A mechanism was developed explaining why bubbles can enter a capillary
of diameter smaller than the diameter of the bubble. A cocurrent flow
of liquid creates a hydrodynamic pressure field that deforms the bubb
le surface and promotes the formation of a saddle-like interface near
the capillary mouth. A mechanistic model for the bubble to deform is g
iven. Even small bubbles would not enter the capillary if there is no
flow of liquid. Bubbles of a diameter larger than the diameter of a ca
pillary - or the side of a capillary of square cross section - can be
forced to enter the capillary only if there is a cocurrent flow of liq
uid. A high-speed video imaging system was used to record interesting
aspects of this phenomenon. On the average, the entire process for the
bubble to enter the capillary lasts less than 200 ms. This problem ha
s interesting applications in two-phase flows inside ceramic monoliths
, the flow of foams inside porous media and in the flow of blood into
capillary vessels. (C) 1997 Elsevier Science Ltd.