Drops That Change Their Mind: Spontaneous Reversalfrom Spreading to Retraction
2019
A liquid
drop may spread faster on surfaces when surfactants are
added. Here we show that after some time the spreading in such systems
can, under certain conditions, spontaneously reverse to retraction
and the droplet pulls itself back, receding from areas it has just
recently wetted, elevating its center of mass in a jerklike motion.
The duration from drop placement to the onset of retraction ranges
from hours to less than a second primarily as a function of surfactant
concentration. When the retraction is asymmetric, it results in drop
motion, and when it is symmetric, the mass of the drop collects itself
on its spot. This phenomenon, which was predicted theoretically in
2014, is apparently a general one for drops with surfactants; however,
other factors, such as evaporation and contamination, prevented its
observance so far.
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