Interfacial rheology of coexisting solid and fluid monolayers

• Published in: Soft matter Vol. 13; no. 7; pp. 1481 - 1492
• Main Authors: Sachan, A. K, Choi, S. Q, Kim, K. H, Tang, Q, Hwang, L, Lee, K. Y. C, Squires, T. M, Zasadzinski, J. A
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 15.02.2017
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/c6sm02797k

Biologically relevant monolayer and bilayer films often consist of micron-scale high viscosity domains in a continuous low viscosity matrix. Here we show that this morphology can cause the overall monolayer fluidity to vary by orders of magnitude over a limited range of monolayer compositions. Modeling the system as a two-dimensional suspension in analogy with classic three-dimensional suspensions of hard spheres in a liquid solvent explains the rheological data with no adjustable parameters. In monolayers with ordered, highly viscous domains dispersed in a continuous low viscosity matrix, the surface viscosity increases as a power law with the area fraction of viscous domains. Changing the phase of the continuous matrix from a disordered fluid phase to a more ordered, condensed phase dramatically changes the overall monolayer viscosity. Small changes in the domain density and/or continuous matrix composition can alter the monolayer viscosity by orders of magnitude. Two-phase coexistence in phospholipid-fatty acid-cholesterol monolayers determines interfacial rheology.