Journal of Colloid and Interface Science 412 (2013) 82–88

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Journal of Colloid and Interface Science

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Interferometry of surfaces with well-defined topography in the surfaceforce apparatus

0021-9797/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jcis.2013.09.008

⇑ Corresponding author. Fax: +1 (410) 516 5510.E-mail address: jfrechette@jhu.edu (J. Fréchette).

1 Present address: Department of Chemical and Biomolecular Engineering, Univer-sity of Pennsylvania, Philadelphia, PA 19104, United States.

Rohini Gupta 1, Joëlle Fréchette ⇑Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, MD 21218, United States

a r t i c l e i n f o

Article history:Received 16 July 2013Accepted 4 September 2013Available online 17 September 2013

Keywords:Surface forces apparatusTopographySurface structureMultiple beam interferometry

a b s t r a c t

We studied the multiple beam interferometry of surfaces with well-defined microscale features in thesurface force apparatus (SFA). The structures investigated consist of hexagonal arrays of microscale cylin-drical posts made out of the photoresist SU-8. The ability of the SFA to visualize the profile and topogra-phy of the interacting surfaces leads to the observation of discontinuities in primary fringes of equalchromatic order that are caused by the microscale structural features. The shift in wavelengths has beenanalyzed to extract the post height and compared to independent profilometry measurements. The anal-ysis based on the shift in wavelength is shown to be viable only when the order of the fringe and the posi-tion of the discontinuity is precisely known and within the field of view. Analysis of the full profile of theinteracting surface for two orthogonal 2-dimensional slices can be used to determine how the surface lat-tice is oriented within the point of closest approach. Finally, we discuss cases in which the structural fea-tures detrimentally affect the spatial resolution of the SFA.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

The surface force apparatus (SFA) [1–5] is a technique to mea-sure the interactions between two macroscopic surfaces. Centralto the SFA is its exquisite ability to resolve absolute surface sepa-ration within 0.1–0.3 nm using multiple beam interferometry(MBI) [6,3,7]. The traditional Fabrey–Perot interference filters em-ployed for MBI in SFA consist of a medium bound by two thin (2–4 lm), transparent, and molecularly smooth mica pieces coatedwith 50 nm of silver on the back. The presence of the two semi-transparent bounding (silver) films facilitates analysis of transmit-ted light [3]. Alternatively, reflected light can be analyzed for inter-ferometers where one of the bounds is opaque or non-transparent[8]. Furthermore, the surfaces that can be investigated using theSFA are not limited to mica, and may not necessarily be molecu-larly smooth. Over the years, substrates such as mica coated withmetal [9–15], inorganic [16,17], organic [18–20], and polymer[21–27] thin films, and standalone polymer [28] and inorganic[29] surfaces, have also been explored. The use of generic surfacesin the SFA has been exploited for the investigation of rough[22,23,28,30,31] or patterned [22,32,33] surfaces and has helpedaddress scientific and technical concerns such as surface adhesion,deformation, friction, and wear that are relevant in engineering

applications. Most generic surfaces including metal films bearnanoscale surface roughness, and MBI can be employed not onlyto resolve absolute surface separation but also to estimate surfaceroughness (for example how the roughness varies with appliedload or when in contact with another surface) [9,10]. Furthermore,non-optical capacitance-based estimation of surface separation ina modified surface force apparatus can be employed for opaqueor non-transparent surfaces, further extending the capabilities ofSFA (this method does not provide topographical informationabout the surfaces) [34–40]. Both interferometry and capaci-tance-based surface force apparatus offer similar resolution forsurface separation [37].

Surfaces with well-defined topographical features (lengthscales significantly larger than surface roughness) are at the heartof the design of biomimetic adhesives, superhydrophobic surfaces,and metamaterials, where topography plays a central role in sur-face adhesion, friction, wetting, and optical properties. The SFAwith MBI has so far been limited to the investigation of surfaceswith intrinsic random nanoscale roughness or nanoscale patternse.g., friction caused by a structured surface inspired by the geckohas been studied using SFA [41–44], but without interferometry-based surface separation estimation. It is important, to demon-strate the compatibility of structured surfaces with MBI, especiallyconsidering the advantages of measuring absolute surface separa-tion and bearing in mind a possible loss of resolution in surfaceseparation caused by the surface features. The ability to introducesurfaces with microscale topographical features in the SFA and re-solve absolute surface separation using MBI would allow us to

R. Gupta, J. Fréchette / Journal of Colloid and Interface Science 412 (2013) 82–88 83

expand the range of materials and surfaces that can be investigatedwith this technique and help uncover how surface structures mod-ulate surface interactions.

