Journal Club February 2010: Mechanics of Patterned and Structured Interfaces

Welcome to February 2010 Journal Club!  This month, I look forward to a lively discussion on the mechanics of structured and patterned interfaces in which geometry is used to tailor adhesion.  Much of the work in this area has been inspired by a variety of animals and insects that have feet covered with small structures (often hierarchical and sub-micrometer in size at the end) that allow them to strongly adhere to a broad range of surfaces.  The best known example is the Tokay Gecko (Gekko gecko), which can achieve an adhesion force much greater than its body weight and can quickly form and separate adhesive contacts between their feet and a range of smooth and rough surfaces.  In the journal club this month I do not want to delve into the mechanics of Gecko adhesion specifically, but instead would like to highlight a few papers that elucidate the basic mechanics of the adhesion of patterned interfaces through modeling and experimental work on structured interfaces.

The essential problem that is considered in much of this work is the adhesion of two elastic bodies, one of which has a surface that is patterned or structured.  The pattern on the surface may consist of shallow grooves that disrupt crack propagation but do not alter the compliance or, more commonly, may be arrays of fibrillar or high-aspect ratio structures that change both the contact area and compliance.  The structured surface is typically a polymer (E~1-10 MPa) and is usually contacted to a flat glass, silicon, or sapphire surface that is much stiffer (E~70-400 GPa).  The adhesion is generally non-specific (e.g., van der Waals forces) and the two bodies adhere after being brought into contact at room temperature.  Adhesion enhancement due to the presence of structuring is typically measured by contact probe adhesion tests in which a sphere or flat punch is contacted and then retracted from a surface while measuring the force.  

The first two papers that I suggest are short review papers from the MRS Bulletin.  These papers are a quick read and give a nice overview of the basic mechanics and some experimental measurements of engineered structured interfaces.  If you only have 30 min to devote to Journal Club this month, I suggest you start with these.

E. Chan, C. Greiner, E. Arzt, and A.J. Crosby, MRS Bulletin, 32, pp. 496-502 (2007)
This paper reviews the basics of adhesion enhancement due to patterning and reviews a number of key experimental measurements in the field.   The authors specifically discuss the role of aspect ratio of the structures at the interface and note that there are multiple benefits to high-aspect-ratio structures: (1) such features reduce compliance and in turn improve the adhesion to rough surfaces by allowing for greater contact through elastic accommodation, (2) such features reduce the stress concentration along the interface and effectively “blunt” the crack.   The authors also review a series of experimental measurements and note that choosing a quantity to describe the adhesion is not straightforward and that multiple values, including separation force, contact strength, and an effective work of adhesion, are frequently used (see Table 1 in paper).
 

A. Jagota, C.-Y. Hui, N.J. Glassmaker, and T. Tang, MRS Bulletin, 32, pp. 492-95 (2007)
This paper discusses the essential mechanics concepts in bioinspired and biomimetic fibrillar interfaces.  The authors itemize a number of key mechanics phenomenon that play a role in controlling the adhesion of fibrillar interfaces (see Table 1 in paper).  Specifically, they note that fibrillar interfaces can be stronger than a flat surface because the features can (1) arrest and blunt the crack, (2) create more equal load sharing in the material near the crack tip, and (3) increase energy dissipation during failure.  Furthermore, they also note that higher aspect ratio features generally lead to a higher adhesion because of a lower stress concentration, but that high-aspect ratio features also are more prone to buckling and lateral collapse, which are detrimental to adhesion.

The two papers above provide a broad perspective on the field and introduce a number of key concepts in the behavior of structured interfaces.  The two papers given below are more recent reports that highlight some of the additional issues and unique features of structured interfaces.

A.V. Spuskanyuk, R.M. McMeeking, V.S. Deshpande, and E. Arzt, Acta Biomaterialia, 4, pp. 1669-76 (2008)
The shape of the tips of the structures in a fibrillar interface can strongly affect adhesion.   The structures at the end of the gecko toes terminate in spatulae and a number of researchers have fabricated structured interfaces that consist of mushroom-shaped or film-terminated pillars.  The presence of a terminating structure that is larger than the pillar size greatly increases the design space for engineered fibrillar interfaces.   In this paper, finite element analysis is used to investigate the stress distribution in mushroom-shaped and simple punch-shaped pillars.  A key finding of this work is that the mushroom-shaped pillars are much less sensitive to the presence of edge defects than a punch shaped-pillar due to the lower stresses at the edge in the mushroom-shaped structures.

M. Murphy, B. Aksak, and M. Sitti, Small, 5, pp. 170-5 (2009)
This final paper is included to provide an example of a complex engineered structured interface.  A unique fabrication approach (see Fig. 6 in the paper) has been used to realize arrays of angled pillars with angled mushroom tips.  The experiments demonstrate that such complex structures can be used to create interfaces that exhibit different responses based on the applied loading.  For example, in Fig. 4 of the paper, the authors show that the adhesion force of their system can be controlled by the direction of a shear displacement that is applied in their detachment experiments.

These four papers provide a sampling of the research in a field that is active and growing.  Please feel free to suggest other references and to initiate discussion on any of the topics covered here.