K. Mathwig
Transport von Teilchen in asymmetrisch modulierten Poren
Dissertation, Martin-Luther-Universität Halle-Wittenberg (2009).
[pdf]

The separation of mesoscopic particles by their size has numerous applications in industry, medicine and biology. Brownian motion plays an important role in the filtration or separation of small particles, but generally leads to an undirected motion or intermixing by diffusion. The present work deals with the microfluidic transport of particles by the drift ratchet effect, which utilizes Brownian motion instead to aid in the separation. In the experiment, suspended particles are microfluidically driven through an array of asymmetrically modulated microchannels fabricated out of macroporous silicon. A new interpretation of the first report on experimental demonstration of the drift ratchet effect (Matthias & Müller Nature 424, 53–57, 2003) is offered: New experiments, which allow to distinguish between particles of different sizes, as well as a reexamination of the original work leads to the conclusion that the measured particle transport does not result from a drift ratchet effect. It is demonstrated that convective mixing instead causes the transport.