A study on the interaction force between two small bodies on a liquid for micro self assembly and separation

The paper presents a detailed study of the mechanism by which a MEMS plate forms a meniscus with a liquid, and the corresponding solid-liquid interaction force. The study is than extended to investigate the interaction force between two small thin plates forming meniscii with a liquid. It is shown that the force is attractive for similar meniscii and repulsive for opposite meniscii. If the meniscii are of the same type but not identical (e.g., one solid makes a steeper meniscus than the other) then the force is attractive at long distances, repulsive at shorter distances, with a stable equilibrium at a finite distance between the solids. The study reveals that MEMS (hydrophilic or phobic) can probe objects in a liquid without inundating themselves. Furthermore, capillary forces can be harnessed to self assemble or separate small solids (micro chips or living cells) on a liquid surface noninvasively.