  Wet-on-wet printing is frequently used in inkjet printing for graphical and
industrial applications, where substrates can be coated with a thin liquid film
prior to ink drop deposition. Two drops placed close together are expected to
interact via deformations of the thin viscous film, but the nature of these
capillary interactions is unknown. Here we show that the interaction can be
attractive or repulsive depending on the distance separating the two drops. The
distance at which the interaction changes from attraction to repulsion is found
to depend on the thickness of the film, and increases over time. We reveal the
origin of the non-monotonic interactions, which lies in the appearance of a
visco-capillary wave on the thin film induced by the drops. Using the thin-film
equation we identify the scaling law for the spreading of the waves, and
demonstrate that this governs the range over which interaction is observed.
