Ph.D. Dissertation Defense by Jie Zheng
Friday, December 10, 2004

(Dr. Jeffrey L. Streator, Chair)

" Effects of Capillarity on the Mechanical Stability of Nanoscale Interfaces"


Interfacial adhesion and friction are significant factors in determining the reliability of small-scale mechanical devices such as with MEMS and the computer head/disk interface (HDI). As the interface spacing becomes smaller, operational failure via stiction has become a growing concern in these systems. Fundamentally, interface failure is related to mechanical instability of the interface caused by capillary effects.

In MEMS and the HDI, the desired spacing between solid surfaces is often on the order of a few tens of nanometers. When liquid is present in such confined geometries, large concave meniscus curvatures often develop at the liquid-vapor interface, leading to negative pressures in the liquid film and large tensile forces on the surfaces. When the elastic restoring force can not balance the capillary force, the interface will lose its stability and collapse into full, intimate contact. The stability of the interface depends on the liquid surface tension (γ), the liquid volume in the bridge, the mechanical properties of the solid materials, and the undeformed interface geometry. On the other hand, owing to the concept of tensile strength of liquid, when the negative pressure in the liquid decreases below a certain threshold, cavitation should occur in the liquid bridge. The approaching surfaces may then be suddenly released. According to the crevice theory (a heterogenous nucleation theory to determine the tensile strength of liquid), the rupture of the liquid bridge depends on the surface roughness and surface wetting. The intent of this study is to provide a systematic description of the role of capillarity in the interface stability, through both theory and experiment. To achieve this goal, the proposed study includes: 1) Theoretical investigation on the influence of capillarity and elasticity on interface stability; 2) Development of a model for the rupture of a liquid bridge; 3) Design and implementation of an apparatus for experimental verification.

The proposed work has the potential to make important contributions to the body of knowledge as well as to engineering practice. Not only will we know more about the fundamental nature of liquid capillarity, but we will also acquire practical information that can guide the design of nano- and micro- scale devices.