The smoothest surface ever created

[Translation by Dr. Moshe Nachmani]

The smoothness of a surface can be adjusted by changing its roughness at the molecular level - this is how researchers from Finland demonstrate

This research helps explain how water droplets manage to easily slide off hydrophilic surfaces, if they are chemically uniform. In addition, this research opens a window to new possibilities for the development of low-friction surfaces - which will allow researchers to develop in the future the most slippery surface ever created. Such surfaces could be used as self-cleaning coatings following the rejection of dirt.

The smoothness of a surface depends on its topographic heterogeneity, or actually its roughness, and several studies have shown that this dependence is maintained all the way down to the molecular level. At the same time, droplets do not slide easily over most hydrophilic surfaces, with a few exceptions such as those based on polyethylene glycol. "There are a number of such studies, but the researchers have not really been able to explain the origin of this phenomenon," says researcher Robin Russ from Aalto University in Finland. 

Ras and his colleagues decided to systematically examine how the slipperiness of a surface with a drop of water on it changes as the hydrophobicity changes by placing pure nitrogen dioxide in a vacuum chamber and using vapor deposition to create a partially hydrophobic monolayer of octyltrichlorosilane on the surface. By varying the stacking time, the researchers created surfaces ranging from a nearly completely hydrophilic surface to hydrophilic and hydrophobic islands to a completely hydrophobic surface. When they tested the slipperiness of the surfaces, the researchers found that water easily slid off surfaces densely coated by the hydrophobic octyltrichlorosilane monolayer. However, water droplets slid just as easily from very hydrophilic surfaces that were lightly coated with this material. When the surface coverage was intermediate coverage, the water droplets experienced higher friction than on the completely hydrophilic and hydrophobic surfaces. The researchers then focused on molecular dynamics simulations to explain this finding.   

In the case of a very hydrophobic surface, a drop of water touches the surface at a very sharp angle, so that only a small area of ​​the drop makes contact with the surface and thus it slides easily. This principle in which the contact area of ​​the drop is extremely limited is known in the scientific literature and is even frequently used for superhydrophobic surfaces. The other cases surprised the researchers more: for the case of little coverage, the case of a hydrophilic surface, the water drop flattens to form a thin coating that covers the surface. Other water droplets make wider contact with this thin layer, but given the fact that the layer was extremely mobile, it helped reduce the friction with the surface and helped these droplets pass the surface along it. In the intermediate case, however, this result of a hydrophilic surface was distorted by the presence of hydrophobic "islands". Instead of spreading along the surface, the water drops get stuck on the surface again and again. "Actually, we didn't observe this phenomenon when we started researching the issue, but looking back the phenomenon is very consistent with the idea that the uniformity of the surface affects its level of friction, so this hypothesis (the relationship between the uniformity of the surface and its level of friction) was quite logical," said the researcher the main 

The researchers extended their research to create a super-hydrophobic and super-slippery surface. They "grew" their hydrophobic monolayer on top of a layer of aluminum oxide coated with black silicon, which was treated at the micrometric level in order to reduce as much as possible the contact area of ​​the water drop and create an outstanding super-hydrophobic surface. "The usual approach is to reduce the area of ​​the area where the drop can make contact with the surface by making the surface as uniform as possible, but the researchers did not really focus on changing the chemistry of the surface," said the researcher. In the present case, however, the researchers combined hydrophobicity both physically and chemically. "The contact area is very small, and at the point where the water rinse does touch the surface, the effect of friction comes into play as explained earlier," says the researcher. The result, the researchers believe, is the creation of the most slippery surface ever created by man. The lead researcher believes that their approach is scalable and says that he has already held discussions with many companies that have expressed interest in his research findings.  The article is about the research  The news about the research

Computer simulations showed that changing the coverage of the monolayer changed the smoothness of the surface. Both low coverage and high coverage produced very slippery surfaces, but for different reasons. [Source: Sakari Lepikko et al 2023]