Who hasn't blown bubbles outside on a sunny day? If you haven't, devote some time to blowing bubbles the next time the sun is out - it's fun. As a bubble floats gracefully through the air, sunlight creates a virtual rainbow (actual rainbows are formed by a different process) of colours across the bubble's surface. The colour-making phenomenon at work is the same as what creates the colours on a slick of oil, a rooster's tail, a cardinal tetra or hummingbird's gorget – it's iridescence.
Bubble walls are constructed of several thin layers, two soap layers sandwich a layer of water between them. This wall encases a volume of air. As sunlight shines on a bubble, some of it reflects off the surface and some enters the soap film. Inside the bubble wall, light travels slower because both water and soap are denser than the air. At the interface between the soap film and the water, again some light reflects and some passes through. The reflected light may bounce back and forth between the two surfaces a few times or it may just pass back out of the bubble. Reflection or transmission of light occurs at every interface. Most of the light emerging from the interior of the bubble wall will be out of phase with the light that reflects off the interface. Out of phase means that the troughs from one wave line up with the crests from another so that the waves cancel each other out. Some of the emerging light will be in phase, that is, the crests and troughs line up with each other. These two light waves amplify each other, resulting in brilliant iridescent colours.
Layer thickness determines what wavelengths (thus colour) will be amplified. If you move and look at the bubble from a different angle, the colours will change. This is because your viewing angle has changed in relation to the layers. From different angles the distance the light has to traverse to reach you changes, thus the wavelengths that amplify each other also change.
Unfortunately, soap bubbles last only a short time. It doesn't take long before gravity pulls the liquid to the bottom and evaporation whisks fluid away. Bubble colours change as the bubble changes. When the bubble walls are thick, only red gets canceled out leaving blues and greens. As the walls thin, yellow is also canceled out leaving blue. Next green is removed and the bubble looks magenta. Blue goes last making the bubble look golden yellow. As the bubble wall's continue to thin, all the waves in the visible region cancel out and the bubble looks just clear. When the walls reach about 25 nanometres thick the bubble is in serious risk of popping.
Since the thickness of a bubble's walls aren't constant - the walls thicken towards the bottom (remember gravity acts to pull the water down), bands of colours seem to fall downwards on the surface. That pesky gravity also prevents us from dyeing bubbles. The dye will only mix with the water and drain to the bottom of the bubble. However, when gravity is absent dyeing bubbles becomes possible – so if you head out on a long voyage to Mars bring lots of bubble making supplies as you'll have years to perfect your bubble dyeing technique.
Showing posts with label bubbles. Show all posts
Showing posts with label bubbles. Show all posts
Friday, February 11, 2011
Wednesday, January 5, 2011
Another type of scattering
Clouds are mists drawn up by the heat of the sun, and their ascension stops at the point where the weight they have gained is equal to their motor power
- Leonardo da Vinci
The weather has changed outside. We had almost a week of beautiful but cold, sunny days. Now, clouds have rolled in, changing the sky from beautiful blue to drab gray. Have you ever looked up at a cloudy sky and wondered why the clouds are white? Alternately, have you ever looked at the foam of a dark beer and wondered how the foam could be white while the beer is dark? The answer lies in how light interacts with water droplets in clouds and tiny bubbles in beer foam.
The average size of a water droplet is between 0.01 and 0.02 mm, with the largest ones about 0.15 mm (from 'The Field Guide to Natural Phenomenon' by Keith Heidorn and Ian Whitelaw) and they are transparent. Cloud colour results from an optical phenomenon. Since water droplets are similar in size to visible light wavelengths, when light passes through the water droplets all wavelengths are affected the same way. This optical phenomenon is very different to the preferential blue scattering from gas particles in Rayleigh scattering that make the sky appear blue. The effect of scattering each wavelength of light in the same way is called Mie scattering after Gustav Mie, the German physicist who figured this out (there are others who independently came to the same conclusion but didn't get the phenomenon named after them). In Mie scattering all wavelengths scatter equally, making clouds appear white since all the wavelengths are present in the same amounts. In a thick bank of clouds, no direct light makes it through; instead all colour results from diffuse radiation. Thick clouds may appear in menacing shades of gray.
The foam atop of a freshly poured beer is composed of uniformly sized bubbles suspended in beer (from 'Does Anything Eat Wasps? And 101 Other Questions' edited by Mick O'Hare). Each tiny bubble is filled with air with a lower refractive index than the liquid around it. As a result, the bubbles act like magnifying glasses in reverse, where light that enters the bubbles is scattered in different directions – another example of Mie scattering. Reflections off the bubble's surface adds another layer of scattering. Both scattering effects created by each bubble is compounded in the foam. Since each wavelength of light is affected the same way, the fraction of light that makes it out will appear white, that is all wavelengths are equally present (The end result might be slightly yellow if the beer surrounding the bubbles absorbs some of the light).
