In the conventional picture of a quiescent liquid, all the molecules play an equivalent role: all move in the same way. Mounting indirect evidence suggests that this equivalence may not always hold: there may be transient mobile and frozen regions. The nature of these heterogeneous regions has remained a puzzle. Our recent experiment on a two-dimensional colloidal fluid reveals the heterogeneity decisively and also shows that the motion of the mobile particles is very co-operative: particles follow each other in single file as shown in the picture below.
The figure shows a 13.3-second time exposure of a monolayer of 1.6 micron silica spheres suspended in water between two glass plates, viewed in an optical microscope. Video data was processed to show only the tracks of the centers of the spheres. Tracks that intersected within the exposure time are shown as heavier lines. Longer trains of tracks are shown in color. Different colors distinguish nearby but distinct tracks.
This striking heterogeneity suggests ways of controlling the rheology and molecular diffusion of viscous industrial fluids as well as dense colloidal suspensions as used in paints.
by Seth B. Darling, Stuart Rice
Bianxiao Cui, Binhua Lin and Stuart A. Rice, submitted to Journal of Chemical Physics 2/01.