![]() In this periodic structure, known as a photonic crystal, interference among waves traveling different routes prevents transmission at certain frequencies. The range of suppressed transmission was centered at about 150 kHz for a median bubble size of 35 μ m and at about 100 kHz when the bubble size increased to 45 μ m.Īn ability to block transmission in a certain frequency range was first demonstrated in 1991 for microwave radiation passing through a structure with a periodic pattern of nanosized holes (see 23 August 2013 Focus Landmark). The discovery that foam could totally block transmission came as a surprise. They plotted the attenuation versus bubble size and ultrasound frequency, which ranged from 60 to 600 kilohertz (kHz). Leroy and his colleagues measured the attenuation of ultrasound pulses that passed through a layer of foam sandwiched between two polymer films spaced 0. Once a liquid foam had formed, its median bubble size grew steadily from 15 to 50 micrometers ( μ m) over a period of about 90 minutes. The team made foams using a two-syringe method that injected air saturated with the insoluble gas perfluorohexane ( C 6 F 14) into water mixed with sodium dodecyl sulfate, a surfactant that stabilizes bubbles. Valentin Leroy of the University of Paris-Diderot and his colleagues reasoned that measuring the transmission of sound waves through foams would be a good way to probe their structure. The size range and distribution of bubbles in foams influence their properties and behavior, but the fragility and opacity of such foams make them hard to study. Liquid foams are used in industry, including oil spill recovery, and as consumer products, such as shaving foam and whipped cream. The researchers suggest that such foams may have practical value as acoustic insulators. Their experiments are the first demonstration that a foam can act as a metamaterial-a material whose complex internal structure endows it with unusual physical properties. In Physical Review Letters, they report that the foam completely blocked the transmission of ultrasound waves in a certain range of frequencies. While investigating the use of ultrasound to probe the structure of a liquid foam, researchers in France came across an unexpected result. Experiments with this foam (shown under a microscope) showed that at certain frequencies, the bubbles and the surrounding liquid vibrate out of synch in response to an ultrasound wave, completely suppressing the transmission of sound.
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