|Abstract or Summary
- Infiltration of highly concentrated solutions into unsaturated sand is suspected
to be affected by the liquid-gas interfacial tension between the resident water and
that of the infiltrating solution. The wetting of non-porous solid surfaces by liquids
is commonly quantified by contact angle measurements. However, it is well known
that wettability of porous solids cannot be accurately obtained by optical inspection
because the liquid is penetrating into the pores of the solid in question.
The main objective of this investigation was to find an effective method to
measure contact angle in coarse porous media such as sandy soils. In this study, we
compare both static and dynamic methods to estimate the contact angle formed by
solutions of varying surface tension on silica sands. In addition, the contact angle
of the imbibing solutions is estimated in both dry and water-wetted sand.
Experiments in this study employed three clear acrylic columns of known
volume to determine contact angles using two methods, one static method and one
dynamic method. The three acrylic columns were packed with the same mass of
each grade of Accusand® (40/50, 30/40, 20/30, and 12/20 grades respectively) for
triplicate measurements. The solutions used in this study included (1) pure water
and (2) 5 molal NaNO₃ and (3) n-hexane as a reference.
The static method estimated contact angles in initially dry sand of 23° for
40/50 sand, and 30° to 33° for 12/20, 20/30, and 30/40 sands, with the same values
obtained for both solutions. Contact angles of these solutions observed in the
dynamic test, were twice those found in the static test (averaging 45° and 62°
In the case of pre-wetted sands, dynamic imbibition with water provided an
estimated contact angle of 2°, while the NaNO₃ solution yielded 21° contact.
Based on relative surface tensions of water and the 5 molal NaNO₃, the Young's
equation predicts a contact angle of 25°. These results strongly support recent
claims of effective contact angles between these miscible, but contracting, fluids.
The observed data suggest that the zero contact angle assumption is a poor
one even for clean dry silica sand. In a dynamic system, gravitational forces cannot
be ignored in course porous media. An analytical method, used to model
imbibition of the solutions into the silica sands, was both accurate and useful for
estimating contact angle.