Crystallization Kinetics and Morphology in Phase Separating and Sedimenting Mixtures of Colloidal Spheres and Rods S. M. Oversteegen,* J. G. E. J. Wijnhoven, C. Vonk, and H. N. W. Lekkerkerker Van’t Hoff Laboratory for Physical and Colloid Chemistry, Debye Research Institute, Utrecht UniVersity, P.O. Box 80051, 3508 TB Utrecht, The Netherlands ReceiVed: February 25, 2004; In Final Form: July 23, 2004 The crystallization of sedimentating silica spheres in the presence of silica-coated boehmite rods in low-salt dimethylformamide is studied by means of confocal scanning laser microscopy. As expected, addition of rods gives rise to a net attraction due to the depletion effect. Upon increasing rod volume fractions, below a predicted equilibrium binodal, crystalline ordering of the spheres takes place faster but gives cause for more grain boundaries. Addition of rods at volume fractions in the theoretically predicted two-phase region gives rise to aggregation and glasslike sediments. We explain these results on the basis of the different gravitational lengths and sedimentation rate of both species: higher rod concentrations drive the system quicker into the two-phase region of the predicted phase-diagram. I. Introduction The stability of suspensions such as coatings, containing spherical pigments, can change drastically when rodlike par- ticles, frequently used as rheology enhancers, are added. Mixtures of rods and spheres may also arise naturally in the synthesis of pigments. Their phase behavior may greatly influence the way the pigments can be processed in, e.g., color filters or fillings. The mutual asymmetry of the colloids alone may already induce a net attraction between alike particles by the so-called depletion effect. 1-3 The closer the hard spheres are together, the more volume is available to the rods. In this manner, the system may gain more conformational freedom and hence a higher, more favorable, entropy. This depletion effect has been well-established both theoretically and experimentally for bimodal mixtures of colloidal spheres, 4 colloidal mixtures of spheres and polymers, 5 platelets and polymers, 6-8 and rods and platelets. 9-11 When a rod enters the gap between two hard spheres that are nearer than the length of the rod, the former can no longer assume all conformations, as illustrated in Figure 1. This unfavorable loss of entropy can be overcome by depleting the rod from the gap thereby gaining orientational realization probabilities. When outnumbered, the consequent lower density of rods between the spheres may give rise to an unbalanced osmotic pressure amid the spheres and forces the latter together. The strength of this “attraction through repulsion” or depletion potential W reads up to first order in the rod density φ r 12-14 Here h is the face-to-face distance of the spheres of diameter σ, whereas L represents the length of the rod and D its diameter, as indicated in Figure 1a. Moreover, k B is the Boltzmann constant and T the absolute temperature. In the appropriate limits, i.e., low density and relatively small rods (L , σ), eq 1.1 has been confirmed experimentally. 15-17 For certain rod concentrations, the net attraction of the spheres may be large enough to induce phase separation. This has been predicted by means of the free-volume theory 18 applied to spheres and rods 19,20 and has indeed been observed in colloidal mixtures of spheres and rods. 21-24 All these experimental studies observe the depletion interaction “as is”. However, the kinetics in these phase-separating systems in relation to sedimentation has not been studied yet but is nevertheless of great importance to the above-mentioned applications. In this paper, we therefore have a closer look on the kinetics and the final sediments of phase-separating sedimenting colloidal spheres in the presence of colloidal rods. II. Experimental Section We prepare silica spheres and silica-coated boehmite rods. When dispersed in dimethylformamide (DMF), the van der Waals interactions between the spheres are negligible since the * To whom correspondence may be addressed. E-mail: m.oversteegen@ chem.uu.nl. W(h) k B T )- 1 3 φ r L D σ D ( 1 - h L ) 3 (1.1) Figure 1. An unbalanced pressure between two spheres arises since the rods that are closer to a sphere than half their length cannot rotate freely. This creates a net attraction between the spheres. 18158 J. Phys. Chem. B 2004, 108, 18158-18163 10.1021/jp0491515 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/28/2004