Monthly Archives: October 2013

What’s in a flame?

We see fire all around us. Well, hopefully not all around us, but in fireplaces and candles and bonfires. So what’s in a flame?

A flame is the visible part of a sequence of chemical reactions called combustion, like the tip of an iceberg that indicates much more is present below the surface. To start combustion, we need oxygen, heat, and a material sensitive to both which acts as a fuel. When the oxygen and fuel are raised to a high enough temperature, the oxygen molecules interact with the molecules of fuel, exchanging electrons and swapping atoms while releasing energy. These atomic changes can make the material look very different, as you can see if you compare a piece of wood with a piece of charcoal.

The released energy, which comes from the changing configuration of atoms and chemical bonds in the burning material, can be in the form of heat or in the form of light. It’s only the hottest part of the flame that emits light, the part which is a plasma. What color the light appears as to us depends on the energy of the particles of light being released, which in turn depends on what elements were present in the fuel. If you have a propane flame and you add sodium or copper, you can see the color of the flame change. This happens because combustion of those materials releases different amounts of energy than combustion of propane.

As materials undergo combustion in the flame, the resultant gas expands. This means that the molecules move further apart, and the gas becomes less dense. Here on earth, the presence of gravity means that denser gases are pulled downward and lighter gases rise upward, so the hottest gas products will rise up and away from the fire. This is why flames often look like they are rippling upward, even though their base never moves. But experiments have been done in space to show that, without gravity, hot gases do not rise away from the flame. So flames in space are more or less spherical, and are usually all blue which would indicate the hottest part of a flame on earth.

What a flame looks like can tell us a lot about what a material is made of, how much oxygen is in the air, and even whether there is gravity or not! But fundamentally, flame is a visual sign that the burning material is being chemically changed into something new.

The Colors of Noise

Talking about the ‘science’ of something often evokes ideas like precision, or a very controlled study where every parameter is meticulously accounted for. But the natural world is not necessarily precise or controlled; evolution gives the quickest functional solution, rather than the most elegant. And as with entropy, management of disorder and messiness is often simpler to achieve than full control. So even as we attempt to study and engineer our natural environment, we have to accept some level of randomness and chaos, or even perhaps take advantage of it as nature does.

It’s with that in mind that I’d like to talk about noise. Colloquially, noise refers to sounds, but specifically disordered or cacophonous ones. This comes from the idea of noise as a sort of randomness that’s part of a signal carrying information, like static on a phone line or a video. Those examples of noise are like random factors that affect the timbre or perception of the signal itself, but one can also think about noise more generally in nature as a perceivable, measurable result of some inherently random physical process. Such processes are called ‘stochastic’, as opposed to ‘deterministic’ processes where the probability is determined or fixed by the initial conditions.

But as you might imagine, there are lots of different physical phenomena that might lead to noise. So the noise resulting from stochastic processes has a sort of fingerprint that can be read out and may give useful information about the system the noise came from. For example, noise on a telephone line may be different from noise in a photodetector, even if both come from random things that electrons are doing in the material. But how to tell the two apart? Variations in noise are usually analysed by separating out the noise by frequency, which is to say looking at what parts of the signal occur once per second, versus twice per second, versus a hundred or a thousand times per second. This is described as the ‘power spectrum’ of the noise, but more lyrically, the most common power spectra for naturally occurring noise are called the colors of noise.

The power spectrum above shows noise that has the same power at all frequencies, which is called white noise, because of the idea that white light contains all the visible colors in equal proportions. White noise comes from thermal processes, which is to say atoms using the heat and kinetic energy around them to jump up to higher energy states and then back down. And since thermal processes don’t have any preference for one frequency of activity over another, the frequency spread of thermal noise is flat.

