  Liquids spreading over a solid substrate under the action of various forces
are known to exhibit a long wavelength contact line instability. We use an
example of thermally driven spreading on a horizontal surface to study how the
stability of the flow can be altered, or patterns selected, using feedback
control. We show that thermal perturbations of certain spatial structure
imposed behind the contact line and proportional to the deviation of the
contact line from its mean position can completely suppress the instability.
Due to the presence of mean flow and a spatially nonuniform nature of spreading
liquid films the dynamics of disturbances is governed by a nonnormal evolution
operator, opening up a possibility of transient amplification and nonlinear
instabilities. We show that in the case of thermal driving the nonnormality can
be significant, especially for small wavenumber disturbances, and trace the
origin of transient amplification to a close alignment of a large group of
eigenfunctions of the evolution operator. However, for values of noise likely
to occur in experiments we find that the transient amplification is not
sufficiently strong to either change the predictions of the linear stability
analysis or invalidate the proposed control approach.
