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In situ Measurement of the AdhesionStrength and Effective Elastic Stiffness ofSingle Soft MicropillarJi Yeong Lee a b , Won Kyung Seong a , In-Suk Choi c , Ranajay Ghoshd , Ashkan Vaziri d , Cheol-Woong Yang e , Kwang-Ryeol Lee a &Myoung-Woon Moon aa Institute for Multi-disciplinary Convergence of Matter, KoreaInstitute of Science and Technology , Seoul , Republic of Koreab Advanced Analysis Center, Korea Institute of Science andTechnology , Seoul , Republic of Koreac High Temperature Energy Materials Research Center, KoreaInstitute of Science and Technology , Seoul , Republic of Koread Department of Mechanical and Industrial Engineering ,Northeastern University , Boston , Massachusetts , USAe School of Advanced Materials Science and Engineering,Sungkyunkwan University , Suwon , Republic of KoreaAccepted author version posted online: 28 Apr 2014.Publishedonline: 17 Oct 2014.

To cite this article: Ji Yeong Lee , Won Kyung Seong , In-Suk Choi , Ranajay Ghosh , Ashkan Vaziri ,Cheol-Woong Yang , Kwang-Ryeol Lee & Myoung-Woon Moon (2015) In situ Measurement of theAdhesion Strength and Effective Elastic Stiffness of Single Soft Micropillar, The Journal of Adhesion,91:5, 369-380, DOI: 10.1080/00218464.2014.908123

To link to this article: http://dx.doi.org/10.1080/00218464.2014.908123

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In situ Measurement of the Adhesion Strengthand Effective Elastic Stiffness of Single Soft

Micropillar

JI YEONG LEE1,2, WON KYUNG SEONG1, IN-SUK CHOI3,RANAJAY GHOSH4, ASHKAN VAZIRI4, CHEOL-WOONG YANG5,

KWANG-RYEOL LEE1, and MYOUNG-WOON MOON1

1Institute for Multi-disciplinary Convergence of Matter, Korea Institute of Science and

Technology, Seoul, Republic of Korea2Advanced Analysis Center, Korea Institute of Science and Technology, Seoul,

Republic of Korea3High Temperature Energy Materials Research Center, Korea Institute of Science and

Technology, Seoul, Republic of Korea4Department of Mechanical and Industrial Engineering, Northeastern University,

Boston, Massachusetts, USA5School of Advanced Materials Science and Engineering, Sungkyunkwan University,

Suwon, Republic of Korea

We report the deformation behavior and mechanical properties ofa polymeric micropillar, which measures approximately 10 lm by30lm in size by measuring the loading=unloading response usingan in situ force measurement system. When the single poly(dimethylsiloxane) (PDMS) micropillar was subjected to com-pression, we observed a periodic wrinkle and global (Euler) buck-ling at the sidewall. During unloading, we found the pull-off force(adhesion force) to increase for higher values of preloading andalso for lower loading=unloading rates. From the slope of theload–displacement curves measured in situ, we calculated theeffective elastic stiffness of the PDMS micropillar to be about2.03MPa. In addition to the current work, we report that thismethod can be used more broadly for in situ measurement of the

Received 3 December 2013; in final form 21 March 2014.Address correspondence to Myoung-Woon Moon, Institute for Multi-disciplinary

Convergence of Matter, Korea Institute of Science and Technology, Hwarangno 14-gil, Seong-buk-gu, Seoul 136–791, Republic of Korea. E-mail: mwmoon@kist.re.kr

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gadh.

The Journal of Adhesion, 91:369–380, 2015

Copyright # Taylor & Francis Group, LLC

ISSN: 0021-8464 print=1545-5823 online

DOI: 10.1080/00218464.2014.908123

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intrinsic mechanical and adhesion properties of polymers andother relatively soft materials.

