Residual Fluid as a Source of Error in Bubble Point Testing

The bubble point test (ASTM D6767) is a method of calculating the pore size distribution of geotextiles. ASTM D6767 allows a variety of wetting fluids to be used in the test. However, different fluids are found to produce different pore size distributions, even though relevant fluid properties are supposedly accounted for in the data reduction (Elton, D. J., Hayes, D. W, Adanur, S., Geotech. Test. J., Vol. 30, No. 1, 2007, pp. 9 16). The ASTM D6767 data reduction is found to be theoretically incorrect. The effect of residual wetting fluid on the pore size distribution is examined. Residual fluid is defined as fluid that remains on the walls of a previously saturated pore after the critical pressure for the fluid in the pore has been reached (after the bubble point pressure is reached), Since the phenomenon of residual fluid is not considered in ASTM D6767, it presents a Source of error. Bretherton's law suggests residual fluid errors will be reduced by using fluids with a small viscosity to surface tension ratio.