In this extended abstract, we briefly describe a thermodynamic model to treat capillary adhesion energies in MEMS. We first determine the constitutive laws of a capillary pendular ring bridging an asperity to a substrate. We find that the work of adhesion, W, depends on the surface separation rate. For the constant volume case (rapid surface separation), W = 2γ (where γ is the surface energy per unit area of the liquid-air interface), but if thermodynamic equilibrium is maintained (slow surface separation), W = γ. Thermodynamic analysis indicates that heat from the system walls can lower the work of adhesion at slow separation rates. We extend these constitutive laws to a simple multi-asperity surface in which the asperities are all of constant height. At low vapor partial pressures (p/psat), adhesion can be several orders of magnitude below γ because of incomplete wetting. As vapor partial pressure increases, condensing liquid fills in the geometric irregularities of the surface. As this filling takes place, W approaches 2γ for both the slow and rapid separation rates.

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