Non-contact friction at miscroscopic scale

Friction is a daily phenomena and everybody has been used to it and learned how to use it by instinct. And the macroscopic law is usually stunningly simple and universal and relative motion is a necessary condition. On the other hand, how it comes about microscopically has been one among the mootest topics that arouse curiousities. Here is a review on a recent work in this field:

To unravel this tangle of vdW, electrostatic and phononic friction, Kisiel et al. incorporated a second experimental approach that aims to distinguish between phononic and electronic dissipation. In both vdW and electrostatic friction, dissipation ultimately takes place through forced motion of charge in a resistive medium. To separate this effect from phononic dissipation, one can vary the electrical resistivity of one of the two mating materials. This can be done most elegantly in a temperature-dependent experiment in which resistivity is switched on and off by going through a superconductivity transition. An experiment of this type was performed in 1998 by Dayo and colleagues⁵, who used a quartz microbalance to observe increased slip times of nitrogen adhered to a metal surface. Their work triggered significant debate, which continues to this day, because the observed behaviour did not show the predicted temperature dependence around the critical temperature.

In their experiments, Kisiel et al. used a nanoscale cantilevered tip vibrating in close proximity to the surface of a conductor. An atomic force microscope allowed both precise positioning and friction measurements. By positioning a silicon tip close to a niobium surface at temperatures above niobium's superconductivity transition temperature, T_c, they measured friction coefficients down to ~10⁻¹² kg s⁻¹. Moreover, they collected evidence of an electromagnetic origin of friction by verifying the dependence on the distance and applied voltage as predicted by Volokitin and Persson². Going through T_c to lower temperatures, the friction dropped to one third of the initial value. Below T_c, they argue, electronic dissipation is excluded and phononic interaction should govern the friction. Again, the dependence on the distance and applied voltage fitted the prediction, and even the temperature dependence they found is somewhat more gradual than that in the experiments of Dayo et al. [http://www.nature.com/nmat/journal/v10/n2/full/nmat2947.html]

Friction not so simple

Friction is certainly a standard part of middle school physics courses. It is observed that, to move an object in contact with another one, a force must be applied larger than the static friction, which is supposed to be uniform across the interface. However, this picture is inadequate. Actually, it was perceived that non-uniformity occurs at least locally. Understanding the nature of friction and how to model it better is not only theoretically interesting but practically imperative, because friction is relevant to a plenty of phenomena, such as rampant earthquakes and snow ruptures. Friction is the force that holds those events from bursting out. On the hand, it is also desirable to gain insight into how slip occurs locally when friction fails. This is key to modeling. This latest publication investigated this problem.

The way in which a frictional interface fails is critical to our fundamental understanding of failure processes in fields ranging from engineering to the study of earthquakes. Frictional motion is initiated by rupture fronts that propagate within the thin interface that separates two sheared bodies. By measuring the shear and normal stresses along the interface, together with the subsequent rapid real-contact-area dynamics, we find that the ratio of shear stress to normal stress can locally far exceed the static-friction coefficient without precipitating slip.
Moreover, different modes of rupture selected by the system correspond to distinct regimes of the local stress ratio. These results indicate the key role of nonuniformity to frictional stability and dynamics with implications for the prediction, selection, and arrest of different modes of earthquakes.

confined water

The properties of water under conventional conditions are largely known to scientists. But those under unusual cases are rarely revealed. One example is, what happens to the viscosity and elasticity of water confined to two solids in thin layer of nanometer? According to a recent study[1], there may happen a solid-like transition with respect to the rate at which the two solids approach each other, that is, elasticity increases while viscosity decreases.
[1]Phys. Rev. Lett. 105, 106101 (2010)

Schematic illustration of a confined fluid. Imagine that a liquid droplet is placed between a ball and a flat surface, and a ball is allowed to fall (right panel) onto it. When the thickness of the liquid is plotted schematically against time after the ball begins to fall, the film thickness remains finite at equilibrium (bottom left panel). This is because fluid tends to layer parallel to the solid surfaces. When the local liquid density is plotted against the distance between the solid boundaries, it shows decaying oscillations with a period of about a molecular dimension (top left panel). When these density waves shown in the bottom panel come sufficiently close to interfere with one another, the liquid can support force at equilibrium.

Imaging the coffee blend with cream

This is so cool !
http://www.insidescience.org/research/coffee_cup_secrets

The unknown aspects of friction

Perhaps every one has some knowledge of friction, which he gains through experience and/or science education. As a part of daily life, friction is as common as gravity. Many people think that, its thorough understanding must have been attained. The reality is nonetheless not so satisfactory. Here [1] is a piece of work attempting to resolve an issue concerning the onset of sliding under a force at the trailing of a slider, which rubs against a track.



The author set up a phenomenological model and solved it numerically. The interaction between the slider and the track is modeled by spring contacts, whose rupture and formation are governed by a simple law set by a threshold force. Their study showed that, the sliding is always preceded by crack-like fronts, which signifies the propagation of a broken contact.

[1]PRL, 103:194301(2009)