This paper is published as part of Faraday Discussions volume 146:

Wetting Dynamics of Hydrophobic and Structured Surfaces

Introductory Lecture Exploring nanoscale hydrophobic hydration

Peter J. Rossky, Faraday Discuss., 2010

DOI: 10.1039/c005270c

Papers

Dynamical superhydrophobicity Mathilde Reyssat, Denis Richard, Christophe Clanet and David Quéré, Faraday Discuss., 2010 DOI: 10.1039/c000410n

Superhydrophobic surfaces by hybrid raspberry-like particles Maria D'Acunzi, Lena Mammen, Maninderjit Singh, Xu Deng, Marcel Roth, Günter K. Auernhammer, Hans-Jürgen Butt and Doris Vollmer, Faraday Discuss., 2010 DOI: 10.1039/b925676h

Microscopic shape and contact angle measurement at a superhydrophobic surface Helmut Rathgen and Frieder Mugele, Faraday Discuss., 2010 DOI: 10.1039/b925956b

Transparent superhydrophobic and highly oleophobic coatings Liangliang Cao and Di Gao, Faraday Discuss., 2010 DOI: 10.1039/c003392h

The influence of molecular-scale roughness on the surface spreading of an aqueous nanodrop Christopher D. Daub, Jihang Wang, Shobhit Kudesia, Dusan Bratko and Alenka Luzar, Faraday Discuss., 2010 DOI: 10.1039/b927061m

Discussion

General discussion Faraday Discuss., 2010 DOI: 10.1039/c005415c

Papers

Contact angle hysteresis: a different view and a trivial recipe for low hysteresis hydrophobic surfaces Joseph W. Krumpfer and Thomas J. McCarthy, Faraday Discuss., 2010 DOI: 10.1039/b925045j

Amplification of electro-osmotic flows by wall slippage: direct measurements on OTS-surfaces Marie-Charlotte Audry, Agnès Piednoir, Pierre Joseph and Elisabeth Charlaix, Faraday Discuss., 2010 DOI: 10.1039/b927158a

Electrowetting and droplet impalement experiments on superhydrophobic multiscale structures F. Lapierre, P. Brunet, Y. Coffinier, V. Thomy, R. Blossey and R. Boukherroub, Faraday Discuss., 2010 DOI: 10.1039/b925544c

Macroscopically flat and smooth superhydrophobic surfaces: Heating induced wetting transitions up to the Leidenfrost temperature Guangming Liu and Vincent S. J. Craig, Faraday Discuss., 2010 DOI: 10.1039/b924965f

Drop dynamics on hydrophobic and superhydrophobic surfaces B. M. Mognetti, H. Kusumaatmaja and J. M. Yeomans, Faraday Discuss., 2010 DOI: 10.1039/b926373j

Dynamic mean field theory of condensation and evaporation processes for fluids in porous materials: Application to partial drying and drying J. R. Edison and P. A. Monson, Faraday Discuss., 2010 DOI: 10.1039/b925672e

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Molecular dynamics simulations of urea–water binary droplets on flat and pillared hydrophobic surfaces Takahiro Koishi, Kenji Yasuoka, Xiao Cheng Zeng and Shigenori Fujikawa, Faraday Discuss., 2010 DOI: 10.1039/b926919c

Discussion

General discussion Faraday Discuss., 2010 DOI: 10.1039/c005416j

Papers

First- and second-order wetting transitions at liquid–vapor interfaces K. Koga, J. O. Indekeu and B. Widom, Faraday Discuss., 2010 DOI: 10.1039/b925671g

Hierarchical surfaces: an in situ investigation into nano and micro scale wettability Alex H. F. Wu, K. L. Cho, Irving I. Liaw, Grainne Moran, Nigel Kirby and Robert N. Lamb, Faraday Discuss., 2010 DOI: 10.1039/b927136h

An experimental study of interactions between droplets and a nonwetting microfluidic capillary Geoff R. Willmott, Chiara Neto and Shaun C. Hendy, Faraday Discuss., 2010 DOI: 10.1039/b925588e

Hydrophobic interactions in model enclosures from small to large length scales: non-additivity in explicit and implicit solvent models Lingle Wang, Richard A. Friesner and B. J. Berne, Faraday Discuss., 2010 DOI: 10.1039/b925521b

Water reorientation, hydrogen-bond dynamics and 2D-IR spectroscopy next to an extended hydrophobic surface Guillaume Stirnemann, Peter J. Rossky, James T. Hynes and Damien Laage, Faraday Discuss., 2010 DOI: 10.1039/b925673c

