A new approach for studying friction on ice helps explain why the ease of sliding depends strongly on temperature, contact pressure, and speed.
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In Latvia, where bobsledding, luge, and skeleton are popular, ice can be more than just “slippery.” The local language has another term slīdamība—roughly translated as “slideability”—which refers to the ease of movement on a surface. This terminology signifies the awareness that sliding on ice depends on multiple factors—something physicists have had trouble explaining despite 160 years of effort. Previous work has focused on the water layer that forms between the ice surface and the sliding object, say, an ice skate. However, this model does not show why friction is higher near the ice melting point than it is at lower temperatures (Fig. 1). A new study of the solid properties of ice may provide a solution. Rinse Liefferink from the University of Amsterdam and colleagues have performed a series of experiments, in which they measure both the friction of a sliding object and the hardness of ice over a wide range of conditions [1]. The observations show that the hardness decreases as the temperature increases, leading to a high-friction “ploughing” behavior once the sliding object is able to penetrate the softer ice surface. This novel approach to studying ice friction could help in developing technologies that improve safety for winter drivers or give an edge to winter athletes.
The traditional oil paints popular up to the 19th Century were made by grinding pigments with linseed, walnut or poppy seed oil – but while they produced stunning hues, their drying time meant that it could could take months or even years to complete a painting if several layers of colour were used.
Artists like JMW Turner understandably didn’t want to spend their lives watching paint dry, so they collaborated with chemists to produce gels that could be added to oil-based paints to shorten drying times.
The supramolecular structure of the gel is revealed by freeze fracture electron microscopy. Aa frozen specimen is fractured along natural planes, making an impression or replica of the exposed surface,then examined using transmission electron microscopy. Credit: LAMS (CNRS/UPMC)
They found that lead, in its acetate form, is essential to the formation of these gels. But other questions remained – how do they bind with the paint? How do they age?
The researchers reconstituted the original paint formulas and were able to reproduce the gels using lead and mastic to study their rheological properties, such as flow and deformation behaviour. They found that even minute amounts of the gels would modify the characteristics of the paint, yielding superior elastic properties.
On canvas, the consistency of gels and gel-paint mixtures differs greatly from that of paint alone, which spreads without retaining volume. Credit: Hélène Pasco, LAMS (CNRS, UPMC)
Using spectroscopic techniques, they defined the molecular interactions of the hybrid organic-inorganic gels and the mechanisms of the gelling process. They found that the lead not only catalysed the gelling process but contributed to the structure of the medium itself.
The challenge, now, is to understand how the lead binds with the resin and which conditions are best and worst for their conservation.
Recipes for three-dimensional (3D) printing, or additive manufacturing, of parts have required as much guesswork as science. Until now.
Resins and other materials that react under light to form polymers, or long chains of molecules, are attractive for 3D printing of parts ranging from architectural models to functioning human organs. But it’s been a mystery what happens to the materials’ mechanical and flow properties during the curing process at the scale of a single voxel. A voxel is a 3D unit of volume, the equivalent of a pixel in a photo.
Now, researchers at the National Institute of Standards and Technology (NIST) have demonstrated a novel light-based atomic force microscopy (AFM) technique – sample-coupled-resonance photorheology (SCRPR) – that measures how and where a material’s properties change in real time at the smallest scales during the curing process.
