Hole Growth Instability in the Dewetting of Evaporating Polymer Solution Films.
Hole Growth Instability in the Dewetting of Evaporating
Polymer Solution Films.
Gu, X.; Raghavan, D.; Douglas, J. F.; Karim, A.
Journal of Polymer Science: Part B: Polymer Physics,
Vol. 40, No. 24, 2825-2832, December 15, 2002.
dewetting; Hele-Shaw flow cell; instability; stability;
fingering patterns; atomic force microscopy (AFM); spin
We investigate the dewetting of aqueous, evaporating
polymer [poly(acrylic acid)] solutions cast on glassy
hydrophobic (polystyrene) substrates. As in ordinary
dewetting, the evaporating films initially break up
through the nucleation of holes that perforate the film,
but the rapidly growing holes become unstable and form
nonequilibrium patterns resembling fingering patterns
that arise when injecting air into a liquid between two
closely spaced plates (Hele-Shaw patterns). This is
natural because the formation of holes in thin films is
similar to air injection into a polymer film where the
thermodynamic driving force of dewetting is the analogue
of the applied pressure in the flow measurement. The
patterns formed in the rapidly dewetting and evaporating
polymer films become frozen into a stable glassy state
after most of the solvent (water) has evaporated,
leaving stationary patterns that can be examined by
atomic force microscopy and optical microscopy. Similar
patterns have been observed in water films evaporating
from mica substrates, block copolymer films, and modest
hole fingering has also been found in the dewetting of
dry polymer films. From these varied observations, we
expect this dewetting-induced fingering instability to
occur generally when the dewetting rate and film
viscosity are sufficiently large.