FIG. 24 Dependencies of flow rate v on pressure difference ∆P in a hydrophobed quartz capillary, r=3.6 µm, when D-8 solution displaced silicon oil (v>0) and when silicon oil displaced surfactant solution (v<0).

The capillary counterpressure, which is proportional to the wetting tension γ12 cos θR≈10 mN/m, is about three times lower than when silicon oil was displaced by pure water. The value of wetting tension measured in hydrophobed capillaries equals in this case 30 mN/m. Therefore, use of the D-8 trisiloxane surfactant may facilitate displacement of oils from hydrophobic pores. Besides, when water displaces silicon oil a wetting film of the oil remains on the hydrophobed capillary surface. Because the thickness of the remaining dynamic film grows according to Derjaguin [7] with flow rate, the low rates of displacement are preferable in these cases.