Keeping One's Head

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Science  12 Nov 2004:
Vol. 306, Issue 5699, pp. 1105
DOI: 10.1126/science.306.5699.1105b

Foam consists of a mass of bubbles—air trapped within thin liquid shells—that forms on agitation, such as when beer is poured into a glass. The stability of a foam depends on the nature of the liquid: evanescent in some cases, hardier in others because of additives that extend their lifetime. Surfactants, occasionally in conjunction with solid particles, are used as stabilizers because they reduce the surface tension of the liquid, preventing the bubbles from coalescing.

Alargova et al. have found that polymer microrods made from an epoxy-based photoresist can stabilize foams so that they resist collapse even when most of the liquid is allowed to evaporate. In contrast to foams made with the common household detergent sodium dodecyl sulfate, which survived for 2 days, the polymer rod foams were stable for more than 2 weeks. The authors speculate that the greater stability is due to two factors. First, the rods induce a much thicker liquid layer between the air bubbles, and these layers sterically repulse each other, thus preventing coalescence of the bubbles. Second, the rods within a layer form an intertwined network, thus increasing its overall strength and also imparting to the bubbles a spherical shape, which tends to be highly unstable in ordinary foams. — MSL

Langmuir 10.1021/la048647a (2004).

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