Surface Tension Driven Transport Using Roughness

Surface tension, a line force, is a unique type of force, which scales directly to length. When the dimension of interest shrinks down to sub-millimeter range, the size range typical for MEMS devices, surface-tension force becomes dominant over most other forces, such as those based on pressure (surface force) or mass (body force), e.g., see [1,2]. This dominance of surface tension has most typically been the source for a serious hindrance against successful fabrication and operation of micro devices, calling for elaborate analyses and fabrication techniques in MEMS [3-6]. A completely different approach would be to design micro devices, which use surface tension. Indeed there have been a few attempts in MEMS to utilize surface tension, some passively using surface tension to block unwanted movements and some actively controlling it to induce movements. Many examples can be found that use surface tension to stabilize small objects. A few examples of utilization of surface tension are the bubble check valve that increases ejection frequency of a droplet from the inkjet head by several folds and the passive valve to stop liquid flow using an abrupt geometry change in the channel.