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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Shear Adhesion of Tapered Nanopillar Arrays Younghyun Cho,†,§ Helen K. Minsky,‡ Yijie Jiang,‡ Kaiyang Yin,† Kevin T. Turner,*,‡ and Shu Yang*,† †
Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104, United States ‡ Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, 220 South 33rd Street, Philadelphia, Pennsylvania 19104, United States § Energy Efficiency and Materials Research Division, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea S Supporting Information *
ABSTRACT: Tapered nanopillars with various cross sections, including cone-shaped, stepwise, and pencil-like structures (300 nm in diameter at the base of the pillars and 1.1 μm in height), are prepared from epoxy resin templated by nanoporous anodic aluminum oxide (AAO) membranes. The effect of pillar geometry on the shear adhesion behavior of these nanopillar arrays is investigated via sliding experiments in a nanoindentation system. In a previous study of arrays with the same geometry, it was shown that cone-shaped nanopillars exhibit the highest adhesion under normal loading while stepwise and pencil-like nanopillars exhibit lower normal adhesion strength due to significant deformation of the pillars that occurs with increasing indentation depth. Contrary to the previous studies, here, we show that pencil-like nanopillars exhibit the highest shear adhesion strength at all indentation depths among three types of nanopillar arrays and that the shear adhesion increases with greater indentation depth due to the higher bending stiffness and closer packing of the pencil-like nanopillar array. Finite element simulations are used to elucidate the deformation of the pillars during the sliding experiments and agree with the nanoindentation-based sliding measurements. The experiments and finite element simulations together demonstrate that the shape of the nanopillars plays a key role in shear adhesion and that the mechanism is quite different from that of adhesion under normal loading. KEYWORDS: shear adhesion, tapered nanopillars, anodized aluminum oxide templates, nanoindentation, bending stiffness
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INTRODUCTION
Since HAR pillar arrays can be significantly deformed during shear adhesion experiments, it is often challenging to interpret the experimental results. Therefore, there has been less attention on the behavior of pillar arrays under shear load than normal loading. Furthermore, most studies on dry adhesion have focused on micrometer-sized and larger pillars due to the difficulty in fabrication of nanopillars as well as the lateral collapse issues originating from surface energy minimization. In addition, as the pillar diameter decreases, it is increasingly difficult to achieve uniform contact to the surface of HAR pillars during typical adhesion measurements. So far, only limited shear adhesion or friction studies have been carried out, e.g., using stiff carbon nanotubes, which nevertheless show relatively poor adhesion compared to that of nanofibrils in nature.31−35 Previously, we have fabricated tapered epoxy nanopillar arrays of three different geometries (cone-shaped, pencil-like,
The ability of geckos, flies, and spiders to adhere to a broad range of surfaces has been attributed to the high-aspect-ratio (HAR) micro- and nanofibrillar structures on their feet.1−7 The difference in dry adhesion of structured surfaces under normal and shear directions and their relative strength plays a key role in bioadhesion for attachment and detachment. For example, the locomotion of the gecko originates from the combination of direction-dependent shear adhesion and tunable normal adhesion controlled by direction and magnitude of the shear force applied.8−14 Normal adhesion is maximized when a shear force is applied onto a gecko’s foot in one direction while it decreases when the shear force is applied in the opposite direction or without shear force. This range of adhesion allows for attachment or detachment of the gecko by controlling the magnitude of normal force. There have been numerous studies on the adhesion behavior of individual pillars and structured surfaces that have investigated the effects of properties such as tip shape, tilting angle, materials properties, and hierarchy of fibrils.15−30 © XXXX American Chemical Society
Received: February 7, 2018 Accepted: March 20, 2018
