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1400 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 53, no. 8, august 2006

Letters

Characterization of Superhydrophobic MaterialsUsing Multiresonance Acoustic Shear Wave

Sensors

Sun Jong Kwoun, Ryszard M. Lec, Richard A.Cairncross, Pratik Shah, and C. Jerey Brinker

AbstractVarious superhydrophobic (SH) surfaces, withenhanced superhydrophobicity achieved by the use ofnanoparticles, were characterized by a new acoustic sens-ing technique using multiresonance thickness-shear mode(MTSM) sensors. The MTSM sensors were capable of dif-ferentiating SH properties created by nano-scale surfacefeatures for lm, exhibiting similar macroscopic contact an-gles.

I. Introduction

In recent years, a variety of synthetic approaches havebeen developed to create so-called superhydrophobic(SH) surfaces characterized normally by high static con-tact angles of water (> 150) [1][4]. Superhydrophobicitydepends on surface roughness and surface chemistry, butto date rigorous structure-property relationships have notbeen established, especially the relationship between staticand dynamic properties and how superhydrophobicity isinuenced by nanoscale structural features. In this pa-per we use high-frequency shear acoustic waves generatedby a piezoelectric quartz resonator thickness-shear mode(TSM) sensor to interrogate SH surfaces loaded with liquidmedia. For the TSM operating in the frequency range of 1to 100 MHz, the depth of penetration is on the order oftens to thousands of nanometers [5]; therefore, these sen-sors are sensitive to nanoscale interfacial phenomena andprocesses. Moreover, because the depth of penetration de-creases with increasing TSM frequency, a multiresonanceexcitation of the sensor allows spatial interrogation of theinterface with controllable interrogation depth. The pur-pose of this investigation is to evaluate the multiresonanceTSM sensing technique to study the dynamic behavior ofthe SH/H2O interface and to correlate the multiresonanceTSM (MTSM) response with microscopic and nanoscopicfeatures of SH surfaces. Although all the SH surfaces in thisstudy had similar macroscopic wettability (optical contactangle 150), MTSM showed dierent responses, depend-ing on the surface treatments and lm morphology. Thus,MTSM sensing may provide a new means of probing thefunctional behavior of SH lms and help establish neededstructure-property relationships.

Manuscript received January 10, 2006; accepted February 16, 2006.S. J. Kwoun, R. M. Lec, and R. A. Cairncross are with Drexel

University, Philadelphia, PA 19104 (e-mail: sunkwoun@gmail.com).P. Shah and C. J. Brinker are with the University of New Mexico,

Albuquerque, NM 87131.

II. Experiment

The TSM sensors were 10 MHz fundamental resonantfrequency quartz crystals AT-cut and coated initially withgold electrodes on both sides. Five samples of varying hy-drophobicity were prepared by coating the sensors andsubjecting the coating to various surface treatments (Ta-ble I summarizes the sample preparation). Sample 1 wasa bare TSM sensor and Samples 2 to 4A were coated withSH coatings of increasing hydrophobicity. Sample 2 wascoated with TFPTMOS (triuoropropyltrimethoxysilane)to produce a low-surface, free energy with submicron-scaleroughness [4]. Sample 3 also had a TFPTMOS coating butwas further treated with HMDS (hexamethyldisilizane) toderivatize any remaining hydroxyl groups with hydropho-bic trimethyl silyl Si(CH3)3 groups on the surface. Sample4A was prepared by the same techniques as Sample 3 butwith the addition of silica nanoparticles (2% by weight)to the TFPTMOS coating, followed by treatment withHMDS. Sample 4B is the same MTSM sensor and coatingas Sample 4A but after exposure to ultraviolet (UV)/ozoneto reduce its hydrophobicity. All the SH samples were pre-pared in the laboratory at the University of New Mexico.

The samples were characterized for macroscopic con-tact angle and surface roughness. Contact angle was mea-sured by the sessile drop method [6]. The surface topologyand roughness of samples was measured using atomic forcemicroscopy (AFM) [7]. The AFM images of the SH sur-faces were obtained in 3 m 3 m areas and are shownin Fig. 1. The AFM software enables analysis of relativechanges of surface area (Table I).

