Two-Dimensional Snowflakes – Dynamic and Equilibrium Domain Morphologies in Lung Surfactant Monolayers
Speaker
Joseph A. Zasadzinski
3M Harry Heltzer Professor of Multidisciplinary Science and Technology
Department of Chemical Engineering and Materials Science
University of Minnesota
Abstract
Two-Dimensional Snowflakes – Dynamic and Equilibrium Domain Morphologies in Lung Surfactant Monolayers
Competition between the intra-domain electrostatic pressure, 2, and line tension, (2-D version of the surface tension) at the domain boundary leads to elaborate shape morphologies in phase-separating lipid monolayers. The question remains if these morphologies are energy minima or are kinetically trapped metastable states. Here we show dynamic periodic growth instabilities are followed by a reversible evolution of equilibrium uniform width stripes in model lung surfactant monolayers of dipalmitoylphosphatidylcholine (DPPC), hexadecanol (HD) and dihydrocholesterol (DChol). The initial semi-circular domains grow at a fixed 2:1 DPPC:HD stoichiometry, depleting the liquid phase of HD, leaving behind a liquid enriched in DPPC and DChol. As the domains grow, periodic fingering instabilities occur similar to those in snowflakes and metal dendrites. We derive a two-dimensional version of Mullins-Sekerka (MS) linear instability theory to relate the instability wavelength to the line tension at these domain boundaries for the first time. The line tensions calculated are consistent with theoretical predictions and decrease with increasing cholesterol fraction, showing that cholesterol acts as a 2-D line-actant. The fingering wavelength depends on the compression rate which is directly related to the local supersaturation in the melt. However, with time, the domains evolve into extended stripes of uniform width and the instabilities anneal away. The final domain shapes minimize the free energy, and the stripe widths are only a function of 2, the ratio of line tension to the internal electrostatic pressure. We show that the stripe width is independent of the approach to equilibrium. By analyzing these morphologies, we can determine values of both and 2 use this information to rationalize both the dynamic and equilibrium shapes of monolayer domains and how cholesterol and other lipid species alter these morphologies.
Bio
Joseph Zasadzinski is the 3M Harry Heltzer Professor of Multidisciplinary Science and Technology in the Chemical Engineering and Materials Science Department of the University of Minnesota for the past 13 years. Prior to this, Dr. Zasadzinski was a professor of chemical engineering at UC Santa Barbara for 24 years. He did a postdoc at AT&T Bell Laboratories after his Ph. D. at the University of Minnesota. His BS is from the California Institute of Technology, where he played basketball and was instrumental in ending a 99 game losing streak by beating a team coached by the legendary Greg Popovich, now of the San Antonio Spurs. He lives on Lotus Lake and routinely lands trophy size bass (in his dreams). Dr. Zasadzinski research is on the properties of surfactants, both synthetic and biological, with emphasis on treating human respiratory diseases caused by insufficient or degraded lung surfactant. He also works to replace perfluorinated surfactants in fire fighting foams by understanding the mechanisms behind low surface tensions.