Intermolecular forces become important in small scales due to the small spacing present and high surface area to volume ratio. Liquid-mediated adhesion is one of the intermolecular forces which is prevalent in both natural and engineered systems. Familiar examples in nature include: plants and trees, which transport fluid from roots to leaves in opposition of gravity through xylem conduits; and soils whose strength characteristics depend on the way water interacts between solid particles. Among engineered systems, there are several small scale devices such as nano/micro-electro-mechanical devices (NEMS/MEMS), magnetic storage head/disk interface (HDI), the tip of atomic force microscope (AFM) for which liquids are present in confined regions during fabrication or during operation. The liquid film could be present in these devices due to condensation (humid environment), contamination, or lubrication. In many cases, the presence of the liquid film causes excessive adhesive forces and device failure. In the contacting surfaces, the wetting liquid film located in small spacing causes large concave curvatures at the interface, which in turn, causes large pressure drops within the liquid film. The pressure drop can be obtained using the famous Laplace-Young relation. This negative pressure drop induces tensile stresses between the contacting surfaces, which tend to reduce the spacing between the surfaces. Opposing these tensile stresses, are the compressive stresses developed at solid-solid contact spots. A comprehensive study of the interaction between liquid films and elastically deforming solid surfaces in contact is performed through both theory and experiment.
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Liquid-mediated adhesion between contacting rough surfaces
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