Scientific Internship: The origin of adhesive friction
In this project you will revisit lateral force microscopy experiments and systematically study the nature of the friction reductions.
The research field of tribology, devoted to contact formation, friction and wear phenomena down to the atomic scale, is of direct and pressing relevance to the manufacture of semiconductor devices. Friction-induced stresses and deformations on the scale of only a few atomic spacings are starting to challenge the future of nanolithography technology, limiting the achievable feature size in semiconductor chips.
Consider the contact between a single, atomically sharp tip and a perfect crystalline surface. What is the origin of the friction force that resists sliding motion at this interface? Arguably, most of the energy that is released when the tip slips past a surface atom will be transformed into lattice vibrations; phonons. Ultimately the energy invested into phonons will heat the contacting bodies – as each of the excitations created by the sliding will eventually be thermalized. However, the relatively long timescale associated with thermalization of locally invested kinetic energy does not necessarily match that of the seemingly instant damping experienced by the slider. This discrepancy suggests that the friction force that resists the sliding motion should be the consequence of a damping mechanism that is faster than the thermalization – or scattering – of well-defined lattice vibrations. A recent theoretical study indeed explicitly shows that the oscillation that follows when a displaced surface atom of a harmonic crystal is released – just like during atomic scale stick slip sliding – is critically damped. Interestingly, the damping in a theoretical crystal that consists of particles connected by springs is an emergent phenomenon. The initial displacement of the surface atom is a superposition of a number of the basic natural mechanical vibrations of the crystal. When the atom is released each of these phonons will continue to oscillate at its own frequency. Because these frequencies differ, the initial set of lattice vibrations – that define the displacement of the atom that resists the sliding – will soon run out of phase, effectively damping the oscillation of the displaced surface atom.
In other words: the friction force between an atomic force microscopy tip and a perfect crystalline surface exists because whenever the tip slips past an atom on the substrate it launches a multitude of phonons that run out of phase before the displaced surface atom can complete a single oscillation. If this damping mechanism is indeed responsible for adhesive friction, the system size – which naturally limits the phononic eigenstates – is an important parameter in defining the rate at which an interface can damp frictional slip and thus generate friction. Indeed recent experiments have demonstrated a phenomenal reduction in adhesive friction of up to 99% when only a limited number of atoms surround the contact, thereby confining the phonons. In this project we will revisit these lateral force microscopy experiments and systematically study the nature of the friction reductions; is the dimensionality of the materials important (layered/amorphous/crystalline)? Is there a relation between damping and interface stiffness? How can we exploit phonon dephasing to obtain tunable (and directional) friction?
Recommended articles for further reading on this subject:
- Wada, M. Ishikawa, T. Shiga et al. Superlubrication by phonon confinement. Phys. Rev. B 97, 161403 (2018).
- Y. Park and M. Salmeron. Fundamental aspects of energy dissipation in friction. Chem. Rev. 114, 677-711 (2013).
- Y. Krylov and J. W. M. Frenken. The physics of atomic-scale friction: Basic considerations and open questions. Phys. Status Solidi B 251, 711-736 (2014).
- V. Dedkov. Nanotribology: experimental facts and theoretical models. Phys.-Uspekhi 43, 541-572 (2000).
About the group
At the Advanced Research Center for Nanolithography (ARCNL) we carry out exciting fundamental physics research at the highest possible level with relevance to key technologies in nanolithography. We contribute to the production of ever smarter and smaller electronics, while at the same time pushing the boundaries of our fundamental insight into the workings of nature.
You will be embedded in the Contact Dynamics team at ARCNL but will also be closely associated with the University of Amsterdam and ASML, the world leading manufacturer of high tech lithography machines for chip making.
You have or will soon have a Bachelors degree in physics or a related field and participate in a Master study during the entire internship duration. The internship must be a mandatory part of your curriculum. You have a nationality of an EU-member state and/or you are a student at a Netherlands University. Please note: as from 01-01-2021 the UK is no longer an EU-member state. You must be available for at least 5 months.
Terms of employment
At the start of the traineeship your trainee plan will be set out, in consultation with your ARCNL supervisor, including a small allowance.
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