Light hitting dust, smoke or pollen can also experience Mie scattering. This effect also explains why milk is white.
- Leonardo da Vinci
The weather has changed outside. We had almost a week of beautiful but cold, sunny days. Now, clouds have rolled in, changing the sky from beautiful blue to drab gray. Have you ever looked up at a cloudy sky and wondered why the clouds are white? Alternately, have you ever looked at the foam of a dark beer and wondered how the foam could be white while the beer is dark? The answer lies in how light interacts with water droplets in clouds and tiny bubbles in beer foam.
The average size of a water droplet is between 0.01 and 0.02 mm, with the largest ones about 0.15 mm (from 'The Field Guide to Natural Phenomenon' by Keith Heidorn and Ian Whitelaw) and they are transparent. Cloud colour results from an optical phenomenon. Since water droplets are similar in size to visible light wavelengths, when light passes through the water droplets all wavelengths are affected the same way. This optical phenomenon is very different to the preferential blue scattering from gas particles in Rayleigh scattering that make the sky appear blue. The effect of scattering each wavelength of light in the same way is called Mie scattering after Gustav Mie, the German physicist who figured this out (there are others who independently came to the same conclusion but didn't get the phenomenon named after them). In Mie scattering all wavelengths scatter equally, making clouds appear white since all the wavelengths are present in the same amounts. In a thick bank of clouds, no direct light makes it through; instead all colour results from diffuse radiation. Thick clouds may appear in menacing shades of gray.
The foam atop of a freshly poured beer is composed of uniformly sized bubbles suspended in beer (from 'Does Anything Eat Wasps? And 101 Other Questions' edited by Mick O'Hare). Each tiny bubble is filled with air with a lower refractive index than the liquid around it. As a result, the bubbles act like magnifying glasses in reverse, where light that enters the bubbles is scattered in different directions – another example of Mie scattering. Reflections off the bubble's surface adds another layer of scattering. Both scattering effects created by each bubble is compounded in the foam. Since each wavelength of light is affected the same way, the fraction of light that makes it out will appear white, that is all wavelengths are equally present (The end result might be slightly yellow if the beer surrounding the bubbles absorbs some of the light).
Light hitting dust, smoke or pollen can also experience Mie scattering. This effect also explains why milk is white.
Thursday, October 7, 2010
Sea foam – and why it can be bad for birds
The word “sea-foam” has a feminine mystique to it. I picture it used for frilly prom dresses and girly princess rooms. In the real world, sea foam is never the pretty light aqua colour of paint chips, instead it looks like dirty cappuccino foam – a yellowed off-white often with chunks of stuff in it. As the sea sloshes, salts, chemicals, dead plants, decomposing fish and sea weeds are churned into sea foam. Individual bubbles link together, and when a surface wave passes these bubbles they mass together as they swirl upwards to make foam on the sea surface.
Sea foam on the water's surface changes how wind energy is transferred into the water. This transfer of energy is a type of friction, called the wind stress, and it can play a part in important oceanographic processes such as currents and upwelling. What sea foam does is add another layer between the wind and ocean, so instead of the wind pushing the water, the wind has to push the foam and then the foam pushes the water, which is a much less efficient process. Sea foam can do much worse things.
In the coastal ocean red tides occur, which is a dangerous form of algal bloom. A red tide is better described as a Harmful Algal Bloom or HABs, because HABs aren't all red and not all red tides are HABs. Specific phytoplankton (little water plants) full of toxins are the culprit and are bad news for shell fish and anything that eats them, like us. Recently, migrating birds along the Washington State coast were found dead in large numbers at the same time as an HAB occurred. The birds weren't poisoned; instead they were freezing to death. Why? One or more of the HABs causing phytoplankton were getting churned up into the sea foam. Birds would rummage around in this foam getting themselves covered in it, because normally sea foam isn't a danger to them. But in this case, the phytoplankton-laced foam coated the bird's feathers is such a way that they lost waterproofing and insulation, so that the poor birds froze.
Sea foam on the water's surface changes how wind energy is transferred into the water. This transfer of energy is a type of friction, called the wind stress, and it can play a part in important oceanographic processes such as currents and upwelling. What sea foam does is add another layer between the wind and ocean, so instead of the wind pushing the water, the wind has to push the foam and then the foam pushes the water, which is a much less efficient process. Sea foam can do much worse things.