Discussion

General discussion Faraday Discuss., 2010 DOI: 10.1039/c005417h

Papers

The search for the hydrophobic force law Malte U. Hammer, Travers H. Anderson, Aviel Chaimovich, M. Scott Shell and Jacob Israelachvili, Faraday Discuss., 2010 DOI: 10.1039/b926184b

The effect of counterions on surfactant-hydrophobized surfaces Gilad Silbert, Jacob Klein and Susan Perkin, Faraday Discuss., 2010 DOI: 10.1039/b925569a

Hydrophobic forces in the wetting films of water formed on xanthate-coated gold surfaces Lei Pan and Roe-Hoan Yoon, Faraday Discuss., 2010 DOI: 10.1039/b926937a

Interfacial thermodynamics of confined water near molecularly rough surfaces Jeetain Mittal and Gerhard Hummer, Faraday Discuss., 2010 DOI: 10.1039/b925913a

Mapping hydrophobicity at the nanoscale: Applications to heterogeneous surfaces and proteins Hari Acharya, Srivathsan Vembanur, Sumanth N. Jamadagni and Shekhar Garde, Faraday Discuss., 2010 DOI: 10.1039/b927019a

Discussion

General discussion Faraday Discuss., 2010 DOI: 10.1039/c005418f

Concluding remarks

Concluding remarks for FD 146: Answers and questions Frank H. Stillinger, Faraday Discuss., 2010 DOI: 10.1039/c005398h

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Hierarchical surfaces: an in situ investigationinto nano and micro scale wettability

Alex H. F. Wu,a K. L. Cho,a Irving I. Liaw,a Grainne Moran,b

Nigel Kirbyc and Robert N. Lamb*a

Received 22nd December 2009, Accepted 2nd February 2010

DOI: 10.1039/b927136h

Two scales of roughness are imparted onto silicon surfaces by isotropically

patterning micron sized pillars using photolithography followed by an additional

nanoparticle coating. Contact angles of the patterned surfaces were observed to

increase with the addition of the nanoparticle coating, several of which, exhibited

superhydrophobic characteristics. Freeze fracture atomic force microscopy

and in situ synchrotron SAXS were used to investigate the micro- and

nano-wettability of these surfaces using aqueous liquids of varying surface

tension. The results revealed that scaling different roughness morphologies result

in unique wetting characteristics. It indicated that surfaces with micro, nano or

dual scale roughness induced channels for the wetting liquid as per capillary

action. With the reduction of liquid surface tension, nano-wetting behaviour

differed between superhydrophobic and non-superhydrophobic dual-scale

roughness surfaces. Micro-wetting behaviour, however, remained consistent.

This suggests that micro- and nano-wetting are mutually exclusive, and that the

order in which they occur is ultimately governed by the energy expenditure of the

entire system.

1. Introduction

The influence of roughness on the surface wetting behaviour, first reported by Wen-zel,1 highlighted a reduction in contact angle as a result of the increase in surfacearea. The postulation of a fully dewetted state, as reported by Cassie and Baxter,2

revealed, beyond a certain level of roughness, the existence of a composite interface(air and solid) in contact with the liquid.

The combination of low energy surface with surface roughness results in superhy-drophobicity, where water contact angle exceeds 150� with a sliding angle of lessthan 10�. This is an extreme case of the Cassie–Baxter dewetted state. Since itsdiscovery,3 efforts into synthetically replicating this phenomenon boomed with thedevelopment of various techniques to induce stable superhydrophobic characteris-tics on surfaces through irregular roughness,4–11 hierarchical roughness12,13 and ar-rayed nanotubes.14–19 Recent studies have shown that, regardless of fabricationtechnique, superhydrophobic characteristics are stabilized by hierarchical rough-ness.13,20–22 This suggests that both micro-roughness (RMS roughness 100 nm –several microns) and nano-roughness (RMS roughness <100 nm), and its

aSchool of Chemistry, The University of Melbourne, Melbourne, Victoria, AustraliabUNSW Analytical Centre, The University of New South Wales, Sydney, New South Wales,AustraliacThe Australian Synchrotron, Clayton, Victoria, 3168, Australia

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corresponding wetting behaviour, contributes to observable macroscopic wetta-bility.

