Role of Surface Charge and Hydrophobicity in the Three-Phase

Article pubs.acs.org/JPCC

Role of Surface Charge and Hydrophobicity in the Three-Phase Contact Formation and Wetting Film Stability under Dynamic Conditions Anna Niecikowska,† Marta Krasowska,*,‡ John Ralston,‡ and Kazimierz Malysa† †

Jerzy Haber Institute of Catalysis and Surface Chemistry Polish Academy of Sciences, ul. Niezapominajek 8, 30-239 Cracow, Poland Ian Wark Research Institute, University of South Australia, Mawson Lakes SA 5095, Adelaide, Australia

‡

S Supporting Information *

ABSTRACT: Determination and control of the parameters influencing threephase contact formation and wetting film stability under dynamic conditions is important for both industrial applications and fundamental studies. Hydrophobicity profoundly affects not only the critical thickness of rupture but also the subsequent rate of the three-phase contact expansion. On the other hand, surface charge has a significant impact on the wetting film stability. In the present study, by comparing collision of air bubbles of different sizes with hydrophobized amorphous titanium dioxide (TiO2) surfaces immersed into solutions at different pH values, we demonstrated that for an intermediate degree of hydrophobicity (advancing contact angle ≤ 60°), the surface charge becomes a decisive factor determining the time of the threephase contact formation, tTPC, and the time of wetting film drainage, td. For pH values below the isoelectric point (IEP) of TiO2, the tTPC and td were significantly shorter than for pH values above the IEP. Moreover, the tTPC and td values decreased with the bubble diameter.

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INTRODUCTION The interaction between gas bubbles and particles is critical for various processes that occur in mineral processing and wastewater treatment as well as in the food, cosmetic, and pharmaceutical industries. In mineral processing, flotation is used to separate mineral particles with selectivity controlled by differences in surface wettability.1 During the flotation separation, as particles approach a gas bubble, three fundamental processes determine whether or not the bubble−particle aggregate can be formed. These are collision, attachment, and detachment.1 For aggregate formation, a liquid intervening film formed between the bubble and the particle surface needs to be ruptured, and a liquid/gas/solid three-phase contact (TPC) line must be formed.1,2 The formation of the bubble−particle aggregate upon collision is determined by the stability of the thin wetting film between the bubble and the particle. Stable films remain intact between the bubble and the particle making attachment impossible, while unstable films rupture spontaneously at a definite thickness (known as the critical thickness, hcr). The larger the hcr, the easier the film ruptures, and hence, the smaller is the film stability. The critical thickness depends both on the wettability of the solid and the properties of the film interfaces. The wettability of solid surfaces is influenced by the surface energies of the contacting phases as well as by surface heterogeneity and surface electric charge.3 When an aqueous © 2012 American Chemical Society

solution spreads over a solid surface as a thin liquid film, its thickness and stability depend on the interfacial forces which act across the film. In the simplest case of hydrophilic solids, the total force is just the sum of van der Waals and electrostatic interactions.4 For wetting films, for example, on metal oxide surfaces the Hamaker constant is negative.5 Therefore, a repulsive van der Waals force acts within the film preventing the film rupture. Electrostatic interactions depend on the properties of the overlapping electrical double layers at both interfaces.6 When the pH of the aqueous phase exceeds the point of zero charge, pHPZC, of the metal oxide and that of the liquid/vapor interface, both surfaces are negatively charged, which results in a repulsive electrostatic force between them. When the pH is below the pHPZC of the metal oxide (but still above that of the liquid/vapor interface), the metal oxide surface is positively charged and an attractive electrostatic force arises between the two surfaces. The way in which the van der Waals and the electrostatic contributions combine determines whether or not the wetting film remains stable. Wetting films, formed between a bubble and a solid, have been investigated for several decades. Most of the experiments have been carried out under equilibrium or semiequilibrium conditions7−13 with only recent experiments performed under Received: November 26, 2011 Revised: December 21, 2011 Published: January 3, 2012 3071

