▁into ▁other ▁spaces ▁behind ▁obst acles , ▁as ▁they ▁simply ▁hit ▁the ▁t ube ▁wall . ▁This ▁implies ▁conventional ▁p umps ▁cannot ▁be ▁used , ▁as ▁they ▁rely ▁on ▁vis c ous ▁flow ▁and ▁fluid ▁pressure . ▁Instead , ▁special ▁sor ption ▁p umps , ▁ion ▁p umps ▁and ▁momentum ▁transfer ▁p umps ▁are ▁used . ▁ ▁Free ▁mole cular ▁flow ▁occurs ▁in ▁various ▁processes ▁such ▁as ▁mole cular ▁dist ill ation , ▁ult ra - high ▁vac u um ▁equipment ▁such ▁as ▁particle ▁acceler ators , ▁and ▁naturally ▁in ▁outer ▁space . ▁ ▁The ▁definition ▁of ▁a ▁free ▁mole cular ▁flow ▁depends ▁on ▁the ▁distance ▁scale ▁under ▁consideration . ▁For ▁example , ▁in ▁the ▁inter plan et ary ▁medium , ▁the ▁pl asma ▁is ▁in ▁a ▁free ▁mole cular ▁flow ▁regime ▁in ▁scales ▁less ▁than ▁ 1 ▁A U ; ▁thus , ▁plan ets ▁and ▁mo ons ▁are ▁effectively ▁under ▁particle ▁bomb ard ment . ▁However , ▁on ▁larger ▁scales , ▁fluid - like ▁behavior ▁is ▁observed , ▁because ▁the ▁probability ▁of ▁coll isions ▁between ▁particles ▁becomes ▁significant . ▁ ▁See ▁also ▁Kn ud sen ▁equation ▁Kn ud sen ▁flow ▁ ▁References ▁ ▁Category : Fl uid ▁dynamics <0x0A> </s> ▁This ▁is ▁a ▁list ▁of ▁players , ▁past ▁and ▁present , ▁who ▁have ▁been ▁c apped ▁by ▁their ▁country ▁in ▁international ▁football ▁whilst ▁playing ▁for ▁C elt ic ▁Football ▁Club . ▁As ▁well ▁as ▁Scotland , ▁ 3 9 ▁other ▁nations ▁have ▁field ed ▁C elt ic ▁players ▁in ▁their ▁international ▁sides