Despite comprehensive theoretical studies such as thermodynamic23–27 and math-ematical28,29 modelling, experimental measurements in controlled texturing,30–32

SANS33 and AFM force measurements,34,35 the mechanism of hierarchical wettingbehaviour and its contribution to macroscopic wettability is still unclear. In anattempt to elucidate the effects of hierarchical wetting behaviour, our previousstudies have revealed a correlation between nanoroughness and antifouling behav-iour.36 We have also demonstrated that two superhydrophobic surfaces (SHS)with identical contact angle and hysteresis underwent different nanowettingphenomena,37 which also correlated with antifouling characteristics.38 This isa strong indication that both micro- and nanowetting behaviour govern differentmacroscopic properties of SHS, which is indistinguishable using macroscopic wetta-bility measurements.

We recently adapted two complimentary, non-invasive techniques; freeze fracturemicroscopy39 and synchrotron small-angled X-ray scattering (SAXS)37 to observethe fluid/surface interface of SHS at both the micro- and nanoscale.

Freeze fracture microscopy (FFM) has long been used in morphological studies inmicrobiology.40–42 A recent adaptation of this technique was conducted by Switkes43

to observe nanobubbles at the interface of a smooth hydrophobic surface. FFMachieves this by rapidly cryofixating the fluid/surface interface followed by repli-cating this interface using metal sputter coating, thus imprinting the fluid’smorphology at the interface onto a thin metal replica. Applying this technique toSHS, we have been able to quantify the morphology of air inclusion at the micro-scale.39

SAXS is a technique where the scattering of X-rays through a material is causedby electron density inhomogeneities due to the presence of particles or voids. Thescattered X-rays form a pattern which is commonly represented as scattering inten-sity (I) as a function of the scattering vector (q¼ 4psin(4/2)/l). Where 24 is the anglebetween the incident and scattered X-ray beam and l is the wavelength of the X-ray.

In a dual media system of differing electron densities (r1 and r2), the scatteringintensity (I12) is directly related to the electron density contrast (dP):44

I12 f (r1 � r2)2 ¼ dP2 (1)

Wetting involves the replacement of an air/surface interface with a liquid/surfaceinterface. Since liquid has a much higher electron density than air, a reduction in dP,and subsequently, the scattering intensity is observed. To quantify this effect, wemake use of the invariant Q, which is a measure of the overall scattering power ofthe system, and is approximated by

Q ¼Ð

I(q) q2 dq (2)

The raw calculated value of invariant Q is affected by the scattering media’s thick-ness, and is subsequently a source of error as the thickness between each SHS mayvary. By calculating the percentile changes in invariant Q between dry and wetsubstrates (Qwet/Qdry), we can remove the effects of thickness variations and stan-dardize invariant Q, thus allowing the direct comparison of the percentage interfacethat undergoes nanowetting between SHS.

In this work, photolithographically patterned micron-sized pillars with thin layersof nanoparticles are used systematically to vary micro- and nano-roughness. Theeffect of varying pillar size, pitch and nanoparticle size on micro- and nano-scaledwetting behaviour by immersing the surfaces in fluids with different surface tensionwas studied. Both FFM and synchrotron SAXS were performed in situ to revealchanges in the wetting behaviour at the micro- and nanoscale, respectively.

224 | Faraday Discuss., 2010, 146, 223–232 This journal is ª The Royal Society of Chemistry 2010

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2. Materials and methods

2.1 Preparation of microrough surfaces

Microroughness on silicon wafers (8 in. diameter, P/N doped, <100> oriented, 35mm thick) was fabricated by standard photolithography techniques.30 The desiredpatterns of a master mask were printed (Bandwidth Foundry Pty Ltd.) on a 4½in. square quartz plate. The masks were used to irradiate a photoresist coated waferwith a maximum precision of about 1 mm. After irradiation and development, thewafers were etched using reactive ion etching (RIE) with SF6 gas for 30–45 min de-pending on the desired heights of the pillars.

The surface chemistry of photolithographically patterned silicon substrates wasaltered by immersing in piranha solution (3 : 1 H2SO4 : H2O2) for 30 min at 90 �C,followed by rinsing with MilliQ water (0.05 mS cm�1, Millipore), drying with a gentlestream of nitrogen, then refluxing in a 1 M solution of trimethylchlorosilane inhexane for 24 h. The functionalized surfaces were cleaned of excess silanes by soni-cation in n-hexane for 10 min, and stored under argon until use.