Here, we demonstrate the design, fabrication, and use of sur-faces with well-defined microscale features that are compatiblewith the surface force apparatus. The structured surfaces, here,consist of hexagonal arrays of SU-8 cylindrical posts preparedusing standard photolithography procedure on the silver side ofthe mica pieces. We show how microscale structural features (withlength scales significantly larger than the nanoscale roughnesspresent on the surfaces) are mirrored as discontinuities in primaryfringes and that the shift in their wavelengths is consistent with adifference in optical path. Using analysis proposed by Zeng et al.[32] for rough polymer films, we highlight how MBI can be em-ployed to extract topographical dimensions of the structural fea-tures and discuss limitations of the method.

1.1. Background

When white light is incident normally through a Fabrey–Perotinterference filter, multiple beam interferometry (MBI) [6,3,7] re-sults in transmission of discrete wavelengths that satisfy the crite-ria for constructive interference, upon being reflected repeatedlybetween the two reflective films. The resulting fringes of equalchromatic order (FECO), or an infinite series of alternating sharpbright and dark bands (see Fig. 1a as an example of FECO), are dis-persed via a grating spectrograph. The positions of the known dis-crete spectral wavelengths of mercury are used to calibrate thespectrograph. The wavelengths of ‘‘primary’’ fringes are sensitiveto the thickness and index of refraction of the components of theinterference filter. The nanoscale roughness associated with gener-ic surfaces has been shown to result in small yet discernible shift inthe wavelengths of the primary fringes due to a change in the opti-cal path [9,10]. Zeng et al. also employed arguments based on thedifference in optical path to characterize the height of surface fea-tures in polymer films [32]. The use of highly reflective silver sup-presses the secondary, tertiary, and gap fringes. The secondaryfringes, if observed, do not undergo wavelength shift with a changein the optical path between the two reflective surface, but theirshape is susceptible to the topography and deformation of the sur-face�medium interface [45]. MBI, therefore, allows for accurateand independent determination of the thickness and/or refractiveindex and roughness of the component(s) of interference filter un-der investigation.

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The interference filter is mounted onto rigid and optically trans-parent cylindrical lenses (radius of curvature, R � 1–2 cm) ar-ranged orthogonal to each other, a configuration that isgeometrically equivalent to that of a sphere interacting with aplane. In this configuration the interference filter acquires curva-ture, which makes the medium thickness and, consequently, wave-lengths of the FECO a function of the lateral position as shown inFig. 1b. The use of macroscopic surfaces enables the visualizationof the 2-dimensional geometry of the interacting surfaces, whichis reflected in the shape of the FECO. For example, the sphere-planegeometry is visible in the curved fringes observed for two crossed-cylinders as shown in Fig. 1a, with the vertex of the parabola cor-responding to the point of closest approach (PCA) between the twosurfaces. For two crossed-cylinders in contact with each other, theFECO exhibit a flat section that corresponds to the zone of contactand reflects the elastic deformation inside and outside the interfer-ence filter.

Due to the finite width of the FECO, their corresponding wave-lengths are represented by the center of mass (CoM) of the trans-mitted light intensity measured at a given lateral pixel position,as shown in Fig. 1c for the lateral pixel position corresponding tothe point of closest approach indicated by the white line inFig. 1a. The wavelengths of primary fringes are sensitive to thethickness and index of refraction of the components of the interfer-ence filter. The FECO undergo red shift with an increase in the opti-cal path via an increase in either the thickness or index ofrefraction of either of the components of the interference filter,and vice versa. The shift in the wavelengths of the FECO can, there-fore, be used to resolve changes in medium thickness within 0.1–0.3 nm and index of refraction within 0.001–0.01 [3,46]. Sharpchanges in index of refraction result in discontinuities in the FECOonly in even (and not odd) fringes for small medium thickness,which can be used to visualize and investigate the presence of het-erogeneities in the optical path e.g., capillary condensation result-ing in an annular liquid meniscus between surfaces in the presenceof condensable vapors [47–51].

