  The structure of liquid foams follows simple geometric rules formulated by
Plateau 150 years ago. By placing such foam on a microtextured hydrophilic
surface, we show that the bubble footprint exhibits a morphological transition.
This transition concerns the liquid channels, also called pseudo Plateau
borders, which are straight between vertices on a smooth surface. We
demonstrate experimentally that for a sufficiently large roughness size
compared to the width of the liquid channels, the footprint adopts a zigzag
shape. This transition is associated with the absence of a wetting film between
the pillars caused by capillary suction of the foam, observed by confocal
microscopy. We rationalize the number of zigzag segments by a geometric
distribution describing the observations made with the footprint perimeter and
the mesh size of the asperities.
