The long-standing observations that different amorphous materials exhibit a pronounced enhancement of viscosity and eventually vitrify on compression or cooling continue to fascinate and challenge scientists, on the ground of their physical origin and practical implications. Glass formation is a generic phenomenon, observed in physically quite distinct systems that encompass hard and soft particles. It is believed that a common underlying scenario, namely cage formation, drives dynamical arrest, especially at high concentrations. Here, we identify a novel, asymmetric glassy state in soft colloidal mixtures, which is characterized by strongly anisotropically distorted cages, bearing similarities to those of hard-sphere glasses under shear. The anisotropy is induced by the presence of soft additives. This phenomenon seems to be generic to soft colloids and its origins lie in the penetrability of the constituent particles. The resulting phase diagram for mixtures of soft particles is clearly distinct from that of hard-sphere mixtures and brings forward a rich variety of vitrified states that delineate an ergodic lake in the parameter space spanned by the size ratio between the two components and by the concentration of the additives. Thus, a new route opens for the rational design of soft particles with desired tunable rheological properties.

Star polymers with a high number of arms, f 263, become kinetically trapped when dispersed in an athermal solvent at concentrations above the overlapping one, forming physical gels. We show that the addition of linear chains at different concentrations and molecular weights reduces the modulus of the gel, eventually melting it. We explain this linear polymer-induced gel-liquid transition in terms of effective interactions and star depletion. In the limit of very high linear-chain molecular weight a ''reentrant gelation'' is detected and attributed to bridging flocculation, analogous to that observed in colloidal dispersions. DOI: 10.1103/PhysRevLett.89.208302 PACS numbers: 82.70.Gg, 61.20.-p, 61.25.Hq, 82.70.Dd One of the most intriguing features of colloidal dispersions is the wide range of rheological behavior they exhibit, from liquidlike to solidlike, and which depends primarily on their volume fraction [1][2][3][4][5]. This behavior is well established for hard spheres, which have received a great deal of attention; on the other hand, for soft spheres, such as hard colloids with grafted polymeric layers, it is intrinsically related to the changing thickness of the layer and thus to the strength and range of the repulsive interactions [6]. Star polymers with high functionality have emerged as a novel class of ultrasoft colloidal particles, characterized by wide ranging interactions with Yukawatype repulsions at long distances and logarithmic repulsions at short ones [7,8]. At high volume fractions, achieved via manipulation of their effective thickness in conditions of varying solvent quality, these systems exhibit a liquid-solid transition, which is of kinetic, rather than thermodynamic, origin [9][10][11]. This type of dynamic arrest is described as glasslike gelation, bearing many similarities with both colloidal glass formation (crowding of single particles) and colloidal gelation (formation of clusters) [11], and represents a manifestation of jammed colloidal particles [12]. The great challenge with such transitions is how to achieve molecular control by tuning the dynamic response. This will have a significant scientific and technological impact as it will open the route for the rational design of soft materials with desired properties in a variety of situations. Mixtures of star polymers with linear polymers are an obvious choice (as many soft matter systems occur as mixtures), which, however, has been overlooked so far.In this Letter, we demonstrate the dramatic and unexpected effects of the addition of linear chains to a star polymer gel. We show that the added polymer reduces the modulus of the gel and in the limit of high polymer molecular weight or concentration, the gel melts. Within the liquified region, the reduced star viscosity drops upon further addition of linear polymer. The effective starlinear polymer interactions are shown to be responsible for the observed counterintuitive phenomena, via a depletionlike mechanism which explains the gel-liquid transition. Eventually, for very high ...

The dynamics of colloidal star-linear homopolymer mixtures is investigated by photon correlation spectroscopy. In dilute star solutions, osmotic forces due to the added polymers lead initially to a shrinkage of the stars and eventually, at higher polymer concentrations, to stable star clusters. Furthermore, concentrated glassy star solutions melt upon addition of small amounts of linear polymers, as manifested by the remarkable speed-up of the star selfdiffusion. Quantitative description of the experimental findings is provided by calculations of the effective star-star pair potential.