Separating substances with a 100-year-old idea
16 Jan 2013
Dutch researcher Wijnand Germs has built an electronic variant of a Brownian ratchet that allows microscopic particles to be separated by size.
Particles in a fluid are constantly thrown backwards and forwards by collisions with the molecules of the fluid. This is called Brownian motion.
In his doctoral thesis, Wijnand Germs of Eindhoven University of Technology focuses on a Brownian ratchet that can make polystyrene beads with diameters between 300 and 500 nanometers move.
His ratchet consists of a very narrow water channel in which the beads are suspended. Under the channel are carefully designed electrodes that create an asymmetric energy landscape - in other words electronic ’hills and valleys’.
When Germs switches off the electrical voltage, the beads distribute themselves through the collisions with water molecules (Brownian motion) in both directions of the channel.
Then, when the voltage is switched on again, the beads are captured in the ’valleys’. But since the landscape is asymmetrical, more beads are captured on one direction than on the other. Which means that, on average, the beads have moved.
Germs’ research shows that the size of the beads determines the extent of this movement. That means this type of ratchet can be used to separate particles - or molecules in general - by size.
The research could prove useful in the development lab-on-chip systems or in the production of nanoparticles.
