Non-Contact Microrheology of Monolayers and Membranes

Abstract

Understanding the mechanics/rheology of surface monolayers and lipid bilayers is of fundamental biological importance. One technique used to explored these questions is membrane microrheology, in which the observed thermal fluctuations of a tracer particle in the monolayer is used to extract the rheological data. This technique is challenging for at least two reasons. On the one hand, in fragile monolayer systems the presence of the tracer can locally perturb the monolayer. On the other hand, in sufficiently stiff monolayers/membranes it has proved problematic to embed the particle in it. In this talk, we develop a noncontact microrheological approach to avoid these issues by exploring the effect membrane rheology on the thermal fluctuations of a bead in the fluid near the monolayer. Specifically, we develop the theory of the force response function of a spherical particle submerged below either a Langmuir monolayer or a lipid bilayer. We show that one can use the observed thermal fluctuations of that submerged particle to extract rheological properties of the essentially two-dimensional monolayer or membrane. We also present experimental results on application of this technique to surfactant monolayers and monolayers bound to an F-actin network on the aqueous side of the Langmuir monolayer.