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Capillary replacement in a tube pre-filled with a viscous fluid.

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 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 horizontal geometry. 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. As a result, 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 derived a simple unified expression describing the three distinct dynamics, which accounts well for the observations. We also demonstrated a thorough experimental confirmation on the initial velocity of the replacement and the replacement time, the time required for the invading fluid to replace completely the pre-existing fluid in the horizontal geometry.