Determining contact angle of solutions with varying surface tension on dry and pre-wetted silica sands Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/pz50gz833

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  • 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° respectively). 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.
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