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DOI: 10.1021/acsami.8b02303 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematics of the process to prepare tapered nanopillar arrays. Illustration of the (b) cone-shaped, (c) stepwise, and (d) pencil-like pillars and their dimensions. purchased from Goodfellow (Huntingdon, UK). Diglycidyl ether of bisphenol A-based epoxy resin (DER 354) and photoinitiator (Cycracure UVI 6976) were purchased from Dow Chemical (Newark, DE). Preparation of Tapered Nanopillar Arrays. Tapered epoxy nanopillars were prepared following the procedure described in our earlier publication.36 First, organic impurities were removed from the aluminum by sonication in acetone for 20 min. Then the electropolishing process was performed on the aluminum plates immersed in a mixture of perchloric acid and ethanol (1:3 v/v) at a constant applied voltage of 20 V (Agilent Technologies 6035A) for 4 min at 10 °C. The plates were subsequently hard anodized at an applied voltage of 160 V for 2 h. During the reaction, the temperature was maintained at 3 °C using a cooling bath (RTE 7, NESLAB). The aluminum oxide layer grown onto aluminum plate was chemically removed by an aqueous mixture of acid solutions (1.8 wt % of chromic acid and 6 wt % of phosphoric acid) for 8 h at 50 °C in a water bath. Then the combination of mild anodization (140 V at 10 °C) and pore etching (in a 10 wt % phosphoric acid solution at 30 °C) was carried out as desired for different pillar shapes. Epoxy resin (D. E. R. 354, Dow Chemical Co.) mixed with 3 wt % photoinitiator (UVI 6976, Cycracure) was dropped onto AAO templates and covered with a glass slide. It was UV cured (365 nm, total dosage of 17 J/cm2) for 40 min. The remaining aluminum bottom layer and AAO template were removed in an aqueous mixture of copper chloride (3 wt %) and hydrochloric acid (50 wt %) at room temperature (for aluminum bottom layer) and in phosphoric acid solution (10 wt %) at 45 °C for 2 h (for the AAO template), respectively. Shear Adhesion Test Using Nanoindentation. Sliding tests were performed on the epoxy nanopillar samples using a nanoindenter (TI 950 TriboIndenter, Hysitron Inc., Eden Prairie, MN) with a conospherical diamond tip (radius, 100 μm). The tip was brought into contact with the sample at a rate of 10 nm/s until it reached a prescribed indentation depth: 20, 50, 100, 150, 200, 250, 300, 400, and 500 nm. The tip was subsequently moved in the lateral direction for 6 μm at a rate of 0.133 μm/s. The tip was retracted from the sample at a rate of 10 nm/s. The normal and shear loads were recorded throughout the tests. The shear adhesion was obtained from the threshold of motion in the lateral force vs the lateral displacement curves (see Figures S1 and S2) and averaged over at least ten sliding tests per sample. Characterization. The morphologies of the pristine and indented nanopillar arrays were investigated using field emission-scanning electron microscopy (FE-SEM) (JEOL 7500F HRSEM). The area outside of the center indented region in the normal direction (in the middle of the sliding track) was imaged to confirm the deformed nanopillars during shear indentation.
and stepwise) templated from the anodized aluminum oxide (AAO) membranes (Figure 1). All of the pillars had a diameter of 300 nm at the base and an aspect ratio (height/diameter) of 3.67. We showed that both the normal dry adhesion and the stability of nanopillar arrays are highly dependent on the pillar shape by performing nanoindenatation experiments with loading normal in the direction normal to the surface.36 The short cone-shaped nanopillars exhibit the highest normal adhesion force and are stable even under a high load due to the uniform stress distribution throughout the nanopillars. In contrast, the normal adhesion of stepwise and pencil-like nanopillars with discontinuities in diameter along the pillar height dropped sharply when the indentation depth is greater than 150 nm. This results from fracture at the step where the pillar diameter changes and where stress is most concentrated, thus degrading the adhesion performance. Here, we investigate the shear adhesion of these tapered nanopillar arrays (coneshaped, stepwise, and pencil-like, diameter from 100 nm (top) to 300 nm (bottom) and height of 1100 nm) by performing sliding nanoindentation experiments. Our results show that the shear adhesion is strongly governed by pillar geometry; however, the trend is different from that of the normal adhesion. Pencil-like nanopillars show the highest shear adhesion compared to cone-shaped and stepwise nanopillars without a significant drop in adhesion as the indentation depths increase, whereas under normal loading the adhesion for the pencil-like nanopillars decreased with increasing depth in our previous observation.36 To confirm the experimental results and elucidate the mechanism of shear adhesion, we perform finite element simulations of the sliding process, where the indenter tip is indented to different depths in the nanopillar surface, slid, and then retracted, similar to the process used in the experiments. The simulation results corroborate the experimental observation, confirming that different deformation processes occur depending on the shape of the nanopillars and thus differences in shear adhesion.
