Influence of Surface Topography on the Interactions between

It was later suggested that cavitation or bridging bubbles between the surfaces ..... The force curves that fall into Population 1 share the same feat...
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Influence of Surface Topography on the Interactions between Nanostructured Hydrophobic Surfaces Petra M. Hansson,† Agne Swerin,†,‡ Joachim Schoelkopf,§ Patrick A. C. Gane,§,∥ and Esben Thormann*,‡ †

YKI, Ytkemiska Institutet AB/Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden Department of Chemistry, Surface and Corrosion Science, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden § Omya Development AG, CH-4665 Oftringen, Switzerland ∥ School of Chemical Technology, Department of Forest Products Technology, Aalto University, P.O. Box 16300, FI-00076 Aalto, Finland ‡

ABSTRACT: Nanostructured particle coated surfaces, with hydrophobized particles arranged in close to hexagonal order and of specific diameters ranging from 30 nm up to 800 nm, were prepared by Langmuir−Blodgett deposition followed by silanization. These surfaces have been used to study interactions between hydrophobic surfaces and a hydrophobic probe using the AFM colloidal probe technique. The different particle coated surfaces exhibit similar water contact angles, independent of particle size, which facilitates studies of how the roughness length scale affects capillary forces (previously often referred to as “hydrophobic interactions”) in aqueous solutions. For surfaces with smaller particles (diameter < 200 nm), an increase in roughness length scale is accompanied by a decrease in adhesion force and bubble rupture distance. It is suggested that this is caused by energy barriers that prevent the motion of the three-phase (vapor/liquid/solid) line over the surface features, which counteracts capillary growth. Some of the measured force curves display extremely long-range interaction behavior with rupture distances of several micrometers and capillary growth with an increase in volume during retraction. This is thought to be a consequence of nanobubbles resting on top of the surface features and an influx of air from the crevices between the particles on the surface.

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a conclusion that is thermodynamically consistent.26,27 Evidence for capillary evaporation being the main source behind the hydrophobic interaction has been obtained using evanescent wave atomic force microscopy (AFM)28 and confocal Raman microscopy.29 The similarity of the interaction between hydrophobic surfaces in water and hydrophilic surfaces in humid air has provided further evidence that both are due to capillary condensation.30 The complexity of this type of interaction is further emphasized by the observation that the hydrophobicity of the surfaces involved, as described by the static contact angle, is not solely responsible for the range and magnitude of the interaction.31,32 Surface roughness and chemical heterogeneities are known to be of vital importance and to have a significant influence on the long-range attractive capillary force.33 The stabilization of air pockets and the extra air reservoir within the rough and heterogeneous surface are likely reasons for this observation.34 The wettability of a surface is an important factor in nature and it is closely related to interactions between two such surfaces. The nonwetting phenomenon of superhydrophobicity is exhibited by many plants, such as can be observed on the lotus leaf.35,36 The leaf is characterized by its intrinsic

INTRODUCTION The first direct measurements of the attractive interaction between hydrophobic macroscopic surfaces were reported in 1982.1 The study showed that a long-range force, considerably stronger than the van der Waals force, dominated the interaction. It was later suggested that cavitation or bridging bubbles between the surfaces could be responsible for the longrange nature of the attractive force observed on bringing the surfaces together.2−5 Even though there is still an ongoing debate about the origin of the interaction, this mechanism seems to explain many of the results reported in the literature.6−12 The existence, stability, and shape of nanoscopic or microscopic gas bubbles bridging the surfaces are still a frequently discussed topic. Numerous studies have given evidence for nanobubble formation as well as apparent stability of the nanobubbles for hours.13−18 Studies by Zhang and coworkers suggest that gases change their solubility in different solvents such as water and ethanol if the solvents are exchanged during a measurement, hence creating a “recipe” for nanobubble formation.19,20 Most reports show isolated bubbles with a spherical shape,21,22 but studies by Tyrell and Attard demonstrate irregularly shaped close-packed bubbles like a continuous film.23,24 Also, Singh et al. claim that no visible nanobubbles were present during force measurements between superhydrophobic surfaces.25 Instead they argue that cavitation occurs when water is confined between two nonpolar surfaces, © 2012 American Chemical Society

