A fluidic rectifier is a special kind of device in which the flow in one direction does not experience the same resistance as the flow in the opposite direction. Acting in a similar way to a diode in an electronic circuit, a rectifier that can control the flow of fluids through microscopic channels would open up the possibility of developing more complex integrated microfluidic circuits for biotechnology applications.
The problem is that the necessary behaviour for a fluidic rectifier only applies to flows that have a large Reynolds number – the ratio of inertial and viscous forces in the fluid. As the Reynolds number is proportional to both the size of the system and the average velocity of the flow, and inversely proportional to the viscosity, a large Reynolds number means either a large system (e.g. the turbulent flows in the Earth’s atmosphere), high velocities (e.g. the flows behind fast moving cars and planes) or low viscosities (e.g. the flow of water compared with thick oil).
In microfluidics, however, the Reynolds number is very small, which means that the flows are supposed to be perfectly reversible and display exactly the same behaviour when moving from left to right and vice versa. This is because a low Reynolds number implies a time-reversible flow: changing the sign of time is equivalent to changing the sign of the velocity, and this gives exactly the same flow in the pipe but in the opposite direction. As a result, one usually concludes that it is impossible to build a fluid rectifier that operates on small scales.
However, these arguments are only valid for common or Newtonian fluids. Alexander Groisman and Stephen Quake of the California Institute of Technology have now devised a special pipe and fluid combination in which the flow is non-symmetric. The device – the basic geometry of which was patented by Nikola Tesla in 1920 – has no moving parts, and could find applications in microfluidic pumps and valves (Phys. Rev. Lett. 92 094501).
In the June issue of Physics World Nicolas Garnier at the Ecole Normale Supérieure de Lyon in France describes this work in more detail.
