istence ▁impos es . ▁ ▁The ▁mechanical ▁response ▁includes ▁trans itions ▁between ▁solid - like ▁and ▁fluid - like ▁behavior ▁as ▁well ▁as ▁fl uct u ations . ▁ ▁Their ▁mechanical ▁properties ▁can ▁be ▁attributed ▁to ▁characteristics ▁such ▁as ▁high ▁dis order , ▁c aging , ▁and ▁clust ering ▁on ▁multiple ▁length ▁scales . ▁ ▁Example ▁ ▁Sh aving ▁cre am ▁is ▁an ▁example ▁of ▁a ▁complex ▁fluid . ▁ ▁Without ▁stress , ▁the ▁fo am ▁appears ▁to ▁be ▁a ▁solid : ▁it ▁does ▁not ▁flow ▁and ▁can ▁support ▁( very ) ▁light ▁ ▁loads . ▁ ▁However , ▁when ▁ade qu ate ▁stress ▁is ▁applied , ▁sh aving ▁cre am ▁flows ▁easily ▁like ▁a ▁fluid . ▁ ▁On ▁the ▁level ▁of ▁individual ▁b ub bles , ▁the ▁flow ▁is ▁due ▁to ▁re arr ang ements ▁of ▁small ▁collections ▁of ▁b ub bles . ▁ ▁On ▁this ▁scale , ▁the ▁flow ▁is ▁not ▁smooth , ▁but ▁instead ▁consists ▁of ▁fl uct u ations ▁due ▁to ▁re arr ang ements ▁of ▁the ▁b ub bles ▁and ▁releases ▁of ▁stress . ▁ ▁These ▁fl uct u ations ▁are ▁similar ▁to ▁the ▁fl uct u ations ▁that ▁are ▁studied ▁in ▁earth qu akes . ▁ ▁Dynam ics ▁The ▁ ▁dynamics ▁of ▁the ▁particles ▁in ▁complex ▁flu ids ▁are ▁an ▁area ▁of ▁current ▁research . ▁Energy ▁lost ▁due ▁to ▁fr iction ▁may ▁be ▁a ▁ ▁non linear ▁function ▁of ▁the ▁velocity ▁and ▁normal ▁forces . ▁ ▁The ▁topological ▁in hib ition ▁to ▁flow ▁by ▁the ▁crow ding