All of us have seen it: a raindrop running down the windowpane. It stops at a certain point, is met by a second raindrop and the two join up before continuing to run down the pane. Very small irregularities or dirt on the windowpane appear to stop the course of the raindrops. If the surface was entirely smooth and chemically clean, the raindrops would be able to flow unhindered. Surface defects such as small bumps and dimples as well as chemical contaminants stop the liquid drops.
These are everyday phenomena everyone knows and can observe with the naked eye.
However, for years, progress in science and technology has been directed towards ever more finely structured surfaces of solids which can be used for a wide variety of applications. Here the typical structure dimensions are in the micrometer or even the nanometer range (a nanometer is one-millionth of a millimeter).
But how is the flow behavior of a drop influenced by such fine surface structures, and how is the transportation of tiny amounts of liquid on extremely narrow paths impeded by tiny surface defects along this path? Here the surface defects in question are not much larger than the molecules or atoms which form he liquid or the surface of the solid.
The influence of such small surface defects on the transportation of liquids cannot be studied with the naked eye. Even with sophisticated modern experimental methods, it is currently not possible to observe and investigate the transportation of liquids across such small surface defects. Theoretical methods and model calculations overcome this challenge.
The research department “Theory of inhomogeneous condensed matter” headed by Prof. Dr. Siegfried Dietrich at the Max Planck Institute for Intelligent Systems in Stuttgart has developed a model calculation which incorporates the relevant molecular structure on the nanometer scale. This theo-retical model makes it possible to calculate the resistance against the transportation of liquids caused by irregularities or contaminants of a few nanometers in size.
Dr. Alberto Giacomello and Dr. Lothar Schimmele along with Professor Dr. Siegfried Dietrich recently published these results in the journal “Proceedings of the National Academy of Sciences” (PNAS).
This multidisciplinary journal of international renown only publishes studies of extraordinary scien-tific importance. They also have to be of general interest for other groups of specialists, in this case for example scientists in the fields of microfluidics, nanostructure physics and surface chemistry.
For several years, Lothar Schimmele has developed the corresponding computer program. Alberto Giacomello combined it with a novel algorithm for this project. The program makes it possible to calculate how liquids behave under the influence of external forces caused for example by confining walls.
Alberto Giacomello and Lothar Schimmele chose a simple model for the issues discussed here: two smooth walls parallel to each other, forming a channel with a diameter of a few nanometers. On the lower wall of this narrow channel, the liquid encounters an obstacle such as a contaminant or an irregularity. Results obtained with this simple system can be transferred to other geometries using theoretical considerations.
“So far the scientific community has assumed that an obstacle smaller than one nanometer is too weak to stop a liquid. Our calculations refute that,” Dr. Lothar Schimmele explains.
The results of these studies can be used to explain another phenomenon as well. Tiny gas bubbles which form on surfaces, for example during catalysis or electrolysis, often have an unexpectedly long lifespan. However, these gas bubbles reduce the effectiveness of electrolysis processes and disturb them.
The pinning of a gas bubble on the surface prevents the continuous increase of pressure in the bubble, thereby stabilizing it. Pinning can be explained by the observations of the Dietrich research group: It is caused by irregularities on the surface in the range of a few nanometers.
The insights gained through this work can also be of importance for other practical applications. One example is the use of liquid bridges for the artificial assembly of nanostructures. Nanoparticles are positioned and oriented with the help of these bridges. Once again pinning on irregularities plays an important role.
The scientists have already set additional goals for themselves: They want to investigate various types of irregularities on surfaces in order to find out what respective influence the material composition or geometric shape of an irregularity in the nanometer range has on stopping a drop of liquid.
The researchers are interested in collective phenomena as well. “Next we want to investigate the influence of multiple defects which are close to each other forming a group. We are also interested in the flow behavior of liquids across obstacles in the form of surface holes, rather than the bumps studied so far,” Dr. Lothar Schimmele explains.
Annette Stumpf | Max-Planck-Institut für Intelligente Systeme
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