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MATERIALS AND METHODS
Materials. Oxalic acid (98%), perchloric acid (70%), and phosphoric acid (85%), electrolytes for hard anodization, electropolishing, and mild anodization, respectively, were purchased from Sigma-Aldrich Corporation (St. Louis, MO) and used as received. High purity (99.999%) aluminum plates (25 mm in diameter) were B
DOI: 10.1021/acsami.8b02303 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
nanopillars was fixed at 1.1 μm, the diameter of the pillars at the base was 300 nm, and the center-to-center distance was maintained at 350 nm. Therefore, we only investigate the effect of pillar shape on the shear adhesion behavior. To do so, we performed sliding nanoindentation experiments and subsequently observed the morphology of the indented nanopillars by FE-SEM to investigate nanopillar deformation behaviors and their contribution to shear adhesion. A conospherical-shaped tip with a nominal radius of 100 μm was used, and specimens were indented to depths ranging from 20 to 500 nm, followed by the lateral sliding of the indenter tip while maintaining the indentation depth, providing measurements of shear adhesion between the tapered nanopillars and the indenter tip (see Figure S1 for bulk and Figure S2 for nanopillars). The overall shear adhesion response includes the static and kinetic friction of structured surfaces, and it is generally lower than that of a flat bulk sample due to the deformation of pillars during sliding and the smaller contact area.8,39 As observed in Figure S2, the sliding tests on the pillars generally exhibit (1) an increase in compressive force at the beginning of the test when the tip is indented into the nanopillar array, (2) a decrease in compressive normal force as the tip is slid and the nanopillars deform, and (3) the normal force acting on the tip is reduced and changes from compression to tension as the tip is retracted; similar behavior has been observed in other studies.39,40 The lateral force is measured throughout the sliding test on the nanopillar arrays, as seen in Figure. S2, and decreases during sliding. We believe that the oscillation for stepwise nanopillars comes from fracture at the “joint” locations where pillar sections of different diameters meet. As the indenter tip slides laterally at a fixed penetration depth (i.e., 400 nm in Figure S2), the stress on the pillars increases; when a critical stress is reached, fracture occurs. When a pillar fractures, the lateral force decreases slightly but then increases when the tip contacts the next pillars. During sliding, this process repeats, resulting in oscillation of the lateral force. On the other hand, for pencil- and coneshaped nanopillars, deformation occurs via bending rather than fracture generation, similar to the findings in our previous report.34 The shear adhesion force is defined as the threshold of motion in the lateral force vs the lateral displacement curves. The shear adhesion force varies significantly with indentation depth and the shape of nanopillars, as shown in Figure 3.
Finite Element Simulation. Finite element (FE) simulations of nanopillars under different indentation conditions were performed using ABAQUS/Explicit (Dassault Systèmes Americas Corp., Waltham, MA). The material was considered ideal elastic−perfectly plastic with properties measured from the bulk epoxy by nanoindentation. The Young’s modulus was 5.09 GPa, and the Poisson’s ratio was assumed to be 0.4. The hardness was measured to be 408 MPa. The yield stress was estimated as 1/3 of the measured hardness, i.e., 136 MPa.37 The three-dimensional continuum element, C3D6R, was used, and the models had a characteristic element length between 10 and 50 nm. The indenter tip was modeled as a rigid spherical cap with a radius of 100 μm. An array of 76 × 4 representative pillars located beneath the indenter tip was considered in each simulation. Contact elements were used to define the interaction at indenter tip−pillar contact and pillar−pillar contact. The friction coefficient was set to 0.3; this value was obtained from sliding tests on a bulk epoxy sample with an indentation depth of 20 nm. The indentation process in the finite element model mimicked the experiments. Mass scaling factors in ABAQUS/Explicit were set between 2500 and 10 000 in order to reduce the simulation time. The ratio of kinetic energy to internal energy for each simulation was verified to be small (2.14% on average for all cases). Therefore, a semistatic process was simulated with these factors. The deformation of the pillars indicated that the pillars were either plastically deformed or bent mostly along the tip in the direction of motion. Thus, the external boundaries of the pillars were free surfaces, with the exception of the fixed bottom surface.
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RESULTS AND DISCUSSION Various tapered epoxy nanopillars were prepared by infiltration of epoxy monomers into the AAO templates with different pore geometries, followed by UV curing and chemical etching of AAO template (see Figure 1).36,38 The pore shape of AAO template was controlled by the combination of anodization conditions including vertical pore growth, lateral pore widening, and the number of repeating cycles. Three different epoxy nanostructures, including cone-shaped, stepwise, and pencil-like nanopillars (Figures 1 and 2), were prepared. The height of the
Figure 2. Cross-sectional FE-SEM images of (a) cone-shaped, (b) stepwise, and (c) pencil-like AAO templates. (d−f) FE-SEM images of the epoxy nanopillar arrays templated from the corresponding AAO membranes shown in (a−c).
