Measurements of contact angle between fine, non-porous particles with varying hydrophobicity and water and non-polar liquids of different viscosities

The three phase contact angle (theta) between liquids and fine solids is a key design parameter used in the control of the mineral separation, foam stabilisation or antifoaming processes, and therefore the accurate determination of theta is critical. Commonly used techniques produce variable results and are sensitive to the liquid type or viscosity. Different size ranges of non-porous, spherical glass beads were used to determine the applicability of the sessile drop method on particles forming 'flat' surfaces using different adhesives. Also investigated were the capillary rise and thin layer wicking techniques for the measurements of the contact angle between particles and water or oils of variable viscosity. The measured contact angles between particles and all liquids were compared with the contact angles obtained on the flat glass, made of the same material as the particles. Additionally, some of the particles were chemically modified in order to produce variable range of particle hydrophobicity for further comparison of the technique applicability. The measured contact angles were also qualitatively verified with the particle attachment to the bubble formed in the investigated liquids. Very good agreement between the measured water contact angles on particles using the sessile drop method and the water contact angles measured on flat glass was found for both degrees of glass hydrophobicity. The water contact angles measured of the particles with the capillary rise and thin layer wicking methods were found to be consistently larger, although reliable if considered as advancing contact angles. The contact angles between particles and three different viscosity oils were measured correctly only with the sessile drop method. By comparison, the thin layer wicking and the capillary rise technique produced significant errors. To conclude, the sessile drop on the adhesive slide is the most accurate technique for the measurements of contact angles between particles and water or oils, demonstrating that the technique is independent of the particle size or degree of hydrophobicity. (C) 2013 Elsevier B.V. All rights reserved.