Modification of the effective contact angle by means of particle collection and the generation of armored bubbles

We report on an experimental work where hydrophobic particles are collected by a moving water-air meniscus in a capillary tube. The results show that under certain conditions the collected particles can position themselves on a granular monolayer at the liquid-air interface and move with it. A continuation in the particle collection by the meniscus results in the decrease of the effective liquid-water contact angle (CA) in the capillary tube. At zero CA an ``air finger'' with its walls covered by particles forms. The finger displays stick-slip behavior in its motion due to solid friction of the particles with the wall. This friction eventually provokes its pinch-off which results in the formation of a cylindrical armoured bubbles.