Lifelike Droplets Do The Dissolving Dance

Drops of dichloromethane develop varying shapes, from circles to polygons, and show complex and dynamic behavior patterns as they dissolve into aqueous solution, according to a study (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201104261). A group led by Véronique Pimienta of France’s Paul Sabatier University and Oliver Steinbock of Florida State University added 25-µL drops of dichloromethane into aqueous cetyltrimethylammonium bromide (CTAB) solutions. They found that what happens next depends on the CTAB concentration. On the surface of 0.5-mM CTAB, for example, a circular dichloromethane drop initially pulsates and ejects smaller droplets from its edge. It then starts rotating, developing two arms from which more droplets release, before eventually breaking apart and disappearing. On 30-mM CTAB, in contrast, the spherical rim of the drop morphs into polygonal structures with mobile vertices that release smaller droplets when they collide. The researchers believe drop movement is driven by thermal gradients that develop with evaporation, whereas drop rotation comes from the difference in surface tension between the drop and CTAB solution.