IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 43 (2010) 045301 (5pp) doi:10.1088/0022-3727/43/4/045301

A novel and inexpensive technique forcreating superhydrophobic surfaces usingTeflon and sandpaperMichael A Nilsson, Robert J Daniello and Jonathan P Rothstein1

Department of Mechanical Engineering, University of Massachusetts Amherst, 160 Governors Drive,Amherst, MA 01003-2210, USA

E-mail: rothstein@ecs.umass.edu

Received 10 June 2009, in final form 14 August 2009Published 12 January 2010Online at stacks.iop.org/JPhysD/43/045301

AbstractConsiderable efforts have been spent over the last decade developing hydrophobic surfacesexhibiting very large contact angles with water. Many of these methods require complex andexpensive fabrication techniques. We demonstrate that sanding Teflon can producesuperhydrophobic surfaces with advancing contact angles of up to 151◦ and contact anglehysteresis of less than 4◦. Furthermore, we show that a wide range of both advancing contactangles and contact angle hysteresis can be achieved by varying the grit size of the sandpaper,allowing for future hysteresis and contact angle studies. Scanning electron microscopy imagesof the roughened surfaces depict the range and amplitude of length scales imparted on thesurface by the sandpaper, which leads to deeper understanding of the state of wetting on thesurface.

A great deal of effort has been spent over the last decadedeveloping hydrophobic surfaces exhibiting very large contactangles with water [1–17]. A number of strategies have beenemployed to increase the average contact angle between asubstrate and water, including chemical modification of thesubstrate to lower the surface energy between the water and thesurface [1]. However, it has been shown both theoretically [8]and experimentally with water [12] that chemical alteration ofa smooth surface can only achieve advancing contact angles,θA, with water up to about θ � 130◦ [11]. A surfaceis considered to be superhydrophobic if it has both a largeadvancing contact angle, greater than θA > 150◦, and minimalcontact angle hysteresis [17]. The contact angle hysteresis isdefined as the difference between the advancing and receding,θR, contact angles. Reducing contact angle hysteresis iswidely accomplished in practice by introducing either randomor precisely patterned surface roughness to a hydrophobicsubstrate. For an excellent review of modern techniques pleasesee Nakajima et al or Zhang et al [11, 18].

Superhydrophobic surfaces were first inspired by thecharacteristic water repellency of the lotus leaf [19].Additionally, there are a number of other superhydrophobic

1 Author to whom any correspondence should be addressed.

surfaces found throughout nature in both the plant and insectworld [20]. When properly designed, these bio-mimeticsurfaces can reduce the contact angle hysteresis of waterby maintaining an air–water interface above the depressionsbetween the peaks of the surface roughness. Moreover, ithas been shown that it is often advantageous to have multiplelength scales of roughness in order to increase both advancingand receding contact angles while simultaneously minimizinghysteresis [21].

Superhydrophobic surfaces have demonstrated the abilityto be self-cleaning. As water droplets move alongthese surfaces, they roll, collecting dust and particulatesfrom the surface [22]. Droplets move very easilyalong superhydrophobic surfaces because the drag forceis proportional to the contact angle hysteresis, FD ∝cos (θA − θR). Minimizing the hysteresis allows drops tobe easily dislodged by even the smallest perturbations [9,16, 23, 24]. The high level of water droplet mobility onsuperhydrophobic surfaces is desirable in many industrialapplications. Some notable areas where the application ofsuperhydrophobic surfaces is emerging include automotive,transportation, communication hardware, marine technology,textiles and biological applications. Specific examples of these

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J. Phys. D: Appl. Phys. 43 (2010) 045301 M A Nilsson et al

applications being self-cleaning windows or antennas, stain-proof and water-repellent clothing and snow repellent satellitedishes [5, 11, 14, 15, 25, 26]. These surfaces also facilitate thecontrol and manipulation of individual drop-directed motion,allowing for the possible use of these surfaces in microfluidicapplications.

The two major models for describing water contact angleson rough surfaces are the Cassie and the Wenzel models. Inthe Cassie state, an air–water interface is supported betweenthe surface roughness [27]. This phenomenon modifies thecontact angle, θC, such that

cos (θC) = γ (cos (θ) + 1) − 1. (1)

Here θ is the equilibrium contact angle between a smoothsurface sample and the liquid as defined by Young’s law andγ is the area fraction of the solid–liquid interface. In order tomaximize the equilibrium contact angle, the percent coverageof the air–water interface must similarly be maximized. Itis also important to note that the shape and size of the surfaceprotrusions affect the contact angle and the resulting hysteresis[13]. At sufficiently large microfeature spacing and liquidpressures, the air–water interface can collapse, resulting in afully wetted Wenzel state [28]. The equilibrium contact anglefor the Wenzel state, θW, is given by

cos (θW) = r cos (θ) . (2)

Here r is the ratio of the wetted area to the projectedsurface area. For a smooth surface, r = 1, and increaseswith increasing roughness. For a hydrophobic surface, theequilibrium angle will increase in the Wenzel state; however,the contact angle hysteresis is also typically very large. This isdue to the contact line being pinned at each wetted featureas it recedes [29]. Furthermore, it has been shown that,even with proper spacing of surface features, at a smallscale the roughness cannot sufficiently ‘trap’ air, and anincreased hysteresis is experienced [10]. Therefore, truesuperhydrophobic surfaces exist in the Cassie, not the Wenzelstate.