Received: February 13, 2012 Revised: April 23, 2012 Published: May 3, 2012 8026

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Sweden), and chloroform (99.9% pure, Merck) were used as solvents and for cleaning purposes. To hydrophobize the particle surfaces, dodecyldimethylchlorosilane (Sigma, U.K.) was used. Polystyrene particles with a diameter of 10 μm (G. Kisker GmbH, Germany) were used as probes in AFM force measurements. A thermosetting resin (Epikote 1004, Resolution Europe B. V., The Netherlands) was used to glue the polystyrene particles to calibrated cantilevers. NaCl p.a. (Merck, Germany) was used as received. The water used in all experiments was purified by means of a Milli-Q Plus Unit (Millipore) and had a resistivity of 18.2 MΩ cm. Sample Preparation. Preparation of the particle coated films using LB deposition was performed following a protocol originally developed by Lee et al.43 and Tsai et al.40 and later refined by Hansson et al.42 Briefly, water was removed from the silica particle suspensions by heating them to just below 100 °C in an oven. Next, the particles were redispersed in 2-propanol by sonication for 2 h followed by addition of 1000 ppm CTAB surfactants followed by sonication for another hour. The dispersion was left to equilibrate for 24 h followed by evaporation of the solvent. Finally, chloroform was added to give a final particle concentration of approximately 20 mg mL−1. The particles were deposited in a Langmuir trough (KSV 5000, KSV Instruments, Finland). They were first spread on a clean water subphase using a microsyringe (705N, Hamilton Co.), followed by a 15 min waiting period to let the chloroform evaporate. Glass microscope slides with dimensions of 15 × 15 mm2 × 1 mm were thoroughly cleaned in ethanol and water and used as substrates for film deposition. The particles were deposited at a speed of 1 mm min−1 in the upstroke direction at the selected surface pressure, which was approximately 30% of the measured collapse pressure. The particle coated surfaces were sintered in a furnace for 30 min at 600 °C immediately after LB deposition. After sintering and plasma cleaning (PDC-3XG, Harrick Scientific), the surfaces were hydrophobized by silanization with dodecyldimethylchlorosilane (Sigma, U.K.) for 12 h in gas phase, and were subsequently rinsed in chloroform, ethanol, ethanol/water 1:1, and water. Flat hydrophobic surfaces were prepared from a glass microscope slide without particles by sintering, plasma cleaning, and silanization, as described above. Surface Characterization. AFM topographical images of the surfaces were recorded using a Nanoscope Multimode V (Bruker) instrument operated in PeakForce mode using ScanAsyst software and silicon nitride cantilevers (ScanAsyst Air, Bruker). In order to compare the roughness between the surfaces, the scan size was scaled so that all images contained approximately the same number of particles, thus the scan size for the 800 nm particles was 15 × 15 μm2 while that for the 30 nm particles was 1 × 1 μm2. The AFM images were further used to obtain the Ra and Rq roughness parameters for all the surfaces. By utilization of an AFM based robustness test, we have also demonstrated that the surfaces are stable and robust enough for the force measurements.42 Macroscopic contact angle measurements were measured using a DataPhysics OCA40 micro (DataPhysics GmbH, Germany) system. The system includes a high-speed CCD camera (2200 fps) with 20× magnification and automatic syringe dispenser. The static water contact angles and the roughness characteristics of the particle coated surfaces are displayed in Table 1 and show, as expected, that the increase in surface roughness, when going from a flat to a particle coated surface, gives an increased hydrophobicity in terms of a higher macroscopic contact angle. The measured value of the flat hydrophobic surface is 108° as compared to 122−124° for the particle coated films. However, the hydrophobicity does not vary with respect to the size of the deposited particles. This is due to a constant roughness factor even though the length scale of the surface roughness features changes.37 Force Measurements. The forces acting between a hydrophobic colloidal probe and the hydrophobized particle coated surfaces were measured using an atomic force microscope (Nanoscope III Pico Force, Bruker) equipped with a liquid cell. The cantilevers (b-lever with k = 20.8 and 23.3 N m−1, NSC12/tipless/No Al, Mikromasch, Estonia) were calibrated using the method proposed by Sader et al.46