Figure 3. Shear adhesion between the arrays of tapered epoxy nanopillars with different geometries and the nanoindenter tip as a function of the indentation depth. C
DOI: 10.1021/acsami.8b02303 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 4. FE-SEM images of (a) cone-shaped, (b) stepwise, and (c) pencil-like epoxy nanopillars before and after sliding on the indenter tip at different indentation depths.
In our previous study,36 when indentation depth was increased, the change of normal adhesion strength of the nanopillars was highly dependent on the pillar geometry and thus deformation modes. For example, the pull-off force of the cone-shaped nanopillars gradually increased with indentation depth due to their high stability whereas the adhesion of the pencil-like and stepwise nanopillars decreased sharply at the indentation depth greater than 150 nm due to the unrecoverable mechanical deformation. The previous work showed that cone-shaped nanopillars with continuous geometric changes could accommodate the normal stress more efficiently;36 they bend under the larger indentation depth, exhibited a gradual increase in normal adhesion force at the higher indentation depth, and had the highest adhesion among the three types of nanopillars. In the present study, we also observe shape-dependent change of shear adhesion strength with the indentation depth (see Figure 3); however, the trend is different from the prior results on normal adhesion. The adhesion force of the bulk sample increased linearly with the indentation depth. This is because the normal force gradually increased with indentation depth as shown in Figure S3. For the bulk sample, indentation was performed up to 300 nm due to a limitation on the force of the instrument could apply. For the cone-shaped nanopillars, the shear adhesion force gradually increased with indentation depth up to 657 μN at 500 nm (the maximum indentation depth in our experiments) without significant decrease. The shear adhesion force of pencil-like nanopillars showed a similar trend but was always higher than that of cone-shaped and stepwise nanopillars. At 500 nm indentation depth, the shear adhesion force of the pencil-like nanopillars increased to 1438 μN, which is 3 and 10 times higher than that of the cone-shaped and stepwise nanopillars, respectively. In comparison, a significant drop in shear adhesion
force was observed for stepwise nanopillars when the indentation depth exceeded 300 nm. The transition seen in stepwise nanopillars is similar to that of normal pull-off force from our previous study,36 in that fracture occurs due to high localized stress in the regions where the pillar diameter changes. We hypothesize that the difference in shear adhesion between indenter tip and nanopillars of different geometries can be attributed to two reasons. First, when the indenter tip moves in the lateral direction at a fixed indentation depth, the bending stiffness (or elastic restoring force) of an individual nanopillar, FE, which determines the resistance to the tip movement, is highly dependent on the pillar dimension.41−43 If the pillar is viewed as a cantilever beam, the elastic restoring force provided by an individual pillar is
FE =
3πEd 4δ 64h3
(1)
where E is the Young’s modulus of the pillar material, d is the diameter of the pillar, δ is the deflection, and h is the height of the pillar at which the force is applied. Therefore, nanopillars with smaller diameter bend more easily when other parameters are fixed (see Figure S4). We note that this relation only provides basic scaling though as the pillars do not have a sufficiently high aspect ratio to be modeled as a beam. Second, depending on the nanopillar geometry, the interpillar spacing varies from pillar bottom to top even though the center-tocenter distance is kept constant. At the bottom of all nanopillars, the interpillar spacing is 50 nm regardless of geometry. At the upper positions, however, the interpillar spacing increases up to 255 nm for stepwise and cone-shaped nanopillars but remains ∼50 nm in the cylinder region (715 nm tall) for the pencil-like nanopillars. Therefore, the neighboring pencil-like nanopillars contact each other when they are D
DOI: 10.1021/acsami.8b02303 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
experiments (Figure 3), as well as unloading of the indenter tip after sliding in the FE (Figure 6), corresponding to the deformed nanopillars observations in FE-SEM (Figure 4).
