Monday, 23 January, 2012
Tidy up the nanoparticle mixture
High-resolution particle separation in two directions
Speed, convenience, and accuracy: these are the most important features of a good analytic device. Currently, a variety of instruments already exist which are able to detect even very faint traces of a given substance. But in all existing methods there remains the potential for improvement. Recently, a team around the NIM scientist Professor Peter Hänggi from the University of Augsburg and Professor Jose Miguel Rubi from the University of Barcelona developed a very accurate process to separate DNA fragments of different sizes. To do this, they took advantage of several entropy potentials of the particle dynamics. Computer simulations demonstrate that this is the reason why the particles divide go in two directions. Furthermore the calculations demonstrate that the new technique can reach a degree of purity of 99,9 percent -a significant improvement on other common methods.
The central element of this technique is a tiny tube with a diameter of only few micrometers. The tube is lined with an asymmetrically serrated structure, similar to saw teeth. Seen from left to right the geometry corresponds to slowly ascending ramps. But from the top of the ramps it descends quite steeply (see Fig.). The new method of particle separation is based on exactly this changing geometry. In general all particles inside the tube move forwards and backwards according to Brownian motion. But the physicists exposed the Brownian particles additionally to an oscillating and constant force. In this case the constant force is only strong enough to influence the movement of the small particles to the left side. The force has no impact on the orientation of the bigger particles. As a consequence the small particles surmount the steep slopes in the left direction, whereas the bigger ones are only able to take the slowly increasing slopes towards the right direction.
As a result, the smaller particles move left on average, while the bigger particles move to the right. Working with these forces as an additional tool the physicists demonstrated in an elegant manner how particle separation can function much faster and more accurately.
The new method is appealing, first of all, because of its extremely high purity of separation. In addition, it opens up the possibility to easily adapt the system´s geometry to various particles sizes. The breadboard construction is only usable for small volumes but it is possible to attach several tubes in parallel on a chip. And due to the 99,9 % particle separation there should not be any sample left inside the tube. That is why the next analysis can start immediately and operate continuously.
„The idea of separating particles of various sizes based on entropic forces, Brownian motion and external force field establishes a new scenario. Now we are able to quickly purify a heterogeneous mixture of particles with the highest sensitivity,” says Peter Hänggi. “This new principle might replace the common methods and start a revolutionary phase within the preparation of heterogeneous substances. But I guess we have to overcome several unpredictable technical hurdles until we will reach this level.”
This paper was highlighted as "Editor´s choice" in the APS journal Physics: physics.aps.org/articles/v5/6
Entropic Splitter for Particle Separation
D. Reguera, A. Luque, P.S. Burada, G. Schmid, J.M. Rubi, and P. Hänggi. PRL 108, 020604 (2012)
Prof. Peter Hänggi
Institut für Physik, Universität Augsburg