ABSTRACT
The large surface-to-volume ratio intrinsic to nanoscale materials, structures, and devices ensures that surface forces are dominant. The dominance of surface forces at small scales is present wherever we look. Dust, insects, and other small entities can stick to walls and ceilings, while large objects obviously do not. Forces often thought of as weak, including van der Waals, electrostatic, and capillary interactions, are suf-‘cient to overcome gravity at small scales. Other bulk-related or mass-determined forces, including inertial effects, are often not enough to displace such small entities, and this has proven to be a critical issue in small devices. For example, the Digital Micromirror DeviceTM developed by Texas Instruments required signi‘cant effort to
4.1 Introduction .................................................................................................... 51 4.2 Surface Energy ...............................................................................................54 4.3 Surface Tension: Liquids ................................................................................ 55 4.4 Laplace Pressure ............................................................................................. 57 4.5 The Kelvin Equation ....................................................................................... 59 4.6 Interfacial Energy and Work of Adhesion ...................................................... 61 4.7 Surface Energy of Solids ................................................................................ 61
4.7.1 Surfaces as Defects ............................................................................. 61 4.7.2 Relaxation ........................................................................................... 62 4.7.3 Reconstruction .................................................................................... 63 4.7.4 Rules of Thumb for Surface Energy ................................................... 63
4.8 Contact Angle .................................................................................................64 4.9 The Relation between Surface Forces and Surface Energies .........................65 4.10 Adhesion in Dry Environments ...................................................................... 67 4.11 Adhesion in Wet Environments ......................................................................68 Acknowledgment ..................................................................................................... 73 References ................................................................................................................ 73
overcome adhesion problems [1] (Figure 4.1). The device, which is widely used for commercial digital image projection equipment, consists of an array of microfabricated mirrors, each on a ¤exible hinge. Each mirror corresponds to one pixel of the image to be projected. Electrostatic forces can de¤ect each mirror individually to different angles to re¤ect the chosen color of light for the pixel. The stopping position of the mirror is crucial, and so its yoke is stopped by making contact with a landing area. As originally fabricated, the designers found the yoke and pad stuck together and could not be released reliably. Coating the surfaces with an organic ‘lm, reducing the contact time by vibrating the mirror, adding a spring tip that pushes back when the mirror is released, and hermetically sealing it in a clean, dry environment to prevent condensation of water and contaminants all contributed to solving the problem [1], and the device is now a commercial success, sold in the tens of millions.
