text: The Kelvin equation gives solubility as a function of this property.
full_question: The Kelvin equation gives solubility as a function of this property. Dynamic light scattering is used to measure this quantity, which continuously increases due to Ostwald ripening. The mean free path is proportional to the negative second power of this quantity. This property of a polymer is determined using chromatography where a gel is the stationary phase. The cross section in (*) collision theory is roughly the sum of this quantity for the molecules. The difference between a colloid and a suspension is this property for the particles. The b term in the van der Waals equation accounts for this property, countering the ideal gas law's assumption that this property for gas molecules is zero. For 10 points, name this quantity which is small for particles that pass through a filter.
first_sentence: The Kelvin equation gives solubility as a function of this property.
raw_answer: particle size [or volume; or surface area; or radius; or diameter; or chain length; do not accept or prompt on "mass" or "weight" or "molar mass"] 