The wild and crazy world of the quantum
Back in 1801 (or thereabouts), Thomas Young—physician, physicist, linguist, member of the Royal Society, etc.—performed an experiment intended to prove the validity of the wave theory of light. He used a single slit to render incoming sunlight coherent. Then he allowed the coherent sunlight to strike a card positioned so as to divide the beam. Young succeeded in producing interference fringes on a waiting screen surface. The existence of those fringes did help prove that light was composed of waves. Of course, other experiments, such as those dealing with photoelectric phenomena, helped prove that light was composed of particles. In reality, Young's experiment was a huge step on the road to demonstrating the wave-particle duality of electromagnetic radiation. The experiment, modernized and modified, reverberates even today within the field of quantum mechanics. Many hail Young's work as the most beautiful experiment in physics. But why?
Richard Feynman is often quoted as saying that the double-slit experiment, as it came to be known, "contains the only mystery...of all quantum mechanics." What mystery is Feynman referring to? Using a ripple tank, it's easily demonstrated that water waves emerging from two slits will interfere generating fringes like those Young produced. This is often done in elementary science classes. So what's the big deal?
Now we get to the beautiful part. Instead of the sun, let's use a laser. Since its beam is already coherent, all we need to add is a closely spaced double slit and a screen. Let's reduce the light intensity way, way down until only individual photons are zipping on by. Even though only single photons pass through one or the other slit at a time, and even though any particular photon may have passed through the apparatus and struck the screen before another photon enters, over time interference fringes form. It's as if individual photons were interfering with themselves. How is that possible? That's the really big mystery.
I had to see this for myself. Below is a diagram of my setup. I use a randomly polarized green HeNe laser (wavelength = 543 nanometers) as the coherent light source. I chose green because the image intensifier has an increased sensitivity to green light and because the green HeNe laser I have only produces a 0.85 milliwatt beam. (This means that a quantity of, roughly, two thousand million million photons are being produced each second. A low-gain laser doesn't require quite so much filtration to reduce the photon rate down to where it needs to be for the experiment.) Next in the optical train is a pair of polarizers. The first element polarizes and absorbs some of the green beam's intensity. The second polarizer offers a further reduction of light intensity that is controllable by simply rotating the element. The laser beam is considerably dimmer as it strikes the angled, first-surface mirror. The beam then passes through a sandwich of neutral density filters and a pinhole. The laser beam is now totally invisible to the eye. As shown, the weakened beam diverges from the pinhole and strikes two very closely spaced slits. (I made these slits by taping two safety razors together and carefully scratching a glass disk that had been spray painted black. The pinhole was made by pressing a fine needle against a disk of thin aluminum foil placed in contact with a sheet of glass. It took me a number of tries before getting satisfactory results.)
Calculations showed that only a few dozen photons pass through my apparatus each second. Further calculations revealed that only one photon at a time passes through either slit and zips away long before the next photon enters. The photons next strike the image intensifier/CCD camera. The output signal is transferred to the Argus 20 image processor module and PC. The image processor allows me to selectively detect and display individual photons instantaneously with or without storage. Both display techniques are shown in the video. (Note: the video was recorded using time-lapse to speed up the otherwise slow accumulation by a factor of 15.)
In addition to the totally baffling appearance of self-interference, it's easy to demonstrate that simply knowing which slit a photon passes through destroys interference. I couldn't readily modify the above apparatus to do this. That experiment required constructing a special Mach-Zehnder interferometer. (However, it was much simpler and far less costly to build the interferometer than the double-slit system shown. Any good college physics book will show the way.) The double-slit experiment can also be carried out with electrons and atoms. When you look at the video, I hope that you are as amazed as quantum physicists who ponder the impossible phenomenon. It's easy to see why so many agree that Young's experiment truly is the most beautiful experiment in physics!
Cyclosis as a means of getting around
One benefit of living in Florida is ready access to plants considered pests. Two such plants are Hydrilla verticillata and Sagittaria spp. Hydrilla closely resembles both Brazilian elodea and the American waterweed, Elodea canadensis. Both hydrilla and sagittaria were introduced here by fresh-water aquaria enthusiasts who, apparently, lost their enthusiasm at some point. Hydrilla, especially, is a nuisance weed that requires continuous harvesting to clean up. These plants are bad for boaters but great for microscopists, as you will see. I gathered up several plants lining the edge of a nearby canal in a bottle of canal water. After giving the plants a chance to accommodate to air conditioning, I placed one or two leaves of each plant on microscope slides along with a few drops of water. After adding cover slips, the slides were examined and photographed using my Leitz Metalloplan. Both standard white light and intense blue light from an LED were used for the photography. Time-lapse recording was used throughout as indicated in the video.
