Abstract
Janus-faced pillar structures are a new class of bioinspired dry adhesives based on the exploitation of unique physical and chemical properties when two different materials are placed on two opposite faces. Using Janus-faced high aspect ratio (HAR) pillars, it is possible to control the geometry and surface chemistry that are key factors in designing gecko-inspired artificial dry adhesives with high performance. In this review, first, we revisit the conventional fabrication techniques for homogeneous, angled HAR structures. Second, we present the fabrication methods for Janus-faced micro and nanopillars via physical and chemical modification of one of the faces. Third, we discuss the geometric features of Janus-faced structures with bending mechanisms of the pillars and the selection of contacting faces. Finally, we cover the theoretical and practical viewpoints of adhesion behavior with a particular focus on the role of work of adhesion, tilt angle, and adhesion hysteresis. We emphasize that Janus-faced structures potentially provide powerful systems to control anisotropic adhesion behavior by taking advantage of both physical and chemical aspects of the angled structures.
Acknowledgments
We gratefully acknowledge support from National Research Foundation of Korea (NRF) grant (No. 20110017530), WCU (World Class University) program (R31-2008-000-10083-0), and Basic Science Research Program (2010-0027955). This work was also supported, in part, by the Korea Research Foundation Grant (KRF-J03003), Korea Institute of Science and Technology (KIST) Internal Project, and the Institute of Biological Engineering of Seoul National University.