Optically observed imbibition and drainage of wetting fluid in nanoporous Vycor glass

The light scattering and absorption of monolithic nanoporous Vycor glass during imbibition and drainage of wetting fluid such as water exhibit the following two optical hysteretic characteristics: one is the hysteretic response of the transient white turbidity in the visible (Vis) region and the other the hysteretic response of the absorbance peak in the near-infrared (NIR) region. We analyzed the effect of increasing and decreasing humidity in ambient air on the transmission in the glass with emphasis on its response to the humidity change, with or without holding at the maximum constant humidity, and its response to the humidity change up to various values of maximum attained humidity. We show that both the light scattering in the Vis region and the absorption in the NIR region are strongly affected by the duration and the maximum values of humidity, which implies that the amount of water in the pore space determines saturated and unsaturated responses of optical hystereses in both regions. We also show that the duration decreases the white turbidity while the immediate change of humidity from increasing to decreasing rather increases the turbidity. These facts verify that the nonequilibrium inhomogeneous distribution of the imbibed water in the pore space results in the optical inhomogeneity that causes the scattering, which is subsequently observed as the transient white turbidity, and that hysteresis loops of absorption are caused by the imbibed water in the pore space of Vycor glass. The existence of a threshold humidity of about 40% relative humidity (or corresponding pore-filling fraction of about 0.4) below which the two optical hystereses are suppressed and the fact that the maximum of optical turbidity occurs at about f=0.6 where the capillary condensation takes places imply that the appearance of a long-range capillary bridge between pores causes the transient white turbidity phenomenon.