Introduction Mineral ¯akes are commonly used in plastics as reinforcing agents to enhance stiness (Busigin et al. 1984; Xavier and Sharma 1986; Faulkner 1988; Bouti et al. 1989) or dielectric strength (Xanthos et al. 1978) as well as to reduce warpage (Hawley 1987), thermal expansion (Debnath et al. 1988) or permeability to liquids (Cussler 1990; SoÈdergard et al. 1996). Apart from the adhesion between the matrix and the platelet particles (Garton et al. 1982; Vu-Khan and Fisa 1986; Chiang et al. 1994), the most important parameter in¯uencing the ®nal properties is the aspect ratio of the particles, p, which is equal to the thickness of the particles divided by their diameter. A very low aspect ratio (p<<1) is necessary to maximise the improvement in mechanical properties. The second important param- eter is the orientation of the particles, which is deter- mined by the material rheology and by the processing conditions (Fisa and Utracki 1984; Utracki and Lara 1984; Utracki et al. 1984; Rockenbauer et al. 1985; Malik and Prud'homme 1986; Utracki 1986; Vu-Khanh and Fisa 1986; Suh and White 1996). Since the work of Jeery (1922), the rheological behaviour of dilute rigid spheroid suspensions has been the subject of numerous theoretical investigations (Khun and Khun 1945; Goldsmith and Mason 1967; Cox 1971; Hinch and Leal 1972; Okagawa et al. 1973, 1974; Zuzovsky et al. 1980; to cite only a few). One of the more extensive reviews of the work done on this subject was proposed by Brenner (1974). The behaviour of a semi-dilute suspension in which interactions decrease the degrees of freedom of the particles has been explored, for example by Batchelor (1970, 1971) or by Dinh and Armstrong (1984). Experimental con®rmation of the theoretical has been carried out for spherical particles, as well as for prolate spheroids using ®bre suspensions (Goldsmith and Mason 1967) or other rod-like particles (Lauer 1944), but the suspension of oblate spheroids has received little attention in spite of the important practical implications of the subject. The most probable reason for this is the diculty of obtaining model suspensions of oblate particles (or plate- lets as a ®rst approximation) with perfectly de®ned characteristics and in a sucient amount for rheological testing. An attempt to make such model suspension was Rheol Acta 38: 437±442 (1999) Ó Springer-Verlag 1999 ORIGINAL CONTRIBUTION Andre Luciani Yves Leterrier Jan-Anders E. MaÊnson Rheological behaviour of dilute suspensions of platelet particles Received: 30 September 1998 Accepted: 1 December 1998 A. Luciani á Y. Leterrier J.-A.E. MaÊnson (&) Laboratoire de Technologie des Composites et PolymeÁres Ecole Polytechnique FeÂdeÂrale de Lausanne CH-1015 Lausanne, Switzerland Abstract The rheological behaviour of Newtonian suspensions of plate- lets with an aspect ratio ranging from 0.006 to 0.2 is studied in this work. Using model particles to cor- relate the aspect ratios square- shaped particles with their sedimen- tation behaviour, the results ob- tained were used to estimate the average aspect ratio of mica parti- cles. A comparison is made between the aspect ratio measured by sedi- mentation experiments and that de- termined from the rheological behaviour of dilute suspensions. Their good agreement shows that the average aspect ratio of a poly- dispersed suspension of platelets es- timated using packing experiments correlates well with the apparent aspect ratio given by the intrinsic viscosity of the suspension. Key words Platelets á Packing á Viscosity