  Natural or industrial flows of a fluid often involve droplets or bubbles of
another fluid, pinned by physical or chemical impurities or by the roughness of
the bounding walls. Here we study numerically one drop pinned on a circular
hydrophilic patch, on an oscillating incline whose angle is proportional to
$\sin(\omega\,t)$. The resulting deformation of the drop is measured by the
displacement of its center of mass, which behaves like a driven over-damped
linear oscillator with amplitude $A(\omega)$ and phase lag $\varphi(\omega)$.
The phase lag is ${\cal O}(\omega)$ at small $\omega$ like a linear oscillator,
but the amplitude is ${\cal O}(\omega^{-1})$ at large $\omega$ instead of
${\cal O}(\omega^{-2})$ for a linear oscillator. A heuristic explanation is
given for this behaviour. The simulations were performed with the software
Comsol in mode Laminar Two-Phase Flow, Level Set, with fluid 1 as engine oil
and fluid 2 as water.
