▁el astic ity . ▁ ▁S cope ▁▁ ▁In ▁practice , ▁r he ology ▁is ▁princip ally ▁concerned ▁with ▁extending ▁continu um ▁mechan ics ▁to ▁character ize ▁flow ▁of ▁materials , ▁that ▁exhib its ▁a ▁combination ▁of ▁el astic , ▁vis c ous ▁and ▁pl astic ▁behavior ▁by ▁properly ▁combining ▁ ▁el astic ity ▁and ▁( New ton ian ) ▁fluid ▁mechan ics . ▁It ▁is ▁also ▁concerned ▁with ▁establish ing ▁predictions ▁for ▁mechanical ▁behavior ▁( on ▁the ▁continu um ▁mechanical ▁scale ) ▁based ▁on ▁the ▁micro - ▁or ▁nan ost ruct ure ▁of ▁the ▁material , ▁e . g . ▁the ▁mole cular ▁size ▁and ▁architecture ▁of ▁polym ers ▁in ▁solution ▁or ▁the ▁particle ▁size ▁distribution ▁in ▁a ▁solid ▁susp ension . ▁Material s ▁with ▁the ▁characteristics ▁of ▁a ▁fluid ▁will ▁flow ▁when ▁subject ed ▁to ▁a ▁stress ▁which ▁is ▁defined ▁as ▁the ▁force ▁per ▁area . ▁There ▁are ▁different ▁sorts ▁of ▁stress ▁( e . g . ▁she ar , ▁t ors ional , ▁etc .) ▁and ▁materials ▁can ▁respond ▁differently ▁under ▁different ▁str esses . ▁Much ▁of ▁theoretical ▁r he ology ▁is ▁concerned ▁with ▁associ ating ▁external ▁forces ▁and ▁tor ques ▁with ▁internal ▁str esses ▁and ▁internal ▁stra in ▁grad ients ▁and ▁flow ▁veloc ities . ▁ ▁R he ology ▁un ites ▁the ▁seem ingly ▁un related ▁fields ▁of ▁pl astic ity ▁and ▁non - New ton ian ▁fluid ▁dynamics ▁by ▁recogn izing ▁that ▁materials ▁under going ▁these ▁types ▁of ▁de formation ▁are ▁unable ▁to ▁support ▁a ▁stress ▁( part