dx.doi.org/10.1021/jp211378s | J. Phys. Chem. C 2012, 116, 3071−3078

The Journal of Physical Chemistry C

Article

dynamic conditions,15−17 which are much closer to the real systems (e.g., flotation separation). The very first experiments on wetting film stability on hydrophilic solids were carried out by Derjaguin and Kussakov18 with an air bubble pressed against a clean hydrophilic glass plate. They observed that once the equilibrium thickness was reached, the film remained stable and did not rupture. The first quantitative measurements of the thickness of wetting films between a hydrophilic silica surface and an air bubble were obtained by Platikanov19 followed by Read and Kitchener20 who used monochromatic light interferometry and calculated film thickness from the interference pattern. Read and Kitchener21 reported that electrostatic interactions are responsible for the wetting film stability at separations greater than 20 nm. Zorin22 et al. observed that a thick wetting film (up to 60 nm) stabilized by electrostatic interactions was formed on quartz surfaces at pH values greater than 3, that is, above the pHIEP of the silica and liquid/vapor interface. At pH 2.7, however, the film became unstable and, within a fraction of a second, collapsed to a thin film (thickness of a few nm), which was stabilized by a shortrange repulsive van der Waals force. Such ultrathin films, called α-films, have been investigated recently by Krasowska et al. by means of colloid probe atomic force microscope (AFM).23 They showed that in the case of 3.6 nm thick electrolyte films formed between the surfaces of a negatively charged bubble and a positively charged titania colloid probe, stabilized by van der Waals repulsion, the hydrodynamic contribution was very substantial and acted over a much more extensive region of the film providing additional stabilization. Such ultrathin stable films prevent intimate contact between the air bubbles and oppositely charged, fine, hydrophilic particles during the flotation process. On the other hand, such particles are held in close proximity by attractive electrostatic forces and can be recovered. The theoretical concept of such contactless flotation originally proposed by Derjaguin et al.24 was recently proved to work for fine (smaller than 5 μm) hydrophilic titania23 and αalumina25 particles. Pushkarova and Horn26 used a surface force apparatus (SFA) to measure the interaction between a hydrophilic mica surface and an air bubble in different salt solutions at pH 5.6 (when both interfaces were negatively charged). They observed that even though the deformation of the bubble surface increased with decreasing separation between the bubble and the mica surface, the wetting film remained stable because of repulsive van der Waals and electrostatic forces acting between the two bodies. The stability of the wetting films formed between a hydrophilic metal oxide and a bubble was investigated by Parkinson and Ralston,27 who used high-speed, dynamic film drainage interferometry to measure the thickness of the thin film between the microbubble and a hydrophilic titania surface as a function of the solution composition. They demonstrated that the wetting films were stable irrespective of whether or not the electrostatic contribution between the interacting surfaces was repulsive or attractive. The properties of the wetting films between a negatively charged bubble and negatively charged solids at equilibrium have also been studied by colloid probe AFM.7−11 Only repulsive forces, indicating the presence of a stable wetting film, occur between a hydrophilic particle and an air bubble. In contrast, when a hydrophobic particle is pressed against an air bubble,12,13 the wetting film ruptures, which is indicated by an abrupt jump-into-contact on the approach part of the AFM

force curve. The stability of thin liquid films at hydrophobic surfaces can be judged in terms of thin film drainage, rupture, and TPC formation time. So far, most of the studies carried out in bubble-plate geometry have investigated individual effects of hydrophobicity,15 surface charge,27,28 or adsorbed polymers29,17 on the attachment of a rising air bubble to such solid surfaces. In this paper, we investigate the interrelated effect of the hydrophobicity and the surface charge in the bubble-solid attachment under dynamic conditions. The time of the wetting film drainage, rupture, and TPC line expansion as well as the critical thickness of rupture is studied as a function of surface hydrophobicity and surface electric charge.

