Composite Posts For Enhanced And Tunable Adhesion

Tunable adhesion is the ability for the same surface to have high adhesion under one set of conditions and low adhesion under another. It has a variety of applications, including transfer printing of micro- and nano-scale components, climbing and perching robots, and material handling in manufacturing. Approaches to tunable adhesion, including the work in this dissertation, often rely on van der Waals forces to achieve dry adhesion. Previous strategies for dry tunable adhesives have generally exploited complex fibrillar structures that are inspired by nature. The work in this dissertation investigates a different strategy for enhanced and tunable adhesion based on composite structures with simple geometries. This dissertation examines the use of composite posts, consisting of stiff insets surrounded by a compliant shell, as an approach for achieving enhanced and tunable adhesion. This composite structure has a high effective adhesion strength under normal loading and low adhesion when shear is applied. Experiments as well as finite element (FE) analysis are used to understand the mechanics of these posts under both types of loading. The adhesion of composite posts is affected by the stress distribution at the contacting surface. Homogeneous posts have concentrated stress near the edge, facilitating crack initiation, while the composite post can result in a redistribution of this stress towards the center, resulting in higher adhesion. The basic mechanics of these posts are demonstrated through experiments on mm-scale posts. The composite mm-scale composite posts have 3x higher adhesion than homogeneous posts under normal loading and shear displacement was shown to significantly decrease the effective adhesion strength. Micro-scale posts are studied and used in micro-transfer printing applications. These posts have an effective adhesion strength of 1.5 MPa, and the pull-off force of the composite post is 9x that of a homogeneous post. In both the mm-scale and micro-scale studies, the experimental results are supported by FE simulations. Arrays of micro-scale posts were fabricated and their adhesion behavior characterized. In an array, the contact of each individual post becomes less critical and can contact diverse surfaces. This work established the mechanics of composite posts for achieving enhanced and tunable adhesion

Mechanics of an adhesive tape in a zero degree peel test: effect of large deformation and material nonlinearity

International audienceThe common pressure sensitive adhesive (PSA) tape is a composite consisting of a stiff backing layer and a soft adhesive layer. A simple and common way to test how adhesive tapes respond to large shear deformations is the zero degree peel test. Because the backing is very stiff compared to the adhesive layer, the region where the adhesive layer is subjected to large shear can be hundreds of times its thickness. We use a large deformation hyperelastic model to study the stress and deformation fields in the adhesive layer in this test. We present a closed-form solution for the stress field in the adhesive layer and use this solution to determine how load is transferred from the backing layer to the adhesive. Our analytical model is then compared with finite element results, and except for a small region near the peel front, the predicted stress and deformation agree well with the finite element model. Interestingly, we find very different results from the classical linear theory established by Kaelble. In particular for large deformations, our analysis shows that the lateral stresses (parallel to the rigid substrate) are much larger than the shear stress in the adhesive layer. The discrepancy in the stress state and the deformation state with the linear theory is particularly large near the peel front, which we study with a finite element model. These new results will be very useful to interpret experiments and in particular to identify the high stress regions where failure is likely to initiate in zero-degree peel tests also called shear resistance tests in the PSA industry

Employer of Last Resort: Strategies for combating poverty

Randall Wray looks at how to solve the problem that market economies do not provide jobs for all who want to work. He argues that joblessness is usually concentrated among groups that suffer other disadvantages: racial and ethnic minorities, immigrants, younger and older individuals, women (especially female-headed households with children), people with disabilities, and those with lower educational attainment. He underlines the need to generate a broad public understanding of the responsibility of government to secure the right to employment. Development (2007) 50, 96–102. doi:10.1057/palgrave.development.1100365