In the coastal ocean red tides occur, which is a dangerous form of algal bloom. A red tide is better described as a Harmful Algal Bloom or HABs, because HABs aren't all red and not all red tides are HABs. Specific phytoplankton (little water plants) full of toxins are the culprit and are bad news for shell fish and anything that eats them, like us. Recently, migrating birds along the Washington State coast were found dead in large numbers at the same time as an HAB occurred. The birds weren't poisoned; instead they were freezing to death. Why? One or more of the HABs causing phytoplankton were getting churned up into the sea foam. Birds would rummage around in this foam getting themselves covered in it, because normally sea foam isn't a danger to them. But in this case, the phytoplankton-laced foam coated the bird's feathers is such a way that they lost waterproofing and insulation, so that the poor birds froze.
Tuesday, October 5, 2010
Foamy frothy fun
Most of my days end with a long soak in a deliciously hot bubble bath. I don't need fancy scents, instead I enjoy the heat of the water and playful texture of the bubbles. Bubbles rise up as a mound ringing the stream of water from the faucet. Further away, bubbles slide into irregular shapes reminiscent of fictitious moon bases and futuristic homes. If it's really quiet, the muffled pops as multiple bubbles end their existence is audible. In addition to a relaxing end to a day, my bubble bath is an example of a foam.
Foams form when billions of tiny bubbles are packed together within a solid or a liquid. Irregular sized bubbles are common in all foams except the most idealized ones. Like what happens for individual bubbles (check out my bubble post), it's surface tension that helps keep a foam stable. Liquid foams break down eventually – there are even chemicals on the market to make this process go faster. Gas can diffuse from small bubbles into large ones and eventually out of the foam, or gravity can drain liquid out the bottom making bubbles so weak they pop on top.
A Belgian physicist, Joseph Plateau (1801-1883), figured out the basis of what we know about soap films and foams. His diverse interests also included spending time with moving image illusions like the action one sees when using a "flip-book". Back to soap film, Plateau came up with a series of laws to describe stable foam structures (foams that don't follow these laws tend to rearrange themselves until they do). They are:
1.Soap film surfaces are smooth.
2.The soap film curvature is continuous and constant along the entire a surface.
3.When three or more bubbles connect together, they will shift around until each line only contains three bubble-wall intersections – called a Plateau Border. With matching surface tensions all three angles are 120 degrees, the most efficient option.
4.Only four Plateau Borders can meet at a point.
Foams form in nature. Examples include: sea and river foams, and the foaming at the mouth of a rabid dog. There are fish, such as gourami and Siamese fighting fish, that blow a mass of bubbles coated in saliva to house their eggs. In the same spirit, some species of frogs make foam nests to lay their eggs in. These nests may be constructed in crevices, on the surface of water, or on forest floors.
Beyond bubble baths, soaps can be whipped to form a lather with bubbles so small they hardly can be seen. This idea was extended into modern shaving foams where a compressed gas is rapidly decompressed to expand a cream into a foam. Foams can also be hardened into permanent structures like insulation and flotation devices. Ceramic can be made into foams useful for acoustic insulation, absorption of environmental pollutants, and the filtration of molten metal alloys among other applications. Cement foams are used as a light-weight building material with good insulating capability. Even metals can be manipulated into becoming a foam. Metal foams are used in exhaust mufflers as they a great at dampening noise. They also make efficient materials for heaters and heat exchangers since they have so much surface area.
We eat a lot of foams: they can add a light texture to an angel food cake, or be tasty in the form of whipped cream and meringues. Breads are often a foam as the yeast produces tiny bubbles of gas which causes the dough to rise. One of the best foams is the head that forms when a beer is poured into a glass; this foam is made by which is made by carbon dioxide bubble rising to the surface. For the best head on your glass of beer, chose a wheat beer instead of a barley beer.
Note: there is an optimum foam combination of wheat beer consumed in a bubble bath – use with caution.
Foams form when billions of tiny bubbles are packed together within a solid or a liquid. Irregular sized bubbles are common in all foams except the most idealized ones. Like what happens for individual bubbles (check out my bubble post), it's surface tension that helps keep a foam stable. Liquid foams break down eventually – there are even chemicals on the market to make this process go faster. Gas can diffuse from small bubbles into large ones and eventually out of the foam, or gravity can drain liquid out the bottom making bubbles so weak they pop on top.
A Belgian physicist, Joseph Plateau (1801-1883), figured out the basis of what we know about soap films and foams. His diverse interests also included spending time with moving image illusions like the action one sees when using a "flip-book". Back to soap film, Plateau came up with a series of laws to describe stable foam structures (foams that don't follow these laws tend to rearrange themselves until they do). They are:
1.Soap film surfaces are smooth.
2.The soap film curvature is continuous and constant along the entire a surface.
3.When three or more bubbles connect together, they will shift around until each line only contains three bubble-wall intersections – called a Plateau Border. With matching surface tensions all three angles are 120 degrees, the most efficient option.
