J. Drelich et al., PREPARATION OF A COAL SURFACE FOR CONTACT-ANGLE MEASUREMENTS, Journal of adhesion science and technology, 11(11), 1997, pp. 1399-1431
Coal specimens of different ranks were polished using silicon carbide
abrasive papers (with a grit from #60 to #1200) and alumina powder of
varying size (from 5 to 0.05 mu m). The coal surface roughness and con
tamination (by alumina powder) were examined with both scanning electr
on microscopy and atomic force microscopy. The water advancing and rec
eding contact angles were measured on such surfaces by varying the bub
ble size, using the captive bubble technique. It was found that silico
n carbide paper abraded all components of the coal surface, i.e. both
organic and inorganic matter, to a similar depth. The roughness of the
coal surface due to polishing with silicon carbide abrasive papers af
fected the contact angle hysteresis and the contact angle vs. bubble s
ize relationship. Polishing of coal specimens with alumina powder redu
ced the microroughness of the coal surface but produced rough features
at the macro level and caused mineral inclusions rising above the smo
oth organic matter. This phenomenon results from the heterogeneity of
coal specimens consisting of minerals and macerals with different hard
ness values. The roughness at the macro level was easily distinguishab
le and had a significant impact on the measured contact angles when th
e coal surface was polished with coarse alumina powders, 5 and 1 mu m
in diameter. The effect of surface roughness on the advancing and rece
ding water contact angles was significantly reduced (if not completely
eliminated) when the coal surface was polished with a fibrous cloth (
CHEMOMET) in the final step, after having been polished with 0.05 (0.0
6) mu m alumina powder. Microscopic observation of the coal surfaces r
evealed that an appropriate ultrasonic treatment (8-10 min in an ultra
sonic bath filled with water) and mechanical cleaning (polishing with
a CHEMOMET cloth) of coal samples were required to remove the alumina
particles left on the surface due to the previous polishing procedure.
An improved methodology for coal surface preparation, prior to contac
t angle measurements, as proposed in this paper, includes polishing wi
th a series of abrasive papers and 0.05 (0.06) mu m alumina powder, po
lishing and cleaning with a fibrous cloth (e.g. CHEMOMET), and, finall
y an extended cleaning in an ultrasonic bath filled with water.