KEYWORDS Dry adhesion; Dynamic mechanical analysis;Mechanical properties of adhesives; Microscopy; Polymers

1. INTRODUCTION

A highly efficient natural design for smart adhesives can be found on gecko’sand some spider’s footpad that have high-density hierarchical fibril structures[1]. The footpad of gecko is covered with hundreds of hair-like structures,which are responsible for the directional adhesion and friction properties,allowing the organism to move rapidly on almost any surface [2]. This naturalfunction has been mimicked for practical applications using a combination oftop-down technologies such as lithography and responsive polymer materialswhich enable to fabricate microstructured pillar arrays with switchableadhesion. The performance of adhesives is related to the actual value ofYoung’s modulus, but fibrillar structures overcome this relationship and areable to increase adhesion using fibril made of stiff polymers [3]. This designcan be incorporated into a wide array of applications such as pick-and-placemanipulators, transfer printings, and biomedical smart patches [4–7]. The geo-metrical and mechanical properties of micro and nanopillars, that are theconstituents of these high-performance dry adhesives, have been investigatedin the past using atomic force microscopy (AFM), indentation equipment, andcustom-made equipment for adhesion strength measurements [8–11]. How-ever, these methods generally estimate the properties based onmeasurementson multiple, and in some cases hundreds and thousands of pillar, rather thandirect measurements on single stand-alone pillar thereby inevitably yieldingsomewhat overall or average values [9–12]. However, in situ measurementof the single micropillar behavior would be essential to achieve optimizedadhesion performance as well as proper evaluation of mechanical propertiesof dry adhesives not provided in detail by the abovementioned techniques.

In this study, we utilized an in situ experimental method for both directin situ observation of the deformation behavior and measurement of theadhesion force and mechanical properties of a single poly(dimethylsiloxane)(PDMS) micropillar under uniaxial loading and unloading. To this end, theexperiments were performed using a force measurement system (FMS)equipped with a flat Si microcantilever (FMT) spanning 40mm in width and400mm in length to compress the micropillars. The FMS was installed in thestage of dual-beam focused ion beam (FIB) to allow in situ observation ofthe deformation and contact. Several factors of preload and loading–unloadingrates of the PDMS pillar were considered to characterize the deformation andadhesion behavior of pillars. The direct experimental results were analyzed

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using the mechanics of deformation of a single pillar to understand theadhesion mechanism. It is noteworthy that we selected PDMS as it is widelyused in dry adhesives due to chemical inertness, optical transparency, andrelatively low surface free energy, which allows the stamp to release fromthe printed structures. However, the measurement method used in this studyis applicable in measuring the behavior and properties of pillars made of otherpolymers and soft materials as well.

2. MATERIALS AND METHODS

2.1. Fabrication of PDMS Micropillars

We fabricated a micropillar array on the surface of PDMS using the softlithography method. PDMS networks were prepared by mixture of elastomerand cross-linker in mass ratios of 10:1 (Sylgard-184, Dow Corning, Midland,MI, USA). The mixture was poured on a negative-patterned Si mold. Thetrapped air bubbles were removed in a vacuum chamber. The samples werecured on a hot plate at 75�C for 75min, resulting in cross-linked PDMSnetwork with straight micropillars. The diameter and length of the PDMSmicropillars are about 9.8 mm and 30mm, respectively. The spacing betweentwo micropillars is about 10mm. We prepared the flat tip shape of PDMSmicropillars for optimizing the strongest adhesive properties as reportedin previous study [13].

2.2. Force Measurement System

We carried out the in situ measurement of the adhesion force of the PDMSpillars using a combination of FMS (Kleindek, Reutlingen, Germany) andnanomanipulator (MM3A, Kleindek) installed on the stage of the FIB chamber.The FMS consists of a cantilever, a resistor bridge, an amplifier, an analog out-put, and a display. The resistor bridge and other components are integratedinto the cantilever and the FMS controller, respectively. The FMT is coveredwith piezo-resistive materials. When it contacts the sample, which is one ofthe components of the resistor bridge, the material generates an electricalsignal. By applying a voltage to the bridge, the change of the resistance isconverted into voltage variations. The potential difference induced by thechanged resistance is measured by means of the galvanometer and conse-quently translated into the displacement of the cantilever using an electricalconversion factor [14–16]. The force resolution and sensitivity of FMT are1 nN and 3.1� 10�3mV=nm, respectively.