combination of micro- and nanoscale roughness on its surface, and it is generally known that superhydrophobicity can only be obtained by introducing a certain degree of surface roughness, that is, a low surface energy is not enough. There exist two basic models describing the wetting of structured surfaces. The first model, the Wenzel state, describes the case in which the liquid penetrates the surface roughness features.37 This penetration results in a higher contact angle of a rough surface compared to the corresponding flat surface due to the increase in the liquid−solid contact area. In the second model, the Cassie−Baxter state, the liquid droplet rests on top of the roughness features with air trapped underneath.38 In this case, the contact angle of a rough surface increases with a decrease in the liquid−solid contact area due to a corresponding increase in the liquid−air area. The Wenzel and the Cassie−Baxter states should be viewed as two extreme cases, and situations with partial penetration of liquid into the surface features are also possible. Surfaces coated with particles arranged in a dense and wellordered structure can provide a valuable model for a natural rough surface with a roughness length scale which depends on the particle size. Langmuir−Blodgett (LB) deposition has previously been used to prepare surfaces with particles arranged in a monolayer with a close-packed hexagonal structure.39−41 As previously demonstrated, such surfaces prepared by LB deposition of varying sized silica particles followed by silanization can provide a set of hydrophobic surfaces with different roughness length scales while having similar macroscopic contact angles as a consequence of nearly complete surface feature filling (Wenzel state).42 This study addresses a characterization of how the interaction between hydrophobic surfaces is affected by surface roughness. The atomic force microscopy colloidal probe technique was used to measure forces between micrometersized polystyrene particles and particle coated surfaces with different surface roughness length scales created by using different sized particles varying from 30 to 800 nm in diameter. A schematic figure of the relative sizes of the colloidal probe and the particles is displayed in Figure 1. This choice of system

Figure 1. Schematic illustrating the relative sizes of the probe and the particles deposited on a surface.

thus allows a systematic study of how the roughness length scale affects surface interactions for surfaces with the same hydrophobicity in terms of macroscopic contact angle.

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METHODS

Materials. Silica particles of 30, 60, 90, 200, and 800 nm in diameter were purchased from G. Kisker GmbH, Germany. The cationic surfactant hexadecyltrimethylammonium bromide (CTAB, Sigma, U.K.) was used to physically modify the particles. 2-Propanol (99.8% pure, Solveco, Sweden), ethanol (99.7% pure, Solveco, 8027

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Table 1. Surface roughness, Contact Angles, and Roughness Factors for the Flat Hydrophobic Surface, Particle Coated Surfaces, and the Polystyrene Colloidal Probe Rq (nm)a Ra (nm)b water CA (deg) roughness factor, rc

flat

800 nm

200 nm

90 nm

60 nm

30 nm

probe

5.3 2.8 108

81.2 48.8 123 1.76

25.5 15.2 124 1.81

16.2 7.4 122 1.71

10.5 5.1 122 1.71

8.1 4.6 123 1.76

5.2 2.9 62d/64e/82f/85g

Rq = ((∑i =N 1(Zi − Zave)2)/N)1/2 where Zave = average Z value within the given area, Zi = local Z value, and N = number of points within the given area. bRa = ((∑i =N 1|Zi − Zave|)/N) where Zave = Z value at the central plane, Zi = local Z value, and N = number of points within the given area. cIn the Wenzel regime, the contact angle, θ′, on a rough surface is related to the contact angle, θ, on a flat surface by, cos θ′ = r cos θ, where the roughness factor, r, gives the ratio between the actual surface area and the projected surface area.37 dReceding CA measured with microsphere tensiometry.44,45 eReceding macroscopic CA measured on a flat surface made from melted polystyrene particles. fStatic macroscopic CA measured on a flat surface made from melted polystyrene particles. gAdvancing macroscopic CA measured on a flat surface made from melted polystyrene particles a

