Contact time dependence of adhesion force at silica/silica interface on AFM: Influence of relative humidity and contact history

Abstract

In the micro-nano adhesion force study, discrepancies in contact time dependence require clarification. Adhesion forces between hydrophobic silica cantilevers and partially hydrophobic silica were measured to investigate the dependence on an atomic force microscope (AFM) at 10%∼90% relative humidities (RH). Experimental results demonstrate that the dependence is related to RH and contact history. The behavior with dwell time is logarithmically increasing with saturation time of 80 ∼ 200 s at 10%∼80% RHs and independent at ∼90% RH. However, once the interface is exposed at ∼90% RH for some time, the instability (the change of magnitude with repeated contacts) occurs, and the behavior can change to logarithmical increase or larger forces with long dwell time. These behaviors at ∼90% RH were attributed to the condensation polymerization of orthosilicic acid to form silica sols and gels. With the exposure, the subsequent behaviors can also be changed to: (1) increasing–decreasing or stable-increasing-stable at 60%∼80% RHs, and (2) short saturation time at 10%∼30% RHs. The effect of the exposure can be eliminated with repeated contacts at a low RH. Therefore, the contact history matters. The results may deepen the understanding of adhesion mechanisms and provide solutions to stiction problems for small-scale silicon-based structures.