Capillary Kinetics Between Multi Asperity Surfaces

Capillary bridge formation between adjacent surfaces in humid environments is a ubiquitous phenomenon. Capillary forces are important in nature (granular materials, insect locomotion) and in technology (disk drives, adhesion). Although well studied in the equilibrium state, the dynamics of capillary formation merit further investigation. Here, we show that microcantilever crack healing experiments are a viable experimental technique for investigating the influence of capillary nucleation on crack healing between rough surfaces. To demonstrate the effects, a custom micromachine characterization system is built that allows for full environmental control (pressure, humidity, and gas composition) while retaining full micromachine characterization techniques (long working distance interferometry, electrical probe connectivity, actuation scripting capability). The system also includes an effective in situ surface plasma cleaning mechanism. The average spontaneous crack healing velocity, ̅, between plasma-cleaned hydrophilic polycrystalline silicon surfaces of nanoscale roughness is measured. A plot of ̅v versus energy release rate, G, reveals log-linear behavior, while the slope |d[log(v)]/dG| decreases with increasing relative humidity. An interface model that accounts for the nucleation time of water bridges by an activated process is developed to gain insight into the crack healing trends. This methodology enables us to gain insight into capillary bridge dynamics, with a goal of attaining a predictive capability for this important microelectromechanical systems (MEMS) reliability failure mechanism. A variety of alcohol vapors significantly reduce or perhaps eliminate wear in sliding micro-machined contacts. However, these vapors may increase adhesion due to the capillary forces. Equilibrium adhesion energies at various partial pressures are found for n-pentanol (long chain molecule) and ethanol (short chain molecule). For low partial pressures (p/ps=0.3), adhesion energy of n-pentanol is even larger than water.