There are many methods by which roughness may beintroduced into a material, including chemical or mechanicalprocesses, oftentimes in concert with each other [1, 9, 10, 12,13, 16, 22–24, 29, 30]. These methods generally necessitateextensive material or facility costs; however, there have beenefforts made in minimizing these requirements [6, 7]. Thispaper focuses on the simple mechanical surface alterationof polytetraflouroethylene (PTFE), commercially known asTeflon, renowned for its water-repellent properties. Biological,commercial and industrial applications comprise a few ofTeflon’s various uses. Teflon is naturally hydrophobic,featuring a smooth surface with contact angles, θA/θR, of128◦/78◦, the average contact angle being 103◦. The unalteredsurface has large hysteresis, θA − θR = 50◦, restricting dropmovement. Teflon’s low coefficient of friction with manymaterials and consequently its ‘non-stick’ properties are itsprimary appeal for its wide use in cookware. Teflon is alsoapplied to fabrics (Goretex) and structural components (domeroofs) which take advantage of its water repellency.

Table 1. Characterization of both the sandpaper used and theresulting surface roughness and wetting properties of water and thesanded Teflon surfaces.

Grit Mean particle RMS roughness Contactdesignation diameter (µm) Teflon (µm) angle (θA/θR)

Smooth — 5.6 128◦/78◦

600 25.8 7.6 132◦/71◦

400 35.0 4.6 140◦/80◦

360 40.5 5.5 136◦/99◦

320 46.2 10.9 150◦/119◦

240 58.5 13.7 150◦/146◦

180 82 15.4 148◦/141◦

120 125 16.3 151◦/134◦

80 201 14.5 146◦/129◦

60 269 18.2 146◦/125◦

40 425 17.5 143◦/108◦

This paper presents a simple and inexpensive methodof creating a superhydrophobic surface, possessing a highadvancing contact angle, and a hysteresis comparable tosurfaces produced via more complicated and costly methods.The primary method used was simple sanding and subsequentcleaning of solid Teflon. We mounted each Teflon sample topieces of aluminium using epoxy, in order to ensure a generallyflat and secure Teflon surface. After letting the epoxy set, eachsample was held stationary, and sanded by hand. Reasonableforce is applied to impart a good representation of the grit sizeof the sandpaper onto the Teflon. Sanding is performed in arandom manner, as to show no preference in any particulardirection. After approximately 20 s of sanding, the sandedsurface is then cleaned with acetone briefly, then rinsed withreverse-osmosis deionized water and dried with pressurizedair. It is important to note that the final hydrophobic propertieswere not sensitive to additional sanding, excess pressure whensanding or the use of a mechanical sander. The cost of aTeflon surface and sandpaper is on the order of dollars, whilemore complicated methods can be on the order of hundreds tothousands of dollars for the materials and equipment requiredfor production.

A range of commercial sandpaper grit designationsbetween 40-grit and 600-grit were used, with each samplebeing sanded by only one grit designation. Table 1 showsthe common grit designation for the sandpaper used, theaverage particle size on the paper and the average RMSroughness of the Teflon surface following sanding. Surfaceroughness measurements were taken using a Zygo 7300 opticalprofilometer, with each surface measured three times at threedifferent locations over an area of 0.045 mm2. The resultingsurface roughness imparted by the sanding exhibits a widedisparity between the various grit sizes. The smooth Teflonhad the lowest average RMS roughness of 5.6 µm. As coarsersandpapers are used, the RMS roughness increased as isexpected. As seen in equations (1) and (2), this increase insurface roughness is a mechanism by which contact angle canbe increased.

The contact angle was measured photographically usingan in-house constructed goniometer and is listed in table 1and presented graphically in figure 1. Each contact anglemeasurement was taken three times, each at a different location

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J. Phys. D: Appl. Phys. 43 (2010) 045301 M A Nilsson et al

Figure 1. Hysteresis as a function of advancing contact angle for smooth Teflon, as well as a series of Teflon surfaces, sanded withsandpaper of grit designation • 600, � 400, � 360, � 320, � 240, ♦ 180, � 120, � 80, ◦ 60 and �� 40.

on the Teflon surface. The volume of each drop was keptconstant, and in each case the drop was well below a Bondnumber of one to ensure gravitational effects were negligible.