Both plants nicely exhibited cytoplasmic streaming or cyclosis. The chloroplasts were seen moving along with the stream. These organelles may benefit through increased exposure to light as well as being exposed to varying concentrations of nutrients. Also, the plants probably move cell nutrients around to aid in growth, waste disposal, etc. Some conjecture that the streaming disperses cell contents so that when cells divide, each cell gets a fair share. Motor proteins are involved in cyclosis along with microfilaments of actin mentioned in the video. Probably, microtubules also play a part.
(I originally produced this video for display at the very first Society for Amateur Scientists conference—held in Philadelphia in 2002— to help demonstrate LED fluorescence in action. If you've had a chance to look at other sections of my Web site, you will probably recognize the voice of my wife Joanie narrating.)
In the video, we first see hydrilla leaves under normal white light. This is followed by both hydrilla and sagittaria being viewed under intense, blue LED illumination. For the blue light recording, a deep red filter was placed between microscope and camera. This allowed the camera to record the deep-red fluorescence characteristic of chlorophyll while blocking the blue excitation light. Time-lapse software permitted speeding up the cyclosis movement either 7.5 or 90 times normal as indicated in the video.
Elodea, mentioned above, also exhibits cyclosis. Many plants, protists, and diatoms do as well. Light intensity, distribution of ambient light, temperature, and pH also have significant roles in cyclosis.
Cyclosis Video
Ball lightning?
A rarely seen form of lightning, ball lightning has been described as floating balls of plasma, typically around 6-12 inches in diameter. Their colors range from yellow, white, blue, through blue-white. Lifetimes can range from seconds to minutes. Velocities range from 1 meter/sec to 240 meters/sec. They have been observed during earthquakes. (Check the References section for the book on ball lightning by Stanley Singer.) They can float along the ground, travel up and down chimneys, or pass through closed windows as if there were no glass. Sometimes they vanish with nary a whimper and sometimes quite explosively, producing sulfurous smells (ozone). During these explosions, animals have reportedly been killed.
As depicted on the cover of the January 1916 issue of The Electrical Experimenter, it is believed that ball lightning killed Georg Wilhelm Richmann in 1753 during one of his experiments with natural lightning —probably his last one. Richmann, a German physicist, was carrying out studies on atmospheric electricity in St. Petersburg, Russia at the time. He rushed home to his lab when he realized a thunderstorm was on its way. Richmann wanted to see how the insulated iron rod he set up, running from the roof to his lab, would behave during the storm. He frequently took measurements of spark length to get an idea of the state of atmospheric electricity. An eyewitness filled in details after his death, but it appears that ball lightning struck Richmann on his head leaving a red welt and blowing off his shoes. (Thanks to Wikipedia for filling in some details on Richmann's demise. However, there is another version of the story stating that Richmann was startled by his assistant walking into the lab, thus causing the experimenter to carelessly move too close to the rod.)
In the issue of The Electrical Experimenter dedicated to ball lightning, there is an article describing work performed by M. Stéphane Leduc in attempting to replicate the phenomenon in his lab. Dr. Leduc, much like Thomas Young, was a physician trained in physics.
Using a static generator similar to a Wimshurst machine, Leduc electrified two sharp needles. The needles were placed in perpendicular contact with the emulsion side of a silver bromide, gelatinized, glass photographic plate. Under the plate, Leduc situated a rectangular piece of foil insulated from the front surface by a gap all around. When the static machine was activated, small globes of plasma glided across the surface of the plate. According to Leduc, sometimes the globes exploded.
(More recently, a number of experimenters have attempted to produce ball lightning using very large Tesla coils, microwaves, etc. In the past, it had been noted that large plasma spheres could be seen while switching off heavy-duty submarine batteries. But these may have been exploding metal fragments that flew off the throw switch.)
I was able to reproduce Leduc's experiment. An unexposed, unprocessed holographic plate served very nicely in lieu of Leduc's standard photographic plate.
