Microstructure and Rheology near an Attractive Colloidal Glass Transition T. Narayanan, 1, * M. Sztucki, 1 G. Belina, 1 and F. Pignon 2 1 European Synchrotron Radiation Facility, F-38043 Grenoble, France 2 Laboratoire de Rhe ´ologie, CNRS, UMR 5520, F-38041, Grenoble Cedex 9, France (Received 22 December 2005; published 27 June 2006) Microstructure and rheological properties of a thermally reversible short-ranged attractive colloidal system are studied in the vicinity of the attractive glass transition line. At high volume fractions, the static structure factor changes very little but the low frequency shear moduli varies over several orders of magnitude across the transition. From the frequency dependence of shear moduli, fluid-attractive glass and repulsive glass-attractive glass transitions are identified. DOI: 10.1103/PhysRevLett.96.258301 PACS numbers: 82.70.Dd, 61.10.Eq, 64.70.Pf, 82.70.Gg Recently, colloidal systems interacting via short-ranged attractive potentials have received considerable attention in terms of their dynamical properties [1–6]. Mode-coupling theory and computer simulation have successfully pre- dicted two different glass transitions: the conventional repulsive colloidal glass where the ergodicity is lost due to blocking of the particle diffusion by the dense surround- ing cages formed by their nearest neighbors, and the at- tractive glass in which the particle motion is jammed even at low volume fractions () by the short-ranged attraction or stickiness [2]. These two glass transition lines meet at high defining a reentrant transition of repulsive glass– fluid–attractive glass as the attractive interaction is pro- gressively increased [1,2]. The attractive glass line extends beyond the reentrant region to a higher order singular point (A 3 ) delineating a glass-glass transition [2]. In experi- ments, similar dynamical features as that predicted by theory and simulation have been observed in a diverse class of short-ranged attractive colloidal systems [7–16]. The interparticle potential, V r, in short-ranged attrac- tive colloidal systems can be approximately described by hard-sphere repulsion (HS) with an attractive square-well (SW) [1,15]. V r1, for 0 <r<, V ru for <r< , and V r 0 for r> , where is the hard-sphere diameter, u and are depth and width of the attractive well. The strength of attraction is character- ized by the stickiness parameter, B 1=12" expu=k B T, where " = [17]. The phase behavior and microstructure of this model can be readily obtained using the Ornstein-Zernike integral equation and the Percus-Yevick approximation (PYA)[1,17]. At low , the system shows a gas-liquid type phase separation with the liquid phase having gel-like dynamics [15]. The distinguishing features of attractive and repulsive glasses are in their dynamical behavior [2,4,5]—the me- chanical properties especially have not been investigated so far. This Letter presents a study of microstructure and rheological behavior near attractive glass transitions in a thermally reversible model short-ranged interacting colloi- dal system. The microstructure was obtained by ultra small-angle x-ray scattering (USAXS) and the rheological properties were derived from bulk rheology. The results demonstrate subtle changes in the static structure but co- lossal variations in the rheological parameters in the neigh- borhood of the reentrant region where the two glass lines meet. The experimental system consisted of stearyl grafted silica colloids suspended in n-dodecane. This system undergoes a reversible aggregation below a well-defined temperature, T A , which is attributed to a lyotropic ordering transition of the grafted stearyl chains [18]. The x-ray contrast of grafted stearyl layer (thickness about 1.85 nm) is very closely matched with dodecane. As a result, the x-ray scattering essentially originates from the silica core and the core volume fraction can be determined from the absolute scattered intensity. The USAXS measurements were performed at the High Brilliance beam line (ID2) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, using a Bonse-Hart camera [19]. The crossed analyzer configura- tion in the setup permitted us to obtain intensity profiles, Iq, directly in an absolute scale without any smearing. Additional, SAXS measurements were made using a 10 m pinhole SAXS instrument [19]. The incident x-ray wave- length () was 1 A in all experiments. Samples for USAXS were contained in thin walled flat glass capillaries with sample thickness 0.5 mm. High resolution rheology was performed using a stress-controlled rheometer (Haake, RS300 with microtorque option) with plate-plate geometry thermostated to 0:01 C. Typical sample diameter and thickness were 8 mm and 0.5 mm, respectively. Special care was exercised for reducing the evaporation losses using a solvent trap and correcting for the real size of sample between the plates. However, the maximum dura- tion of the experiment was still limited to less than 24 h. The applied low frequency oscillatory stress () was well within the linear viscoelastic range. To establish the short-ranged nature of attraction in this system, Fig. 1 displays the typical SAXS intensity, Iq, in the vicinity of T A for a sample with 0:06. Here, the PRL 96, 258301 (2006) PHYSICAL REVIEW LETTERS week ending 30 JUNE 2006 0031-9007= 06=96(25)=258301(4) 258301-1 2006 The American Physical Society