▁cannot . ▁K etch up ▁is ▁a ▁she ar - th in ning ▁material , ▁like ▁y og urt ▁and ▁em ulsion ▁paint ▁( US ▁termin ology ▁late x ▁paint ▁or ▁ac ry lic ▁paint ), ▁exhib iting ▁th ix ot ropy , ▁where ▁an ▁increase ▁in ▁relative ▁flow ▁velocity ▁will ▁cause ▁a ▁reduction ▁in ▁vis cos ity , ▁for ▁example , ▁by ▁stir ring . ▁Some ▁other ▁non - New ton ian ▁materials ▁show ▁the ▁opposite ▁behavior , ▁r he o pect y : ▁vis cos ity ▁increasing ▁with ▁relative ▁de formation , ▁and ▁are ▁called ▁she ar - thick ening ▁or ▁dil at ant ▁materials . ▁ ▁Since ▁Sir ▁Isaac ▁Newton ▁origin ated ▁the ▁concept ▁of ▁vis cos ity , ▁the ▁study ▁of ▁liqu ids ▁with ▁stra in - rate - dependent ▁vis cos ity ▁is ▁also ▁often ▁called ▁Non - New ton ian ▁fluid ▁mechan ics . ▁ ▁The ▁experimental ▁character isation ▁of ▁a ▁material ' s ▁r he ological ▁behaviour ▁is ▁known ▁as ▁r he ometry , ▁although ▁the ▁term ▁r he ology ▁is ▁frequently ▁used ▁syn onym ously ▁with ▁r he ometry , ▁particularly ▁by ▁experimental ists . ▁The oret ical ▁aspects ▁of ▁r he ology ▁are ▁the ▁relation ▁of ▁the ▁flow / de formation ▁behaviour ▁of ▁material ▁and ▁its ▁internal ▁structure ▁( e . g ., ▁the ▁orientation ▁and ▁el ong ation ▁of ▁polym er ▁mole cules ), ▁and ▁the ▁flow / de formation ▁behaviour ▁of ▁materials ▁that ▁cannot ▁be ▁described ▁by ▁classical ▁fluid ▁mechan ics ▁or