Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)
Article
Composite Micro-Posts with High Dry Adhesion Strength Helen K. Minsky, and Kevin T. Turner ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Apr 2017 Downloaded from http://pubs.acs.org on April 30, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
Composite Micro-Posts with High Dry Adhesion Strength H.K. Minsky and Kevin T. Turner∗ Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia E-mail:
[email protected] KEYWORDS: tunable adhesion, dry adhesion, transfer printing, composites, microscale Abstract Interfaces with enhanced and tunable adhesion have applications in a broad range of fields, including microtransfer printing of semiconductors, grippers on robots, and component handling in manufacturing. Here, a composite post structure with a stiff core and compliant shell is used to achieve enhanced adhesion under normal loading. Loading the composite structure in shear significantly reduces the effective adhesion strength, thus providing tunability. The composite posts can be used as stamps in microtransfer printing processes or as building blocks of large area tunable surfaces composed of arrays of posts. Experimental measurements on composite posts with diameters of 200 µm, show a peak adhesion strength of 1.5 MPa, a nine times enhancement in adhesion relative to a homogeneous post under normal loading, and that the adhesion can be reduced by nearly a factor of seven through the application of shear. The adhesion behavior of these composite structures was also examined using finite element analysis, which provides an understanding of the mechanics of detachment. Finally, the composite adhesive posts were used as stamps in a microtransfer printing process in which 5 µm thick silicon membranes were retrieved and subsequently printed. ∗ To
whom correspondence should be addressed
1
ACS Paragon Plus Environment
ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Paragon Plus Environment
Page 2 of 19
Page 3 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
setae in synthetic structures. Previous work has focused on achieving enhanced adhesion by tailoring the geometry of individual posts or fibers 5,16–18 and by exploiting hierarchical structures. 6,19 Many bioinspired adhesives have fibers with a flared end (i.e., mushroom shape) to alter the stress distribution at the interface and reduce the stress around the edge. 20 Reduced stress at the edge of a post makes it more difficult to initiate an interface crack and thus increases the pull-off force. The composite post geometry investigated here has only recently been investigated, 10,21 however there are related examples in nature. For example, the ladybird beetle’s setae have a gradient in material properties where the tips are significantly more compliant than their stiff base. 22 While the composite posts presented here require a two material fabrication process, the absence of reentrant features, which are common in many bioinspired adhesives, makes the geometry amenable to large-area and high volume manufacturing processes based on molding. Tunable adhesion has been realized through fibers with asymmetric geometries 9,19,23,24 or through other means, including changing the contact area, 11,25 wrinkling 26 and magnetization. 27,28 Shear loads have been used to tune the adhesion of arrays of bioinspired asymmetric posts 6,9 and to facilitate detachment in microtransfer printing processes. 12,29 In general, a shear load applied on the top of a post or fibril produces a shear force and moment on the adhered interface. The shearinduced moment is particularly significant in many cases and causes the tensile normal stress along one edge of the post to increase, while reducing the tensile stresses on the opposite edge. The high stress along one edge facilitates crack initiation and propagation and reduces pull-off force. 10
Mechanics Modeling In the experiments, we investigated the adhesion of composite posts with circular and square crosssections. The composite posts are 45 µm tall and have a radius of 100 µm if they are circular or have a half side length of 100 µm if they are square. The ratio of the core radius, Ri , to the overall radius, R, is fixed at 0.9. The ratio of shell material under the inset, h, to post radius, R, is varied from 0.13 to 0.45, where h/R = 0.45 is a homogeneous post made entirely of the compliant material.
3
ACS Paragon Plus Environment
ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The shell material, polydimethylsiloxane (PDMS), has a Young’s modulus of Es = 2.1 MPa and a Poisson’s ratio of νs = 0.49, 30 while the core, which is made of SU-8, has a Young’s modulus of Ec = 2.0 GPa and a Poisson’s ratio of νc = 0.35. 31 Finite element (FE) simulations (Abaqus 6.9, Providence, RI), described in the methods section, were used to investigate the effect of post geometry on the stress distribution at the interface and single post pull-off forces of composite posts. The stress distribution at the adhered interface plays a critical role in the initiation of a crack at the interface and, thus, the pull off force. For a homogeneous post (Ri /R = 1.0) that has a sufficiently large height to radius ratio, H/R, the stress is significantly higher at the edge of the post than in the center (Figure 2). For the two homogeneous posts examined here with the largest h/R ratios (h/R = 0.4 and h/R = 2.0), the stress is clearly highest at the edge. For h/R = 2.0, the stress is high at the edge and the stress in center region of the post is uniformly low. For h/R = 0.4, the peak stress is still at the edge, however the reduced aspect ratio leads to an increase of the stress in the center region. When the homogeneous post has lower aspect ratio (h/R = 0.1), the stress in the center is similar to that of the h/R = 0.4 case, but the stress in the region just inside of the edge is lower than it is in the h/R = 0.4 case. The stress rises at the very edge due to the corner singularity. As these stress distributions are found through FE simulations and there is a singularity at the edge, the actual stress value at the edge of the post is dependent on the mesh size, however the overall shape of the distribution provides insight into the effect of geometry on composite post design. When comparing an inset post (Ri /R = 0.9) to a thin homogeneous post (Ri /R = 1.0), both with h/R = 0.1, the general shape of the stress distribution for both posts is similar, however the stress in the near edge region is lower for the inset post than it is for the homogeneous post. The pull-off force depends on both the stress as well as the presence of defects at the edge of the post. The reduced stress in the near edge region of the inset post (Ri /R = 0.9) may lead to enhanced adhesion over the homogeneous post (Ri /R = 1), but this depends on the defect distribution at the edge of the post. Finally, we note that if the radius of the inset is too small (e.g., Ri /R
