  Sessile droplets of aqueous poly(ethylene oxide) solution, with average
molecular weight of 100 kDa, are monitored during evaporative drying at ambient
conditions over a range of initial concentrations $c_0$. For all droplets with
$c_0 \geq 3%$, central conical structures, which can be hollow and nearly 50%
taller than the initial droplet, are formed during a growth stage. Although the
formation of superficially similar structures has been explained for
glass-forming polymers using a skin-buckling model which predicts the droplet
to have constant surface area during the growth stage (L. Pauchard and C.
Allain, Europhys. Lett., 2003, 62, 897-903), we demonstrate that this model is
not applicable here as the surface area is shown to increase during growth for
all $c_0$. We interpret our experimental data using a proposed drying and
deposition process comprising the four stages: pinned drying; receding contact
line; bootstrap growth, during which the liquid droplet is lifted upon
freshly-precipitated solid; and late drying. Additional predictions of our
model, including a criterion for predicting whether a conical structure will
form, compare favourably with observations. We discuss how the specific
chemical and physical properties of PEO, in particular its amphiphilic nature,
its tendency to form crystalline spherulites rather than an amorphous glass at
high concentrations and its anomalous surface tension values for MW = 100 kDa
may be critical to the observed drying process.
