV and ψparticle ) -75 mV. The dashed line represents the interface deformation modeled according to ref 4 with a contact angle of 93° for a water drop on Teflon.

Figure 6. Typical approaching force curves for the interactions between two hydrophilic silica surfaces in pure DI water. Table 1. Evolution of the Investigated Parameters parameter

pure DI water

time, min critical rupture force, nN jump-in force, nN kD-1, nm penetration depth (Dr), nm receding contact angle (eq 5) interfacial spring constant (kb), mN/m

30 4.03 -81.9 48 923.6 24.8° 68.5

60 3.02 -76.6 45 816.78 23.3° 66.6

120 2.64 -64.3 36 775.36 22.7° 66.1

10 mM MIBC 30 2.10 -67.1 38 734.41 22.0° 52.1

60 3.11 -58.3 38 681.05 21.2° 52.1

120 3.7 -46.3 36 620.56 20.2° 51.9

Regarding the film rupture, Attard and Miklavcic15 used eq 4 to roughly estimate the maximum force which can be applied until bubble breakage.

Fmax e 2πRσ

(4)

where σ is the surface tension. Using the same expression, we found Fmax ) 4521 nN. Figure 1 shows that Frupture is 4 nN for the curve taken after 30 min from bubble generation. Therefore,

Frupture/Fmax ) 0.8 × 10-3, which is in good agreement with previous reports stating that this ratio should be 0.001-0.01.9,12 The ratio is much less than unity due to the colloidal attractive forces and capillary waves causing rupture, which are not taken into consideration in the theory. The attractive double layer force that occurs at small separations due to charge regulation or constant potential boundary conditions35,36 could be responsible for the rupture of the film and the fact that this rupture occurs at much smaller forces than the theoretical upper bound because van der Waals forces are repulsive between a particle and an air bubble. Of course, whether one uses constant potential or constant charge boundary conditions has no effect at larger separations, and this will not change the calculated forces there. From the deformation analysis represented by the dashed line in Figure 2 it can be observed that the Hookean response of the air-water interface matches the model.4,14 As shown in Figure 2, the experimental data at short distances lie between the hard wall model and the deformable model. The data recorded after 60 and 120 min are closer to the deformation model, which indicates that the bubble becomes more deformable. On the other hand, the interfacial spring constant, kb,37 was found to be approximately equal to the surface tension, which agrees with previous reported data.2,4,15 The results from the modeling are presented in Table 1. Previously reported data for more deformable (29) Nguyen, A. V.; Evans, G. M.; Schulze, H. J. Int. J. Miner. Process. 2001, 61, 155. (30) Attard, P. J. Phys.: Condens. Matter 2007, 19, 1. (31) Attard, P. Phys. ReV. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1993, 48, 3604. (32) Karakashev, S. I.; Nguyen, A. V. Colloids Surf., A 2007, 293, 229. (33) Karakashev, S. I.; Nguyen, A. V. Colloids Surf., A 2007, 293, 229. (34) Hunter, R. J. Zeta Potential in Colloid Science; Academic Press: London, 1981. (35) McCormack, D.; Carnie, S. L.; Chan, D. Y. C. J. Colloid Interface Sci. 1995, 169, 177. (36) Nguyen, A. V.; Schulze, H. J. Colloidal Science of Flotation; Marcel Dekker: New York, 2004. (37) Attard, P.; Miklavcic, S. J. Langmuir 2003, 19, 2532.

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Figure 7. (A) Bubble shape evolution in time and (B) bubble relative dimensions as a function of time. Here, the relative change in pressure was calculated from the fit of the bubble shape by a spheroid.

Figure 8. Schematic representation of the possible bubble evolution due to water droplets formed inside the pendant bubble on the PTFE surface within the experimental time interval. The radius of interface curvature at the apex initially decreases with time and then remains constant.

bodies, such as PDMS or oil droplets, show a better demarcation from the hard wall model and fit the deformation model more accurately.30,38,39 Another observation which can be extracted from Figure 1 is that the jump-in force decreases with time. The jump-in force is the maximum attractive force experienced by the colloidal probe after rupture of the film. Even though the point of film rupture was found to be a statistical event, which agrees with previous reported results for an oil-water interface,40 the jumpin force was always decreasing in time. Furthermore, the penetration depth (Dr) decreases in time. The results are presented in Table 1. The experiments were repeated several times, (38) Attard, P. J. Adhes. Sci. Technol. 2002, 16, 753. (39) Gillies, G.; Prestidge, C. A.; Attard, P. Langmuir 2002, 18, 1674. (40) Filip, D.; Uricanu, V. I.; Duits, M. H. G.; Agterof, W. G. M.; Mellema, J. Langmuir 2005, 21, 115.