A constant approach and retract velocity of 400 nm s−1 was used. Measurements were performed in aqueous 10 mM NaCl that had been heated to 50 °C just prior to use in order to reduce the amount of dissolved air. The surface forces were recorded over a 10 × 10 μm2 area, on 10 by 10 regularly spaced spots. One force curve was recorded on each spot. The order in which the surfaces were measured was randomized. The same polystyrene colloidal probe, and hence cantilever, was used for one set of measurements (100 force curves) on all surfaces. To test the variation between the surfaces, a second set of measurements was thereafter performed using a different set of surfaces, a different cantilever, and a different polystyrene probe. Receding contact angle measurements of the polystyrene colloidal probe were performed using an AFM based method developed by Preuss and Butt.44,45 The same AFM instrument as employed for force measurements was used. The force versus distance curve between the probe and an air bubble in aqueous solution was measured, and the receding contact angle was calculated from the jump-in distance. The macroscopic static and dynamic water contact angles of polystyrene were measured on a flat surface made from melted polystyrene particles. All the measured contact angles for polystyrene are given in Table 1.

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Figure 2. Typical force curve recorded from a measurement between a flat hydrophobic surface and a hydrophobic probe. The gray line is the approach curve, and the black line is measured on retraction. The solid red line represents a force profile calculated from eq 1.

RESULTS AND DISCUSSION

We start by describing the general features of the force curves determined between a flat hydrophobic surface and the hydrophobic probe. Thereafter, we will consider how these features differ as the roughness length scale of the surface is increased. Force Curves between Flat Hydrophobic Surfaces. A typical force curve measured between a flat hydrophobic surface and a hydrophobic probe is illustrated in Figure 2. No detectable force is acting between the surfaces until they are about 25 nm apart. At this point, an attraction suddenly appears. In conformity with previous literature, we will refer to the separation at which there is a discontinuity in the force curve on approach as the jump-in distance. Previous studies, using a range of hydrophobic surfaces, have shown both continuous long-range attractions as well as more discontinuous jumps to an attractive force.12,30,47,48 Strong attraction upon retraction is observed when the surfaces are separated, where the maximum attraction force, in this case about 110 mN m−1, is referred to as the adhesion. As the surfaces are separated from direct surface-to-surface contact, they remain connected by a vapor/air cavity. This cavity results in a long-range attraction that diminishes as the surfaces are separated further and the cavity is thinned. Eventually the cavity breaks, and when this occurs, the attractive force suddenly disappears. The distance at which the cavity breaks will be referred to as the rupture distance. The solid line in Figure 2 is a theoretical fit,

obtained by assuming that the capillary volume is constant during the separation process (it takes ∼0.3 s to move the surfaces from the attractive minimum to the rupture distance) and calculated using ⎛ ⎜ F = 4πγcR ⎜1 − ⎜ ⎝

⎞ ⎟ ⎟ + D2 ⎟⎠

D V πR

(1)

where cos(θ1 + β) + cos θ2 (2) 2 Equation 1 describes how the capillary force, F, between a sphere, with radius, R, and a surface can be calculated.49 D is the distance between the probe and the surface, V is the volume of the capillary, θ1 and θ2 are the contact angles of the capillary against the surface and the probe, respectively, and β is the angle between the line starting from the center of the spherical probe going in the normal direction and toward the contact point of the capillary at the probe. In this case, β ≪ θ1 and is therefore omitted. Force Measurements from Particle Coated Surfaces. AFM images of the particle coated surfaces are displayed in Figure 3, and values from contact angle measurements and roughness analyses are given in Table 1. Two sets of c=