We carried out the mechanical experiment by the displacement controlmethod. By increasing the strain (displacement control), the yielding in incre-ments of the force (stress) reveals instability attributed to dissipation of storedstrain energy. The force-induced deformations of the sensor were then

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converted into actual force values via the elastic properties of the cantilever[17,18]. We used the flat plane of the FMT for measuring the mechanicalbehavior of a single PDMS micropillar. The flat FMT was vertically loadedon the top surface of single PDMS micropillar. Nanomanipulator was usedto move the position of FMT for measuring the mechanical properties of a sin-gle PDMS micropillar using piezoelectric motor. The two ways of the manipu-lator were fine mode and coarse mode by controlling the moving speed. Theloading rate of the FMT with a resolution of 1 nN� 80 mN was automaticallycontrolled at constant speeds of 0.47 and 2.5 mm � s�1 to investigate the effectsof loading rate.

2.3. Adhesion Measurements

To carry out the adhesion measurements, FMS was employed in the stage ofthe FIB chamber as shown in Fig. 1. The PDMS micropillar samples werecut to a size of 0.25 cm2 and attached on the aluminum sample stub by usingsilver paste as shown in Fig. 1(a) and 1(b). This small size was proper to pre-vent the charging effect of PDMS during scanning electron microscope (SEM)observation without any metal coating. The compression and tensile tests of asingle PDMS micropillar were performed at room temperature and 4� 10�5

torr of vacuum. The PDMS micropillars were vertically loaded to the e-beamdirection for observing the buckling phenomena by in situ, as shown inFig. 1(c)–1(e). During the loading and unloading procedure, we simul-taneously observed and recorded the deformation behavior of a PDMSmicropillar using a SEM installed in the FIB system.

FIGURE 1 The side view (a) and top view (b) of chamber images with FMS system inFIB. (c) Schematic diagram of red rectangle box in (b). The speed and direction of FMT areautomatically controlled by piezomotor.

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Figure 1(c) shows the schematic diagram of the measurement process.The FMT is first attached to the top surface of the single PDMS pillar andthereafter moved downwards with a constant speed during the loading stage.This cantilever movement was reversed during the unloading phase. Interest-ingly, during the unloading phase, the pillar stretches more than the length ofthe pristine pillar. The increased length is caused by the pull-off force due tothe adhesion force between the flat FMT and the PDMS pillar head.

2.4. In situ Measurement

The deformation and the effective elastic stiffness of an individual PDMS pillarwere characterized during the loading–unloading process using a FMT in theFIB by in situ. This loading–unloading sequence can also be depicted usingthe time–force curve, as shown in Fig. 2(a). The force in Fig. 2(a) correspondsto the FMT moving continuously at a constant loading and unloading rate of0.47mm=s. As seen in Fig. 2(a), when the force reached �8 mN, the load wasset to hold at this value called the preload for 60 s. Thereafter, the unloadingprocess started, and the force was found to transit from negative to positivesign since the pristine length of the pillar was recovered. In the final stageof the unloading, a strong adhesion force of more than 6 mN was measuredwith an increase in the size of the PDMS micropillar, which is attributed tothe adhesion strength between the FMT and the top surface of the PDMSmicropillar. The magnitude of this maximum force is called the pull-off force.

FIGURE 2 (a) Time–force curve showing the loading–unloading sequence; loading, preloadvalue, holding time, unloading, and pull-off force. Sequential SEM images by in situ measure-ment during loading process (b)–(e), unloading process (f)–(g), pull-off force process (h)–(j).

