For each phase, you can optionally set a minimum volume fraction. A minimum value is used for numerical robustness purposes, as the volume fractions are not allowed to go to machine zero at any location. A default value of 1.0E-15 is used if you do not set a value; this is almost always appropriate. If this causes numerical problems, then set the value to be several orders of magnitude less than the minimum expected physical value, but much larger than machine zero.