Institute of Physics AS CR

Friction is hard to predict and control, especially since surfaces that come in contact are rarely perfectly flat. New experiments demonstrate that the amount of friction between two silicon surfaces, even at large scales, is determined by the forming and rupturing of microscopic chemical bonds between them. This makes it possible to control the amount of friction using surface chemistry techniques. Friction is hard to predict and control, especially since surfaces that come in contact are rarely perfectly flat. New experiments demonstrate that the amount of friction between two silicon surfaces, even at large scales, is determined by the forming and rupturing of microscopic chemical bonds between them. This makes it possible to control the amount of friction using surface chemistry techniques. The new results are published in Physical Review Letters. "There is a lack of quantitative understanding of friction, despite its crucial role in tackling challenges as diverse as predicting earthquakes and reducing energy consumption in mechanical devices," says PhD researcher Liang Peng, who conducted the research project. This is no small matter: friction is estimated to be responsible for more than 20% of our global energy consumption. Controlling friction in machinery is also important for reducing material wear and increasing positioning precision. Peng worked together with other researchers of the Institute of Physics and the Van 't Hoff Institute of Molecular Sciences at the University of Amsterdam as well as the Advanced Research Center for Nanolithography (ARCNL). The research is part of an ongoing collaboration to investigate how large-scale friction emerges at a microscopic level. In recent years, new research methods have allowed researchers to zoom in to what exactly happens when two surfaces make contact and slide over one another. Crucially, surfaces are never perfectly smooth. At the scale of a nanometre, one-billionth the size of a metre, they look like mountainous landscapes with pronounced peaks and valleys. Previous experiments and numerical simulations have demonstrated that at this small scale, friction is largely determined by the formation and rupturing of bonds between surface atoms. This is affected not just by the roughness of the sliding surfaces, but also by which atoms or molecules (such as water) are present at the interface. "We decided to extend and apply these nanofriction mechanisms to larger, industrially relevant scales," explains Peng. Using a special instrument called a rheometer, the researchers studied how the amount of friction between a relatively rough silicon ball and a smooth silicon wafer depends on the density of microscopic chemical bonds at the interface. Silicon, with chemical symbol Si, is a particularly interesting material to study thanks to its widespread use in the semiconductor industry. Its abundance in the Earth's crust also makes it relevant to the study of earthquakes. After cleaning the surfaces of contaminants, the researchers discovered that much less force is needed to slide the ball over the wafer -- in other words, there is less friction -- when the surfaces are dried for longer in pure nitrogen gas. Further experiments showed what happens at the level of atoms: longer drying reduces the number of hydroxyl (OH) groups exposed at the silicon surface. When brought into contact with another silicon surface, the presence of these groups results in the formation of silicon-oxygen-silicon (Si-O-Si) bonds between the two surfaces. The research demonstrates that there is a striking relation between the friction force measured at large scales and the density of microscopic Si-OH groups present on the two silicon surfaces before contact, which controls the number of Si-O-Si bonds made during contact. The density of these chemical bonds is regulated by setting the length of time for which the cleaned surfaces are dried. Excitingly, this means that it is possible to predict and control the friction force between silicon surfaces. "Our result is quite remarkable because it demonstrates a quantitative understanding of macroscopic friction from first principles. Our findings can thus bridge the knowledge gap that hampers understanding-based control over friction," concludes Liang.