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In addition to the force measurements, in situ sequential SEM imageswith a side view were also recorded to analyze the overall mechanicalbehavior of a single PDMS pillar during the loading–unloading process. In situsequential SEM images were recorded during measurement of the time–forcecurve, as shown in Fig. 2(b)–2(j). The wrinkles are formed like as bamboostructure at the side surface of PDMS pillar, as shown in Fig. 2(c) due to thecompression force. The wrinkle direction is inversely changed at high preloadvalue, as shown in Fig. 2(e). This direction of wrinkle is recovered afterunloading process, as shown in Fig. 2(h). The length of PDMS pillar after forcemeasurement is recovered to the pristine PDMS pillar, as shown in Fig. 2(i).

3. RESULTS

The pull-off behavior of the micropillar was characterized pull-off strengthwhich is defined as pull-off force per unit actual contact area. Figure 3(a) plotsthe pull-off strength across a range of preloads. From Fig. 3 it is clear that thepull-off strength showed a marked dependence on the loading–unloadingrate. In Fig. 3, preload was varied from 1 to 16mN at two different loading–unloading speeds of 0.47 and 2.5 mm=s. Figure 3(a) indicates that for a given

FIGURE 3 (a) Average pull-off strength as a function of loading rate. (b) Time–force curveof a single PDMS pillar depending on the holding time. (c) Pull-off force measurement of asingle PDMS pillar as increasing the preload value. The holding time duration was fixed to1min. (d) Average pull-off force as a function of preload value.

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loading rate, the pull-off strength steadily increased with an increase in thepreload till a critical value of preload was reached after which any increasein preload caused a marked decrease in the pull-off strength. The rapidreduction of pull-off strength can be explained by the global (Euler) bucklingof micropillars when the preload is higher than the critical value, over whichthe global buckling induces the adhesion loss of the contacting area betweenthe pillar head and the FMT. This maximum value of pull-off strength wasfound to be about 9N=cm2 for a loading rate of 0.47mm=s and 4N=cm2 fora loading rate of 2.5mm=s. This set of measurement thus indicates anotherimportant characteristic of this system, which is a pronounced dependenceof the pull-off strength on the loading rate. Thus, when the sensor approachedthe PDMS pillar with a low speed of 0.47mm=s, the pull-off strength wasalmost two or three times larger than that of high speed of 2.5mm=s, as shownin Fig. 3(a). It was suggested that the loading rate plays also a critical role ininfluencing the adhesion strength due to the viscoelastic behavior of softpolymeric material [19]. We also note that the maximum pull-off strengthof a single PDMS pillar as noted above of about 9N=cm2 (for the lowestloading rate), is higher than general values of polyimide hairs (3N=cm2)and polyurethane (5N=cm2), which are measured by flat surface of AFMcantilever and by glass sphere, individually [19,20].

Interestingly, as shown in Fig. 3(b), we found that when the preload isheld constant at �20mN, the pull-off force increased steadily with a corre-sponding increase in the holding time from 30 s to 1min thereafter saturatingat 10mN in spite of increasing the holding time up to 3min. Thus, in the sub-sequent set of experiments, the holding time was fixed at the saturation valueof 1min when inquiring into other mechanical characteristics of the system.

Furthermore, Figs. 3(c) and 3(d) show the preload dependence of thepull-off force with a holding time of 1min when the loading=unloading rateis 0.47mm=s. Figure 3 indicates that when preload value increased, the pull-offforce also increased till it saturated at about 14mN. We carried out the multipleexperiments for measuring the preload dependence of the pull-off force witha holding time of 1min. The pull-off force was saturated at about 16 mN andthen decreased, as shown in Fig. 3(d).