2.2 Preparation of nano and dual-scaled roughness surfaces

Flat and photolithographically patterned silicon substrates were both spin-coatedwith a dilute solution of nanoparticles to generate nano-scaled and dual-scaledroughness, respectively. To investigate the effect of nanoroughness on wettingbehaviour, three nanoparticle sizes (7 nm, 20 nm, 40 nm) were used. The solutionswere prepared by mixing silica nanoparticles (Degussa, 0.25 g), n-hexane (AR, 40mL), methyltrimethoxysilane (MTMS, Aldrich, 0.36 g) and HCl (10 M, 0.2 mL)in a reaction flask and sonicating the mixture for 30 min at 40 kHz. Prior to coating,the mixture was diluted with additional n-hexane (80 mL). The coated substrateswere baked in a furnace at 150 �C for 15 min.

2.3 In situ synchrotron transmission small angled X-ray scattering measurements

All transmission SAXS experiments were performed at the SAXS/WAXS beamlineat the Australian Synchrotron, Clayton, Australia. Measurements were taken witha camera length of 7000 mm, providing a data collection q range of 0.001–0.1�A�1. All samples were mounted on a sealed fluid cell (Specac) with a pathlengthof 1 mm, which was bolted tightly onto a sample stage, such that the same positionwas analyzed for each test fluid used. The wetting fluids used were aqueous solutionsof sodium dodecyl sulfate (SDS, Aldrich) with different concentrations (0%, 20%,60% and 100% critical micelle concentration (CMC)).

2.4 Freeze fracture atomic force microscopy measurements

The freeze fracture technique was performed using a Moor-type freeze fracture unitmanufactured by Balzers AG. Cryogens used in the process were liquid nitrogen andFreon 22 (chlorodifluoromethane, DuPont). Further details of this technique aredescribed elsewhere.39 The resulting surface replicas were analyzed using AFMsection analysis (Digital Instruments 3000 AFM) to determine the penetration depthof the fluids into the troughs of textured surfaces.

3. Results and discussion

3.1 Microrough surfaces

The micro-scale roughness was fabricated by photolithographically patterning thesurface with pillars (Fig. 1). The roughness of the surface was then geometricallycontrolled through the variance of pillar size and pitch whilst maintaining a consis-tent height throughout.

This journal is ª The Royal Society of Chemistry 2010 Faraday Discuss., 2010, 146, 223–232 | 225

Fig. 1 Schematic of photolithographically fabricated microrough surfaces.

Table 1 Contact angle and roughness characteristics of test surfaces

Micro-roughness Nano-roughness Contact angle/� Hysteresis/�

1 mm � 1 mm, 1 mm pitch — 110 18

1 mm � 1 mm, 2 mm pitch — 108 28

1 mm � 1 mm, 5 mm pitch — 94 50

1 mm � 1 mm, 20 mm pitch — 96 >70 (Pinned)

5 mm � 5 mm, 1mm pitch — 92 15

10 mm � 10 mm, 1 mm pitch — 102 17

— 12 nm 126 6

— 20 nm 94 8

— 40 nm 85 32

1 mm � 1 mm, 1 mm pitch 12 nm 157 4

1 mm � 1 mm, 2 mm pitch 12 nm 155 6

1 mm � 1 mm, 5 mm pitch 12 nm 121 12

1 mm � 1 mm, 20 mm pitch 12 nm 115 23

5 mm � 5 mm, 1 mm pitch 12 nm 123 10

10 mm � 10 mm, 1 mm pitch 12 nm 119 9

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Photolithograpy affords a control over the process ensuring the consistency acrossthe surface with little or no variance. The penetration depth, as measured by freezefracture AFM, is defined by the depth of which the wetting fluid penetrates into thetroughs between each pillar. Ideally, the walls of the pillar are synonymous to theboundaries of a capillary tube with a diameter equivalent to the pitch.

Subsequent functionalization with trimethylchlorosilane rendered the surfacefeatures hydrophobic as shown in Table 1.

3.1.1 Wetting of microrough surfaces. The observed microwetting phenomenaconforms to behaviour described by capillary action in eqn (3), where h is the heightof a liquid in a column, g is the liquid–air surface tension, q is the contact angle ofthe surface chemical group, r is the density of the fluid, g is gravity and r is the radiusof the tube, or in this case, pitch.

h ¼ 2gcosq/rgr (3)

226 | Faraday Discuss., 2010, 146, 223–232 This journal is ª The Royal Society of Chemistry 2010

Fig. 2 Correlation between penetration depth and pitch of microrough surfaces with 1 mm� 1mm pillar size, under fluid immersion of varying surfactant concentration.

