  The impact of a sessile droplet with a moving meniscus, as encountered in
processes such as dip-coating, generically leads to the entrapment of small air
bubbles. Here we experimentally study this process of bubble formation by
looking through the liquid using high-speed imaging. Our central finding is
that the size of the entrapped bubble crucially depends on the location where
coalescence between the drop and the moving meniscus is initiated: (i) at a
finite height above the substrate, or (ii) exactly at the contact line. In the
first case, we typically find bubble sizes of the order of a few microns,
independent of the size and speed of the impacting drop. By contrast, the
bubbles that are formed when coalescence starts at the contact line become
increasingly large, as the size or the velocity of the impacting drop is
increased. We show how these observations can be explained from a balance
between the lubrication pressure in the air layer and the capillary pressure of
the drop.
