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Bathtub Physics: Using Bubbles to Battle Gravity

Don't try this in the tub. Scientists applied an electric field across a soap bubble and discovered that the electric field could draw fluid up through the soapy film – against gravity. The researchers also found that the strength of the electric field controls the thickness of the soap film and changes the rate of fluid flow up the bubble. The results were published February 1, 2013 in the journal Physical Review Letters. 

O. Bonhomme / University of Lyon / CNRS

Progress at making electronic circuits smaller and smaller have dramatically reduced the size, energy, and cost of data storage and calculations. Scientists are similarly interested in scaling down the scientific laboratory by developing new methods of rapidly performing several simultaneous experiments with the minimum of each experimental ingredient. Microfluidics is science at the microLiter scale.

Previous work in microfluidics found that applying an electric field to a narrow channel filled with an electrolyte liquid will cause the fluid to move through the channel through electro-osmosis.

Fill a channel with an electrolyte liquid and some of the ions in the liquid will collect along the surface of the channel walls. This thin ion layer will create a slight net-charge between the bulk of the liquid and the wall surface. Applying an electric field to the fluid, the field applies a force to the charged ions lining the walls. As the ions move, they drag the rest of the liquid in the channel with them. This fluid movement is called electro-osmosis and is particularly strong in tiny micro-channels and nano-channels where the ratio of the channel's surface area to fluid volume is high.

The new research by physicists at the University of Lyon in France proposed a new type of micro-channel with soft walls made of soap, water, and potassium chloride ions.

Using the soapy solution, the researchers drew a bubble between two electrodes about half a centimeter apart. The bubble acted as two thin, concentric cylinders of soap with water and ions flowing up and down between the walls. The slight positive charge of the soapy walls attracted the negatively charged chloride ions to form a slightly charged layer around the water. Applying a voltage across the electrodes, the researchers were able to move the ions and water up and down the bubble channel.

In past research, scientists found that when they increased the applied electric field, the fluid rate increased linearly. But, when the French team increased the strength electric field, they saw the fluid flow nonlinearly – much faster than expected. The increased field was causing the channel between the soap walls to widen, allowing much more liquid to flow through. The team could adjust the thickness of the channel by a factor of ten, to range from less than 100 nanometers to almost a micron.

The researchers think that the ability to control the flow through soft nano-channels might have applications in microfluidics as diode-equivalents: the fluid flow would remain off until the applied electric field surpassed a threshold voltage. The researchers also hope to explore whether the counter-gravity fluid flow could stabilize foams, the category of semi-fluid-but-kind-of-solid bubbles that continues to fascinate physicists.


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