Dynamics of grainy liquid crystalline monolayers: visible and hidden grain boundaries and chiral rheology
AbstractCo-authors: Kyuhan Kim (UCSB ChE), SiYoung Choi (UCSB ChE), Joe Zasadzinski (Minnesota CEMS)
While the equilibrium properties of fluid interfaces have been manipulated and studied for centuries, their dynamic, rheological properties (e.g. viscosity and elasticity) have proven more elusive. Despite the dominant role that even molecularly-thin interfaces can play in multiphase flows, the viscosity of the bulk fluids on either side of the interface can easily overwhelm any attempt at measuring surface rheology. I will describe a technique we have developed to measure the interfacial rheology -- the viscous and elastic properties -- of fluid-fluid interfaces, typically laden with some surface-active species (molecular surfactants, copolymers, colloids, etc.). A novel feature is our ability to visualize the interface during the measurement, enabling us to directly relate the measured response to the microstructure of the interface.
In particular, we study model lung surfactant monolayers that consist of liquid-condensed phases of the phospholipid DPPC and, in some cases, cholesterol. We directly visualize the deformation of liquid-crystalline domains under both linear and nonlinear deformations. Despite the simplicity of the system -- a single-component, 2 nm-thick molecular monolayer -- we find an extraordinarily rich rheological response, including a soft, glassy response, elastic strain energy that is stored over a shockingly long time, two-dimensional yielding behavior, aging, rejuvenation, and anisotropically chiral rheology, exhibiting either ductile plasticity or brittle fracture, depending on the sense of the shear. We relate these rheological responses to observed boundaries between individual DPPC crystals, as well as hidden boundaries where tail group tilt orientations change rapidly.