Application Note: Beating the Bulge

Capillary valves

Most often we think of surface tension as a wetting process that pulls liquid into a channel, like a sponge. It can also be non-wetting, making it difficult to get liquid into a region. For instance, a powder such as flour can be difficult to wet uniformly (hence the need to make a roux for gravy). This property suggests that surface tension might be used as a type of micro-valve without moving parts.

In fact, some medical test devices-referred to as labs-on-a-chip-use surface tension svalve to control the progress of liquid samples through successive stages of processing. The simplest concept is to place an abrupt expansion in a channel. If the liquid does not wet the walls of the channel, it cannot move from the smaller to larger channel without first overcoming a surface tension force proportional to the bulge of the liquid surface (see figure a).

A similar flow resistance occurs when a non-wetting liquid tries to pass into a smaller channel; the radius of curvature in the smaller channel must be smaller and this generates a greater surface tension force to be overcome.

An illustration of the latter behavior is shown in the adjacent figures, b1-b3, where FLOW-3D has simulated liquid progressing through a series of are reducing junctions. Each junction requires a pressure increase (hotter color in figures) to keep the liquid moving, proving that, at small scales, bulges are hard to beat.