were devoted to the shear induced fragmentation of aggre-The aggregation of spherical colloids partially stabilized by a gates previously formed in the absence of shear (4). In the thin hairy surface layer was investigated under peri-and orthokipresent work, we were interested in the behavior of a particunetic conditions. The weight and number average sizes of the lar class of colloidal particles bearing a hairy surface layer aggregates were derived from the aggregate size distribution of very small thickness (5).
which was provided by particle counting. Perikinetic aggregation
As for previous works, a laminar capillary flow was conditions were obtained in batches where polystyrene latex parused to model the passage of the suspension through one ticles were suspended in a deuterium oxide/ water mixture in pore of the porous medium. This experimental situation order to prevent sedimentation. Periodically, small samples were has been reviewed by Gregory who correlated experimentaken from the medium and immediately analyzed. Orthokinetic aggregation conditions were established by flowing the suspen-tal results and theoretical analysis ( 6 ) . Gregory considsion through a 1.25-m-length capillary of various internal diameered the situation of fully destabilized colloids and the ter. Samples were collected at the outlet of the tube and immediobserved decrease of the collision efficiency term in ately analyzed. At any time the aggregate size was found to sheared suspensions. This effect has been predicted by decrease with the colloid concentration. Aggregate aging over Mason and attributed to purely hydrodynamic factors ( 7 ) . periods of the order of the average time elapsed between two van de Ven and Mason calculated the collision efficiency successive collisions was found to exert a major effect on the factor for smooth, spherical particles in simple shear, aggregate sizes in peri-and orthokinetic processes. This effect which was found to decrease with shear rate and particle was increased by the particle concentration showing typical radius for a given system ( 8 ) . The correlation between the threshold effects such as a fast aggregation in dilute suspensions, number average size of the colloids, the flow and capillary a reduced influence of the concentration in the intermediate docharacteristics was investigated and a good correlation main and a strong inhibition of the aggregate growth in more concentrated systems. Orthokinetic aggregation of hairy colloids was found between theory and experiment ( 2 ) .
was found to produce a narrower aggregate size distribution than
We focused on the fate of a colloid which cannot be perikinetic aggregation. α§ 1997 Academic Press represented by a hard sphere of polystyrene which may Key Words: aggregation/fragmentation of hairy colloids; shear be almost nonexistent in the aqueous natural environment. induced aggregation; delayed colloid destabilization; collision effi-The most usual situation may correspond to particles charciency; reversible aggregation. acterized by surface heterogeneity or roughness induced by adsorbed small colloids ( of either identical or different chemical nature ) decorating the surface of larger colloids medium ( 13, 14 ) . This situation was considered in the 1 To whom correspondence should be addressed.
present work and we chose a model colloid for which the 312