The effect of rank, lithotype and roughness on contact angle measurements in coal cleats

We report the effect of rank and lithotype on the wettability of coal in microfluidic experiments in two types of artificial microchannels: (1) reactive ion etched (RIE) channels and (2) die-cast channels prepared by pressing powdered lithotype concentrates. Five coals from the Bowen Basin with ranks in the R-max% range 0.98-1.91% were examined. Contact angles and entry pressures of air and water in the artificial cleats were measured in imbibition experiments performed with a Cleat Flow Cell (CFC). The relative contact angles measured in CFC imbibition experiments were in the range 110-140 degrees in the RIE channels and 85 degrees-115 degrees in the pressed discs, which are larger contact angles than measured on the flat bulk surfaces of these samples by the conventional sessile drop technique (58 degrees-85 degrees). The CFC observations also show the surface roughness of coal in inertinite-rich dull bands effects contact angle and the entry pressure of the air-water interface differently to the vitrinite-rich bright bands. Drainage experiments in the CFC revealed a thin residual water film on the inertinite cleat wall, yet not on the smooth vitrinite channel. The experimental observations are used to present a modified Cassie Equation model to predict coal contact angles based on the fractions of dull and bright bands, mineral content, and cleat surface roughness. The results of this study provide the basis to consider an improved relative permeability model that explicitly accounts for the effect of coal lithotype.