review of monolayers - Couette trough  

(additional support had been provided from Research Corporation, the Petroleum Reserach Fund, and the Sloan Foundation)

Our two areas of active research are:

(1)   microrheology of monolayer systems

(2)   protein/monolayer interaction

Our past work includes:

(1)  impact of divalent ions

Langmuir monolayers: Amphiphilic molecules confined to an air-water interface.

Langmuir monolayers are an ideal system to study behavior in two-dimensions. They are composed of amphiphilic molecules that are confined to the air-water interface. As such, they are an ideal system for studying behavior in two-dimensions.

They possess a rich equilibrium phase behavior. In addition to the usual isotropic gas and liquid phases, there are a large number of phases that are analogous to three-dimensional smectic liquid crystals. Our current interest in Langmuir monolayers is in their response to shear. Experiments have shown that the viscoelastic properties of a number of the Langmuir monolayer phases are not well understood. Furthermore, Langmuir monolayers can be used as a model system to understand the flow of foams, emulsions and other systems consisting of random domains.

In order to study the effect of shear, we have designed an Couette trough that has a number of capabilities:

  • a torsion pendulum for measuring stress versus strain
  • an "outer cylinder" of individual teflon "fingers" for centrosymmetric compression of the monolayer
  • The outer cylinder can  be rotated to generate Couette flow.
  • a Brewster angle microscope (BAM) and a flourescence microscope can both be mounted over the trough to image the films

The pictures here show the Couette trough and a close-up of the central region of the trough with the torsion pendulum and the mirrors of the BAM:

The central region consists of an "outer cylinder" of twelve fingers, the torsion pendulum and attached disk for measuring stress/strain and the Brewster angle microscope. The angular position of the disk at the end of the torsion pendulum is measured using a magnetic field and coil arrangement. The coil used to generate the magnetic field can be seen in the image. We are often ask, how can you measure a signal with all that water? The disk has a knife edge and is lowered until it just touches the surface of the water. With no film present, rotation of the band produces an angular displacement of the disk that is less than our resolution. Another way to say this, is that the angles that are measured with a typical film present would require that the disk be submerged in 15 cm of water for the signal to be due to the viscous torque from the water. As our trough is only a few cm deep, this is not possible. The microscope consists of a HeNe laser mounted vertically. The light is reflected off the surface of the water at the Brewster angle using a small mirror. The reflected beam is then sent through a series of lenses and the final image recorded using a CCD camera. The basic principle behind the Brewster angle micrscope is that the density of the monolayer and the tilt of the molecules relative to the polarization of the incoming beam change the index of refraction of the surface. This changes the Brewster angle and results in an intensity variation that is a function of density and tilts angle. This allows us to image both domains of constant tilt angle and density.

Microrheology Our lab has just started developing microrheology techniques to measure the viscosity of monolayers. This section is under construction and more will be added soon. This work is being carried out in collaboration with Alex Levine, Fred MacKintosh, and Christoph Schmidt.

Effect of divalent ions: We have found that divalent ions can have very long time scales associated with their interactions with monolayers. This section is under construction and more will be added soon. However, some of the work is published and available as preprints.

Protein Interactions with monolayers: This is work that is being carried out in collaboration with Hudel Luecke in biology and Doug Tobias in the Chemistry Department. The work involves using flourescence microscopy and pressure-area measurements to study the interaction of annexins and phospholipids. This work is relatively new. For a summary of our current research please see our paper. A complete preprint should be available soon.

Some selected references for monolayers:

For reviews of Langmuir monolayers, see H. Mohwald, Annu. Rev. Phys. Chem 41, 441 (1990), and H. M. McConnell, Annu. Rev. Phys. Chem. 42, 171 (1991). For a review of phase transitions in monolayers, see C. M. Knobler and R. C. Desai, Annu. Rev. Phys. Chem. 43, 207 (1992).

For a survey of shear in thin films, see D. A. Edwards, H. Brenner, and D. T. Watson, Interfacial Transport Processes and Rheology (Butterworth-Heinemann, Boston 1991). For a discussion of some particular issues with Langmuir monolayers, see R. S. Ghaskadvi, J. B. Ketterson, and P. Dutta, Langmuir 13, 5137 (1997) and M. L. Kurnaz and D. K. Schwartz, Phys. Rev. E 56, 3378 (1997).

Work on shearing foams in monolayers is reported in M. Dennin, C. M. Knobler, Phys. Rev. Lett., 78, 2485 (1997).

A reference for Brewster angle microscopy is S. Siegel, D. Höning, D. Vollhardt, and D. Möbius, J. Phys. Chem. 96, 8157 (1992).

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