The Teflon prepared with the finest sandpaper, 600-grit,exhibited greater hysteresis than the smooth Teflon, as wellas a larger advancing contact angle, suggesting that the addedroughness exists primarily in the Wenzel state. As the grit sizewas increased from 600-grit to 360-grit, the advancing contactangle continued to increase with little change in the contactangle hysteresis. At a grit size of 320-grit, correspondingto a RMS roughness of 10 µm, the advancing contact anglereached a maximum value of θA = 151◦ and the contactangle hysteresis showed a significant decrease. These data aresuggestive of a transition from a mostly Wenzel to a mostlyCassie state of wetting. The contact angle hysteresis is furtherminimized as the grit size is increased from the 320-grit to the180-grit. Of the surfaces with lower contact angle hysteresis,the 240-grit sanded Teflon has the least amount of hysteresis,4◦, which competes well with many of the published surfacepreparation techniques that possess similar hysteresis but areeither much more expensive or require more complicatedmethods to produce [11]. As seen in figure 1, two distinctregimes in the data become clearly apparent. The first atlarge sandpaper grits where the hysteresis remains fixed andadvancing contact angle varies over a range of about 30◦,and the second at lower sandpaper grits where the advancingcontact angle is nearly constant at θA � 150◦ and the contactangle hysteresis varies over more than 60◦. It is important tonote that contact angle hysteresis is a key component whendetermining whether a droplet will move freely on a surface,or adhere [1, 29, 30]. The large variation of contact anglehysteresis in the second regime suggests that this surfacepreparation technique is well suited for systematically studyingthe effect of hysteresis on drop dynamics.

The advancing contact angle only marginally decreasedas the grit size was increased from the 120-grit to the40-grit; however, the contact angle hysteresis began to

increase for grit designations smaller than 120-grit. Thelargest grit designation tested, 40-grit, still possessed a highlyhydrophobic advancing contact angle, although their contactangle hysteresis approached that of smooth Teflon. Scanningelectron microscopy (SEM) in figure 2 shows that thisdeterioration of superhydrophobic character is likely causedby the large spacing between the surface features resultingfrom the large grit size and a suspected transition back to theWenzel state from the Cassie state achieved for grits between240- and 120-grit.

SEM images were taken of a subset of the surfaces listedin table 1 and are shown in figure 2. The Teflon surfaceswere sputter-coated with gold and measurements were taken at20 kV. While the untreated Teflon is relatively smooth, sandingwith the 600-grit sandpaper introduces surface roughness bycoarsening the Teflon surface. While this roughness servesto increase the advancing contact angles, the valleys are notdeep enough to maintain an air–water interface, giving furthersupport to the hypothesized Wenzel state of wetting. Thisresults in lower receding contact angles because the water ispinned to the surface along the receding contact, increasing thehysteresis substantially, as shown in figure 1. As the sandpaperincreases in grain size (or decreases in grit designations), theamplitude of the surface peaks and valleys increases, while thesecond level of roughness becomes more obvious on the Teflonsubstrate. This is shown with the 320-grit sanded Teflon withnoticeably deeper depressions on the surface, and furthermorewith the 240-grit sanded Teflon. These deeper surface featuresand more pronounced secondary roughness likely allow for theformation of air–water interfaces typical of the Cassie state,resulting in even higher contact angles and reduced contactangle hysteresis. However, as the surface roughness increaseseven further, as seen with the 80-grit in figure 2, some ofthe valleys and depressions become large, making the supportof an air–water interface less likely, resulting in the gradualtransition back into the Wenzel state. The effect of sandingother naturally hydrophobic surfaces will be investigated inthe near future.

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J. Phys. D: Appl. Phys. 43 (2010) 045301 M A Nilsson et al

Figure 2. SEM images of a series of Teflon surfaces sanded with sandpapers of various grit designations: (a) smooth at 100×, (b) smooth at200×, (c) smooth at 1000×, (d) 600-grit at 100×, (e) 600-grit at 200×, (f ) 600-grit at 1000×, (g) 320-grit at 100×, (h) 320-grit at 200×,(i) 320-grit at 1000×, (j ) 240-grit at 100×, (k) 240-grit at 200×, (l) 240-grit at 1000×, (m) 80-grit at 100×, (n) 80-grit at 200× and (o)80-grit at 1000×.

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J. Phys. D: Appl. Phys. 43 (2010) 045301 M A Nilsson et al

In this paper, we have shown that by sanding Teflonwith various grits of sandpaper, it is straightforward to createsurfaces with a wide range of hydrophobic properties. Itis possible to create surfaces with an array of hydrophobiccharacteristics, from low contact angles and high hysteresis,to ones with very high contact angles and low hysteresiscompetitive with those produced my more expensive andcomplicated techniques. It is worth noting that sanded Teflonsurfaces possess relatively short regions of continuous air–water interfaces which are ideal for modifying the ease of dropmotion on the surface.

Acknowledgments

The authors acknowledge the Office of Naval Research for thesupport provided for this research under grant N00014-06-1-0497. The authors also thank Al Crosby for use of his Opticalprofilometer. Finally, the authors thank the UMASS MRSECfor use of its SEM and J Feng for his assistance in performingthe SEM measurements.

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