Let’s put winter behind us — it’s time to think about sand.
Physicists think about sand a lot because they don’t really understand how it works. How can sand — and other granular materials such as grains or rocks — behave both like a liquid that flows through fingers and a solid that forms dunes?
Physicists have a theoretical framework to predict how microscope objects like molecules flow and freeze but lack the fundamental concepts to describe how assemblies of macroscopic objects behave similarly.
Last year, Bulbul Chakraborty, the Enid and Nate Ancell Professor of Physics received a three-year, $1 million grant from the W.M. Keck Foundation to develop the first predictive theoretical framework to characterize the collective behavior of a large number of macroscopic objects.
This theoretical representation of experimental data (Behringer Lab.) provides a quantitative tool for identifying the fluid to solid transition in a granular solid. The fluctuations in the net show the change in strength of the solid. Credit: Sumantra Sarkar, Brandeis University
She and her team are developing quantitative tools for identifying the fluid to solid transition in granular solids in order to build a theoretical framework to describe assemblies of macroscopic objects.
Think finals are stressful? Try being chased by a lion.
Humans evolved in a dangerous, threatening world where stress usually preceded bodily injury. As a result, the evolutionary theory goes, our stress response system jump-starts our immune system, triggering the deployment of white blood cells and increased expression of the inflammatory gene interleukin-6 (IL-6) into the blood stream.
As part of our immune system, IL-6 can help stave off infection and promote wound healing, but if left unregulated, this inflammatory agent can contribute to cardiovascular disease, cancer and Alzheimer’s. Knowing where and how in the body various forms of IL-6 are produced is an important step in understanding how stress relates to disease.
In a recent study published in Brain, Behavior and Immunity, Brandeis researchers discovered that one’s perception of stress directly impacts how genes express stress. The paper was written by graduate student Christine McInnis and professor Nicolas Rohleder, and co-authored by Danielle Gianferante, Luke Hanlin, Xuejie Chen and Myriam Thoma.
Researchers have long known that IL-6 proteins increase in blood plasma after stress but this is the first time scientists have observed increased activation of the IL-6 gene in white blood cells as a stress response.
The size and duration of the increase is closely tied to perception and mood, according to the study. The more a person stresses out, the more IL-6 is expressed.
“Stress perception initiates the gene expression and self-reported mood changes are directly related to the size of the gene expression response,” says McInnis.
In other words, we can, to some extent, control our genetic response to stress by moderating how we perceive stress.
“If you learn to control your stress levels, your genes will follow,” says McInnis.
So next time you’re stressed about a big test, calm down: It might be a bear of a test, but it’s no lion.
Here’s the rub with friction — scientists don’t really know how it works. Sure, humans have been harnessing the power of friction since rubbing two sticks together to build the first fire, but the physics of friction remains largely in the dark.
Ancient Egyptians understood friction well enough to know that heavy objects moved easier across wet sand than dry sand, as can be seen in this picture.
In a new paper in Nature Materials, Brandeis University professor Zvonimir Dogic and his lab explored friction at the microscopic level. They discovered that the frictional force is much stronger than previously thought. The discovery is an important step toward understanding the physics of the cellular and molecular world and designing the next generation of microscopic and nanotechnologies.
Dogic and his team focused on the frictional forces of actin filaments, essential cellular building blocks responsible for many biological functions including muscle contraction, cell movement and cell division. All of these processes require filaments to move and slide against one another, generating friction.
A schematic of the experimental set-up. Actin filaments attached to gelsolin-coated beads are assembled into anti-parallel bundles using optical traps. Bead 2 is pulled at a constant velocity while simultaneously measuring the force exerted on bead.
Scientists assumed that the frictional forces of these movements were minimal, acting more like weaker hydrodynamic friction — like pulling an object through water — than the larger solid friction — pushing an object across a desk.
But Dogic and his team observed the opposite. They developed a new technique to measure friction, and when they dragged two actin filaments against each other, they observed frictional forces nearly 1,000 times greater than expected — closer to solid friction than hydrodynamic friction.
