Researchers have found a way to control remotely and reversibly a material's viscosity using sound. Takeshi Naota and Hiroshi Koori of Osaka University in Japan have created a substance from small, organic molecules containing platinum dissolved in acetone. The result is an oily liquid, which, when zapped with ultrasound at 40 kHz, turns into a white gel.
The gel can be reversed back into a liquid by heating and perhaps also in response to another burst of ultrasound. This behaviour is mysterious, as sound might be expected to cause things to liquefy rather than to gel. While there are theories about the mechanism, precisely what is going on remains a puzzle.
Further reading
Takeshi Naota and Hiroshi Koori 2005 Molecules that assemble by sound: an application to the instant gelation of stable organic fluids J Am. Chem. Soc. 127 9324; DOI:10.1021/ja050809h.