RHEOLOGICAL MODELING OF COMPLEX FLUIDS - II - SHEAR THICKENING BEHAVIOR DUE TO SHEAR-INDUCED FLOCCULATION

The structural model discussed in Part I of the present work is applie d to ''true'' behavior -i. e. shear thinning followed by shear thicken ing (sometimes followed by shear thinning) -very often observed in com plex fluids as the applied shear is increased. This model states that the viscosity eta of these fluids -described as concentrated dispersio ns of several classes of Structural Units (SUs) -is a unique function of a flow-dependent effective volume fraction, phi eff. The latter is expressed in terms of S-i = fraction of ''aggregated'' particles conta ined in all the SU(i)s (as SUs of i-class) and C-i = (phi(i)(-1))= ''c ompactness factor'', directly related to the mean compactness phi(i) o f SU(i)s. Shear induced flocculation (SIF) is the more obvious process capable to explain the shear thickening behavior of partially floccul ated suspensions (obviously, another process should be required for st abilized dispersions). After progressive reduction of SUs submitted to shear forces (i.e. leading to a decrease of S-i), such a behavior sho uld be observed if SIF occurs beyond a critical shear rate; (gamma) ov er dot (c), then resulting in re-increase of S-i, thus of phi(eff) and eta. The simplest ''SIF-model'' will introduce only one class of SUs, with only one variable S governed by a kinetic equation in which the kinetic rate for SU-formation increases with shear, thus giving the ex pected shear-thickening behavior if (gamma) over dot > (gamma) over do t (c). However, at high shear rates, SIF is limited by a (Smoluchowski -like) shear decreasing sticking probability. Effects of varying model parameters on predictions of the resulting SIF-model are discussed. F inally, this model is tested by comparison with observed rheological b ehavior, namely viscosity change vs, pH in aqueous styrene-ethylacryla te dispersions, from Laun's data [2].