Detailed insight into friction: How objects s

The researchers dragged a ball across a glass surface decorated with special fluorescent molecules.

Image: The researchers dragged a sphere across a glass surface decorated with special fluorescent molecules.
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Photo credit: HIMS/UvA

Chemists and physicists from the University of Amsterdam shed light on a crucial aspect of friction: how things slip. Using fluorescence microscopy and special fluorescent molecules, they can pinpoint how and when friction is overcome when two objects come into contact and sliding occurs. They report on the details of this important transition from static to dynamic friction Journal of Physical Chemistry Letters.

Friction accounts for an estimated 25% of global energy consumption. One of the key questions for the stability of many systems is how and when objects start sliding relative to each other; Think about earthquakes or your foot on the ground. When two objects touch, the contact surface is formed by the many microscopic protrusions of the two interfaces touching and interlocking. By applying a shearing force, the objects slide along each other, breaking these initial contacts.

Dragging a ball across a glass surface

At the University of Amsterdam, the groups of Prof. Daniel Bonn (Institute of Physics) and Prof. Fred Brouwer (Van ‘t Hoff Institute of Molecular Sciences) have an ongoing collaboration to study the process of friction down to the microscopic level of roughness . In the newspaper that has just appeared The Journal of Physical Chemistry Letters they report on experiments in which a ball is drawn across a glass surface. The glass surface has been decorated with a special type of molecules (fluorogenic mechanophores) which begin to glow under the stress of shear forces (fluorescence). The moment this force disappears, the molecules return to their stable, non-fluorescent form. This makes it possible to directly visualize and quantify the microscopic shear force down to the microscopic roughness and how it evolves in the transition from the static to the moving state. Among other things, the researchers found that shortly before gliding, a gliding wave propagates from the edge to the center of the macroscopic contact surface. This allows for a quantitative and microscopic local understanding of how surfaces begin to slide.

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