Elastic deformation of non-uniform polymer crusts formed
by droplet evaporation
Department of Applied Physics, Tokyo
University, Tokyo, JAPAN.
Crust formation of an evaporating polymer solution is an important
technical process for micron-scale manufacturing processes. For a droplet
on a wetting surface, the crust takes a cap-shape that elastically
deforms under the osmotic pressure arising from continuing evaporation
through the porous solid. A variety of shapes have been observed in
experiments, including so-called "craters" and "mexican
hats" - however, it is not clear for what parameters (crust
thickness, contact angle etc.) each shape arises. We have investigated this
phenomenon using quasi-static simulations and have constructed a
"phase diagram" of contact angle versus crust thickness to show
where each shape arises. Our protocol is pressure-controlled rather than
volume-controlled, both to simplify the numerical procedure and to allow
us to draw on a significant literature of elastic spherical caps. We have
also looked into the effects of non- uniform shell thickness, since the nature
of droplet evaporation suggests the crust formed will be thicker near the
contact line than the apex. In parallel with these numerical studies, we
will also present some approximate theoretical calculations that
incorporate non-linear strains. This is crucial, as the various shapes
only emerge under non-linear deformation; linear strains always
produce qualitatively similar results. Some recent results on symmetry-
breaking deformations will also be given