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values and standard deviations for the adhesion, rupture distance and jump-in distance for both populations and all surfaces are compiled in Table 2. For the nanostructured surfaces, we note the trend that the higher the adhesion force and larger the rupture and jump-in distances for Population 1 force curves, the more abundant were the Population 2 force curves. Consequently, for the 30 nm particle coated surfaces, almost 30% of the measured force curves belonged to Population 2 while they were scarce for the 200 and 800 nm particle coated surfaces. In the following, statistics for Population 1 and Population 2 force curves are hereby displayed and discussed separately. Population 1: Capillary Evaporation and the Effect of Surface Topography. For Population 1 force curves, the size of the particles deposited on the surface clearly affects the measured forces. This is most easily seen in the mean value plots displayed in Figure 6. In general, the adhesion, rupture distance, and jump-in distance decrease with increasing particle size. This effect is pronounced when increasing the particle diameter from 30 to 200 nm, whereas the difference between 200 and 800 nm particle coated surfaces is smaller. Interestingly, the hydrophobic flat surface displays values that are intermediate, in general between that of the 90 nm particle coated surfaces and the 200/800 nm surfaces. The results for the smaller particles are in qualitative agreement with a study performed with 9 and 40 nm particles disorderly distributed on surfaces.12 The jump-in distance for Population 1 force curves is related to the formation of a capillary bridge. It is well established that water between two hydrophobic surfaces is in a metastable state, when the separation reaches a critical low value, and formation of an air cavity will reduce the free energy of the system.26 Thus, the larger jump-in distance for the surfaces with smaller particles indicates that the energy barrier for going from the unfavorably wetted state to the favored dewetted state (i.e., with the cavity formed) is lower for small length scales of the surface roughness over the range of curvatures investigated here. We suggest that the explanation for this is that a smaller volume of liquid water needs to be evaporated to create the cavity between surfaces with small roughness features (in our case the volume between the particles plus the volume between the particle coated surface and the probe). Since capillary evaporation is a stochastic process, the likelihood for capillary evaporation decreases with the volume that needs to be destabilized for dewetting to occur. Another factor could be the larger crevices between the larger particles, which lead to larger volume for gas/vapor accumulation within the structure that does not directly contribute to formation of a cavity between the nanostructured surface and the approaching colloidal probe. Also, the three-phase contact line needs to move over larger surface features when the particles are larger, which provide energy barriers for capillary growth. It is interesting to note that the flat surface does not follow this trend, but behaves most similarly to surfaces with 90 nm particles (with respect to the Population 1 force curves). This indicates that a sufficiently large (in the nanometer range) roughness scale needs to be present to promote capillary evaporation. However, particles smaller than the 30 nm particles used in our investigation would be needed in order to clarify where the lower limit is located. We suggest that the roughness of the nanostructured surfaces favors capillary evaporation, since the local contact angle at the cavity/liquid/ solid interface can be more easily adjusted to match the shape

Figure 3. Topographical images recorded by AFM of particle coated surfaces: (a) 30 nm, 1 × 1 μm2, (b) 60 nm, 1.5 × 1.5 μm2, (c) 90 nm, 2 × 2 μm2, (d) 200 nm, 4 × 4 μm2, and (e) 800 nm, 15 × 15 μm2.

