hide
Free keywords:
-
Abstract:
In the present work, static liquid morphologies confined to linear micron sized surface
grooves were studied experimentally and analyzed theoretically. Geometries with increasing
complexities, from wedges to trapezoidal grooves, were explored with the main focus on triangular
grooves. In contrast to chemically structured substrates where only liquid morphologies
with positive Laplace pressure are found, topographically structured substrates exhibit liquid
morphologies with both positive and negative Laplace pressure. Depending upon the wettability
and the exact geometry of substrates, either drop-like morphologies or elongated filaments
with positive or negative Laplace pressure represent the generic equilibrium structures on the
substrates. For very high contact angles, drop-like morphologies are dominant irrespective of
the underlying substrate geometry.
Transitions between these liquid morphologies can be triggered by varying the wettability
or the geometry of substrates. In the present work, various cross sections of the grooves
were explored while the wettability was controlled by various self-assembly monolayers or by
means of the electrowetting effect. Upon changing the apparent contact angle of an aqueous
drop by electrowetting, the transition between the drop-like and elongated filament morphologies
could be triggered and thus a liquid can be transported along prefabricated grooves on
demand. A clear threshold behavior for filling of the grooves was observed which corresponds
to the stability boundaries of the static wetting morphologies in the respective groove
geometry. The length of the liquid filament that advances into the groove depends on the exact
geometry of the groove and the electrical properties of the system. An electrical model
is presented to explain this behavior. Unlike liquid filaments in rectangular grooves, liquid
filaments in triangular grooves become unstable when they are quenched from a filling into a
non-filling regime.
This instability of liquid filaments in triangular grooves was studied in detail using homogeneous
filaments of glassy polymer (polystyrene) which have been prepared in a non
equilibrium state by deposition from a solution. At elevated temperature, molten polystyrene
restores its material contact angle with the substrate thus forming filaments with positive
Laplace pressure. After dewetting, this liquid filament decays into isolated droplets with a
characteristic spacing, depending upon wedge geometry, wettability and filament width. This
instability is driven by the interplay of local filament width and Laplace pressure and constitutes
a wide class of one-dimensional instabilities which also includes the Rayleigh - Plateau
instability as a special case. The dynamics of this instability was also studied via in situ AFM
experiments which allows to determine the time constant of the instability. A careful analysis
of the time constant of the instability allows for the quantitative determination of the slip
length in the system.