This video, recording some experiments performed with the apparatus illustrated at left, presents a closeup view of the glass holographic plate. A brass sphere with an attached needle coated with paraffin was placed in contact with the plate.
The paraffin was required to prevent corona discharge from draining off power. Only the point of contact between needle and plate should be discharging. I have never gotten the experiment to work without paying careful attention to the paraffin insulation. If plasma balls failed to form, sure enough there were always brush discharges occurring at different points on the brass electrode. When set up properly, the moment high voltage is supplied dozens of small, blue-white globes can be seen spewing from the needle's tip, scattering to the outside edges of the plate. For all the world, the plasma balls do look like tiny spheres rolling along effortlessly.
Recording the spheres in subdued light for the video was straightforward. CCD cameras are often quite sensitive to the near infrared and the near ultraviolet unless, of course, there are internal blocking filters present. What is happening? My guess is that there are multiple corona discharges occurring at the tips of the silver halide crystals making up the emulsion. The discharges move rapidly from crystal-to-crystal, driven by the cathode's negative charge. After a while, the corona discharge forms tracks of carbonized gelatin. Carbon is a great conductor too. Scratch a thin line on the emulsion surface, and the globes always fail to cross it. Instead, they sit at the edge of the scratch, on the side nearest the cathode, facing across a chasm as wide and insurmountable as the Grand Canyon—or so it seems. They seem unresponsive to magnetic fields. I have never seen them float above the surface of the glass plate. I've never seen them explode. After days of usage, the holographic plate is covered with patterns reminiscent of Lichtenberg figures. Fractal in appearance, the myriad of fine lines can be photographically processed thereby producing a permanent record.
Leduc's experiment is very interesting. But are we really looking at ball lightning? As the people at FOX News like to say, "We report. You decide."
WARNING: Do not attempt to reproduce this experiment unless you are very familiar with high-voltage techniques. Static generators are entirely capable of producing nasty, and even deadly, shocks.
Schlieren and shadowgraphy techniques
I've been looking for highly sensitive flow-visualization techniques that could work in the micro regime for some experiments. I decided to investigate the use of schlieren and shadowgraphic techniques for the microscopic study. There already is an established field of schlieren microscopy. Hoffman modulation contrast is an example of a well-developed technique within that field. But to see whether either of these these methodologies can work for studying ultra-microflows, I wanted to examine their behavior in the macro world first.
We've all seen examples of the effects of temperature gradients producing mirages on the open road on a hot day. Look carefully at the mirage, and the shimmering heat becomes clearly visible. If you are lucky, you might catch a glimpse of the heat exhaust from an overflying jet as it flies past nearby mountains. The dark versus light division between ground and sky presents a tool for observing schlieren in both these instances. Photos of hot, turbulent gases rising or generated shock waves surrounding airplane wings abound. Schlieren and shadowgraphy techniques are often used for studying density gradients and changes in refractive indices of gases, liquids, and solids. Foucault knife-edge mirror testing is the quintessential schlieren technique. Schlieren and its closely related cousin shadowgraphy date back in some form or other to the days of Robert Hooke, Benjamin Franklin, and countless others whose names are lost to history. More recently, scientists such as Toepler, Schardin, North, Holder, Weinberg, Settles, Merzkirch, etc., have further developed the field and restored its luster. Gary Settles and others have even put color back into the science [Settles 2001].
Place the mouse pointer into the photos to see the layout descriptions. The photos were taken at different times and reflect ongoing modifications in my setups.
For some of my schlieren experiments, I use a single parabolic mirror with a coincident light source using a beamspltter (as seen above). Although prone to producing multiple images (especially when used with multiple light sources), the single mirror system offers greater sensitivity, easier alignment, potentially fewer optical aberrations, and lower cost than the more common z-type schlieren arrangement. My light source is a square array of four ultrabright LEDs arranged as follows: red diagonally opposite green and blue diagonally opposite yellow. I place a translucent diffuser (in reality, a ground glass square rubbed with mineral oil to reduce the light scatter) in front of the LEDs to allow filling the mirror more evenly with light. To help alleviate multiple imaging, I position schlieren objects as close to the mirror as possible.
An iris diaphragm is used as the schlieren mask and serves the same purpose as a knife edge but this time for four light sources encircling its aperture. By opening up or stopping down the iris, I can control the system's sensitivity to density gradients along with modifying color effects, etc. The use of colored LEDs will come in very handy for future experiments that I have planned.