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MATERIALS The titania films were prepared using physical vapor deposition to deposit a thin layer (∼20 nm) of titania on a glass slide. The glass slides were carefully cleaned by dipping in Piranha solution (H2SO4:H2O2, 3:1) followed by rinsing in Milli-Q water (water resistivity 18.2 MΩ·cm, total organic carbon less than 2 μg/l, and interfacial tension 72.4 ± 0.1 mN/m at 22 °C; NANOpure Diamond UV system, Barnstead, United States); then, they were dried in a stream of high-purity N2 and finally were plasma treated in air for 60 s (Harrick, PDC-OD2). The glass slides were placed in a Magnetron deposition chamber just below a titanium target. The titania films were sputtered in an argon:oxygen (5:1) environment. Such titania films were amorphous and hydrophilic.3 The titania surfaces were hydrophobized with 10−3 M octadecylsilane ([CH3(CH2)17SiH3], 97%, Sigma Aldrich) solution in cyclohexane (AR, Chem-Supply). The degree of hydrophobicity was controlled by changing the immersion time in the 10−3 M octadecylsilane solution. The hydrophobized samples were washed several times with cyclohexane followed by ethanol (AR, Chem-Supply) and Milli-Q water. Commercial Teflon was used as a model surface of high degree of hydrophobicity. Ultrahigh purity (Milli-Q) water was used in all the experiments. KCl of AR grade (Merck) was calcined at 550 °C over 8 h, was recrystallized, and was calcined again to remove any impurities. The surface tensions of 10−4 and 10−2 M KCl solutions were, within experimental error, equal to the surface tension of pure water at the same temperature (72.4 ± 0.1 mN/m at 22 °C). HCl and KOH, which were used for the pH control, were of analytical grade (Sigma-Aldrich).

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METHODS

Film Rupture and TPC Formation Experiments. Kinetics of the three-phase contact line formation, by a colliding bubble, at various titania and Teflon surfaces was studied in side- and top-view (titania surfaces only) configurations. The setup used for side-view experiments is presented in Figure.1. The main components of the experimental apparatus are (1) a square borosilicate glass column (cross section 50 mm × 50 mm), (2) a borosilicate glass capillary (of different inner diameters) or a microfluidic chip with T-junction mounted at the bottom of the column, (3) syringe pumps (Cole-Parmer) with gastight high precision glass syringes (Hamilton), (4) a high-speed camera (SpeedCam MacroVis, Germany), and (5) a PC with image analysis software. In such a configuration, the titania surface was mounted just below the water or solution surface at a distance ca. 100 mm from the point of bubble formation (an outlet of a 3072



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Contact Angle Measurements. The degree of hydrophobicity of the surface was assessed by measurements of the water advancing contact angle on hydrophobized titania and Teflon surfaces. The sessile drop method (DSA100, KRUSS) was used, and the advancing contact angles of small water droplets (3 μL) were determined by fitting the Young−Laplace equation to the droplet contour. XPS Analysis. X-ray photoelectron spectroscopy (XPS) of the titania samples was carried out to determine their chemical surface composition. The instrument used was a Physical Electronics PHI 5600 ESCA system equipped with a monochromatic Al Kα source. High-resolution O (1s) spectra arising from different bonding configurations and surface composition were identified and then were quantified using CasaXPS Software Element Library, version 2.2.65.32 Zeta Potential Measurements. An electrokinetic analyzer (EKA, Anton Paar) was used to determine zeta potential of hydrophilic and hydrophobized titania and Teflon surfaces. A clamping cell used in the experiments allows for the measurement of streaming potential in an asymmetric system. The channels are formed between the planar titania or Teflon surface and a small polymethylmethacrylate insert having thin grooves along its length. As the pressure is increased, an electrolyte solution is forced through the rectangular channels, and the streaming potential is measured. The zeta potential is determined from the Smoluchowski equation.33 The pH was adjusted in increments of ∼0.5 by addition of specific volumes of either 0.1 M HCl or KOH. Measurements were taken in 10−3 M and 10−2 M KCl electrolyte solutions. AFM Imaging. Hydrophobized titania surfaces were imaged using a tapping mode AFM (NanoScope IIIa, Bruker) in air. For imaging, the NSG-10 silicon probes (NT-MDT, Russia) were used. Several spots of size 5 × 5 μm were imaged to collect topographic information of the surfaces.

