Phospholipid monolayers at the air-water interface serve as model systems for
various biological interfaces, e.g. lung surfactant layers and outer leaflets
of cell membranes. Although the dynamical (viscoelastic) properties of these
interfaces may play a key role in stability, dynamics and function, the
relatively weak rheological properties of most such monolayers have rendered
their study difficult or impossible. A novel technique to measure the dynamical
properties of fluid-fluid interfaces have developed accordingly. We
microfabricate micron-scale ferromagnetic disks, place them on fluid-fluid
interfaces, and use external electromagnets to exert torques upon them. By
measuring the rotation that results from a known external torque, we compute
the rotational drag, from which we deduce the rheological properties of the
interface. Notably, our apparatus enable direct interfacial visualization while
the probes are torqued.
In this fluid dynamics video, we directly visualize
dipalmitoylphosphatidylcholine(DPPC) monolayers at the air-water interface
while shearing. At about 9 mN/m, DPPC exhibits a liquid condensed(LC) phase
where liquid crystalline domains are compressed each other, and separated by
grain boundaries. Under weak oscillatory torque, the grain boundaries slip past
each other while larger shear strain forms a yield surface by deforming and
fracturing the domains. Shear banding, which is a clear evidence of yield
stress, is visualized during steady rotation. Remarkably slow relaxation time
was also found due to slow unwinding of the stretched domains.Comment: 1 page, no figures, gallery of fluid motion 200