PACS. 46.60. -Rheology of fluids and pastes. PACS. 68.10. -Fluid surfaces and interfaces with fluids.RBsumB. -A l'aide d'un dispositif simple, que nous decrivons dans cet article, nous avons mesure les forces #attraction qui s'exercent entre deux spheres lorsqy'elles deforment les interfaces de separation entre deux ou trois fluides. Ces forces capillaires dependent B la fois de la distance entre les centres des spheres et de la position de ces dernieres par rapport aux interfaces.

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2014 Nous présentons une expérience d'agrégation dans une suspension bidimensionnelle de particules sphériques macroscopiques soumises à un cisaillement. L'agrégation est due à des forces interfaciales attractives dont on peut faire varier l'intensité. On introduit une quantité sans dimension A traduisant l'importance relative des forces attractives et de l'effet du cisaillement. On observe diverses configurations liées à la valeur de A. En particulier, dans le cas où les amas présentent une structure fractale, on trouve une dimension d'Hausdorff à deux dimensions égales à 1,7 ± 0,05. Abstract. 2014 We present aggregation experiments in a bidimensional suspension of macroscopic spherical particles undergoing a shear. The aggregation is due to attractive capillary forces the value of which can be changed. We introduce a dimensionless quantity A expressing the relative importance of capillary forces and of the shear effect. We observe various aggregate configurations related to the values of A. In particular when the clusters show a fractal structure we find one Hausdorff dimension equal to 1.7 ± 0.05.

We study the cluster statistics and the viscosity of a two-dimensional suspension of passive macroscopic spheres undergoing shear, The second moment of the finite cluster statistics exhibits a maximum for a 2-D concentration q5 s near 0.67 without measurable anomaly in the viscosity. The results of the cluster statistics are compared to those obtained in percolation.Key words: Suspension, cluster statistics, viscosity, percolation lntroductionIt was suggested by de Gennes [1] that one could try to explain the plug-flow phenomenon in suspensions [2] on the basis of an analogy with percolation. In that analysis, the dynamical clusters induced by hydrodynamical interactions between suspended particles are similar to percolation clusters. Their mean size grows with particle concentration, and one can expect a critical concentration above which an "infinite" cluster appears, a cluster which contains most of the total (finite) number of particles. So, we were led to study macroscopic passive particle suspensions with regard to their rheology (viscosity, velocity profile) as well as to their statistical cluster properties (mean size, number, length). In order to simplify the observation of clusters, we carried out the study on bidimensional suspensions; preliminary results were published elsewhere [3]. Statistical parameters of the suspensionThe suspension is a two-dimensional collection of passive spheres (diameter d = 5 mm) in a layer (thickness t = 5 mm) of a liquid, the density of which is adjusted to that of the spheres. The suspension was studied in laminar steady flow with a low Reynolds number ( R e~0 . 1 ) in a Couette viscometer (inner diameter R1 = 1.7 cm or 3.35 cm, outer diameter R2 = 9.25 cm õr 18 cm). The inner cylinder is fixed by a 911 torsion wire, the outer one rotating with angular speed f~. The 2D-concentration P s of a suspension containing N spheres is defined by d 2 Ps = N 4(R2 _ R{ )and related to the volume concentration qsv by 3 Bs = ~-~v. For 45s between 0 and 0.80, we study the relative viscosity r/r (ratio of the effective viscosity of the suspension to that of the pure liquid) and the statistical grouping of spheres, by counting from photographs of the suspension undergoing the Couette flow the number ns of clusters of s spheres in contact*), the S 2 ---~~s n « -(mass averaged) mean size. s *) Two spheres are said "in contact" when they seem adjoining within the photographic resolution that is in our case about a tenth of a sphere diameter.

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