To facilitate the mechanical analysis, a set of high-resolution SEM imagesof a single micropillar is depicted in Figs. 4(a)–4(c). The PDMS substrate withstraight micropillars was tightly fixed on the SEM holder using a silver paste.The FMT was fully covered and vertically loaded to the top surface of a singlePDMS pillar. These figures indicate that as the FMT moved downwards ata constant speed, the side surface of the PDMS pillar initially formed wrinklesindicated by yellow arrows of Fig. 4(a). The appearance of such pronouncedwrinkle formation on the surface of the PDMS pillar can be attributed toa rather shallow stiff skin formed on the outside pillar, while the insideremains soft [21]. The formation of this stiff skin on the surface is primarilydue to the natural oxidation processes during curing and handling of

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the system. As the preload value increased, approaching a critical value, thePDMS pillar underwent Euler buckling with bending, as shown in Fig. 4(b).This deformation state was gradually relieved as unloading proceeded till theoriginal pristine length was reached. Thereafter, as discussed earlier, due tothe strong adhesion force between the FMT and the PDMS pillar, the lengthof the PDMS pillar increased to more than its original length as shown inFig. 4(c). When the FMT was detached from the PDMS micropillar, the lengthof the PDMS micropillar reverted to the original pristine length of the micropil-lar. This deformation phenomenon gives rise to characteristic pull-off behavioras well as sharp variations in mechanical stiffness of the system.

The global buckling of an individual micropillar observed during theexperiment can also be used to obtain an estimate of the elastic modulusof the pillar material. To this end, we utilized the load traction–strain relation-ship plotted in Fig. 5 at loading rate of 0.47mm=s. The stress is defined as theapplied load per unit area of the pillar and the strain as the displacement perunit original length. From Fig. 5, we can see that at strain of about �20%, theslope of the curve which is a measure of the instantaneous effective elasticstiffness of the pillar changes roughly to a much lower value, indicatingthe onset of buckling. The slope of this load traction–effective elastic stiffnesscurve before this point is almost a constant and can serve as an excellentapproximation of the effective elastic modulus of the pillar material. Thus,from Fig. 5, the average value of the effective elastic modulus at strains lowerthan the onset strain of Euler buckling of the PDMS pillar can be estimated asabout 2.03MPa, which is within the average value of PDMS pillar arraysmeasured by the AFM method [19,22]. The mechanical properties of the

FIGURE 4 (a) High-magnification SEM images of local wrinkle formed at low preload value,(b) global buckle or Euler buckle formed at high preload value, and (c) stretched PDMS pillar (indi-cated by red arrows) due to the pull-off force. Yellow arrows in (a) and (b) show the wrinkles.A yellow arrow in (c) shows partial detachment between the cantilever and PDMS top surface.

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PDMS are estimated in a typical experiment such as AFM, sphere ball byaveraging the response of multiple micropillars including collapsing,bending, and tilting against neighbor pillars [19–22].

The instantaneous effective elastic stiffness can also be plotted as a func-tion of the strain to highlight effective elastic stiffness variations of the indi-vidual pillars. By using this in situ method, we observed the detailed changeof the effective elastic stiffness of single micropillars during compressive andtensile stress. This behavior is depicted in the inset of Fig. 5, which plotseffective elastic stiffness of the pillar with strain when the loading rate is heldconstant at 0.47mm=s. Figure 5 clearly indicates three distinct zones of effec-tive elastic stiffness: Zone (i) which is observed right after the micropillar hasregained its pristine length and before the FMT detachment. In this zone, theaverage effective elastic stiffness is 2.92MPa, which is somewhat higher thanother regions likely due to the van der Waals force during stretching byadhesion force between the FMT and PDMS pillar head; Zone (ii) whichcorresponds to the initial stage of the loading and the unbuckled stageof the unloading process. Here, the average effective elastic stiffness is2.03MPa, which is similar to elastic modulus of the material measured underuniaxial tension [23]; and finally, Zone (iii) which encompasses the Eulerbuckling regime of the pillar and expectedly exhibits the lowest averageeffective elastic stiffness of 0.64MPa due to the buckling of the micropillar.Thus, the inset of Fig. 5 serves as a useful plot to obtain the effectiveelastic modulus of the micropillar material as well as delineate the variousdeformation zones of the micropillar.

