We report morphological evolution and pattern formation
during
evaporative drying of a droplet of polymethylmethacrylate (PMMA) dissolved
in tetrahydrofuran over a soft, swellable cross-linked Sylgard 184
substrate. In contrast to the well-known coffee ring formation due
to the evaporation of a polymer solution droplet over a rigid substrate,
we show that the situation becomes far more complicated over a Sylgard
184 substrate due to solvent penetration and associated swelling.
The combined effect of evaporation and diffusive penetration leads
to significantly faster solvent loss and results in the formation
of an in situ thin polymer shell over the free surface of the evaporating
droplet due to the attainment of local glass-transition concentration.
The diffusive penetration of the solvent also leads to the spreading
of the three-phase contact line (TPCL) of the droplet after dispensing.
The vertical component of surface tension acting at the TPCL results
in the formation of peripheral creases along the boundary of the droplet
after the TPCL pins. With the progressive solvent loss, the shell
eventually collapses, resulting in a buckled morphology with a central
depression. We show that the evolution pathway and the final deposit
morphology depend strongly on the initial PMMA concentration (
C
i) in the droplet as it undergoes
a transformation from a central depression surrounded by peripheral
folds at lower
Ci
to a central
depression along with radial wrinkles at higher
Ci
. During the late stage of the evolution process,
the substrate undergoes de-swelling, which leads to flattening/rearrangement
of the radial wrinkles, the extent of which again depends on
Ci
. We explored how the deposition
pathway and patterns vary over a topographically patterned substrate
and found out that the presence of topographic patterns leads to even
faster solvent consumption due to enhanced diffusive penetration at
the corrugated liquidsubstrate interface, eventually resulting
in deposition with a smaller footprint and partially aligned radial
wrinkles. The results significantly enhance our understanding of droplet
evaporation over a substrate into which the solvent can penetrate
and unravel the complex physics, which is significantly dominated
by swelling rather than evaporation only, which is common over a rigid,
non-interacting substrate.