Figure 1. Experimental apparatus for monitoring bubble collisions with solid planar surface and determination of the time of TPC formation and the film drainage.

microfluidic chip or a capillary orifice). By varying the diameter of capillaries or depth of the channels in the microfluidic chip, bubbles of different diameters were formed. Bubble collisions with the TiO2 surfaces were recorded with a frequency of 1040 Hz. Images of subsequent frames of recorded movies were analyzed using SigmaScan Pro image analysis software. Each experiment was repeated 20 times. The setup used for top-view experiments consisted of (1) a square borosilicate glass column (cross section 20 mm × 20 mm), (2) a microfluidic chip with T-junction mounted at the bottom of the column, (3) syringe pumps (Cole-Parmer) with gastight high precision glass syringes (Hamillton), and (4) a high-speed camera (Photron 1024 PCI, United States) mounted onto an inverted microscope (Olympus BXFM). Film drainage between the colliding bubble and the modified titania surface was recorded at 3000 Hz for monochromatic light using a filter (Olympus, IF550) with a peak intensity at λ = 550 nm. The film thickness at time (h(t)) and the critical film thickness upon the film rupture (hcr) was calculated from the following equation accounting for multiple orders of interference at greater values of film thickness:30,31

h(t ) =

I(t ) − Imin λ arcsin 2πn Imax − Imin

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RESULTS AND DISCUSSION Surface Characterization. The rms roughness of hydrophilic titania surfaces imaged in air was 0.5 nm and the peak-tovalley (PTV) height was 1.3 nm; both values were determined using the AFM software (NanoScope Analysis, v. 1.20, Bruker). In the case of hydrophobized titania surfaces, both values were higher: 0.9 and 2.5 nm for rms and PTV, respectively (for θadv = 40°), and 1.2 and 3.6 nm for rms and PTV, respectively (for θadv = 60°). The advancing water contact angles, θadv, on the hydrophobized titania surfaces were 40 ± 2° and 60° ± 2°. For hydrophilic TiO2 surface, the relative amount of oxygen in hydroxyl environment is 45.3 atomic %. This value, however, decreases significantly with the titania surface hydrophobicity: 37.1 atomic % for titania of θadv = 40° and 23.6 atomic % for titania of θadv = 60° (Supporting Information, Figure 1SIA, B, and C shows high-resolution XPS spectra of O (1s) on hydrophilic (Figure 1SIA) and hydrophobized (θadv = 40°, Figure 1SIB, and θadv = 60°, Figure 1SIC) titania surfaces). Upon contact with water, or electrolyte solution, the titania surface forms a layer of hydroxyl ions by a two-step process involving the chemical adsorption of a monolayer of water and its dissociation. The H+ and OH− ions are potential determining ions for titania surface,3,34 and thus, the titania zeta potential changes as a function of their concentration. Figure 2 shows the zeta potential of hydrophilic and hydrophobized surfaces as a function of pH and surface hydrophobicity in 10−3 M and 10−2 M KCl. The isoelectric

(1)

where n is the refractive index of the solution and I(t), Imin, and Imax are the measured, minimum, and maximum light intensities, respectively. The film lifetime was measured from the last interference minimum, that is, the film thickness 103 nm. The critical thickness of rupture was measured at the point of film rupture as an average value of film thicknesses for the last 3 frames (∼1 ms) for films at more hydrophobic (θadv = 60°) and for the last 10 frames (3.3 ms) for films at less hydrophobic (θadv = 40°) titania surface before the film ruptured. Each experiment was repeated at least 10 times. 3073



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unstable wetting film was formed between the negatively charged bubble and the positively charged hydrophobized (θadv = 40°) titania surface. The time needed for the film to drain and rupture, resulting in the bubble attachment, that is, the time period from a moment of the first collision to the TPC formation, was 94.8 ms. Figure 4 presents data for the local velocity variations during bubble collision with the titania surfaces shown in Figure 3A

Figure 2. Zeta potential as a function of pH for titania surfaces with various degrees of hydrophobicity.

