A two-dimensional Couette viscometer for Langmuir monolayers

We have developed an apparatus that is capable of simultaneously measuring the viscosity of Langmuir monolayers and visualizing their flow. It consists of a circular trough with a nearly circular elastic barrier that can be rotated to generate two-dimensional Couette flow. The "inner cylinder'' is a Teflon knife-edge disk that is hung by a thin wire. The torque on the inner cylinder is determined by measuring the angular displacement of the disk. A stepper motor controls the barrier rotation. Viscosity can be measured in two different ways: by oscillating the torsion pendulum and by generating Couette flow. The dynamic viscosity range of the apparatus is 10(-4) < eta < 10(3) g/s. Typical shear rates range from 10(-4) to 10(1) s(-1). A Brewster angle microscope is mounted on the apparatus. This is used to study various properties of the monolayer such as: velocity profiles, domain shape during shear, domain relaxation after shear, and size distribution of domains. (C) 1998 American Institute of Physics. [S0034-6748(98)04610-3].