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Probing stresses at the nanoscale

Periodic Reporting for period 3 - Nanostress (Probing stresses at the nanoscale)

Período documentado: 2022-10-01 hasta 2024-03-31

Friction is responsible for an estimated 20-30% of the world's energy consumption but remains ill-understood. This is because of the difficulty of observing directly what happens at the interface: how do two (rough) surfaces contact each other, and how does the sliding change this? We develop, in this project, new tools to answer these century-old questions. To this end, we use molecules whose fluorescence depends on the environment, and notably molecules that start to fluoresce when they are confined in a frictional contact. Using optical microscopy, we can then do the contact mechanics, and observe what happens to the contacts when the surfaces start to slide with respect to each other. We also co-developed molecules that are sensitive to the local stress in the frictional contact. We can therefore now for instance understand where the difference between static and dynamic friction comes from, which is very important for understanding and predicting earthquakes. We also investigate whether the environmentally sensitive molecules can be used to measure local viscosities in polymer glasses when these go through the glass transition. Also here, a very heterogeneous (bulk) dynamics is anticipated from theory, but very few local measurements exist. Again, the fluorescent molecules open up a new window into the very old problem of the nature of the glass transition.
Over the reporting period 3 PhD students and 2 Postdocs have been working on the project, see the table. Dr. Grzelka has worked on implementing the environmentally sensititive molecules in different systems, mostly polymeric in nature, as was part of the initial planning, and a few papers have been published or are being submitted. The relatively recently started PhD student E. Mirzahossein is also working on this, Post-doc Dr. Farain and H. Dekker are working on the friction part of this project and have attacked several key problems in the community such as the effects of surface roughness, adhesion and contact aging, and also here a few papers have been published or are being submitted. Mr, Parrenin has worked for a few months on the friction-part of the project also, showing the effect of phase transitions on friction. On the whole, we are well underway of meeting the challenges posed in the original proposal.

Dr. Marion Grzelka

Dr. Kasra Farain

MSc. Hans Dekker

MSc. Elham Mirzahossein

MSc. Antoine Parrenin
The progress in understanding friction and frictional dynamics goes well beyond the state of the art in that we have been able to observe details of frictional contact mechanics and frictional stresses with unprecedented detail. We are currently working on all the implications of this for our fundamental understanding of what friction is and what it does. For the glass transition, we are currently visualizing the heterogeneous dynamics of stressed or sheared polymer glasses, allowing to test glass transition theories locally.