significantly deformed, preventing further bending of the nanopillars at the high indentation depth. Furthermore, they have higher bending stiffness since they have much larger pillar diameter (∼300 nm) from the cylindrical base. The higher stiffness ensures that plastic deformation experienced by the pencil-like pillars is limited only to the region around the conical tip for all indentation depths (see Figure 4c), whereas the stepwise and cone-shaped nanopillars bent the entire pillar height, leading to much more substantial collapse along the shearing direction at the same indentation depth (Figure 4). As the indentation depth increased up to 500 nm, the cone-shaped pillars were plastically deformed and eventually lay down (Figure 4a). In the case of stepwise nanopillars (Figure 4b), the top section that has the smallest pillar diameter (95 nm) with a height of 366 nm bent at the indentation depth of 300 nm because of the lowest bending stiffness in this section, leading to the lowest shear adhesion compared to pencil-like and coneshaped nanopillars. At higher indentation depths, the middle section of the stepwise nanopillars (diameter, 190 nm) became deformed and were fully collapsed at the indentation depth of 500 nm. To further understand the stability of tapered nanopillars during the shearing indentation process and thus contact area and shear adhesion strength, we performed finite element (FE) simulations at different indentation depths that matched the experiments. An array of (76 × 4) pillars (total of 304) located under the sliding track of indenter tip (radius of 100 μm) was considered in each simulation. A direct quantitative comparison cannot be made between the experiments and the simulations as the simulations only consider a small fraction of the area in contact with the indenter tip. The FE simulations do, however, provide a tool to visualize the stresses and deformation in the nanopillar array. Furthermore, FE allows trends across the three different pillar shapes to be understood. To compare the simulations with experiments, we considered both loading and shearing of the pillars in the FE (Figure 5), corresponding to the measurement of shear adhesion during the sliding
Figure 6. Results of FE simulations of various tapered nanopillars (in the middle of the sliding track) after sliding at the indentation depths of 100, 300, and 400 nm, followed by unloading of the nanoindenter tip: (a) cone-shaped, (b) stepwise, and (c) pencil-like nanopillars. The images show the predicted deformation (plastic deformation remaining after unloading) of the nanopillars as well as the residual stresses in the pillars after unloading.
As shown in Figures 5 and 6, at the shallow indentation depth of 100 nm, the deformation in all of the pillars, regardless of geometry, is minimal, in agreement with FE-SEM image (Figure 4). As the indentation depth increases, more regions within the pillars reach the yield strength of the epoxy (136 MPa). As the indentation depth increases to 400 nm, the pencil-like nanopillars, which have the largest pillar diameter, show large stresses within the pillars during indenter sliding. However, because they have higher bending stiffness and the upper portion of the pillars sit closer to each other than the other two types of nanopillars, the pencil-like pillars come into contact and transfer load across the pillars (Figure 5c), thus limiting the degree of overall deformation and resulting in the largest contact area calculated from FE simulation (see Table S1). This suggests that the indenter tip should experience higher resistance to movement when sliding on the pencil-like nanopillars. Furthermore, due to the higher elastic restoring force of the cylindrical section of the pencil-like nanopillars, which is ∼6 times of stepwise- and cone-shaped nanopillars, the pencil-like nanopillars almost fully recover once the load is removed, with the exception of some permanent bending on the cone tips (Figure 6). The cone-shaped and stepwise nanopillars also recover to some degree after unloading the indenter tip (Figure 6); however, large residual stresses exist within the pillars, especially in the stepwise nanopillars that permanently deform at 400 nm indentation depth. This corresponds well with FE-SEM observation (Figure 4). FE simulation results also indicate that the pencil-like nanopillars have the highest shear adhesion force, 278 μN, compared to 96 μN (cone-shaped) and 45 μN (stepwise, see
Figure 5. Results of FE simulations of the shearing process of various tapered nanopillars (in the middle of the sliding track) during sliding of the nanoindenter tips at different indentation depths: (a) coneshaped, (b) stepwise, and (c) pencil-like nanopillars. The images show the predicted deformation (elastic and plastic deformation) of the nanopillars as well as the calculated von Mises stress in the pillars. The von Mises stress is shown as plastic deformation occurs in regions of the pillars. E
DOI: 10.1021/acsami.8b02303 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
the indentation depth. We attribute this to the higher bending stiffness and closer packing of the nanopillars. The FE simulations provide insight into the stresses in and the deformation of the nanopillars during sliding indentation and after removal of the indenter. The FE results are generally consistent with the experimental observations. The detailed investigation on the shear adhesion behavior of nanopillar arrays with different geometries offers new insights to design nanostructures with improved adhesion strength and tunability. Our study can also be extended to better understand the mechanics of epoxy nanostructures for various potential applications, including highly stable and reusable dry adhesive patches and slanted nanopillar array for anisotropic wetting as seen in butterfly wings and on water strider legs.