The optics are securely mounted on a Gaertner 1.6 meter optical bench that is every bit as heavy as it looks!
Schlieren video
This video is a very brief schlieren overview and displays five experiments:
(1) Up first is a laboratory butane/propane gas burner.
(2) The requisite burning candle, always a crowd pleaser, is seen next.
(3) A compressed Freon dust remover used for cleaning cameras and electronic equipment is shown with vapors blasting away at the parabolic mirror.
(4) Next up is a demonstration of a soap film supported in a wire hoop. The interference colors are gone, replaced by the color effects of varying refractive indices and density gradients.
(5) The final demonstration shows the cold end of a small Hilsch (a.k.a. Ranque-Hilsch) vortex tube.
This fascinating device deserves an entire Web site by itself. Connected to a source of compressed gas, nitrogen in this case, this "simple" device can produce hot and cold flowing gas from opposite ends. However, it can only be set to produce the greatest temperature extremes separately. That is, you can adjust the tube to get very cold nitrogen from one end while getting warm nitrogen from the other. Or you can get hot nitrogen from one end while getting cool nitrogen from the other. With compressed nitrogen at 80 psi, I easily achieved -20°F at the cold end along with +120°F at the hot end at the same time! ( I recorded those temperatures by inserting a minute temperature probe into the tube's throat at the cold end. The hot end was measured by placing the probe against the metal surface near that end.)I show only the cold end because the nitrogen exiting the hot end did not produce sufficient density differences to be visible. There is a restricting screw at the hot end that serves as a control valve to alter the balance of hot and cold gas—that's how the vortex tube is temperature tuned. The screw physically interferes with a vigorous flow from the hot end. In the video, you are seeing cold nitrogen gas emptying into an essentially nitrogen atmosphere. Room temperature was 78°F.
After a short period of operation, frost builds up on the cold end. Incidentally, the vortex tube can operate with compressed air or other gases.
Shadowgraphy is a very broad term. It can refer to casting hand shadows for entertainment as well as to multiple methodologies in the sciences. Applied to fluid dynamics, shadowgraphy bears some similarity to schlieren, but there is no knife or restricting aperture required. Shadowgraphy is capable of presenting very useful images that, in some instances, look a lot like those taken with schlieren setups. Even experts sometimes confuse the two. However, the techniques are not equivalent with regard to the nature of the information supplied and the edge, so to speak, definitely goes to schlieren. Shadowgraphy is often much easier to implement though. Accordingly, I'll present several examples in the video.
When liquids (and gases) are subjected to heat, convection occurs. Usually, the heat is applied uniformly to the bottom of a containing chamber thus producing a positive temperature gradient. This is typically done using a highly conductive, highly reflective metal base equipped with resistance coils. Any resulting convection must then be studied through the top. Shadowgraphy is often used for these studies. If the top window of the chamber is in intimate contact with the liquid's surface, Rayleigh-Bénard rolls may be observed. Buoyancy is the cause of the observed convection. If the top window is separated from the surface of the liquid thereby allowing an intervening layer of gas, surface tension may become dominant. In that setup, visible density gradient cells are referred to as Bénard or Bénard-Marangoni type. I found it easier to use evaporation to produce the convection cells that I was after. No top window was used.The bottom surface of my convection chamber is a transparent window of sapphire allowing transmitted light for the experiments. I'll explain why I use sapphire windows in a moment.
In simple evaporation, the temperature is greater at the top of the liquid than at the bottom thus producing a negative temperature gradient. I use volatile liquids, such as pure Freon TF or a mix of hexane and Freon TF, for the experiments shown in the video below. (For additional information on the nature of the boundary enclosed cells, please check the References.)
Convection chamber
I machined my convection chamber out of a delrin plastic rod. I added a step to both top and bottom. This allowed dropping one-inch windows onto the ring. The bottom window was sealed with silicone sealant. Care was taken to prevent any silicone from intruding into the clear aperture. When in place, the windows project slightly above and below the ring to enable contacting cooling and heating surfaces. (Please move the mouse pointer into the image for details.)