Fig. 3 Correlation between penetration depth with pillar size of microrough surfaces with 1mm pitch, under fluid immersion of varying surfactant concentration.

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Due to the surface’s chemical hydrophobicity, the cosq term is negative. Thus, thereduction in height is equivalent to the increase in penetration depth. This is demon-strated in Fig. 2 where the correlation between fluid penetration depth and surfac-tant concentration is obtained using freeze fracture AFM where the wetting of thesurface is illustrated to be proportional to the pitch. The initial penetration of thewetting solution is increased with larger pitch sizes which sits well with the conceptof increased roughness resulting in lower wettability. Standard capillary action

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wetting proceeds as per normal between 20%–60% before, the penetration depth pla-teaued at approximately 2.5 mm, which is the pillar height, indicating complete mi-crowetting. As expected, increasing levels of surfactant concentration also showincreasing levels of fluid penetration.

This is further confirmed by trends observed in Fig. 3, which showed there was noobservable increase in the penetration depth with increasing pillar size as the pitchwas kept constant at 1 mm.

3.2 Nanorough surfaces

Thin layers of nanoparticles were spin-coated onto flat silicon surfaces to generatea rough hydrophobic surface (Table 1). Roughness measurements, as summarizedin Table 2, indicate that these coated substrates are rough only at the nanoscale.

3.2.1 Wetting of nanorough surfaces. Fig. 4 compares the SAXS profile of a 20nm-coated silicon substrate immersed in fluids of increasing surfactant concentra-tion. The percentile changes in Q reflect the nanowetting behaviour of the fluids.It is therefore possible to elucidate the nanowetting behaviour of rough surfacesby observing percentile changes in Q as a function of surface tension.

The effect of varying surface tension on nano-wetting can be seen in Fig. 5. Themost notable trend is the plateauing of Q despite variations in surfactant concentra-tion for 12 nm and 20 nm nanoparticle coatings. This observation can only be

Table 2 Macroscopic wettability and roughness characteristics of nanorough surfaces

Coating Roughness factor Roughness RMS/nm

12 nm 1.28 97

20 nm 1.22 88

40 nm 1.14 64

Fig. 4 Synchrotron SAXS profile of 20 nm-coated silicon substrate under fluid immersion ofvarying surfactant concentration.

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Fig. 5 Percentile change in Q as a function of surfactant concentration of nanorough surfaces.

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interpreted as wetting remained constant throughout reduction in fluid surfacetension after initial contact with the test fluid. Although complete wetting of thenanoroughness (Wenzel regime) was expected, hysteresis measurements of lessthan 8� for both coatings is indicative of a Cassie regime. This suggests thata composite interface exists for both 12 nm and 20 nm coatings. Furthermore, thepercentile reduction in Q for a 40 nm nanoparticle coating is substantially greater,which corresponds a large exchange between air/surface and fluid/surface interfaces.This is in agreement with the hysteresis measurements (Table 1) as a 40 nm coatingwas observed to possess lower roughness (Table 2), thus a higher degree of pinning.

3.3 Dual-scale roughness surfaces

Dual scale roughness surfaces were fabricated through a combination of the twoprevious scaled surfaces. A further layer of nanoparticles is spun onto a lithograph-ically micro-patterned surface. An electron microscope image of this dual-scaledroughness surface, as shown in Fig. 6, reveals that the nanoparticle coating success-fully imparted nanoroughness in the troughs of the substrate, and most importantly,atop pillars.

The contact angle of several photolithographically patterned surfaces increaseddramatically into the superhydrophobic regime after applying a 12 nm nanoparticlecoating as seen in Table 1. Of the tested substrates, only two exhibited superhydro-phobic characteristics after the addition of nano-roughness. The aspect ratio(height : pitch) of the two superhydrophobic surfaces were 1 : 1 and 1 : 2, respec-tively. Typically, microrough surfaces will only exhibit superhydrophobic character-istics at aspect ratios above 4 : 1,30 therefore the addition of nanoroughness can beseen to increase resilience against wetting.

If we consider that the behaviour of micro- and nanowetting are mutually exclu-sive and remains unchanged between single and dual-scale roughness surfaces, theprogression of wetting in dual-scale roughness surfaces can be predicted as follows:

At 0% CMC, the wetting fluid would first partially wet the nanoroughness atopthe microscale pillars and reduce invariant Q from SAXS measurements. Penetra-tion depth would be identical to microrough surface measurements of �250 nm asthe pitch of the microroughness is unchanged.