Figure S5) at 300 nm indentation depth. We note the absolute value of the shear adhesion force obtained from FE simulation is not accurate since it is the force carried by a relatively small number of nanopillars. However, the trends of adhesion force that are predicted agrees qualitatively with the experiments (Figure 3). Compared to pencil-like nanopillars, the cone-shaped nanopillars can bend more easily due to the larger interpillar spacing at the top and the lower bending stiffness. The cones have less support from each other as the diameter decreases from the bottom to the top of the nanopillars. Therefore, they exhibit lower shear adhesion force than the pencil-like ones at all depths. At the low indention depth (e.g., ∼100 nm), the stress is focused at the top of the pillars. As the indention depth increases, the stress moves down the pillars. Again, due to continuous geometric change along the pillar height, coneshaped nanopillars can accommodate the stress more efficiently; they bend but do not fracture,36 providing a gradual increase in shear adhesion with indentation depth. FE-SEM images show that the tops of the cone-shaped nanopillar begin to bend at the indentation depth of 300 nm. As the indentation depth increases up to 400 nm, they bend severely along the sliding direction, and the yield stress is reached over a large fraction of the nanopillars (Figure 5a). This results in significant plastic deformation of the pillars, which cannot be recovered after removal of the indenter (Figure 6a). The deformed structures observed by FE simulations (Figure 6a) agree well with FE-SEM images shown in Figure 4a. In the case of stepwise nanopillars, fracture is likely to occur at the point where the stepwise nanopillars change cross section size, an area of stress concentration, as shown in our prior study of the normal pull-off force of stepwise nanopillars.36 Although FE simulations only show a severely bent top section in the stepwise nanopillars during sliding of the indenter, it validates our analysis by observing stress distribution throughout the pillars at different indentation depths. At an indentation depth of 100 nm, the stress is localized only in the top section (Figure 5b). After unloading the indenter, the pillars nearly completely recover (Figure 6b). As the indentation depth increases to 300 nm, the stress increases in the middle section, and there is a low stress in the bottom section. At indentation depths of 400 nm, the stress in the bottom and middle sections both increase further, and a stress concentration is observed in the joint regions (Figure 5b). A similar trend is seen in the residual stress distributions in the unloaded pillars (Figure 6b). Therefore, above 300 nm indentation depth an indenter tip would experience little resistance from the severely bent or fractured pillars during sliding, resulting in a significant drop in shear adhesion.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02303. Indentation curves obtained from bulk epoxy and various nanopillars, simulated contact area between nanopillars during indentation, and simulated lateral and normal force of various nanopillars (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(S.Y.) E-mail:
[email protected]. *(K.T.T.) E-mail:
[email protected]. ORCID
Younghyun Cho: 0000-0002-8397-6126 Yijie Jiang: 0000-0002-9347-3290 Kaiyang Yin: 0000-0001-7149-2610 Kevin T. Turner: 0000-0003-4963-4568 Shu Yang: 0000-0001-8834-3320 Author Contributions
Y.C. and H.M. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work is partially supported by National Science Foundation (NSF)/MRSEC grant (#DMR-1120901). S.Y. also acknowledge the support by NSF grant, #CBET1264808. Y.C. acknowledge the support by the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B8-2461-07) and partial support by the Center for Advanced Meta-Meterials (CAMM) funded by the Ministry of Science and ICT (MSIT) as Global Frontier Project (CAMM-No. 2014063701, 2014063700). K.T.T. and H.M. acknowledge support from the NSF Award #CMMI1435745 and the Department of Education GAANN program, Grant No. P200A120237. K.T.T. and Y.J. acknowledge support from the NSF Award #CMMI- 1463344. The Laboratory for Research on the Structure of Matter (LRSM), Penn NSF MRSEC, and Nanoscale Characterization Facility (NCF) are acknowledged for access to SEM.
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CONCLUSION We studied effect of pillar geometry on the shear adhesion behaviors of tapered epoxy nanopillar arrays via sliding nanoindentation experiments and FE simulations. At indentation depths ranging from 20 to 500 nm, the stepwise nanopillars had the lowest shear adhesion compared to coneshaped and pencil-like nanopillar arrays. For the stepwise nanopillars, the shear adhesion drops significantly when the indentation depth exceeds 300 nm. Cone-shaped nanopillars showed a gradual increase in shear adhesion with increasing indentation due to the gradual change in the pillar cross section. Pencil-like nanopillars exhibited the highest shear adhesion strength at all indentation depths and increased with
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REFERENCES
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DOI: 10.1021/acsami.8b02303 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
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DOI: 10.1021/acsami.8b02303 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