point (pHIEP) of all surfaces is located at pH ca. 4.6, which is in good agreement with previous results for amorphous titania.35 The zeta potential increases in magnitude on either side of pHIEP or as the salt concentration decreases. The zeta potential decreases in magnitude as the titania hydrophobicity increases. For example, in 10−3 M KCl at pH 5.8, the zeta potential equals −13.6 mV and −3.2 mV for the surfaces of contact angles θadv = 40° and 60°, respectively. The zeta potential response to pH changes is weaker with increasing surface hydrophobicity. This clearly indicates a reduction in the number of potential determining groups at the hydrophobized titania surface. This decrease in the number of −OH groups with increase in surface hydrophobicity confirms the XPS results. Bubble Collisions. Two sets of photographs showing phenomena occurring upon air bubble (db = 0.98 mm) collision with hydrophilic and hydrophobized θadv = 40° titania surfaces in 10−4 M KCl water at pH 4.0, that is, below the titania pHIEP, are presented in Figure 3A and B, respectively. Subsequent rows in Figure 3A and B show the first, second, third, and fourth collision, while two subsequent photos in each row illustrate changes of the bubble positions and shapes within a time interval of 0.9 ms. As can be noted in Figure 3A, the TPC was not formed at the hydrophilic titania surface even after much longer times than those indicated in Figure 3A. This is in agreement with Parkinson and Ralston,27 who observed a stable wetting film of a finite thickness between a positively charged titania surface and a negatively charged bubble. In contrast, an

Figure 4. Variations of the bubble local velocity during collision with the hydrophilic (black triangles) and hydrophobized (θadv = 40°, gray circles) titania surfaces in 10−4 M water (pH 4). Bubble diameter db = 0.98 mm.

and B. In both cases, the bubble bounced four times during the collision with both the hydrophilic and the hydrophobized titania surface. The velocity of each subsequent collision was lowered as a result of the energy dissipation.16 As indicated in Figure 4, the time of the TPC formation, tTPC, is the time interval from the moment of the first bubble collision with the surface until the TPC is formed, while the time of drainage, td, is the time of syneresis of the liquid film between the bubble and the solid surface. The td was determined as the time interval from the moment the bubble stopped bouncing and

Figure 3. Sequences of photos illustrating bubble collisions with (A) hydrophilic and (B) hydrophobized (θadv = 40°) titania surfaces in 10−4 M KCl (pH 4, i.e., below the pHIEP). Bubble diameter db = 0.98 mm. The time interval between two subsequent photos of each row is 0.96 ms. 3074



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factors (bubble diameter and pH variations) have a significant impact on the tTPC and td values. For example, at pH 5.8 (above the pHIEP of titania and air bubble where the electrostatic forces between the interacting surface are repulsive), the tTPC and the td values decreased, respectively, from 462 ± 31 and 382 ± 30 ms (db = 1.48 mm) to 132 ± 31 and 97 ± 31 ms (db = 0.98 mm) and further to 10 ± 2 and 10 ± 2 ms for the smallest (db = 0.40 mm) bubble. As there was no bouncing in the case of the smallest bubble, the tTPC and the td values were identical. The same decreasing trend in the tTPC and the td values is observed at pH 4, that is, just below the titania pHIEP (the electrostatic forces between the bubble and the surface are attractive here): 229 ± 32 and 148 ± 31 ms (db = 1.48 mm), 95 ± 19 and 59 ± 19 ms (db = 0.98 mm), and 8 ± 2 and 8 ± 2 ms for the smallest (db = 0.40 mm) bubble, respectively. Shorter times of the TPC formation and the film drainage at pH < pHIEP of the titania surface indicate that the attractive electrostatic forces between the bubble and the titania surface facilitate wetting film rupture and bubble attachment to the titania surface. The drainage time of the thin liquid film decreases with film radius, and therefore, the td and tTPC values decreased as the bubble diameter decreased. The character of the electrostatic forces (attractive or repulsive) plays a significant role in the time of the TPC formation. Because the electrostatic forces depend on the magnitude of the zeta potentials of the interacting surfaces as well as on the thickness of the electric double layer, the td and tTPC values should be dependent on the ionic strength of the solution. In Figure 6, the tTPC (open

