Interfacial rheology of coexisting solid and fluid monolayers

Abstract

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 composition. Modeling the system as a two-dimensional suspension in analogy to 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 themonolayerviscosity by orders of magnitude.

Interfacial curvature effects on the monolayer morphology and dynamics of a clinical lung surfactant

“…The resultant anisotropic bending energy may alter the balance between the line tension and dipole density difference to cause the domains to elongate into stripes to minimize the overall energy as the interface curves. Anisotropy in the line tension or the dipole density difference (7,26) can lead to noncircular domains on planar interfaces (Fig. S2), but Survanta domains do not show anisotropy until the bubbles have sufficient curvature.…”

“…Contrast in the confocal fluorescence images is due to the exclusion of the Texas Red 1,2-dihexadecanoyl-sn-glycero-3phosphoethanolamine, triethylammonium salt (TR-DHPE) lipid dye (concentration, 0.4 wt%) from the semicrystalline LC phase domains, which appear black; the fluorescent lipid dye is concentrated in the disordered LE phase, which appears red (7). The LC domains consist primarily of saturated DPPC and PA as shown by grazing incidence X-ray diffraction and pack into a tilted hexagonal lattice with short ranged correlations, very similar to the lattice structure of binary DPPC-PA mixtures (26,44,45). The LE phase contains the unsaturated lipids [mainly mono-and di-unsaturated phosphatidylcholines and phosphatidylglycerols (41)] and the SP-B and C proteins.…”

“…The LE phase contains the unsaturated lipids [mainly mono-and di-unsaturated phosphatidylcholines and phosphatidylglycerols (41)] and the SP-B and C proteins. This LE continuous morphology with isolated LC domains is typical for LC-LE phase separation in a number of lipid-only mixtures on the Langmuir trough (25,26). The relative fraction of LE and LC phases is set by the surface tension and temperature as the molecules in the various domains are in exchange equilibrium (7); this guarantees a uniform surface tension across any static or quasistatic interface.…”

“…Survanta monolayers separate into coexisting semicrystalline "liquid-condensed" (LC) and disordered "liquid-expanded" (LE) phases over a wide range of surface tension and temperature (16,23,24). The saturated dipalmitoylphosphatidylcholine (DPPC) and palmitic acid (PA) cocrystallize in the LC domains in a tilted hexagonal lattice leading to anisotropic viscoelastic properties (25,26). On planar interfaces, the LC domains have a long-range electrostatic dipole-dipole repulsion (7)(8)(9)(10)(11) and do not coalesce and remain discrete, even at high area fractions, for hours to days, even though there is a measurable "line tension" that acts to minimize the domain perimeter and favors coalescence (10,11,(27)(28)(29)(30)(31).…”

Interactions of particulate matter and pulmonary surfactant: Implications for human health

“…The overall viscosity of the film (η s ) is denoted in equation (20) where η so is the shear viscosity of the continuous LE phase, A is the area fraction of LC and Ac is the critical LC fraction where the LC domains start to merge and the viscosity diverges [ 145 ]. As a result, the translocation of particles within lung surfactant is greatly influenced by the compression extent of the film [ 146 ]. In addition to AFM, other instrument such as fluorescence microscope [ 147 ] can also be used to study the morphological change of lung surfactant when interacting with PM.…”