The final step is to convert the wavelengths of FECO to surfaceseparation (and/or refractive index). Analytical solutions for simul-taneous estimation of medium thickness (less than 200 nm) andindex of refraction are available for the following interference fil-ters: symmetric three and five layer [3], asymmetric two layer[3], general three layer [52,53], and three layer with two outer lay-ers of the same material but different thicknesses [52]. These ana-lytical solutions require one in-contact fringe wavelength and itsfringe order; the medium thickness and index of refraction can

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Table 1Structured SU-8 surfaces investigated in this work.

Surface Channeldepth (D)(lm)

Postdiameter(d) (lm)

Channelwidth (w)(lm)

Channel depth usingZeng et al. (D) (lm)

A 0.30 65.0 6.5 0.257 ± 0.017B (0�) 0.42 6.50 6.5 0.388 ± 0.007B (90�) 0.42 6.50 6.5 0.381 ± 0.003C 0.80 6.50 3.0 –

84 R. Gupta, J. Fréchette / Journal of Colloid and Interface Science 412 (2013) 82–88

then be estimated from the shift in the wavelength. The multilayermatrix method [54,55] based on the solution of Maxwell equationsto calculate the intensity of each of the transmitted wavelengthshas been used for more complicated multi-layer asymmetric inter-ference filters. To facilitate the transformation or inversion of thetransmitted intensity to a medium thickness and/or index ofrefraction using the multilayer matrix method, Heuberger devisedan elegant numerical algorithm called Fast Spectral Correlation[56], which is employed here. The Fast Spectral Correlation enablesfast and accurate analysis of MBI spectra (multiple fringes at once)wherein the sum of transmitted intensity is calculated for multiplefringes for a given medium thickness and/or index of refraction;the unknown parameter or combination of parameters (here oilthickness or surface separation) that maximizes the sum of trans-missivities is the optimal solution.

2. Materials and methods

2.1. Surface preparation

1 cm � 1 cm muscovite mica pieces (Ruby, ASTM V-1, S&J Trad-ing) were cleaved, cut using hot platinum wire, and placed on a lar-ger and thicker freshly cleaved mica backing sheet in a laminarhood. Thermal evaporation (Kurt J. Lesker Nano38) was used to de-posit 50 nm of silver (99.999% purity, Alfa Aesar) on the cleavedmica pieces (thickness = 2–8 lm) at a rate of 3–4 Å/s.

2.2. Hydrophobic silver (bottom surface)

The backing sheet with silvered mica pieces were immersed in1 mM hexadecane thiol (92%, Aldrich) solution in ethanol (200proof) overnight to render the silver hydrophobic (water contactangle �107�). Thereafter, the backing sheet was removed andrinsed thoroughly with ethanol to remove any excess reagents.The smaller mica piece with hydrophobic silver is removed fromthe backing sheet and glued (Epon 1004 epoxy) mica side downonto the cylindrical lens (radius of curvature, R � 2 cm). The cylin-drical lens is then mounted on the cantilever connected to themicrostepping motor inside the SFA.

2.3. Uniform or structured SU-8 (top surface)

The cleaved and silvered mica piece is used to support and han-dle both uniform and structured SU-8 (negative photoresist SU-82007, 2002, and 2000.5; MicroChem) surfaces. Prior to spin-coatingSU-8 2007 onto the silver side at 5500 rpm for 1 min, the backingsheet with one of the silvered mica pieces is taped to a glass slide.The smaller mica piece with SU-8 is then removed and placed on afreshly cleaved mica backing sheet followed by baking at 65 �C for

(a)

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d = 6.5 µm w = 6.5 µm D = 0.42 µm

(b)

Fig. 2. (a) Schematic of the hexagonal array of cylindrical posts constituting the structure8 Surface B. (c) Schematic for the SFA configuration employed here showing a structure

3 min, 95 �C for 5 min and 65 �C for 3 min. Upon exposing to ultra-violet radiations at 140 mJ/cm2 followed by post-exposure bakeidentical to the previous baking step, spin-coated SU-8 forms acontiguous layer resulting in uniform SU-8 surfaces with nanoscaleroughness. Thinner SU-8 2000.5 or 2002 of appropriate thickness isspin-coated onto the existing SU-8 contiguous layer followed bybaking (same procedure as earlier). The second thinner layer is ex-posed to ultraviolet radiations at 100 mJ/cm2 through a chrome-on-glass mask, followed by the post-exposure bake and developingfor 3 min. Excess SU-8 developer is removed using isopropanol,and the sample is dried with compressed air to form structuredSU-8 surfaces. The silvered mica piece with SU-8 is removed fromthe backing sheet and glued (Epon 1004 epoxy) mica side downonto a cylindrical lens (radius of curvature, R � 2 cm), followedby hard baking at 150 �C for 30 min. The cylindrical lens is thenscrewed onto the fixed (immobile) top mount of the SFA.

