Sidney Nagel received his B.A. from Columbia University in 1969 and his Ph.D in Physics from Princeton University in 1974. He was a Research Associate at Brown University before moving to the University of Chicago in 1976, where he is currently the Louis Block Professor in the Physical Sciences.

Nagel works to make sense out of disordered, non-linear and out-of-equilibrium systems. Initially his research focused on the transition that occurs when a liquid is supercooled into an amorphous solid. His interests subsequently broadened to include granular materials such as sand and coffee grounds. Along a somewhat related line of research, he has been studying the singularities that occur in hydrodynamic flows. A drop falling from a faucet is a common example of singularity formation, as the liquid breaks up into two or more pieces.

Nagel, an honored member of several professional societies, has also received the 1996 Quantrell Award for Excellence in Undergraduate Teaching and the 1999

2.22.01 Michael Dilger asked:
If you pour a Guinness beer into a pint glass, the bubbles appear to go down, which is odd considering the beer seems substantially more dense than the bubbles. If you take a glass which is narrowest in the middle (wider at the bottom and the top) and pour a Guinness into it, the bubbles at the top of the glass go downward, but the bubbles at the bottom of the glass go upward, and they collide at an interference layer at the narrowest part of the glass. I'd love to hear the explanation.

Nagel's response:
I am certainly not an expert on Guinness but have heard a lot about this effect in the past year. A group from France and Australia did a computational study of this problem. I looked up their work on the WEB here. At that location, there is description of what sort of computations were done to show why the bubbles behaved in a non-intuitive way described in the first part of your question. According to this work, fluid flows are set up in the glass by the large number of bubbles rising from the container bottom. As they rise, they carry the surrounding fluid up with them creating a large-scale convection role. As in a spaghetti pot heated from below, such a convection role establishes a return flow along the sides of the glass that brings the fluid back down to the bottom. This return flow contains some small bubbles which is what is observed from the outside as bubbles sinking in the glass.
Although I have not observed the effect in the glass with the thin neck described in your question, I would guess that the convection rolls are severely altered from what they would be in a tall straight glass. No longer will there be a single roll from top to bottom in the entire container and the rolls in the two halves of the glass will have different shapes and directions. This will make it look as if the bubbles are falling in some cases and rising in others. In all cases, I would expect that it is the convection in the liquid which is responsible for the direction of motion of the bubbles and not the bubbles falling with respect to the background fluid.

2.22.01 Catherine Hogan asked:
Moody ashes: I clean the ashes out of my wood stove every 3 days. I've noticed that on some days, the ashes seem to be so airborne it's difficult to get them from the stove into the bucket without covering my whole house with ash dust. But on other days, the ashes seem to be heavier and less apt to fly around. I suspect a difference in atmospheric pressure, but when I questioned a "scientific" friend about this, he just laughed at me. In fact, I don't even understand atmospheric pressure changes so I don't know why I thought that may be causing the ashes to behave differently day by day. Can you help clear up this mystery? Thanks.

Nagel's response:
I can only hazard a guess as to the reason. I assume that the difference occurs because of different amounts of humidity in the air. When the air is more humid and contains more water vapor the small ash particles become slightly damp and this makes them stick to each other. (Think of what happens when you have two slightly damp pieces of plastic which are stuck to one another. They are very hard to separate whereas when they are dry it is much easier.) Humidity also acts to cut down on static electricity. (In winter when there is much less water vapor in the air, just walking on a carpet can charge you up to give a hefty spark when touching a doorknob. In summer you don't have such charging problems.) When the ash picks up small electric charges, it repels other like-charged pieces of ash. As you try to sweep up the ashes, they rub against the walls of the stove and pick up charges. This can make the entire ash heap self-repelled and hard to handle - Trying to clean it up can be like trying to herd cats!

2.22.01 Jonathan Schulman asked:
Well, this question probably is more trivial than the discussion I saw concerning grains of sand and coffee stains (quite interesting!), but it's bugged me for awhile, and as it's at least superficially related. Perhaps someone can enlighten me: why do most (if not all) porous substances, e.g. paper, become darker when wet? It's such a commonplace event, we just take for granted that, when wetness is perceived, it's because the wet spot is darker than the surrounding area. But why? How does the wetness change the way light is reflected so as to make it appear darker? I'm guessing the answer may be simple, but I'd certainly like to know it. Thanks!

Nagel's response:
The reason is the same as why wet sand on a beach near the water's edge is darker than it is higher up, away from the water, where it is dry. The explanation is that many of the things we think of as white look that color because they are made up of small particles which are transparent (like the sand on the beach which is essentially glass) but which scatter light effectively because they are small and rough with a lot of surface area. Because the particles scatter all the wavelengths of light right back at the viewer, the material appears white. If the material didn't scatter the light so strongly, the light could penetrate the material to a deeper level and would travel a longer distance inside the material before it gets back out to the viewer's eye. Along this distance the light can be absorbed by small impurities decreasing its intensity and the material would then appear to be darker. One effective way that one can decrease the amount of scattered light is to get rid or reflections at the surface of the individual particles. This can be done by surrounding the particle entirely by an index matching fluid. (That is by surrounding it with a liquid that has the same index of refraction as the object itself.) Water helps get rid of scattering in just this way. Although it does not have the identical index of refraction as that of the particles, its index is still much closer to it than that of air. Thus when the object is wet, the light does not get reflected or scattered as efficiently at each interface and the light penetrates farther into the object. The wet material then appears to be darker than the dry one.