With methods of so-called geoengineering, the climate could theoretically be artificially influenced and cooled. Researchers have now investigated whether it would be possible to prevent the melting of the West Antarctic ice sheet by artificially 'dimming the sun'. The results show that artificial influence does not work without decarbonization and entails high risks. With methods of so-called geoengineering, the climate could theoretically be artificially influenced and cooled. Bernese researchers have now investigated whether it would be possible to prevent the melting of the West Antarctic ice sheet by artificially "dimming the sun." The results show that artificial influence does not work without decarbonization and entails high risks. Is there an emergency solution that could stop climate change? Technical methods that artificially influence the climate have been discussed for some time under the term geoengineering. However, the majority of climate researchers have been critical of them: high risks, incalculable consequences for future generations. In a study just published in the journal Nature Climate Change, researchers led by Johannes Sutter of the Climate and Environmental Physics Division (KUP) at the Institute of Physics and the Oeschger Center for Climate Research at the University of Bern investigate the question of whether the melting of ice in West Antarctica could be prevented by artificially influencing solar radiation. The researchers also warn of unforeseeable side effects of geoengineering. Avoiding a key climate tipping point "The window of opportunity to limit the global temperature increase to below 2 degrees is closing fast," says ice modeling specialist Johannes Sutter, "so it is possible that technical measures to influence the climate will be seriously considered in the future." That is why, he says, it is necessary to use theoretical models to study the effects and risks of "solar radiation management." Solar Radiation Management (SRM) is a term used to describe various methods of blocking solar radiation in order to make the Earth cooler. A key reason for the increased interest in geoengineering is the avoidance of tipping points at which the climate could change abruptly and irreversibly. These include the melting of the West Antarctic and Greenland ice sheets and the associated meter-high sea level rise. "Observations of ice flows in West Antarctica indicate that we are very close to a so-called tipping point or have already passed it," explains Johannes Sutter, "with our study, we therefore wanted to find out whether a collapse of the ice sheet could theoretically be prevented with solar radiation management." Artificially dimming the sun Specifically, Sutter and his colleagues have investigated what would happen if so-called aerosols -- suspended particles in a gas -- introduced into the stratosphere succeeded in blocking solar radiation from the earth -- a dimming of the sun, so to speak. So far, research has focused on the global effects of solar radiation management (SRM). The Bern study is the first to use ice model simulations to show what effect such a measure would have on the Antarctic ice sheet. The study examines the possible development of the ice sheet under different future greenhouse gas scenarios and yields differentiated results: If emissions continue unabated and the SRM occurs in the middle of this century, the collapse of the West Antarctic Ice Sheet could be delayed somewhat, but not prevented. In a medium emissions scenario, SRM deployed by mid-century could prove to be an "effective tool" to slow or even prevent ice sheet collapse. According to the model calculations, SRM works best when it occurs as early as possible and is combined with ambitious climate mitigation measures. But, the study authors emphasize, "our simulations show that the most effective way to prevent long-term collapse of the West Antarctic Ice Sheet is rapid decarbonization." The chances of a longer-term stable ice sheet are greatest if greenhouse gas emissions were reduced to net zero "without delay." Possible side effects still hardly studied But how should one imagine a dimming of the sun in practical terms? According to Johannes Sutter, a whole fleet of extremely high-flying airplanes would have to spread millions of tons of aerosols in the stratosphere. However, this technical intervention in the climate would have to be maintained without interruption and for centuries. If the intervention were stopped as long as the greenhouse concentration in the atmosphere remained high, the temperature on earth would quickly rise by several degrees. The consequences of such a termination shock, Johannes Sutter points out, are only one of the possible dangers posed by SRM. The potential side effects are still insufficiently researched and range from a shift in the monsoon regime to changes in ocean and atmospheric circulation. Ocean acidification would also continue. Critical voices also caution political and social effects: The use of techniques such as solar dimming could lead to climate protection measures being slowed down or even prevented. Thomas Stocker, professor of climate and environmental physics at the University of Bern and co-author of the study, says: "Geoengineering would be another global experiment and a potentially dangerous human intervention in the climate system, which should in any case be prevented according to Article 2 of the UN Framework Convention on Climate Change."

Scientists generate low-energy electrons using ultraviolet light. Solvated dielectrons are the subject of many hypotheses among scientists, but have never been directly observed. They are described as a pair of electrons that is dissolved in liquids such as water or liquid ammonia. To make space for the electrons a cavity forms in the liquid, which the two electrons occupy. An international research team around Dr. Sebastian Hartweg, initially at Synchrotron SOLEIL (France), now at the Institute of Physics at the University of Freiburg and Prof. Dr. Ruth Signorell from ETH Zurich, including scientists from the synchrotron SOLEIL and Auburn University (US) has now succeeded in discovering a formation and decay process of the solvated dielectron. In experiments at the synchrotron SOLEIL (DESIRS beamline), the consortium found direct evidence supported by quantum chemical calculations for the formation of these electron pairs by excitation with ultraviolet light in tiny ammonia droplets containing a single sodium atom. The results were recently published in the scientific journal Science. Traces of an unusual process When dielectrons are formed by excitation with ultraviolet light in tiny ammonia droplets containing a sodium atom, they leave traces in an unusual process that scientists have now been able to observe for the first time. In this process, one of the two electrons migrates to the neighbouring solvent molecules, while at the same time the other electron is ejected. “The surprising thing about this is that similar processes have previously been observed mainly at much higher excitation energies,” says Hartweg. The team focused on this second electron because there could be interesting applications for it. On the one hand, the ejected electron is produced with very low kinetic energy, so it moves very slowly. On the other hand, this energy can be controlled by the irradiated UV light, which starts the whole process. Solvated dielectrons could thus serve as a good source of low-energy electrons. Generated specifically with variable energy Such slow electrons can set a wide variety of chemical processes in motion. For example, they play a role in the cascade of processes that lead to radiation damage in biological tissue. They are also important in synthetic chemistry, where they serve as effective reducing agents. By being able to selectively generate slow electrons with variable energy, the mechanisms of such chemical processes can be studied in more detail in the future. In addition, the energy made available to the electrons in a controlled manner might also be used to increase the effectiveness of reduction reactions. “These are interesting prospects for possible applications in the future,” says Hartweg. “Our work provides the basis for this and helps to understand these exotic and still enigmatic solvated dielectrons a little better.”