measurements, each involving 100 force curves, with a change to a newly prepared nanostructured surface and probe/ cantilever in between, were performed for each particle size. When looking at the adhesion, rupture distance, and jump-in distance as a function of time, we notice a significant variation between individual force curves, but no systematic trends with measurement number. We argue that this is a consequence of the stochastic feature of capillary evaporation and capillary rupture and also due to minor local variations in the chemistry and topography at the different spots on the surface. Thus, in order to elucidate the effect of surface roughness length scale, we need to use a large number of force curves (200 for each type of particle coated surface in our case) and emphasize statistical changes. Figure 4 shows statistics for the adhesion force and rupture distances for all the different surfaces. From the measured rupture distances it is notable that the surface force curves measured between the particle coated surfaces constitute of two populations, with distinctly different rupture distances. We refer to the force curves displaying the smaller rupture distance, which also are the most frequently appearing, as Population 1, and the less frequent force curves displaying larger rupture distance, as Population 2. The force curves in Population 1 also have, on average, smaller jump-in distance than those in Population 2. Typical examples of force curves in the two populations, both recorded with a 30 nm particle coated surface, are illustrated in Figure 5. The force curves that fall into Population 1 share the same features as those recorded between the flat surface and the probe, where no Population 2 curves were seen. Thus, they can be ascribed to capillary evaporation occurring on approach and the presence of a bridging capillary of constant volume on separation. As seen in Figure 5, the force curves belonging to Population 2 are not only displaying longer jump-in and rupture distances, but they also have distinctly different shape. Upon separation, the force is first continuously increasing until a separation of several hundred nanometers, after which the force is decreasing in an irregular manner until complete separation. The possible mechanisms giving rise to Population 2 force curves will be discussed later. For a measurement series of 100 force curves, obtained between the probe and a particle coated surface, at most 1−2 curves had a shape falling in between Population 1 and 2, which made it difficult to sort them into one of the two groups, and these were thus not included in the statistical evaluation. Mean 8029

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Figure 4. Histograms showing the frequency of occurrence of the given value of the adhesion force, rupture distance, and jump-in distance for force measurements between a hydrophobic probe and both flat and particle coated surfaces. The white and black bins represent data obtained from Population 1 and Population 2 force curves, respectively.

of the formed capillary, providing an additional degree of freedom that can be utilized to minimize the free energy of the small cavities. Just as for the flat surfaces, Population 1 force curves from particle coated surfaces were compared to the theoretical prediction given by eq 1. Two different fits are displayed together with a measured force curve in Figure 5a. The red and green solid lines are calculated using a capillary volume equal to the volume in the flat surface case plus the extra volume available between and beneath the particles on the surface assuming a radius of 500 nm, as estimated from the flat surface case, of the capillary at a short surface separation. One

conclusion from a previous study was that the particle coated surfaces were found to be predominantly in the Wenzel state but with a slight influence of wetting in the Cassie−Baxter state, that is, having air that could hide in the crevices between or underneath the particles just as what is needed to obtain a good fit here.42 The difference between the red and green fits is the water contact angle for the surfaces used in the calculations. The red line is calculated based on the assumption that the contact angle is the same as for the flat surface, since the local position of a point on the three-phase contact line could be regarded as on a flat surface. However, when using the contact angle for the particle coated surface, the fit corresponds much 8030

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better to the measured curve. The advancing contact angles were used in both calculations, since the dewetted area is receding as the surfaces are separated. This equation cannot, however, be used to predict why the magnitude of the adhesion differs for Population 1 force curves on particle coated surfaces with different roughness length scales. The reason is that the equation is valid for smooth surfaces where the capillary is allowed to grow to its optimum size. Even though real surfaces are never perfectly smooth, this assumption works well for flat and homogeneous surfaces.2,50 However, for the nanostructured surfaces used in this study, where pinning of the threephase contact line can occur, it is likely that the cavity may be prevented from growing to its optimal size, as also discussed by Wallqvist et al.12 The lower adhesion and lower rupture distance observed for the surfaces with larger particle sizes would then be a consequence of higher energy barriers for the three-phase line motion that results in formation of smaller cavities. Since the flat surface does not follow this trend, we suggest that small roughness length scales may aid capillary growth due to favorable dewetting of regions within the nanostructured surface (between the particles in our case), whereas larger scale roughness counteracts capillary growth due to pinning of the three-phase line, which leads to lower adhesion. The pinning of the three-phase line may also promote a small rupture distance due to instabilities induced in the cavity as the dewetted area decreases stepwise. Population 2: Force Curves Displaying Unusual LongRange Interactions. The Population 2 force curves exemplified in Figure 5b display some features different from those in Figure 5a. First, a sudden onset of attraction is noted at a very large separation, around 125 nm in this case. Second, just after the onset of the attraction, when the surfaces move closer to each other, the force becomes somewhat less attractive. Third, the surfaces do not jump rapidly into contact due to the attraction but rather approaches slowly toward the induced attractive force, and several data points can be collected during this transition stage. It is noted that the force curve exhibits small steps and a weak repulsion before the surfaces reach direct contact. These observations have previously been described as being characteristic features for cases when a probe interacts with a preadsorbed air bubble on the surface.16,24 The oscillation in the force curve after the onset of attraction could possibly be explained by an oscillation