There’s also brown noise, which isn’t named for the color brown but rather for the scientist Robert Brown. He was a botanist who discovered that pollen grains in water moved in a random pattern when observed under a microscope, and initially could not explain the mechanism. Many decades later, Einstein explained the movement as a result of molecules in the water buffeting the pollen grains, causing a random movement of the grain itself to be visible even though the water molecules were too small to see. This movement is called Brownian motion, or a random walk, and so Brownian noise is characterized by enhanced low frequencies as you’d get in a random walk. Auditory brown noise sounds like a waterfall, softer than the other colors of noise. Here’s an animation of the pollen grains colliding with smaller water molecules, where you can see the randomness that we can call noise:

Pink noise, or flicker noise, decreases in power as the frequency of the noise increases. It’s also called 1/f noise, because the power of pink noise goes inversely as the frequency f. Pink noise comes from the trapping and detrapping of charge carriers like electrons, which can be counted as a stochastic process that is more likely at lower frequencies. Because the human ear is less sensitive to higher frequency noises, pink noise is often used as a reference signal in audio engineering.

But just as with light, our senses and processing of noise can distort it between its physical origin and our perception of it. So for example, we may perceive white noise, which has a flat power spectrum, as louder for frequencies that our ear can better detect. I mentioned before that the human ear is less sensitive to higher frequencies, but overall that sensitivity can be mapped to create a power spectrum that will appear flat given our sensory distortion. This sort of noise is called grey noise.

There are other colors of noise, such and blue and violet noise which increase in power at higher frequencies, green noise which is the selected center of the power spectrum for white noise, and black noise which is a fancy way of saying silence. You can listen to some colors of noise here. There is also a type of noise specific to small numbers of some countable event like trapping and detrapping, where the discreteness of the event becomes important. Fluctuations in a small number matter more than fluctuations in a large number. This is called shot noise, and is common in any signal that has a small number of countable events, like measuring individual electrons or photons. But noise is just a consequence of the inherent randomness in our physical world, an avatar revealing the exact physical mechanism behind its own creation.

Happy Ada Lovelace Day!

Today is Ada Lovelace Day, a day of blogging about women in science! (Not necessarily blogging by women in science, which is every day here.) The day is named for Ada Lovelace, who was an important figure in the nascent days of computer science, back in the 1800s when it was more of a theoretical math field concerned with the creation of calculation engines. The idea of Ada Lovelace Day is to write about a woman in science, technology, engineering, or math, which raises awareness of all the great women, both now and in the past, who have done amazing things in the STEM fields. There will be lots of stories about various role models over at the official site once the day is concluded, but the scientist I wanted to tell you all about here is Mildred Dresselhaus.

For the last twenty years or so, materials made from carbon have been getting exponentially more and more attention. Carbon is an essential building block in many of the chemicals that are important for life, but there are also huge differences between materials made from carbon depending on how the carbon is bonded. Diamonds and coal are both forms of carbon, but with wildly different crystal structure. So many of the hot carbon materials from recent years have come from new ways that the carbon atoms can be arranged. For example, carbon nanotubes are like rolled up sheets of carbon, and graphene is a sheet of carbon that’s only one atom thick. Both carbon nanotubes and graphene have very high mechanical strength, electrical and thermal conductivity, and low permeability for their size. And there are a lot of other ways carbon can be nanostructured, collectively referred to as allotropes of carbon. You can see some of them in the image below, such as (a) diamond, (b) graphite (multiple sheets of graphene),  and (h) a carbon nanotube.

But Dresselhaus was into carbon before it was cool, and has been a professor at MIT since the 60s studying the physics of carbon materials. Her work has focused on the thermal and electrical properties of nanomaterials, and the way in which energy dissipation is different in nanostructured carbon. Her early work focused on difficult experimental studies of the electronic band structure of carbon materials and the effects of nanoscale confinement. And she was able to theoretically predict the existence of carbon nanotubes, some of their electronic properties, and the properties of graphene, years before either of these materials were prepared and measured. Her scientific achievements are extremely impressive, and she has gotten a lot of honors accordingly.

And as you can imagine, things have changed a lot for women in science over the course of her career. When she began at MIT, less than 5% of students were female, and these days it’s more like 40%. But of course, it helps female students quite a bit to see female role models, like Dresselhaus. Which is the entire point of Ada Lovelace Day!

You can read an interview with Mildred Dresselhaus here, and more about her scientific achievement here.

Topic Index

I have written a lot about basic science and technology on this blog! This post is intended to serve as an index, loosed organized by topic, of those posts.

Atomic Physics

Condensed Matter Physics

Electronics and Circuits

Quantum Mechanics and Nanoscience