Imagine stepping off the edge of a swimming pool, only to find that
your foot deflects the surface of the water without breaking it, as if
held by some impenetrable skin. As you walk forward, the water
continues to support you; but if you take a running leap and bring your
full weight down on the surface, then it snaps open to envelop you
without a splash. Rather than plunging to the bottom of the pool,
however, you stop abruptly as your kinetic energy is instantaneously
dissipated in the fluid. You flail your arms in an attempt to return to
the pool’s edge but make no progress, merely bouncing back and forth
with each stroke.

While such an experience would come as some surprise to a human used
to experiencing life on the macroscopic scale, this is precisely how
fluids behave when confined to micrometre-wide channels. On the
micro-scale, surface tension and viscosity dominate fluid dynamics, as
our imaginary swimmer would discover. These phenomena cause the chaotic
turbulence that characterizes macroscopic flow to disappear and be
replaced by “laminar flow” in which fluid flows in parallel layers with
little or no mixing between them.

In the September issue of Physics World , Carl Hansen,
Kaston Leung and Payam Mousavi look at how the novel physical
properties of this “microfluidic regime” are being applied in areas
ranging from materials synthesis to drug discovery.

To read the full version of this article – and the rest of the September issue of Physics World – please subscribe to our print edition.