stayed captured beneath the solid surface until the moment of the TPC formation. Thus

t TPC = tbouncing + td

(2)

where tbouncing is the time period needed for the dissipation of the kinetic energy associated with the bubble motion.16 Since the determination of the moment when the bubble is captured is not unequivocal, the starting point in the td measurements is a little arbitrary. Hence, every experiment was repeated 20−40 times, and the tTPC and td data reported are the mean values. In addition, since the velocity variations were measured by monitoring positions of the bottom pole of the colliding bubble,16 the moment of the dynamic TPC formation was indicated by a rapid change in the position (and, thus, velocity) of the bubble bottom pole (see the bump in Figure 4). As seen in Figure 4, prior to the first collision, the bubble approach velocities were identical and were equal to ca. 25 cm/s. At the moment of the first collision, their velocity was equal to the terminal velocity in clean water of the bubbles of diameter ∼1 mm.36 Thus, it proves that there were no surface active contaminants present in our system and that the bubble surfaces were clean. The velocity variations (amplitude and frequency) were the same for both collisions despite the difference in the titania surface wettability. After dissipation of the initial kinetic energy, the bubble stayed captured letting the wetting film drain. In the case of the hydrophilic titania surface, the wetting film was stable, while at the hydrophobic surface, after the critical thickness was reached, the film ruptured and the TPC was formed. In Figure 5 are presented the tTPC (open symbols) and td (closed symbols) values for the hydrophobized titania (θadv =

Figure 6. Time of the TPC formation (open symbols) and time of the film drainage (closed symbols) at hydrophobized (θadv = 40°) titania surface (squares and triangles) and Teflon (circles and diamonds) as a function of pH and salt concentration. Bubble diameter db = 1.48 mm.

symbols) and td (closed symbols) values at the hydrophobized titania (θadv = 40°) surface are presented as a function of pH and salt concentration for the same bubble size (db = 1.48 mm). As the salt concentration is increased, the significant decrease in the tTPC and td (from 462 ± 31 and 382 ± 30 ms in 10−4 M KCl to 271 ± 44 and 190 ± 45 ms in 10−2 M KCl) is observed above the IEP. This is due to electric double layer compression (1/κ = 3.03 nm in 10−2 M KCl, while 1/κ is 10 times greater in 10−4 M KCl) and to a decrease in the magnitude of zeta potential at the hydrophobized titania (see Figure 2) and

Figure 5. Time of the TPC formation (open symbols) and time of the film drainage (closed symbols) at hydrophobized (θadv = 40°) titania surface as a function of pH and bubble diameter.

40°) surface as a function of the solution pH and the size of colliding bubble. According to Yang et al.,37 the isoelectric point of the bubble surface is located at pH ∼ 3.0; therefore, within the pH range ∼3.0−4.6, the titania and bubble surfaces bear the opposite charges. As clearly seen in Figure 5, both 3075



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in comparison to the tTPC at pH 5.8 when the electrostatic repulsion should retard the TPC formation. The tTPC values below and above the pHIEP of Teflon not only are the same but also are significantly smaller than in the case of the hydrophobized titania (θadv = 40°) surface. This indicates that the wetting films formed between Teflon and the air bubble are less stable and rupture at greater thicknesses, which are beyond the range of the electrostatic interactions. A nucleation mechanism is responsible for the wetting film rupture at the highly hydrophobic surfaces,40 which means that nanometer or submicrometer sized bubbles, attached at the Teflon surface, can cause rupture of the wetting film. We have shown recently that submicrometer sized air features are responsible for the facilitation of the TPC formation on Teflon surface in Milli-Q water.41,42 Wetting Film Drainage and Rupture. If the resultant total force in the film is attractive, the most important consequence is that the film becomes unstable, that is, drainage is followed by rupture. Classically, the rate of film drainage is described by the Stefan-Reynolds equation first applied to thin liquid films by Scheludko. Scheludko described the thinning of the circular plane parallel film between a solid wall and free surface (full mobility) by the relation30,43

