简介：Theresultsofasimplecomputationalmodelfordifferentialsettlingarepresentedillustratingthesignificantrolethatparticlesizedistributionplaysincollisionfrequencyandsedimentationrateofparticlesinaquiescentenvironment.Themodeltracksalargenumberofparticles(order10~5)withlog-normallydistributeddiameters,astheysettleattheirStokessettlingvelocities.Particlecollisionsaredetectedandresultinlargerparticlesthatfallmorerapidly.Anumberofsimplifyingassumptionsaremadeinthemodelinordertoavoidempiricalcorrelationsforphenomenasuchascollisionefficiencyandparticleshape.Thesesimplifyingassumptionswereneededtoisolateandquantifytheroleoftheparticlesizedistribution.Simulatedconcentrationprofilesindicatethat,evenintheabsenceofcollisions,thestandarddeviation(σ_D)oftheparticlesizestronglyinfluencesthebulkmasssettlingrateas,forlargerσ_D,moremassisconcentratedinlarger,fasterfallingparticles.Thecollisionfrequencyisalsoastrongfunctionofσ_D.Foragivenmassconcentrationthecollisionfrequencyfirstincreaseslinearlywithincreasingσ_Dasgreatervariationinparticlesizeleadstogreatervariationinparticlevelocity,andshortertimesforparticlestocatcheachother.Forlargerσ_Dmoremassisconcentratedinlargerparticles,so,foragivenmassconcentration,therearefewerparticlesperunitvolume,increasingthemeandistancebetweentheparticlesandreducingthecollisionfrequency.Theimplicationsoftheseresultsforsedimentationmeasurementusingopticalattenuationtechniquesarediscussed.