and each time the same general trend in the force data over time was observed. Dr is the difference between the jump-in point and the zero force position (Figure 3). This distance is required in order to estimate the receding contact angle of a colloidal particle according to12

cos θ )

R - Dr R

(5)

This approach has been successfully used in the past to determine the contact angle for a single particle.11,41 It has also been found that the contact angle for an individual polyethylene particle changes with the approaching velocities.10 In the current experiments, the approach velocity was kept constant (1 µm/s). (41) Vinogradova, O. I.; Yakubov, G. E.; Butt, H.-J. J. Chem. Phys. 2001, 114, 8124.

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An interesting result reported by Yakubov et al.11 shows that both receding and advancing contact angles decrease with increasing particle radius. In order to check the dependence of the contact angle with bubble size, we did experiments with the same particle interacting with bubbles of increasing radius. The penetration depth and the receding contact angle were found to increase considerably in experiments which are not shown here. The change in the interaction forces presented in Figure 1 could be a result of contamination. To check the contamination hypothesis, a second set of experiments were conducted in which the interfacial tension was altered by adding MIBC, which is a nonionic surfactant readily adsorbed at the air-water interface but not at the silica particle-water interface. The same silica particle used in the pure water experiments was employed for measurements in 10 mM MIBC solutions. The force curves are presented in Figure 4. The surface potential of the air-MIBC solution interface was measured with the Micro-Electrophoresis Apparatus MK II (Brookhaven, U.K.) using microbubbles of 20 µm in diameter. As in the case of pure water, the repulsive forces were recorded, the three-phase-contact (TPC) was established, and the interaction between the particle and the bubble followed a similar trend in time even though the system contains MIBC. The experiments were repeated several times. In different experiments, the magnitude of the maximum force prior to the engulfment varied, but the force versus nominal separation curves always followed the same path. Unlike the water case, where the force required for the film to rupture was randomly distributed, in the case of 10 mM MIBC the critical force required for the film to rupture increased in the investigated time interval. Also, similar to the pure water experiments, the penetration distance and jump-in force decreased with time. Since MIBC and DI water show a similar change of measured force with time, the trends presented in Figure 4 suggest that the contamination hypothesis expressed above for the case of interactions in water should be excluded. Figure 5 shows a comparison between the measured data in MIBC solution and models with deformable and rigid surfaces. The deformable surface model fits the measured data at short distances taken at short time (e.g., 30 and 60 min). The model interfacial spring constants for the MIBC experiments are close to the surface tension of the MIBC solution (Table 1). The Debye length for the MIBC solution is shorter than the value for DI water. The other parameters are slightly different from those of DI water due to the concentration of MIBC used in the experiments. To summarize, the evolution of critical force curve parameters is presented in Table 1. To check whether the particles employed in these experiments are stable in the investigated time interval, interaction forces between a silica particle and a silica wafer in pure water were taken after 30, 60, and 120 min measured from the immersion in fluid, as shown in Figure 6. The results were repeated many times and were found to be consistent. As observed from Figure 6, the interaction forces between silica surfaces do not change with time. The retracting part of the force curve is overlapping the approaching path, without any hysteresis (not shown here). The absence of hysteresis was used as a test of experimental system cleanliness (i.e., impurities would increase the adhesive forces, leading to the force hysteresis). This behavior was reported before by other groups for hydrophilic silica surfaces.42,43 The actual ionic concentration in pure water is calculated to be about 7 × 10-5 M from the decay length of (42) Hartley, P. G.; Larson, I.; Scales, P. J. Langmuir 1997, 13, 2207. (43) Donose, B. C.; Vakarelski, I. U.; Higashitani, K. Langmuir 2005, 21, 1834.