2.4. Surface characterization

The structured SU-8 consists of a hexagonal array of cylindricalposts as shown in Fig. 2a. The diameter of cylindrical posts d andthe channel width w are dictated by the pre-designed chrome-on-glass mask. The post height or channel depth D, however, iscontrolled via the thickness of the second SU-8 layer and is deter-mined by stylus profilometry measurements performed on thestep height in the structured SU-8 layer on top of the thicker con-tiguous SU-8 layer. The topographical features of the different sur-faces investigated here are presented in the Table 1. The epoxy-based negative photoresist SU-8 is chosen as the material for thestructured surfaces because of its ease of fabrication using stan-dard photolithography procedure and transparency. Hydrophobicsilver is chosen as the second surface to reduce the number of lay-ers in the optical filter. Both of the surfaces are submerged in sili-cone oil (loil = 1.4022). A schematic illustrating the topographicalfeatures of structured SU-8 investigated in this work along witha representative scanning electron micrograph and SFA configura-tion of structured SU-8 (Surface B) interacting with a hydrophobicsilver film across viscous silicone oil are shown in Fig. 2. The micaon both top and bottom surfaces is not part of the interferometer,

t = 7 µmd = 6.5 µmSilicone Oil

Hydrophobic Silver

Structured SU-8

(c)

h

w = 6.5 µmD = 0.42 µm

d surfaces and (b) a scanning electron micrograph (tilt angle of 30�) of structured SU-d SU-8 and hydrophobic silver separated by silicone oil.

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but its silver film serves as one of the two reflective layers (the lay-ers constituting the interferometer are shown in Fig. 2c).

2.5. Interferometry analysis

Note that the structural features under consideration are mi-cron scale and are much larger than the nanoscale roughness thatis also present on all the surfaces considered here. The SU-8 andself-assembled monolayer (SAM) thicknesses are estimated usingthe wavelengths of the in-contact FECO at the top of a post, whichmay undergo a small shift (ignored in our measurements) due tonanoscale surface roughness associated with both the self-assem-bled monolayer and the SU-8 [9,10]. For an asymmetric silver/SU-8/oil/SAM/silver interference filter, Fast Spectral Correlation[56] devised by Heuberger in conjunction with the multilayer ma-trix method [54,55] is used to estimate the unknown medium(here, oil) thickness. Out of contact, wavelengths of the FECO canbe translated to surface separation or oil thickness as a functionof the lateral position and used to calculate the radii of curvatureof the interacting cylinders as shown in Fig. 1b. The lateral resolu-tion needed to calculate the radii of curvature is dictated by theoptical magnification e.g., the lateral resolution in our case is2.64 lm/pixel for 7.4 lm-sized pixel, 8 � 8 binning, and 22.42optical magnification. The wavelengths at the vertex of the para-bolic fringes are used to estimate the surface separation at thepoint of closest approach for a sphere-plane configuration asshown in Fig. 1c.

3. Results and discussion

The interferometry of three distinct structured surfaces withdifferent topographical features (Table 1) was investigated usingthe surface force apparatus. The ability of the SFA to visualize thegeometry of interaction of the two surfaces allows for the micro-scale structural features present on the surface to appear as discon-tinuities in the FECO due the difference in optical path. The FECOshown in Fig. 3a, d, and g correspond to asymmetric silver/struc-tured_SU-8/oil/SAM/silver interference filters for Surfaces A andB and consist of hexagonal arrays of SU-8 cylindrical posts of diam-eter d separated by channels of width w and depth D preparedusing standard photolithography (see Fig. 2a for a schematic). Anincrease in optical path, as in the case of light transmitted throughthe SU-8 posts versus the channels filled with oil results in a redshift in the wavelengths of the FECO. For an individual fringe (forexample see the white box in Fig. 3a, d, and g), the wavelength cor-responding to the center of mass (CoM) of the transmitted lightintensity can be plotted as a function of lateral pixel position, suchas shown in Fig. 3b, e, and h. As expected, the wavelengths of FECOcorresponding to the channels filled with oil of a lower index ofrefraction vis-à-vis SU-8 are blue-shifted compared to those corre-sponding to the SU-8 posts. If the index of refraction of oil and SU-8were equal, the discontinuities would disappear.

