Title: On the principles of nonlinear pulse propagation in quasi-2D
biological interfaces as observed in a lipid monolayer.
Abstract: Biological membranes provide a quasi-2D continuum that
outlines the internal architecture as well as the outer peripheries of a
cellular system. These membranes are bilayers of amphiphilic molecules
such as lipids and proteins that self assemble in an aqueous environment.
Physically such an interface is characterized by its state diagram which
determines the microscopic (fluctuations) as well as macroscopic
(propagation) dynamics of the system. Since biological membranes have been
shown to actively maintain prominent nonlinearities in their state
diagrams near physiological conditions, it is crucial to understand the
role of such nonlinearities in the dynamics of membrane systems. Here
using fluorescent probes embedded in a lipid monolayer at the air-water
interface, nonlinear propagation of in-plane electro-mechanical impulses
is investigated optically. It is shown that near a nonlinearity in the
state diagram of the interface (liquid expanded /liquid condensed phase
transition) the excitation of the impulse is all-or-none that
acquires a solitary pulse shape. The relationship between the
compressibility and the velocity of these pulses establishes their
acoustic nature while the curvature of the state diagram is shown to
govern the nonlinear behavior (velocity, amplitude and threshold). The
observed splitting of the propagating impulse into a non-dispersive
forerunner wave followed by a dispersive slower wave confirms the local
phase transition during compression - a well established principle but an
exotic phenomenon in classical shock compression science. Given that the
state diagrams of biological membranes have intricate profiles with
pronounced regions of positive as well as negative curvatures, these
principles established in a simple system but applicable in every case
demonstrate the fundamental role played by the state diagrams in defining
nonlinear pulse propagation in biological membranes.