Tikalon Blog by Dev Gualtieri

Bubble, Bubbles, Foam

November 18, 2013

The bubble pipe was a popular child's toy of my generation . but its popularity has waned along with that of candy cigarettes . A less offensive option is the simple wire (now plastic ) loop found in the same jar as the bubble liquid . My first chemistry experience was the attempt to replicate the bubble liquid formulation with household chemicals when the jar's original contents were depleted. Alas, dish soaps of that era were not as good as those today, so my experiments were not the successful.

All one shape, but many sizes.

A foam of bubbles on a liquid .

Photograph by Friedrich Böhringer, via Wikimedia Commons .

As I wrote in a previous article (Bubble Noise Reduction, June 7, 2011), bubbles are useful for more than just toys. A wall of bubbles in water acts as an effective sound barrier, since there's a large difference in the product of the speed of sound v and density ρ inside and outside the bubbles. This product is the acoustic impedance Z of the material, and an impedance mismatch causes an acoustic reflection , just as an electrical impedance mismatch will cause a signal reflection in a transmission line .

The acoustic impedance of water is more than 3,000 times that of air , so air bubbles in water will reflect much incident sound, inverting the phase to cancel the energy of the incoming sound wave .[1] Since acoustic wavelengths are larger than light wavelengths, large bubbles are preferred, but you can immerse large balloons for the same effect.[2]

Engineers at Rice University's Department of Chemical and Biomolecular Engineering have been investigating bubble foams as a means to enhance petroleum extraction .[3-5] This research group, headed by Rice University associate professor , Sibani Lisa Biswal , is part of the Rice Consortium for Processes in Porous Media . A description of this research has just been published in Soft Matter .[3]

The idea is to use foam as a displacement volume to extract more petroleum from underground petroleum reservoirs . The foam would pack highly permeable channels to force flow through lower permeability channels. The Rice University experiments examined how bubbles transform in a microfluidic channel . The channel is a surrogate for the porous media through which oil must flow to be extracted. Their experimental apparatus, as shown in the photograph, forces a gas -surfactant mixture through a 20 micrometer channel, which is imaged 10,000 times per second.[4]

Imaging apparatus for the study of bubble dynamics in a microfluidic channel .

(Still image from a YouTube Video.[5])

The research discovered two new mechanisms of in situ foam generation in porous media in which an existing bubble is pinched into two bubbles. In one case, a bubble trapped between another bubble and a wall will split as it enters a channel. In the other case, neighboring bubbles break a third bubble into two pieces as it flows through the channel.[4] The width of pinched bubbles was found to obey a power law in time .[3]

A bubble is split as it enters a microfluidic channel while squeezed between a neighboring bubble and a wall.

(Biswal Lab/Rice University image, modified for clarity.[4])

George Hirasaki , an emeritus professor of chemical and biomolecular engineering and co-author of the paper, summarized the research by saying,

"We're trying to understand how foam behaves in porous media because it is a way of making gas act like a more viscous fluid ... Normally, gas has very low viscosity and it tends to flow through rock and not displace oil and water. Once it finds a path, usually along the top of a reservoir, the rest of the gas tends to follow. If there were some way to make gas act more like a liquid, to make it more viscous, then it would contact much more of the reservoir and would push the fluids out."[4]

This research was funded by various agencies, including the Petroleum Institute of the United Arab Emirates and the U.S. Department of Energy .[4]

In my experience, beer , while technically a depressant , is a good stimulant for conversations about physics , and everyone has observed beer foam. Since most beer is today purchased in cans , not bottles , few observe an interesting beer foam effect which is the topic of entry no. V102302 in the American Physical Society Division of Fluid Dynamics Gallery of Fluid Motion 2013.[6-7] As shown in the photograph, tapping one beer bottle vertically with another will result in a beer foam fountain.[6]

A fountain of beer foam (arxiv1310.3747).

Making a beer foam fountain.

Step 1. - Tap an opened bottle at its top with another bottle.

Step 2. - Observe fountain.

(From a video by Javier Rodríguez-Rodríguez, Almudena Casado and Daniel Fuster, via arXiv.[6])

As explained in the video, the vertical impact of another bottle sends a compression wave through the liquid, which resonates between the liquid surface and the bottom of the bottle as an alternating expansion and compression wave. This leads to cavitation of the liquid, and small bubbles form to drive the plume.[6]

Bubble, Bubbles, Foam

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