The shift in the wavelength due to the difference in optical pathfor the post versus the channel can be used to independently deter-mine the channel depth D using the arguments proposed by Zenget al. using Eq. (1) [32].

D ¼ nðkn;p � kn;cÞ2ðln;p � ln;cÞ

; ð1Þ

where kn;p and kn;c are the wavelengths corresponding to post andchannel, respectively, ln;p and ln;c are the index of refraction ofSU-8 and oil, respectively, and n is the fringe order as given byn ¼ kn�1=ðkn�1 � knÞ. We observe excellent agreement between thechannel depth independently measured using profilometry and thatcalculated using Eq. (1) as shown in Table 1. Care must be taken to

insure the wavelengths for the post and channel used are of thesame fringe order. As the depth of the channel increases, the sameorder fringe for the posts and channels will not be present at once inthe field of view of the imaging spectrometer (this critical depthwill be a function of the dispersion of the diffraction grating em-ployed). This resolution limit also depends strongly on the differ-ence in refractive index between the medium and the structuredmaterial, a larger difference in refractive indices will lead to a lowerlimit in the maximum depth that can be resolved (to have the sameorder fringe present in the field of view). In these cases, the struc-tural features are still visible as discontinuities in the FECO butEq. (1) cannot be employed. Alternatively a more detailed multi-layer matrix solution for the differences in thickness can still beachieved or an analysis based on fringes of different order [53]could potentially also be employed. An additional constraint basedon our experimental observation illustrates that this equation can-not be employed for channel depths greater than 0.8 lm, which isthe depth when the channel of a certain order of fringe is observedto overlap/merge with the post of the higher order fringe as in thecase of Surface C (discussed at length later in the article).

To extract the surface separation or oil thickness from wave-lengths of FECO, we need to determine independently the totalSU-8 thickness for the two layers (contiguous thick layer and pat-terned thin layer, if present). To do so, we bring the two surfaces incontact and use the wavelengths of FECO corresponding to the topof a post in the zone of contact. When the two surfaces are not incontact, the vertex of the parabolic FECO with respect to the postscorresponds to the point of closest approach between the two sur-faces in a sphere-plane configuration. The wavelengths of FECOcorresponding to the point of closest approach are transformedto the minimum oil thickness or surface separation for the struc-tured surfaces using Fast Spectral Correlation in conjunction withmultilayer matrix method. Furthermore, the wavelengths of FECOand therefore, surface separation as a function of the lateral posi-tion can be used to calculate the radii of curvature of the interact-ing cylinders as shown in Fig. 3c, f, and i. Note that the surfaceseparation or oil thickness for structured surfaces is measuredfrom the top of a post, and therefore is not related to the channeldepth.

We can use interferometry to reconstruct independently thestructural features of the surface under observation by combiningthe lateral imaging (resolution of 2.64 lm/pixel) and the shift inwavelengths caused by the difference in optical path of SU-8 postsvis-à-vis the channels filled with lower refractive index oil, asshown in Fig. 3a–c. Using this lateral mapping, we ascertain thatthe width of the channels are roughly a tenth of the diameter postsfor Surface A, which is consistent with the size of the patterned fea-tures on the surface as determined by the design of chrome-on-glass mask used. The width of the channels and posts obtainedfrom a 2D interferometry slice is only equal to the microfabricatedfeature sizes if the slice is aligned with the center of the posts inthe h10i direction, any other orientation will give different sizesfor the channels and posts. Therefore, we can determine the orien-tation of the individual 2D slices on the structured surfaces byusing the lateral imaging for two orthogonal directions (SurfaceB: Fig. 3d–f and g–i). For example, on Surface B we show howtwo orthogonal directions are aligned on the structured surface(shown by red lines in Fig. 2a, which is centered on a post). Itcan be a challenge, however, to determine the orientation of indi-vidual slices because the positioning has two degrees of freedom:rotation and translation with respect to a hexagonal array ofcylinders.