This is due, in part, to interfilament interaction. Imagine filaments as two beaded strings, one on top of the other, pulled in opposite directions. As the strings move, the beads must go up and over their counterparts on the opposite string, generating even more friction. By observing this interfilament interaction, Dogic and his team were able to measure the frictional forces and tune them, altering the forces to include more or less friction.
“Before this research, we didn’t have a good way of controlling or understanding friction,” Dogic says. “We still have a lot more to understand but now, one of our oldest sciences is becoming less opaque.”
The paper was coauthored by Brandeis scientists Andrew Ward, Fiodar Hilitski, Walter Schwenger and David Welch; A. W. C. Lau of Florida Atlantic University; Vincenzo Vitelli of the University of Leiden, and L. Mahadevan of Harvard University.
Last week, a poorly lit picture of a dress sparked one of the fiercest — and perhaps most meaningless — debates in Internet history: Was it white and gold or blue and black? Now that the dust has settled, and the definitive answer established it’s blue and black, it’s time to move onto a more important question: Why did people see the dress differently? ReAction asked biology professor Stephen Van Hooser to break down the science of the dress.
We tend to think we are able to understand the world perfectly through our eyes. In reality, our visual system makes best guesses with limited information. Take, for example, the fact that the world is three-dimensional but the images on our retina are two-dimensional. Our visual system has to make guesses as to the locations of objects based on this two-dimensional image.
Visual illusions force us to confront the fact that the brain still makes guesses, or especially when the visual system doesn’t have all the information its needs.
Consider the “Cornsweet effect” (Purves et al., 1999). If we look at the following scene, we perceive that the gray patch above the white rectangle has the same luminance as the gray patch below the white rectangle.
Cornsweet Effect
However, if we fill in the white rectangle with a rounded contour and transition in luminance, our perception changes. We believe the light source must be coming from above, and that it is shining more directly on the upper surface than the lower surface. Yet, because the amount of light that hits our retina from the upper surface and lower surface is the same, we assume that the upper surface is inherently darker than the lower surface. That is, we imagine that if we were to shine a light directly on the two surfaces, the upper surface would appear darker than the lower one.
Our perception is influenced by assumptions that our visual system makes about the light source and reflectance of the materials.
This is what is going on in “the dress,” except that our uncertainty is in the chromatic nature of the illuminating light. Is the illuminating light daylight, which contains a broad range of component colors ranging from blue to red? Or is it an artificial light source, which usually contains much higher percentages of green, yellow and red light?
An object that looks white when illuminated by daylight will reflect more yellow light when it is illuminated by a yellowish light source, simply because of the color of the light that’s being shone on it. However, if we know that the light source is yellow-dominated, and we have some objects in the room that allow us to guess that the light source is yellow-dominated, then our brain will “correct” the raw data it is receiving and we will still perceive the object as white.
Now let’s consider an object that looks blue when illuminated by daylight. When it is illuminated by a yellowish light source, it will still reflect the weak blue light that is present in the yellowish light source, and it will also reflect some weak yellow light simply because the object is getting hit with a lot of yellow light. That is, in terms of “raw data,” our eyes will collect light more broadly across the spectrum — blues, greens, yellows and reds. If we have cues that tell us that the light source is yellow-dominated, we will still see the object as blue.
If we see a blue object illuminated by yellow-dominated light but for some reason our brain thinks that it is actually illuminated by daylight, our brains will perceive the object as white.
The dress illusion is strong because the light source is ambiguous. Some people might imagine that the dress is being illuminated by daylight, and they will perceive the material as white. Others may think it is under strong artificial illumination, and the material will appear blue.
From moment to moment, our impression can change, but we always have a single impression.
It looks pink to me.
This story was written by Stephen Van Hooser.
Update: Rosa Lafer-Sousa at MIT has combined the dress image with images from Beau Lotto that evoke strong sensations of yellow-dominated or blue-dominated illuminating light. Separately zoom in on both images. See if you can see the dress as blue or white in the different figures! http://web.mit.edu/bcs/nklab/what_color_is_the_dress.shtml