Figure 5. Examples of representative force curves for measurements between a hydrophobic particle coated surface with 30 nm particles and a hydrophobic colloidal probe. For the 30 nm particles, about 70% of the recorded force curves had the characteristic appearance (Population 1) shown in (a). In (b) a force curve with nonequilibrium capillary growth is displayed (Population 2). The gray line shows the approach curve, and the black line is the retract curve. The two solid lines in (a) represent force profiles calculated from eq 1.

Table 2. Adhesion force, Rupture Distance, and Jump-in Distance for the Particle Coated Surfaces and the Flat Reference Surfacea P1/P2 % of force curves

flat

800 nm

200 nm

90 nm

60 nm

30 nm

100/0

99.5/0.5

99/1

89/11

79/21

72/28

adhesion mean value (mN m−1) standard deviation (mN m−1) mean value (nm) standard deviation (nm) mean value (nm) standard deviation (nm)

117/− 18/− 168/− 16/− 27/− 7/−

60/97 54/137 14/− 23/− rupture distance 60/1695 16/−

105/1034 18/− jump-in distance

16/144 9/−

19/114 7/−

164/99 39/45

206/97 45/36

157/1252 21/211

183/1190 18/195

40/121 14/58

53/100 12/30

268/86 60/28 204/1840 31/200 57/143 14/39

a

The left values (in bold) correspond to data collected from force curves belonging to Population 1 (P1), while values to the right (normal font) show Population 2 (P2) force curves. 8031

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interpreted similarly as being due to energy barriers that have to be overcome to move the three-phase line of the cavity across the surface. Steps corresponding to movement of the threephase contact line over the surface features can also be seen in the retraction curve in Figure 5b. It is suggested that the size of the steps is influenced by the particle size, the volume of the capillary and the surface separation, whereas a jump of the three-phase line over the surface features alone would likely correspond to a smaller change in separation between the probe and the capillary. The fact that at least three different parameters most probably affect the step size, and also that very few Population 2 force curves were detected, make analysis of the trends in step size between different particle coated surfaces difficult. The single force curve measured with 800 nm particles shows no small steps like those in Figure 5b at all, but we cannot conclude whether this is a reproducible result or just a coincidence. We observe a strong relation between the frequency of appearance of Population 2 force curves and the particle size, while no relation between particle size and adhesion force, rupture distance, or jump-in distance was found. Considering that Population 2 force curves are observed when small air bubbles are present on the surface already as the probe approaches and that large air cavities are formed between the probe and the surface, means that the force profile is unaffected by the roughness length scale. However, the likelihood of, at all, finding such a bubble extending beyond the surface features depends on the size of the particles constituting the nanostructured surface. The presence of air nanobubbles on hydrophobic surfaces has been observed on a number of occasions,13,20 and the dimension of such nanobubbles is normally reported to be around 50−100 nm in diameter and 5−20 nm in height. These small dimensions can probably explain why Population 2 force curves are more common on surfaces coated with the smaller particles. A bubble with a diameter of 100 nm can span over the surface features for the smaller particles and thus make it more likely that the bubble stays on top of the particles and not, as is the case for the large particles, hide in the crevices between them. It is however notable that no Population 2 force curves were detected for the flat surfaces, even though any air nanobubbles present should be easily accessible in this case. Therefore, we propose that it is not only the presence of nanobubbles but also the 3D structure created by the particles and hence the extra volume offered by the crevices between them that are responsible for the extremely long-range force curves. The force curves measured on separation in the Population 2 case cannot be explained by the assumption of constant capillary volume, as could be used for rationalizing the Population 1 force curves. Rather, the volume of the capillary must increase during the initial stage of the separation process. These facts indicate that, in order to have interactions leading to Population 2 force curves, an air reservoir housed in the heterogeneous surface is needed to provide the cavity with an influx of air. Although eq 1 only applies to the constant volume situation, the profile of the Population 2 force curve can qualitatively be assigned to the response of a capillary bridge with a volume which is increasing with the separation (and time). A further indication of the nature of the Population 2 force curves being related to the existence of air pockets that can supply such an in-flux of air, is that similar long-range interactions with a similar force versus separation profile have been measured between superhydrophobic surfaces.25,29 Super-

