Why is oil flowing on the water

Nanochannels: Oil flows better than water

If a chemistry laboratory is to fit on a tiny chip in the future, the experiments have to take place in nanometer-sized tubes. But physical laws of the macro and micro world only apply to a limited extent on the nano scale. Researchers have now shown this again in computer simulations. They determined which capillary forces act in nanotubes that suck in liquids. After that, a viscous substance such as silicone oil glides through nanochannels better than expected, while liquids similar to water do not show such an effect. Such an effect does not occur in microcapillaries that are many times wider, according to the researchers in the current issue of the journal Physical Review Letters.

Capillary forces are vital for trees - they contribute to the fact that water from the roots also rises into the last tip of the leaf. How high and how fast water flows up through a capillary against gravity generally depends on the diameter of the tube. A capillary with a diameter of one micrometer, that is the hundredth part of a human hair, sucks in water at a pressure of 2.8 bar and thus lets it rise a full 28 meters. The pump is replaced by the interplay of forces that act on the one hand on the surface of the liquid and on the other hand at the interface between the liquid and the tube wall.

Lucas-Washburn law adapted to the nanoworld

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A research team led by Kurt Binder, external scientific member of the Max Planck Institute for Polymer Research, and colleagues from the University of Mainz have now found out to what extent these principles also apply in nanocapillaries. The scientists simulated on the computer how liquids penetrate such a tube. In doing so, they just had to adapt a physical law of capillarity, the Lucas-Washburn law, to the physics of the nanoworld by expanding the formula.

"There have been claims in the literature that there are completely different laws for flowing liquids in the nano world than in the macro world," says Binder: brought experimental and theoretical results on this topic. "

In the computer model of the Mainz scientists, 25,000 atoms of two different liquids penetrate into a tube that is ten nanometers in diameter. The physicists have modeled the properties of one virtual liquid on water, which flows very well in microcapillaries with a water-attracting surface. The other consists of short-chain macromolecules and its properties are similar to a silicone oil. The computer calculated how fast the two liquids flow into the capillary. Accordingly, liquids generally rise more slowly in nanocapillaries than in tubes that are opened wider.

Tube wall slows the flow

“With our calculations, we also examined whether the particles are still moving near the wall,” says Binder: “We wanted to find out how well the different liquids flow in such tiny tubes.” That is, takes the speed of the particles towards the wall down only slightly and is not zero even on the wall, the substance flows quite well through the nanotubes. However, if the atoms on the wall are almost stationary, the liquid flows slowly through the nanocapillary. The particles inside the column of liquid are still moving. Because of the small diameter of a nanotube, the braking effect of the wall predominates here.

The Mainz physicists have now observed this effect in their simulations with the water-like liquid. It therefore rises through the nanotube as slowly as the viscous substance, but the particles of which slide over the wall. “Nobody expected that,” says Binder. They had given the nanotube a wall that attracted water, along which water flows well, at least on a larger scale.

Chips reach their limits

Binder can only explain why the two substances in the nanotubes behave differently than physicists from the macro world are used to: “The small molecules of the water-like liquid tend to feel the attractive forces of the capillary wall and stick. And because the column of liquid is so narrow, this braking effect dominates the entire capillary. The large molecules are presumably less sensitive to these forces and therefore move more freely. "

The researchers' results help to vary capillary properties, liquids and, above all, the size of the capillary so that liquids flow better on a small scale. “But it looks like extremely thin and small capillaries are of little use if all the liquids in them flow slowly. The miniaturization of the lab-on-a-chip may have limits, ”says Binder.

(MPG, 08/15/2007 - DLO)

August 15, 2007