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Fundamental Understanding of Adsorbed Water on Silicon Oxide

Seong H. Kim, Penn State University, 118-B Fenske Lab, University Park, PA 16802-4400 and David B. Asay, Pennsylvania State University, 123 Fenske Lab, University Park, PA 16802.

The adsorption of water molecules on the silicon oxide surface from the gas phase significantly alters the adhesion and friction force measurement results in atomic force microscopy (AFM) study. For example, as relative humidity (RH) increases, the adhesion force measured with AFM initially increases, reaches a maximum, and then decreases at high RH. The capillary force alone cannot explain the observed magnitude of the RH dependence. The origin of the large RH dependence is due to the presence of an ice-like structured water adsorbed at the silicon oxide surface at room temperature. A solid-adsorbate-solid model is developed calculating the contributions from capillary forces, van der Waals interactions, and the rupture of an ice-ice bridge at the center of the contact region. This model illustrates how the structure, thickness, and viscoelastic behavior of the adsorbed water layer influence the adhesion and friction force of the silicon oxide nanoasperity contact.