Separating substances with a 100-year-old idea

How can you make use of the energy from the random collisions between molecules, known as Brownian motion? With a 'Brownian ratchet', an idea that dates back to 1912. Doctoral candidate Wijnand Germs built an electronic variant of the ratchet that allows microscopic particles to be separated by size. The principle can be applied in medical self-testers. Germs will gain his PhD on 16 January at Eindhoven University of Technology (TU/e).

Particles in a fluid are constantly thrown backwards and forwards by collisions with the molecules of the fluid. This is called Brownian motion. The Polish physicist Marian Smoluchowski conceived a thought experiment in 1912 in which the energy from those random collisions could be used to make objects move in a well-defined direction. However this 'Brownian ratchet' conflicted with the laws of physics, because it suggested that a wheel could be made to rotate spontaneously. Fifty years later the American physicist Richard Feynman thought of a crucial modification through which the ratchet really could be made to work.

For the past 20 years or so, the Brownian ratchet hasn't just existed on paper -- many variants of it have been created experimentally. This 'scientific curiosity' has proved to be a useful tool in an increasing number of fields. In his doctoral thesis, Wijnand Germs of the Molecular Materials and Nanosystems (M2N) research group at TU/e 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. That can be useful in the medical self-testers (labs-on-a-chip) which are currently under development. In these devices blood, saliva or urine are analyzed in fine microchannels. Separation of particles by size is also of great importance in the production of nanoparticles.