(750a) Morphological Transitions of Capillary Bridges in a Slit Pore Geometry

Morphological
transitions of capillary bridges in a slit pore geometry

David Broesch and
Joelle Frechette

Chemical and
Biomolecular Engineering Department, Johns Hopkins University

Abstract

                Investigations of capillary bridges are
traditionally limited to the idealized geometry of an axisymmetric bridge
between spheres or between a sphere and a plate. As capillary forces are
employed to drive micro- and nanoscale assembly between complex objects, such
as in flip chip technology, there is a need to better understand the shape (and
associated forces) of capillary bridges in complex geometries. Here we
investigate the morphological evolution of non-axysymetric capillary bridges in
a slit-pore geometry. In our experimental system the liquid bridges are formed
between two hydrophobic surfaces patterned with hydrophilic strips. The aspect
ratio of the capillary bridges (length/width) is varied from 2.5 to more than
100 by changing the pore geometry or the fluid volume. As the separation
between the surfaces increases (aspect ratio decreases), we observe a large
increase in the mean curvature of the bridge. More specifically, we see that
the mean curvature of the bridges change sign (goes from
negative to positive for larger pore height). In addition we found a general
scaling for which all the curves mapping the morphological evolution collapse.
We also simulated our experiments using Surface Evolver and observed
quantitative agreements with our experiments. Finally, we show that the
morphology diagrams obtained from our 3D analysis are considerably different
from those in the simpler 2D model presented in the literature.