bubble surfaces with increasing salt concentration. As a result, much weaker and short-ranged electrostatic repulsion occurs between the interacting surfaces. In contrast, at pH 3.5, when the zeta potential of hydrophobized titania is positive (ca. +5 mV, see Figure 2), the td and tTPC increase with an increase in salt concentration: the time of the TPC formation was 92 ± 5 ms in 10−4 M KCl and increased to 171 ± 36 ms in 10−2 M KCl, while the time of the film drainage increased from 14 ± 4 to 92 ± 35 ms. In this case, we have attractive electrostatic interactions enhancing the destabilization of a wetting film. At lower ionic strength, these interactions are long-range and become short-range with increasing ionic strength. In high salt concentration (10−2 M), in addition to the electric double layer compression, the attractive forces are weaker as a result of a reduction in the magnitude of zeta potentials at both surfaces. This results in an increase in both td and tTPC. The data clearly show the importance of the electrostatic interactions in the kinetics of the TPC formation for surfaces of low degrees of hydrophobicity and are in excellent agreement with studies on the influence of the so-called reverse salt effect on kinetics of the particle deposition on collectors.38,39 Adamczyk and coworkers38,39 have shown that the deposition rate of colloidal particles on an oppositely charged collector surface increased when the ionic strength of the suspension was lowered underlining the importance of the electric double layer interactions in particle deposition. At highly hydrophobic solid surfaces, the effect of electrostatic interactions has practically no importance. To illustrate this, we chose Teflon as a model as it has a highly hydrophobic (θadv = 129 ± 3°) and relatively smooth (roughness below 1 μm) surface with a pHIEP (∼4.0−4.2, see Figure 7) located

d

( )= 1 h2

dt

4n 3ηRF2

Δp (3)

where h is the film thickness, t is the time, n is the factor for the interfacial film mobility, η is the viscosity, RF is the radius of the film, and Δp is the difference between pressure in the thin liquid film and pressure in the bulk phase. For the asymmetric wetting film having interfaces of a solid wall with no-slip hydrodynamic boundary condition and a fully mobile liquid/ gas interface, the factor n equals 4, while for a film with zero velocity at both interfaces n equals 1. After integration and assuming that t = 0 and initial thickness, hi, equals 103 nm, the first interferometric minimum can be written as

1 h f2

=

4n Δp 1 t+ 2 3 ηRF2 hi

(4)

The effective radius of the film formed by a bubble at interface is44,45

RF2 =

FR b 2πσ

(5)

where Rb is bubble radius and F is total force causing the film thinning. Taking into account that

Figure 7. Zeta potential of Teflon as a function of pH in 10−2 M and 10−4 M KCl solutions.

Δp =

close to the pHIEP of the titania surface. The tTPC values at the Teflon surface are presented in Figure 6 (open circles and diamonds). As can be seen, irrespective of the pH and salt concentration, for the same bubble size (db = 1.48 mm), the TPC was formed within a similar time. The results presented in Figure 7 clearly show that in the case of highly hydrophobic Teflon surface there is no significant effect of electrostatic interaction per se on the tTPC. At pH 3.5 and 4, the Teflon surface bears a positive charge while the bubble is negatively charged; thus, if the electrostatic attraction is the dominant factor facilitating the TPC formation, the tTPC should be shorter

2σ Rb

(6)

and

F=

4 3 πR b ρg 3

(7)

it can be finally written

σ2 1 = 4 n t+ 2 5 2 hf ηρgR b hi 1

(8)

where RB is the bubble radius, ρ is the solution density, σ is the soluton surface tension, and g is the acceleration due to gravity. 3076



