TRANSIENT EFFECTS IN LIQUID-LIQUID INTERFACE MOTIONS THROUGH GLASS-CAPILLARIES

We present an experimental study of the nonstationary motion of a water-cyclohexane interface in millimiter-size glass capillary tubes initially saturated with cyclohexane. Starting from an initially high value, the interface velocity is observed to relax exponentially toward an asymptotic steady motion. At a given applied counterpressure, the time variation of the interface velocity follows a law independent of the experimental procedure. These effects are particularly strong at very low capillary numbers between 3 × 10-9 and 5 × 10-7; the relaxation time constant increases roughly as the inverse of the final interface velocity in this range and reaches values of several thousand seconds at the lowest velocities. We suggest that these slow relaxations are due to adsorption sites with a wide range of characteristic times: decreasing the velocity increases the number of sites active for adsorption and enhances the wetting of the high-energy glass surface by water. © 1992.