The interpenetration of light and heavy liquids has been studied in a long tube inclined at small angles α to the horizontal. For angles greater than a critical angle αc (whose value decreases when the density contrast measured by the Atwood number At increases), the velocity of the interpenetration front is controlled by inertia and takes the steady value Vf=ki(Atgd)12, with ki0.7. At lower angles, the front is initially controlled by inertia, but later limited by viscous effects. The transition occurs at a distance Xfc, which increases indefinitely as α increases to αc. Once the viscous effects act, the velocity of the front decreases in time to a steady value Vf which is proportional to sinα. For a horizontal tube in the viscous regime, the velocity of the front decreases to zero as t12. At the same time, the profile of the interface h(x,t) only depends on the reduced variable xt12. A quasi-unidirectional model reproduces well the variation of the velocity of the front and the profiles of the interface, both in inclined and horizontal tubes. In the inclined tube, the velocity of the front is determined by matching rarefaction waves to a shock wave.

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