III. Results and Discussion

The measured macroscopic optical contact angles of wa-ter on each of the samples are reported in Table I alongwith images of a water droplet resting on the sample sur-face. The contact angle increases from approximately 80

on the bare MTSM sensor to 140 to 155 for the SH coat-ings; Ultraviolet/ozone treatment reduces the contact an-gle to approximately 90 by changing the surface energywithout aecting morphology. Samples 2, 3, and 4A areall SH with contact angles of 140 or more. The additionof silica nanoparticles in Sample 4A does not signicantlyincrease the contact angle compared to Sample 3.

The surface area of the samples, as measured by AFM(Fig. 1), increases steadily from Sample 1 to 4A. The sur-face area of Sample 4A was approximately 46% larger thanthe initial surface area because of roughness resulting fromthe surface treatment. Samples 3 and 4A had similar sur-face areas and exhibited high contact angles characteristicof SH surfaces. So according to standard static character-ization protocols, both Samples 3 and 4 were equally SH.

08853010/$20.00 c 2006 IEEE

et al.: characterization of superhydrophobic materials 1401

TABLE IPhysical Properties of SH-Fil ms Deposited on MTSM Sensors.

Water drop onFilms and surface treatment 5 A 6 Schematic TSM sensors

1. Gold, no surface treatment 80 0%

2. SH1 lm, no surface treatment 140 17%

3. SH1 lm + HMDS 2 surface treatment 155 43%

4A. NP 3 mixed SH 1 lm + HMDS 2 surface treatment 155 46%

4B. NP 3 mixed SH 1 lm + HMDS 2 + UV 4 treatment 90 These pictures show a water droplet on the various SH surfaces studied; however, for the MTSM experiments thesensors were completely covered by a 4-mm layer of DI water. SH 1 lm:, TFPTMOS. HMDS 2 , (Y) hexamethyldisilizane,enhances hydrophobicity by replacing hydroxyl groups with trimethyl silyl groups. NP 3 , () silica nano particles (sizesbetween 2266 nm) increase the surface roughness and geometrical SH mechanism. UV 4 , Ultraviolet light treatmentreduces the water contact angle by removal of trimethyl silyl and triuoropropyl groups. 5 , contact angle measuredoptically. A 6 , increase in surface area relative to bare surface measured by AFM (%).

A network analyzer measurement system was used tomonitor the frequency response of the MTSM sensor ex-posed to air or submersed in 200 l (about 4-mm depth)of deionized (DI) water [5]. All measurements were per-formed in an air-ow controlled chemical hood at roomtemperature (approximately 25 C 0.1 C). The multi-harmonic frequency response characteristics (i.e., resonantfrequency and attenuation at rst, third, fth, and seventhharmonics) of coated MTSM sensors were measured whenthe sensors were exposed to air (dry) and when coveredwith a 4-mm layer of water (wet). The relative changesof resonant frequency of the wet and dry sensors [f rel.(1)] are plotted in Fig. 2(a):

f rel. =f dry f wet

f dry, (1)

where f dry and f wet indicate resonant frequencies of dryand wet conditions, respectively. Changes in attenuationalso were measured but are not reported here.

Two obvious phenomena can be extracted fromFig. 2(a):

Sample 4A always shows smaller f rel. than the othersamples at all harmonics.

At higher harmonics (fth and seventh), f rel. of Sam-ples 2 and 3 are greater than Sample 1, and f rel. ofSample 4A is still smaller than Sample 1.

Sample 4A exhibits much less response to water loadingat all the tested harmonics.These trends can be explained

based on the hypothesis that, although the macroscopi-cally observed contact angle of water on the SH surfacesare similar, the mechanics of interaction near the water-SHcoating interface dier. It is commonly understood that onrough SH surfaces, water doesnot wet the entire surfaceat the microscopic or nanoscopic level [4]. Rather at thelevel of the roughness, water contacts the peaks protrudingfrom the surface but does not penetrate into the valleys,which are lled with air or vapor. For the SH lms testedhere, the actual penetration depth of the water layer intothe valleys of the rough SH surface are dependent on theconditions (surface wettability) of the SH surface, withSample 4A exhibiting much less interaction (i.e., less wa-ter penetration into roughness). At the macroscopic scale,this reduced interaction could be interpreted as eectiveslip between the liquid and SH surface due to the reductionof the eective contact area [4], [8], [9].

