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April 20th 2019

What exactly happens when you press two objects together? That is an important question within research into resistance. Researchers from this FOM programme are zooming in down to the nanoscale to answer this question.

Surfaces always have a certain roughness. If you slide two surfaces over each other, this roughness will determine the force needed to move the surfaces. That forms the resistance, in other words the friction, between the two objects.

The standard theory of friction assumes that the contact surface increases linearly with the pressure that the two surfaces exert on each other. Yet why this should be true for rough surfaces remains unclear. The physicists in this programme are therefore investigating – first in the static situation – the contact between two surfaces.

Beads and plates
The contact surface between, for example, a glass bead (a glass sphere) and a flat piece of glass is less boring than you might have thought. That is because even the smoothest of glass beads are far from smooth at the nanoscale. If you look with an atomic force microscope at the surface of such a glass bead, you will see a mountainous landscape with mountains and valleys tens of nanometres high (a nanometre is one billionth of a metre).

At the University of Amsterdam physicists are working on a method to measure the contact surface between, for example, a glass bead and a flat piece of glass. For this glass plates are being produced with so-called rigidochromic molecules on the surface. These molecules fluoresce when one of the mountain tops on the surface of a glass bead pushes against them. With this approach the researchers are imaging the contact surface between the glass bead and the glass plate.