マイクロ構造における表面間力に関する理論的検討

抄録

In mirostructures such as microsensors and microactuators, a small surface force can occur between two contacting surfaces which affects their operations. Therefore, it is very important to clarify the generation mechanism of this surface force and to find a way to decrease it in order to realize these devices.
In this study, the surface force between two contacting surfaces in a sample microstructure was evaluated theoretically and the force magnitudes obtained were compared with the measured values which we reported previously. Four kinds of basic physical forces, i.e. van der Waals force; liquid bridge force due to capillary condensation of water molecules from the atmosphere, hydrogen bonding force between adsorbed water molecules, and electrostatic force based on a contact electrification were calculated by modeling the contacting parts in the sample. The sample microstructure has a movable plate supported by cantilever beams and two opposing electrodes. The movable plate and one of the opposing electrodes contact each other when the movable plate is displaced significantly.
We found that the van der Waals force, the liquid bridge force and the hydrogen bonding force could generate surface force magnitudes which were almost equal to the measured surface force.
From the theoretical analysis described above and the earlier experimental results, we concluded that the dominant surface force in usual microstructures is the liquid bridge force due to the capillary condensation of the water when the humidity of the atmosphere around the two contacting surfaces is high; it is the hydrogen bonding force between water molecules adsorbed on the two surfaces when the humidity is relatively low; and it is the van der Waals force when physisorbed water molecules on the surfaces are almost moved out.