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Fig. 6 Electron microscope image of (left) microrough surface and (right) dual-scaled rough-ness surface.

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Increasing surfactant concentration to 20% CMC, according to Fig. 5, theinvariant Q would remain unchanged as no further nanowetting proceeds, due tothe resilience against wetting observed earlier in nano-rough SAXS measurements.However, since the sidewalls of the pillars are uncoated and remains nanoscopicallysmooth, microwetting would continue as described by capillary action.

According to Fig. 2, at 60% CMC, a large jump in penetration depth is observedas a result of increases in the surfactant concentration, where the wetting fluidreached the bottom of the microroughness. Therefore, the same is expected forthe dual-scaled roughness surface as the sidewalls are smooth. Considering thatthe nanocoating is approximately 1 mm thick, the maximum penetration depthattainable for the wetting fluid would be approximately 1.5 mm, at which point,nanowetting would begin at the base of the microroughness, causing a furtherdecline of invariant Q. Since the majority of the surface area is in the nanoroughnessat the base of the substrate, the corresponding drop in invariant Q would be compa-rably larger than the initial wetting observed with 0% CMC atop pillars.

Finally at 100% CMC, considering the resilience of nanoroughness againstwetting, the invariant Q would be expected to remain unchanged.

From Fig. 7a and 7b, the respective micro- and nanowetting behaviour is observedto correspond with the predicted model of mutually exclusive wetting behaviour ofdual-scale roughness surfaces. There is, however, one discrepancy. In Fig. 7b, theonset of the 2nd decline of invariant Q, which is indicative of nanowetting at thebase of the substrate, for 1 mm and 2 mm pitch is earlier than that of 5 mm and 20

Fig. 7 a) Comparison in microwetting behaviour of microrough and dual-scaled roughsurfaces and b) corresponding nanowetting behaviour of varying pitch as a function of surfac-tant concentration.

230 | Faraday Discuss., 2010, 146, 223–232 This journal is ª The Royal Society of Chemistry 2010

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mm pitch. Since both 1 mm and 2 mm pitch dual-scale roughness surfaces are super-hydrophobic, a correlation exists where nanowetting at the base of the substrateoccurs earlier for superhydrophobic surfaces. If we take into consideration thatthe only variable is the pitch between the two sets of nanowetting behaviour, thistransition can be attributed to a competition between micro- and nanowetting toachieve the lowest energy state.

Wetting, as mentioned earlier, is the interaction between interfaces to achieve thelowest energy state. Thus, when wetting occurs on a dual-scale roughness surface,the fluid can either wet microscopically or nanoscopically, or even both simulta-neously. From the observed transition in Fig. 7b, it is believed that the fluid,when in contact with 1 mm and 2 mm pitch SHS, achieves a lower energy state bynanowetting, however, for 5 mm and 20 mm pitch dual-scale roughness surface,a lower energy state is first achieved by microwetting. In other words, the energyexpenditure of microwetting is fine-tuned by the pitch of the microroughness andultimately governs the preferential wetting order of the entire system.

4. Conclusion

Microroughness was imparted onto silicon substrates using photolithographic tech-niques, allowing precise control of microroughness by adjusting pillar size and pitch.By spin-coating a thin layer of nanoparticles, a systematic method of generatingdual-scaled roughness was possible, providing an ideal platform for wetting studies.Micro and nanowetting behaviour was successfully measured using freeze fractureAFM and in situ Synchrotron SAXS, respectively, with fluids of varying surfacetension. The progression of wetting was tracked by correlating the penetration depthand invariant Q analysis of substrates with dual-scale roughness.

By combining the wetting behaviour at both micro- and nanoscale, a wettingmodel was proposed to explain the transition between preferential micro- and nano-wetting behaviour for dual-scaled roughness surfaces, which was observed to betunable according to microroughness. The demonstration provided herein suggestswhile the wetting behaviour at both micro- and nanoscale appears to be mutuallyexclusive, their respective roughness ultimately governs the progression of wettingof a hierarchical roughness surface.

Acknowledgements

The authors would like to thank Dr. Simon Lam and Dr. Jia Du (CSIRO Sydney)for their assistance provided. This research was undertaken on the SAXS/WAXSbeamline at the Australian Synchrotron, Victoria, Australia. This research was sup-ported under Australian Research Council’s Discovery Projects funding scheme.

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232 | Faraday Discuss., 2010, 146, 223–232 This journal is ª The Royal Society of Chemistry 2010