  We present a novel experimental system that can be used to study the dynamics
of picoliter droplet spreading over substrates with topographic variations. We
concentrate on spreading of a droplet within a recessed stadium-shaped pixel,
with applications to the manufacture of POLED displays, and find that the
sloping side wall of the pixel can either locally enhance or hinder spreading
depending on whether the topography gradient ahead of the contact line is
positive or negative. Locally enhanced spreading occurs via the formation of
thin pointed rivulets along the side walls of the pixel through a mechanism
similar to capillary rise in sharp corners. We demonstrate that a thin-film
model combined with an experimentally measured spreading law, which relates the
speed of the contact line to the contact angle, provides excellent predictions
of the evolving liquid morphologies. We also show that the spreading can be
adequately described by a Cox-Voinov law for the majority of the evolution. The
model does not include viscous effects and hence, the timescales for the
propagation of the thin pointed rivulets are not captured. Nonetheless, this
simple model can be used very effectively to predict the areas covered by the
liquid and may serve as a useful design tool for systems that require precise
control of liquid on substrates.
