Three molecules thick, or two, or one: how does an extremely thin layer of trapped liquid behave when we make it even thinner? Measurements made using the Atomic Force Microscope show that the forces of friction increase with each step. Liquids begin to behave more like a gel. This is the conclusion presented by Sissi de Beer in her PhD thesis. The thesis reveals new findings about forces that play a role in countless biological processes, for example.
Liquids organize themselves into layers of molecules next to a solid object. They are denser close to a wall than they are further away. This means that they behave differently next to the wall. Maybe even more like a solid? Many theories have been published about this over the years. After making measurements and carrying out simulations, De Beer has made some new discoveries about friction and viscosity. "I made measurements using two different methods, as well as carrying out 'molecular dynamics' simulations. I obtained the same results three times, which was quite remarkable, especially when you consider the range of different results that had been obtained in the past."
No longer a liquid
She worked using the tip of an atomic force microscope. The liquid is trapped between the tip and a flat plate. By reducing the distance between the tip and the plate, the molecules are forced to reorganize into three layers, then two, until finally only one layer of molecules remains. The measurements revealed not only the familiar oscillating forces -- the molecules move in a kind of flat wave movement near the wall -- but also significant friction, which becomes greater as the distance becomes smaller. If one of the surfaces moves relative to the other, they will not slide smoothly as would normally happen with liquid lubrication, but there will be a jolting, jerky stick-slip movement. In fact, the liquid is not really behaving like a liquid any more, De Beer says, but more like a gel or like a soft, glassy material.
The results can be applied in biology when evaluating the forces that are exerted on DNA, or to understand the lubrication required in joints, for example. The findings are also relevant to the lubrication of microscopic devices under development in microtechnology and nanotechnology.
De Beer carried out her research in the Physics of Complex Fluids Group of Prof. Frieder Mugele at the MESA+ Institute for Nanotechnology at University of Twente. Numerical work was done in close cooperation with the Computational Biophysics Group of Prof. Wim Briels. This project was funded by the FOM Institute in The Netherlands. She defended her PhD thesis on 27 May at University of Twente.
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