Light scattering microscopy reveals previously undetected topographic insta
bilities in phospholipid monolayers at the air/water interface far below th
e collapse pressure. Following compression through the fluid --> condensed
phase transition in monolayers of dipalmitoyl phosphatidylcholine, after th
e disappearance of the fluid phase, the contact regions between condensed d
omains acquire static roughness as indicated by enhanced light scattering.
With further compression, a nanoscale budding process occurs within the rou
ghened regions, while the interiors of the condensed domains remain flat an
d retain their domain shapes. At monolayer collapse, the buds proliferate a
cross the entire interface, suggesting that the buds detected at lower pres
sures represent spatially confined fluctuations into the collapse phase. Th
e confinement of static roughness formation and budding to domain contact r
egions indicates that these topographic instabilities originate from packin
g defects created where adjacent domain edges with conflicting molecular or
ientations grow together juring the fluid --> condensed phase transition.