The MTSM responses displayed in Fig. 2(a) show thatSH surfaces with similar contact angles can exhibit dier-ent mechanical interactions with water. The contact angleof water droplets on the surface of Samples 2, 3, and 4Aare similar with approximately 150 , but the response ofMTSM sensors to DI water loading of each SH sampleare dierent and are dependent on the harmonics. Dier-ent harmonics probe dierent a coustic penetration depthsinto the liquid, with higher harmonics more sensitive toliquid trapped in submicron valleys.

Sample 4B was produced from Sample 4A by treatmentwith UV light for about 30 minutes. This UV/ozone treat-

1402 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 53, no. 8, august 2006

Fig. 1. AFM images of SH lms on MTSM sensors. (a) Sample 1, surface of gold electrode on the MTSM sensor; (b) Sample 2; (c) Sample3; and (d) Sample 4A.

ment does not change the surface morphology; rather it re-places hydrophobic CH3 and CF3 groups with hydrophilichydroxyl groups via an ozone mediated, photo-oxidativeprocess. The contact angle of water on Sample 4B wasapproximately 90 (see Table I). Again, the frequency re-sponses of Sample 4B were monitored and compared withthose of the SH lm before UV treatment (Sample 4A)and the bare MTSM (Sample 1). As shown in Fig. 2(b),the relative changes of resonant frequency (frel.) of theUV treated sample (Sample 4B) in response to water load-ing approaches that of the bare MTSM (Sample 1). Thefrel. curves for Samples 4B and 1 overlap, except for therst harmonic. Although the morphology of the surface ofSample 4B is virtually identical to that of 4A, the surfacefree energy of Sample 4B is higher than 4A due to the UVtreatment as evidenced by the much lower contact angle ofwater. Higher surface free energy allows water to penetratedeeper into the valleys. Sample 4B senses the additionalmass eect from the trapped water and additional viscousdamping from the water load on the SH lms. Sample 4Bshows a similar response to Sample 1 at larger values ofresonant frequency.

IV. Summary and Conclusions

Three superhydrophobic (SH) and two control surfaceswere fabricated by ve dierent processes to produce sur-

faces with varying wettability and submicron scale rough-ness. The surfaces were characterized with three methods:the acoustic response of MTSM sensors to water loading,nano-scale surface morphology by AFM, and optical mea-surements of contact angle of water droplets. The AFMmeasured increases in surface area of 17 to 46% due toroughness of surface treatments. The dierences in datawere next supported by MTSM results, which also showedthe signicant dierences between those coatings. How-ever, the optical measurements of macroscopic contact an-gles did not detect dierences between the SH coatings.Specically, the three SH coatings (Samples 2, 3, and 4A),fabricated with and without nanoparticles and with dier-ent chemical treatments showed similar macroscopic con-tact angles; the optical method was capable of only mea-suring the dierence between less hydrophobic Samples 1and 4B and more hydrophobic Samples 2, 3, and 4A.

Thus, although the contact angles of water droplets aresimilar on all SH lms (samples 2, 3, and 4A), the acous-tic method using MTSM sensors clearly exhibited dierentresponses in each sample. It is interesting to notice thatthe MTSM responses were dependent on the harmonic fre-quency. Sample 4A showed a much smaller frequency shiftunder water loading than the other samples; this lm in-corporated three techniques to provide SH: low free sur-face energy TFPTMOS coating, HMDS surface treatment,

et al.: characterization of superhydrophobic materials 1403

Fig. 2. Relative changes in f for (a) MTSM-SH sensors at har-monics, and (b) Sample 4A (before UV), Sample 4B (after UV), andSample 1 (bare MTSM).

and addition of nano-particles in TFPTMOS. The combi-nation of these factors produces a sample SH coating withthe optimized SH properties. These MTSM results can be

interpreted as the presence of eective slip on the texturedsurface [4].

Acknowledgments

The authors are pleased to acknowledge the fundingsupport from DOE-Basic Energy Sciences, the Air ForceOce of Scientic Research, the Army Research Oce,Sandia National Laboratorys LDRD Program, and theNational Science Foundation under Grant DBI-0242622.

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