FIGURE 5 The three cycles of stress–strain curves were obtained in a single PDMS micropillarby continuously changing the preload value of 22 (circle), 26 (rectangle), and 29 (triangle) mN,which correspond to the compressive strain of strains of 23%, 31%, and 39%, respectively.Inset shows the effective elastic stiffness (K) of single PDMS micropillar curve as a functionof the strain.

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4. DISCUSSION

We estimated the saturated value of the pull-off force by the JKR model-(Johnson, Kendall, and Roberts) based contact splitting theory [24]. By takingthe elastic modulus (E�) and the pull-off force (P) of the micropillar, theadhesion energy (c) of the micropillar on the FMTwas estimated. By consider-ing the micropillar as a flat punch with a radius of R (4.9 mm) and the FMTas a counter surface, we used the mechanical approach by Kendall [23]to calculate pull-off force:

P ¼ �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi8pE�R3c

p: ð1Þ

We took the elastic modulus E� corresponding to zone-(ii) effective elasticstiffness, i.e., E� ¼ EZone(ii)¼ 2.03MPa as discussed earlier and the typicalvalue for the adhesion energy for the PDMS against Si as c¼ 0.045 J=m2

[23,24]. Above these values in Equation (1), we get the theoretical pull-offforce between the PDMS micropillar and the FMS cantilever as 16.9mN. Thus,this theoretical estimate is slightly higher than our results of 14mN. Note thatthis theoretical estimate is affected by any change in the effective contact areabetween the punch and the pillar which likely takes place in the preloadrange (>20mN) where Euler buckling is present, as shown in Fig. 4(b) andalso reported previously [25].

5. CONCLUSION

In this paper, wemeasured the adhesion or pull-off strength and effective elas-tic stiffness of a single PDMS micropillar by analyzing the loading–unloadingbehavior using an in situmethod for robust measurement. Under compressiveloading of a FMT on a single PDMS micropillar, a periodic wrinkle at the pillarsurface and global buckling of the pillar were observed in situ. The pull-offforce was measured to increase up to 14mN for a high preload of 11mN and9 mN for a low-loading rate of 0.47mN=s, which is close to the predicted valueby using the modified JKR theory. The loading rate plays also a critical role ininfluencing the adhesion strength due to the viscoelastic behavior of soft poly-meric material. The load–displacement curve measured in situ was convertedinto a traction–strain curve which showed three distinct regions of loadingcorresponding to the different states of deformation of the pillar. Identifyingthe linear elastic region and calculating the slope of this curve in this regiongave a proper estimate of the effective elastic modulus for a PDMS micropillarmaterial which was found to be 2.03MPa. The effective elastic modulus curveof a PDMS micropillar had the deformation behavior to three parts: (i)adhesion force region with the van der Waals force, (ii) continuous compress-ive or tensile region, and (iii) Euler buckling region. The effective elastic

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stiffness which was defined as the slope of this stress–strain curve varied from0.64MPa as global buckling set in to 2.92MPa, corresponding to the van derWaals force and adhesive force.

Until now, even though several techniques have been developed formeasurement of adhesion strength and mechanical properties, the dynamicbehavior of the structures or materials under loading–unloading procedurewas not easy to observe or analyze in situ. This technique provides an advan-tageous method for in situ quantitative measurement of adhesion strength ofrelatively soft pillars and as well as the effective elastic modulus, which canbe used for enhancing the functionality of micropillar structures developedfor smart adhesives used in bioinspired machines and clean transportation.

FUNDING

We acknowledge the financial support of grants from internal project of KISTand the National Research Foundation of Korea (NRF) grants funded bythe Korean government (MSIP) (nos. 20110030058, 20110017257, and20110019984). AV and RGh acknowledge the support from NSF CMMI grantno. 1149750.

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