Tracking giant folds in a monolayer.

The collapse dynamics of giant folds in a catanionic monolayer at the air-water interface are examined. A monolayer of dioctadecyldimethylammonium bromide (DODAB) and sodium dodecyl sulfate (SDS) in a 1:1 ratio is the system of study that previously was found to fold upon compression in a Langmuir trough. Carboxylate-coated polystyrene beads (1 microm diameter) are deposited and bound to the monolayer. Displacement of the beads is measured with epifluorescence microscopy and particle image velocimetry, yielding a measurement of the velocity of the monolayer around the fold. Reversibility is confirmed by measuring the amount of monolayer material entering and leaving the fold. Material near folds are found to have a maximum relative velocity on the order of 0.1 mm/s, and fold depths are found to be on the order of 1 mm. The folds exhibit regular unfolding behavior, which can be explained qualitatively by a simple mechanical model.