Volume 79, Number 6, September 2007
Article Number 66005
Number of page(s) 6
Section Condensed Matter: Structural, Mechanical and Thermal Properties
Published online 28 August 2007
EPL, 79 (2007) 66005
DOI: 10.1209/0295-5075/79/66005

Single-particle Brownian dynamics for characterizing the rheology of fluid Langmuir monolayers

M. Sickert1, F. Rondelez1 and H. A. Stone2

1  Institut Curie, Section de Recherche - 26 rue d'Ulm, 75248 Paris cedex 05, France
2  School of Engineering and Applied Sciences, Harvard University - Cambridge, MA 02138, USA

received 27 July 2007; accepted in final form 31 July 2007; published September 2007
published online 28 August 2007

The Brownian movement of a submicron diameter sphere has been used to study the rheology of Langmuir monolayers of short-chain surfactants in the low-density limit (liquid-expanded phase). We show how the shear viscosity can be derived from previously measured diffusion coefficients (SICKERT M. and RONDELEZ F., Phys. Rev. Lett. 90 (2003) 126104) by using recent theoretical models for a sphere of size much larger than the monolayer thickness. We report the surface viscosity as a function of the occupied surface fraction, which we believe are the first such estimates based on physically consistent experiments and theory. The two models yield values of the surface viscosity 1-5$\times$10-10 Ns/m and 2-8$\times$10-10 Ns/m over the entire range of surface densities of the liquid-expanded phase.

68.15.+e - Liquid thin films.
66.20.+d - Viscosity of liquids; diffusive momentum transport.
83.85.Jn - Viscosity measurements.

© Europhysics Letters Association 2007