Certain combination of the dimensions of the topographical fea-tures results in more complicated (or interesting) observations.Surface C (as shown in Fig. 4a) for instance has a channel widththat is comparable to the lateral resolution and the channel depth

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Fig. 3. Surface structure resulting in discontinuities in FECO for Surfaces (a) A, (d) B (0�), and (g) B (90�). Wavelength as a function of pixel position (b, e, and h) converted tosurface separation as a function of lateral dimension (c, f, and i) and their corresponding radius of curvature (RoC).

86 R. Gupta, J. Fréchette / Journal of Colloid and Interface Science 412 (2013) 82–88

is such that the channel of a certain order of fringe overlaps/mergeswith the post of the higher order fringe. In this special case, wecannot isolate the channels from the posts based on both lateralimaging and optical path difference arguments. Consequently,the center of mass (CoM) of the transmitted light intensity as afunction of pixel position does not reflect the structure of SurfaceC as shown in Fig. 4b.

One question that might arise when working with structuredsurfaces is whether the presence of discontinuities in the FECOleads to a loss in our ability to resolve accurately surface separa-tion. Similar to smooth surfaces, sharp fringes obtained for struc-tured Surfaces A and B can be modeled as Cauchy–Lorentzdistribution of intensity of transmitted light with respect to wave-

length as shown in Fig. 5b, implying that the ability to resolve sur-face separation is the same as that for smooth surfaces. On theother hand, broad fringes, such as those obtained with Surface Ccan be modeled as a normal distribution as shown in Fig. 5d. In thisscenario, the breadth of fringes in terms of distribution of intensityof transmitted light adversely affects our ability to resolve surfaceseparation. The mean of the normal distribution of intensity oftransmitted light with respect to wavelength can be used to deter-mine the surface separation such that the standard error of themean corresponds to the error associated with the estimation ofsurface separation. For Surface C, the breadth of the fringes leadsto a ± 3 Å error in the estimation of wavelengths of FECO as indi-cated by the two vertical lines in Fig. 5d. If we assume that the

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mean of the normal distribution of transmitted intensity corre-spond to the posts, the wavelengths of the in-contact FECO canbe used to estimate the SU-8 thickness. Furthermore, when outof contact, wavelengths of FECO at the same lateral pixel positionas the in-contact FECO enables us to resolve the surface separationor oil thickness at the point of closest approach with respect to theposts within ±4 nm, which is an error much larger than what isusually attained in typical SFA experiments but still sufficientlyaccurate for many force measurements.

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Fig. 5. (a and b) Cauchy–Lorentz distribution for sharp fringes that correspond toSurface A. (c and d) Normal distribution for broad fringes that correspond to Surface Cand the associated ±3 Å error in the wavelength as shown by the vertical lines in (d).

4. Conclusions

In the SFA, well-defined microscale features present on surfacesappear as discontinuities in the FECO due to the ability to visualizethe profile and topography of the interacting surfaces and are a re-sult of the difference in optical path of the posts versus the chan-nels. Here, the wavelengths of FECO corresponding to thechannels filled with an oil of a lower index of refraction vis-à-visSU-8 are blue-shifted compared to those corresponding to theSU-8 posts. The shift in the wavelength due to the difference inoptical path for the post versus the channel can be used to deter-mine independently the channel depth D when the fringe orderand the position of discontinuity are precisely known. The compar-ison of 2-dimensional slices of the interacting surfaces at 0� and90� provides valuable information regarding the orientation ofthe interacting surfaces with respect to the periodic array of thefeatures present on the surface. For certain structured surfaces,broadening of fringes may result in larger error in surface separa-tion (here up to ±4 nm), which is an error much larger than what isusually attained in typical SFA experiments.

Acknowledgments

This work is supported by the Office of Naval Research – YoungInvestigator Award (N000141110629), the National Science Foun-dation under Grant No. CMMI-0709187, and partially by the Do-nors of the American Chemical Society Petroleum Research Fundunder Grant No. 51803-ND5. We would like to acknowledge thehelp of Huy Vo during the use of Clark Hall Microfabrication Facil-ity, Mark Koontz for the scanning electron micrograph, and YumoWang for experimental verification.

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