Sessile drop contact angle as affected by the particle size and the drop size

Abstract

Soil water repellency is an important phenomenon that primarily affects the water infiltration rates in soils. The magnitude of soil water repellency can be expressed in terms of soil-water contact angle, in degrees. The purpose of this study is to examine the effects of particle size and drop size (drop volume) on the contact angle of a sessile drop. Silica sand and Glass beads were used to assess the effect of particle size, whereas Acryl, Vinyl tape, Adhesive tape, and hydrophobized silica sand were used to assess the effect of drop size, on sessile drop contact angle. Contact angle was directly measured using a digital microphotograph and calculated using equations using drop parameters as functions. Sessile drop contact angle decreased, from 60° to 20° in silica sand and from 76° to 45° in glass beads, with the increase in particle size from 19 to 250 pm and from 50 to 400 pm, respectively. Calculated contact angle of all the samples decreased continuously with increase in drop size from 5 to 50 pi, which was attributed to the changes in drop parameters with the change in drop size. Measured sessile drop contact angle decreased to a minimum with the increase in drop size up to 15 pi in Acryl, 20 pi in Vinyl tape, and 30 pi in Adhessive tape, and both hydrophobized silica sand samples. This was considered as a result of the decreasing importance in linear tension of a liquid drop with increasing volume. Measured contact angle increased again with increasing drop size up to 50 pi in all the samples, which might be because of the gravity interference to the contact angle after some level.