Rheology of Concentrated Dispersions

In t h i s paper we b r i e f l y p resen t t h e main re levan t physical q u a n t i t i e s i n t h e rheology of concentrated d i spers ions . Main l i n e s o f t h r e e t o p i c s ( s h e a r v i scos i ty , sedimentation ve loc i ty and aggregation processes) a r e then given. The importance o r ordered or random s t r u c t u r e s is emphasized. Nature and i n d u s t i a l processes provide numerous examples of complex systems i n which s o l i d p a r t i c l e s a r e dispersed i n a f l u i d : slip used i n t h e manufacture of ceramics, n a t u r a l muds i n r i v e r s o r e s t u a r i e s , d r i l l i n g and mining f l u i d s , c e l l u l o s i c fibers i n paper pulp, p a s t r i e s , blood, concrete and cements ... Such systems a f ford very var ious behaviours ; t h e i r mechanical, phys ica l o r physicochemical p roper t i es are, at t h e presen t time, widely misunderstood. Such a lack o f understanding is due t o t h e complexity of t h e s e systems cumulating t h e d i f f i c u l t i e s o f random heterogeneous media and t h a t of hydrodynamic systems. Due allowance being made f o r t h e i r eminent importance i n t h e i n d u s t r i a l o r n a t u r a l processes, they have been t h e sub jec t of numerous p r a c t i c a l s t u d i e s and a considerable knowledge has been cumulated. But t h e r e is not , a t t h e present t i m e , a complete t h e o r e t i c a l corpus allowing a deep understanding of t h e s e systems. When one considers t h e case of one i s o l a t e d p a r t i c l e i n a f l u i d , some Simple s i t u a t i o n s as, f o r instance, t h e sedimentation i n a quiescent f l u i d o r t h e behaviour of t h e p a r t i c l e i n a sheared f l u i d may be exac t ly ( o r with a very good approximation) solved i f : i ) t h e f l u i d is unbounded ii) t h e viscous forces p r e v a i l over t h e i n e r t l a l ones (low Reynolds' number regime) i i i ) t h e p a r t i c l e ' s shape is simple enough (sphere /I/, cy l inder /2/, e l l i p s o i d /3/ ) . As soon as one o f these t h r e e condit ions is not f u l f i l l e d one does not know t h e s o l u t i o n except i n few s i t u a t i o n s . Among unsolved problems, some concern a s i n g l e sphere e i t h e r i n f a s t sedimentation i n an unbounded f l u i d /4/ o r i n slow sedimentation ins ide a c y l i n d r i c a l tube / 5 / . So it is not very s u r p r i s i n g t h a t more complex s i t u a t i o n s and e s p e c i a l l y those r e l a t e d t o concentrated d i spers ions have not y e t been solved. I propose i n t h i s paper t o isolate the main parameters re levan t to t h e proper t i es of t h e s e systems and t o put forward some aspec t s of t h e rheology o f d i spers ions c l o s e l y r e l a t e d t o t h e sc ience of ceramics. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986110