Figure 6. Mean values of the measured (a) adhesion force, (b) rupture distance, and (c) jump-in distance plotted versus the diameter of particles of the particle coated surfaces. These mean values are solely based on Population 1 force curves. The horizontal black line represents the value measured with the flat reference surface.

induced in the bubble as the probe penetrates, and the small steps in the force curve on further approach could be assigned to uneven motion of the three-phase line across the surface as the bubble is forced to change shape and spread on the surface. Similar small steps, but less clearly defined, can sometimes also be observed in the Population 1 force curves and they are 8032

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edges the Strategic Science Foundation (SSF) program “Multifunctional pore arrays in silica” for supporting his activities. A.S. thanks the Troëdsson foundation for support of an adjunct professorship at KTH. The authors thank Prof. Per M. Claesson for valuable advice and very useful discussions. Prof. Hans-Jürgen Butt and Dr. Michael Kappl are thanked for introducing P.M.H. to the technique of measuring contact angles on colloidal probe particles.

hydrophobic surfaces are known to be in the Cassie−Baxter state where the aqueous phase is resting on top of air pockets which could act as a reservoir for influx to the capillary bridge. As stated before, even though the surfaces show Wenzel behavior with respect to their similar contact angles and roughness factors, a slight degree of Cassie −Baxter wetting with air trapped between the a particles seems likely. We propose that this extra air is a prerequisite for formation of Population 2 force curves.

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CONCLUSIONS This study has investigated how the surface roughness and topography in the form of particles deposited on a substrate influence interactions between hydrophobic surfaces. LB deposition and silanization of the silica particles having diameters of 30, 60, 90, 200, and 800 nm gave surfaces with similar roughness factors and macroscopic water contact angles. Thus, the surfaces under water contact are predominantly in the Wenzel regime, with a slight influence of Cassie−Baxter wetting with air hidden in the particle structure, and offer the possibility to study forces as a consequence of roughness rather than the individual surface hydrophobicity. It is found that the range and magnitude of the attraction between the nanostructured hydrophobic surfaces are larger the smaller the particles on the surface. Thus, the macroscopic contact angle alone cannot be used to predict the range and magnitude of the interaction forces, which we describe as capillary forces. The larger forces and interaction distances for the surfaces with smaller nanostructures suggest fewer restrictions to capillary growth, such as smaller volume of liquid needed to create the capillary and smaller surface features for the capillary to move over during growth. Some of the measured force curves displayed different features compared to what normally is observed for interactions resulting from capillary forces at constant capillary volume. These force curves showed extremely long-range interaction distances with rupture distances of several micrometers and an increase in capillary volume during separation, and these are suggested to be due to the presence of adsorbed air nanobubbles. In contrast to the more short-range types of force curves described above, no correlation between particle size and adhesion/rupture distance/jump-in distance was found. However, the frequency of appearance of these very long-range forces increased with decreasing size of the particles building the nanostructure surfaces. No extremely long-range force curves were detected for the flat surfaces. Hence, the conclusion is that, in order to form the long-range type force curves, the extra air reservoir formed by the 3D structure found between and beneath the particles on the surfaces is needed to provide the cavity with air during retraction.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: + 46 8 790 9920. Fax: +46 8 208 284. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Omya Development AG is thanked for funding this project and for supporting cooperation between YKI, KTH, and its industrial Mineral and Surface Chemistry R&D. E.T. acknowl8033

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