Viscosity of two-dimensional suspensions

Is similar to that in three dimensions. Many technologically and biologically important interfaces are actually monolayers composed of two coexisting phases: solidlike crystals of the molecules floating in a sea of liquidlike molecules. Researchers at the University of California, Santa Barbara, measured the viscous drag on a magnetic needle in monolayers of human lung surfactant lipids. They found that if the relative fraction of the layer’s area that contained crystals was low, the viscosity was also low and the monolayer spread easily. As the crystals’ coverage increased, so did the viscosity until, at a critical fraction of the total area, the monolayer abruptly became rigid. Moreover, the behavior held for a wide range of surface pressures, temperatures, and monolayer compositions and was the same as that for a 3D dispersion of hard spheres in a solvent with long-range repulsive interactions. The scientists believe their work could lead to better replacement surfactants—for example, for premature infants with respiratory distress syndrome. Human lung surfactant has an important role in replacing the blood’s carbon dioxide with oxygen. It helps keep the lungs’ tiny air sacs properly inflated by controlling their surface tension. (J. Ding et al., Phys. Rev. Lett. 88 , 168102, 2002 http://dx.doi.org/10.1103/PhysRevLett.88.168102 .)