Determining the Colloidal Forces between Bitumen Droplets in Water Using the Hydrodynamic Force Balance Technique

A novel technique using a “hydrodynamic force balance” was introduced to determine the maximum value of attractive forces between two micrometer-sized bitumen droplets in a doublet suspended in water. The technique is based on breaking up a doublet in a gradually increasing wall shear flow and calculating the colloidal force between the two droplets from the breakup shear rate. The measurable force ranges from 10^-13 to 10^-11 N. The upper limit can be further raised after some modifications to the instrument. The validity of the method has been verified by comparing the determined Hamaker constant of bitumen−water−bitumen with both the experimental data obtained from another force-measurement technique and the literature value. The methed is applicable to both emulsion and suspension systems although only a bitumen-in-water emulsion was investigated in this study. A bitumen droplet surface contains isolated “bumps” of 50−100 nm in horizontal diameter according to previous study. A disk−sphere model assuming a single disk-shaped protrusion attached to the bitumen droplet was used to interpret the force data. The calculation yielded the thickness values of the protrusions mostly in the range of 0−20 nm, which are consistent with the previous findings.