Using diffusion-induced growth instabilities to measure line tension at liquid condensed-liquid expanded domain...

Few techniques exist for measuring the line tension between liquid-condensed (L_C) and liquid-expanded (L_E) monolayer phases important to determining monolayer morphology and interfacial mechanical properties. Two-dimensional Mullins-Sekerka (MS) instability theory and non-perturbative fluorescence microscopy and surface pressure-area isotherms are used to evaluate line tension in monolayers of model lung surfactants composed of dipalmitoylphosphatidylcholine, hexadecanol, and cholesterol. The measured line tensions ranging between 7.9 and 41 pN are consistent with theoretical predictions and decrease with increasing cholesterol fraction, confirming cholesterol as a "line-actant". The greatest source of error in our measurements comes from the domain instability spacing. By combining the line tensions determined from the MS instability with a previous analysis of the widths of equilibrium domain stripes in the same system, the dipole density difference was evaluated and ranged between 1.1 and 5.9 pN. The domain shapes and location of the growth instability are consistent with an inhomogeneous line tension that may arise from the tilt of the molecules in the L_C phase. The finger spacing decreased with increasing supersaturation caused by increased monolayer compression rate consistent with MS theory. This compression rate supersaturation may be responsible for a variety of commonly observed variations in domain morphologies in the literature that have not been previously identified as Mullins-Sekerka instabilities.