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indicating either that there is a partial slip at the air/liquid interface or that there are some external factors (such as, e.g., nanobubbles at hydrophobized solid surface) that have not been accounted for. It is clear that the wetting films rupture faster and at greater thicknesses, that is, are less stable, at the more hydrophobic titania surface (θadv = 60°). Moreover, the films formed at the hydrophobized titania surface (θadv = 40°) below the pHIEP of titania (where the electrostatic interactions are attractive) are of comparable thickness to those formed in the presence of repulsive electrostatic interactions (above the pHIEP of titania) even though their lifetime is minimally shorter. Despite the fact that time zero was chosen differently in the bubble striking the plate (the moment when the bubble stays motionless) and in the interferometric (the last interference minimum) experiments, the results for db = 0.40 mm presented in Figure 5 are in fair agreement with those presented in Figure 9. We cannot determine the film thickness in the bubble striking the plate experiment; however, we can speculate that the film may be 1−100 μm thick at the moment when the bubble is trapped beneath the plate. The initial stage of the film drainage is much faster. For example, it would take ∼3 ms for the 1−100 μm thick to thin down to 103 nm assuming full slip at the air/liquid interface.

The sequences of interferometric images at the initial (A) and final (B) stages of the wetting film drainage and rupture at hydrophobized (θadv = 40°) titania surface are presented in Figure 8. The film rupture observed with a high-speed

Figure 8. Sequences of interferometric images at the initial (A) and final (B) stages of the wetting film drainage and rupture at hydrophobized (θadv = 40°) titania. The time interval between subsequent photos in sequence B is 0.33 ms.

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CONCLUSIONS We have shown that in the case of solids of intermediate degree of hydrophobicity the stability of the wetting film formed under dynamic condition and the kinetics of the three-phase contact formation are governed by the interplay of surface electrical charge and surface hydrophobicity. In the case of highly hydrophobic solids, the effect of surface charge is negligible and surface roughness is the key factor determining the kinetics of the wetting film rupture and the three-phase contact formation. We have shown that the wetting film was stable on hydrophilic (completely wetted) titania surfaces even in the presence of the electrostatic attractive forces but ruptured at the hydrophobized titania via a nucleation mechanism. The time of film drainage, td, and the time of the three-phase contact formation depend on the size of the colliding bubble. For wetting films formed under the same conditions, the tTPC and the td values were significantly shorter for the bubbles of smaller diameter.

microscopy (Figure 8B) gives an indication of a nucleation process: the rupture starts from one single hole in the film, probably at the place where the biggest nano- or submicrometer-bubble is located, and leads to a complete dewetting of the whole film area within a few milliseconds (the total dewetted area is not presented). Figure 9 presents the

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ASSOCIATED CONTENT

S Supporting Information *

XPS O (1s) data for hydrophilic, hydrophobized θadv = 40°, and hydrophobized θadv = 60° titania surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 9. The critical thickness of rupture and the film lifetime at hydrophobized (θadv = 40°, gray points, and θadv = 60°, open points) titania in 10−4 M KCl. The dashed line is the calculated curve for nonslip at the air/liquid interface, while the solid line is the calculated curve for full slip at the air/liquid interface (eq 8). Bubble diameter db = 0.40 mm.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS We would like to thank Dr. Mihail Popescu for fruitful discussions and Dr. Craig Priest for his help with microfluidic chip design. The fabrication of microfluidic chips was performed at the South Australian node of the Australian National Fabrication Facility under the National Collaborative Research Infrastructure Strategy. Financial support from the Polish Ministry of Sciences and Higher Education (Grant 1794/B/H03/2010/39), European

critical thickness of rupture and lifetimes of wetting films at hydrophobized titania surfaces (θadv = 40° and θadv = 60°). These experimental results are compared to the theoretical predictions of eq 8 assuming that the initial thickness corresponds to the first interferometric minimum for both no-slip at the air/liquid interface (see dashed line in Figure 9) and full slip at the air/liquid interface (see solid line in Figure 9). All experimental points fall between two limiting cases 3077



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COST P21 Action Physics of droplets, and Australian Research Council Linkage Scheme, AMIRA International, and state governments of South Australia and Victoria is acknowledged. M.K. acknowledges financial support from Australian Academy of Sciences (Grant Europe Scientific Visits).

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