Meso-scale adhesion testing of integrated micro- and nano-scale structures

Abstract

Many insects and lizards display the amazing ability to climb and stick to just about any surface. Recent research has honed in on these systems to better understand how they work, particularly on how fine sub-micron hairs enhance van der Waals, or other short-range interactions. Provided enough intimate surface contact these “weak” forces can add up to produce significant amounts of adhesion. Additionally, the attractive interaction must be much larger than repulsive forces, due to elastic deformation, for the adhesive to be effective. To achieve this, nature has created a hierarchal structure to conform over a range of size scales. In previous work, micro-fabrication techniques were used to create a synthetic dry adhesive modeled after the fine hair adhesive motif found in nature. The artificial structure consists of a silicon dioxide platform, covered with organic looking polymeric nanorods (“organorods”), and supported by a single single-crystal silicon pillar. The multiscale integrated compliant structures (MICS) offer three levels of surface compliance: (1) the organorods on the surface (also necessary for enhancing surface adhesion), (2) the fingers of the platforms, and (3) the flexibility of slender silicon pillar supporting the platform. In this work large arrays, 1 cm × 1 cm, have been batch fabricated across an entire 10 cm wafer. Additionally, to characterize meso-scale adhesion, a nanoindentation adhesion test technique was extended to measure the adhesion between the micro-fabricated samples and a rough 5 mm diameter aluminum flat punch. Results indicate improved adhesion with the integration of the nano- and micro-structures. The multi-scale structures also demonstrated improved wear characteristics over solid supported organorods.