  The sudden arrest of motion due to confinement is commonly observed via the
clogging transition in the flow of particles through a constriction. We present
results of a simple experiment to elucidate a similar transition in the
bidirectional flow of two species in which two species of macroscopic particles
with different densities are confined in a tube and suspended in a fluid of
intermediate density. Counterflowing grains serve as mobile obstacles and
clogging occurs without arch formation due to confinement. We measure the
clogging or jamming probability $J$ as a function of number of particles of
each species $N$ in a fixed channel length for channel widths $D = $ 3$-$7$d$,
where $d$ is the particle diameter. $J(N)$ exhibits a sigmoidal dependence and
collapses on a single curve $J(N/D^3)$ indicating the transition occurs at a
critical density. Data is well-fit by a probabilistic model motivated by prior
constriction flows which assumes grains enter the clogging region with a fixed
probability to produce a clogging state. A quasi-two-dimensional experiment
provides insight into the interface shape and and we identify a Rayleigh-Taylor
instability at large channel widths.