As I mentioned, I used sapphire disks as windows. Sapphire has a thermal conductivity close to that of steel. This is 40 times higher than glass. For the simple evaporation experiments described below, glass would have sufficed. But I constructed the chamber with the intention of heating it from below. For those experiments, I placed the chamber on a disk of indium-tin oxide coated glass [Carlson 1999]. I fabricated the glass disk with two copper electrodes using silver epoxy. One nice thing about electroconductive glass is that it can be adjusted to go from room temperature to the boiling point of water, though I definitely don't recommend driving it that high—it will crack! I operated the glass off a dc power supply. Another benefit of the ITO glass is its transparency, allowing me to use the chamber in a transmitted light mode. Although the ITO disk may not produce the most homogeneous heat distribution, I'm hoping that the sapphire disk will act as a heat reservoir helping to smooth out local temperature variations.
With or without the ITO glass disk, the convection chamber ultimately rests on a thin sheet of glass. About 16 inches above the chamber is an LED light source (not shown in the photo). Positioned below the glass platform is a front surface mirror angled at 45°. The mirror reflects the transmitted light into a lens and CCD camera.
The video to follow displays two experiments performed using my convection chamber. The other clips are intended as demonstrations of shadowgraphy's surprising reach.
Shadowgraphy video
(1) Recorded in time-lapse, a thin layer of Freon TF is seen evaporating in my convection chamber. As the evaporation continues, various geometric shapes are shown forming, growing, shrinking, and coalescing. Hexagons can be viewed morphing into pentagons, rectangles into triangles, and any combination in between. At the end of the clip, you'll notice a region devoid of cells in the upper right half of the chamber. There still was a layer of liquid Freon covering the entire window, but the thickness dropped to such a level that convection cells were no longer being visibly generated. This process continued until the entire surface was clear of cells. Right after that the Freon dried up.
(2) Shadowgraphy reveals itself in the most surprising areas. X-ray images by their very nature are shadowgraphs. Seen here is a short clip that I borrowed from my Web site section on Radioluminescence.
(3) Ozone absorbs light in the shortwave UV. In the clip, ozone (along with hydrogen and oxygen) is being generated by electrolysis of an ice-cold, 3 molar solution of sulfuric acid in water. The acid solution is maintained at a low temperature during electrolysis to improve the ozone yield. Ozone's presence is made visible by its characteristic absorption of light from a shortwave UV lamp. The light is seen striking an x-ray screen placed right behind the flask, fluorescing in response except where interrupted by the shadow cast by ozone.
WARNING: OZONE MAY BE HARMFUL TO THE FREON LAYER!
(4) A layer of n-hexane lying on top of Freon TF produced the intense convection seen in the video that was recorded in real time. Both are nonpolar liquids and will mix completely. However, the mixing of Freon TF and hexane, as with alcohol and water, is very vigorous. The convection generated by the coupling of mixing and evaporation appeared to tear apart any Bénard cells that tried to form.
For the experiment, I was able to produce 3D anaglyphic images using an ordinary, single-lens color CCD camera along with a pair of red and blue ultrabright LEDs. They were placed about 1/2" apart to illuminate the convection chamber.I set the baseline between the two LEDs—optical fibers would also work—horizontal and parallel to the baseline between the eyes. Imagine the eyes positioned to view the chamber from the camera's location. For proper viewing on a color monitor, it must be adjusted to produce red and blue overlapping images that can be properly blocked by the 3D glasses. The monitor's chroma, saturation, contrast, and brightness controls will need to be adjusted. This anaglyphic scheme should work on any scale. While the video clip gives you a taste of the 3D effect, the view on a properly adjusted monitor that is connected directly to the recording camera is remarkable!
Viewing this experiment requires [red/\blue] glasses with red over the left eye.
I've barely scratched the surface on exploring schlieren and shadowgraphy techniques to find out if they can be used for my work with ultra-microflows. Nevertheless, I wanted to post the videos to encourage others to explore these techniques. If you want to examine varying density gradients and refractive indices in flowing or stationary liquids and gases, these incredibly simple tools may be just what you need.
As an example, the evaporation experiment described above produces multiple, short-lived geometric forms including hexagons. However, if a convection chamber is heated from below with other factors well-controlled, it is possible to produce a single, stable hexagon. So, is there any relationship between Bénard cells as produced in convection chambers and Saturn's mysterious polar hexagon? Schlieren or shadowgraphy could help reveal the answer.
