  Capillary invasion of a liquid into an empty tube, which is called capillary
rise when the tube axis is in the vertical direction, is one of the fundamental
phenomena representing capillary effects. Usually, the tube is actually filled
with another pre-existing fluid, air, whose viscosity and inertia can be
practically neglected. In this study, we considered the effect of the
pre-existing fluid, when its viscosity is non-negligible, in a horizontally
geometry. This geometry is free from gravity and thus simpler than the geometry
of capillary rise. We observed the dynamics when a capillary tube that is
submerged horizontally in a liquid gets in contact with a second liquid. An
appropriate combination of liquids allowed us to observe that the second liquid
replaces the first without any prewetting process, thanks to a careful cleaning
of capillary tubes. Furthermore, we experimentally observed three distinct
viscous dynamics: (i) the conventional slowing-down dynamics, (ii) an unusual
accelerating dynamics, and (iii) another unusual dynamics, which is linear in
time. We developed a theory in viscous regimes, which accounts well for the
observations through a unified expression describing the three distinct
dynamics. We also demonstrated a thorough experimental confirmation on the
initial velocity of the replacement. We further focused on the replacement
time, the time required for the invading fluid to replaces completely the
pre-existing fluid in the horizontal geometry, which is again well explained by
the theory.