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the electrostatic interaction fitted at large separation distances, by considering the presence of a 1:1 electrolyte. The ionic strength values come most probably from CO2 dissolved in water from air.43 It has been previously suggested that a gel layer of silanol groups protruding from silica surfaces might be formed at normal pH solutions.44 According to Iler, the dissolution of silica at pH 8.5 lies between 1 and 40 × 10-6 g/mL/day, depending on the type of silica.45 The formation of hairy layers due to the dissolution of silica at high pH was confirmed by friction experiments.46,47 Given that the present experiments show stable silica surfaces on the time scale of 120 min, it is reasonable to assume that the dissolution of silica is negligible. The results presented in Figure 6 were given to simply confirm that the silica particle does not undergo significant changes within the experimental time interval and the time effect is specific to bubbles and is not due to impurities, piezo-crystal drift, or artifacts. During the particle-bubble experiments, the distance between the particle and the bubble had to be increased from one measurement to another as the bubble increased in height over time and engulfed the particle. The increase in the bubble height, along with changes in other dimensions, is believed to be the reason for the effect of time on the force data. In order to confirm the link, the bubble shape evolution in time was examined. Analyses of bubble dimensions on a PTFE surface were performed in a thoroughly cleaned clear plastic cell containing the solution of interest. The bubble was placed onto the surface using the same method as in the case of AFM experiments. The bubble was viewed from the side using a Sony Digital Interface camera (model XCD-SX910) with a Nikon 10× magnification lens. Dimensional analysis (contact radius, height, and radius of curvature) was performed using the Nikon NIS Elements Imaging software. The results are presented in Figure 7. As can be observed in Figure 7A, the bubble height increases with time. After 120 min, even though the increase is not significant (visually), the AFM measurements seem to be sensitive enough to sense these very small variations. Furthermore, it can be observed that some droplets formed inside the pendant bubble on the PTFE surface over time. Although we performed the AFM measurements in a time interval of 120 min, here, for clarity, the snapshots of bubble evolution were taken over a longer time span. The bubble in Figure 7A could not be fitted with a sphere but a spheroid. As the results in Figure 7B indicate, the two axis lengths of the fitted spheroid, especially the vertical axis length, change over the first 30 min and then stabilize. The pressure difference across the interface does not change after 30 min. There appear to be two possibly related mechanisms. If the air that is initially injected to form the bubble is at less than 100% relative humidity,48 then it is thermodynamically favorable for water in the form of vapor to diffuse into the bubble across the air-water interface. In the extreme case, this would lead to an increase in bubble volume of more than 20%. Because of the relatively large surface area to volume ratio of the bubble, one would expect the air inside it to become saturated over the time (44) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. N. J. Colloid Interface Sci. 1994, 165, 367. (45) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (46) Taran, E.; Donose, B. C.; Vakarelski, I. U.; Higashitani, K. J. Colloid Interface Sci. 2006, 297, 199. (47) Taran, E.; Kanda, Y.; Vakarelski, I. U.; Higashitani, K. J. Colloid Interface Sci. 2007, 307, 425. (48) Alternatively, the water coming from the Millipore unit under pressure can be oversaturated with air once at room temperature. However, freshly pored Millipore water was not used in this paper. The Millipore water was equilibrated at room temperature before use.

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scales of the experiment. A possible test of this hypothesis may be to begin the experiment with a bubble formed from saturated or supersaturated air. A second possibility is that the water droplets that are observed to coalesce on the PTFE substrate within the bubble increase the volume of the bubble by an amount equal to their own volume. It is thermodynamically favorable for a droplet of water to adhere to a hydrophobic surface compared to being in the vapor phase as submicroscopic droplets. Such droplets arise from the saturation of the air inside the bubble by water vapor, so this second possible mechanism is not unrelated to the first proposal. As visible droplets grow on the substrate, the relative humidity of the air inside the bubble would drop unless it was replenished by the transport of water vapor across the air-water interface. The schematic representation of the possible bubble evolution within experimental time is presented in Figure 8. The experiments were also performed with a bubble attached to the underside of a Teflon plate (pictures not shown here). It has been found that the bubble also grew over time, which indicates that gravity and detachment do not play an important role in the bubble growth.

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Conclusions The effect of time on the interaction between a hydrophilic silica particle and an air bubble in pure water and 10 mM MIBC solution were investigated over 120 min. It was found that water droplets formed inside the pendant air bubble on the PTFE surface, which resulted in an increase in the apex height of the bubble while the contact base radius stayed relatively constant. This bubble expansion affected the interaction forces, resulting in a reduction of the penetration depth, jump-in force, and receding contact angle over time. In addition, contrasting with the water case where the magnitude of force required for the film to rupture had a random distribution, in the case of 10 mM MIBC solutions the critical force has been found to increase within the investigated time interval. Acknowledgment. The Australian Research Council is gratefully acknowledged for financial support through a Discovery grant (A.V.N.). BHP Billiton Mitsubishi Alliance (BMA) is gratefully acknowledged for funding the BMA Chair of Minerals Processing at the University of Queensland (A.V.N.). LA802638S