4.Only four Plateau Borders can meet at a point.
Foams form in nature. Examples include: sea and river foams, and the foaming at the mouth of a rabid dog. There are fish, such as gourami and Siamese fighting fish, that blow a mass of bubbles coated in saliva to house their eggs. In the same spirit, some species of frogs make foam nests to lay their eggs in. These nests may be constructed in crevices, on the surface of water, or on forest floors.
Beyond bubble baths, soaps can be whipped to form a lather with bubbles so small they hardly can be seen. This idea was extended into modern shaving foams where a compressed gas is rapidly decompressed to expand a cream into a foam. Foams can also be hardened into permanent structures like insulation and flotation devices. Ceramic can be made into foams useful for acoustic insulation, absorption of environmental pollutants, and the filtration of molten metal alloys among other applications. Cement foams are used as a light-weight building material with good insulating capability. Even metals can be manipulated into becoming a foam. Metal foams are used in exhaust mufflers as they a great at dampening noise. They also make efficient materials for heaters and heat exchangers since they have so much surface area.
We eat a lot of foams: they can add a light texture to an angel food cake, or be tasty in the form of whipped cream and meringues. Breads are often a foam as the yeast produces tiny bubbles of gas which causes the dough to rise. One of the best foams is the head that forms when a beer is poured into a glass; this foam is made by which is made by carbon dioxide bubble rising to the surface. For the best head on your glass of beer, chose a wheat beer instead of a barley beer.
Note: there is an optimum foam combination of wheat beer consumed in a bubble bath – use with caution.
Monday, October 4, 2010
Tiny bubbles
Anyone remember the show “That's Incredible!”? It was on in the early 80's. I watched it as a kid (and yes I'm dating myself as the show went off the air in 1984). One episode I remember vividly: The hosts hyped how this man could make a square bubble, which I suppose what hosts are supposed to do. Since all the bubbles I had ever seen were round, I was really curious how a square bubble could be made (and I wasn't yet jaded about TV). I was expecting the square bubble to be free-floating by itself, so I was kinda disappointed in how it was done. The “performance artist” blew a bunch of connected bubbles (I forget how many). Next he took a long inhale from a cigarette, then stuck a straw between the connected bubbles and filled the space with the smoke. The filled space was in the form of a cube, and I felt tricked.
A short lived creation, soap bubbles hold a sphere of air with a thin film of soapy water which is formed by surface tension. Spherical shapes are preferred (really large bubbles can end up forming elongated shapes from air currents) because a sphere is the smallest surface area possible to contain a specific volume of air. The soap film surface tension is strong and flexible enough that waves can travel along the surface and is so thin the surface appears iridescent.
Surprisingly, soapy water has less surface tension that water alone and is needed to keep the bubble stable. As a bubble is formed, the soap film stretches decreasing the concentration of soap which increases the surface tension. This mechanism is called the 'Marangoni Effect' and occurs when a surface tension gradient (that is regions of greater and lesser surface tension) causes liquid to move away from areas of low surface tension. The soap acts as a stabilizer by letting the thinnest parts of the film to have the strongest surface tension thus keeping the bubble together.
What happens when two bubbles stick together? Well, they will arrange themselves in such a way that minimizes the surface area. Bubbles of different sizes will end up with a bulging internal wall into the larger on as smaller bubbles have higher internal pressure. If they are the same size, the internal wall will be flat - a phenomenon exploited by the cube-making bubble performer.
So sneaky internal bubble-wall cubes aren't so impressive. How about antibubbles ….
A short lived creation, soap bubbles hold a sphere of air with a thin film of soapy water which is formed by surface tension. Spherical shapes are preferred (really large bubbles can end up forming elongated shapes from air currents) because a sphere is the smallest surface area possible to contain a specific volume of air. The soap film surface tension is strong and flexible enough that waves can travel along the surface and is so thin the surface appears iridescent.
Surprisingly, soapy water has less surface tension that water alone and is needed to keep the bubble stable. As a bubble is formed, the soap film stretches decreasing the concentration of soap which increases the surface tension. This mechanism is called the 'Marangoni Effect' and occurs when a surface tension gradient (that is regions of greater and lesser surface tension) causes liquid to move away from areas of low surface tension. The soap acts as a stabilizer by letting the thinnest parts of the film to have the strongest surface tension thus keeping the bubble together.
What happens when two bubbles stick together? Well, they will arrange themselves in such a way that minimizes the surface area. Bubbles of different sizes will end up with a bulging internal wall into the larger on as smaller bubbles have higher internal pressure. If they are the same size, the internal wall will be flat - a phenomenon exploited by the cube-making bubble performer.
So sneaky internal bubble-wall cubes aren't so impressive. How about antibubbles ….
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