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.

Crowded solutions of densely branched polymers with starlike architecture undergo a reversible gelation upon heating. This phenomenon is characterized by slow kinetics and is attributed to the formation of clusters causing a partial dynamic arrest of the swollen interpenetrating spheres at high temperatures. A kinetic pseudo-phase diagram of gelation temperature against the effective volume fraction is constructed. Stars with different functionalities exhibit a different sol-gel boundary; small differences in the internal structure of the stars (regular with spherical dense core vs irregular with nonspherical core but spherical overall shape) are presumably responsible for these differences. Thermal gelation is proposed as an alternative route to jamming of soft materials.

Crowded solutions of multiarm star polymers, representing model colloidal spheres with ultrasoft repulsive interactions, undergo a reversible gelation transition upon heating in solvents of intermediate quality (between good and Theta). This unusual phenomenon is due to the kinetic arrest of the swollen interpenetrating spheres at high temperatures, forming clusters, in analogy to the colloidal glass transition. In this work we demonstrate that the choice of the solvent has a dramatic effect on the gelation transition, because of the different degree of star swelling (at the same temperature) associated with the solvent quality. We construct a generic kinetic phase diagram for the gelation of different stars in different solvents (gelation temperature against effective volume fraction, phi) and propose a critical "soft sphere close packing" volume fraction phi(c) distinguishing the temperature-induced (for phiphi(c)) glass-like gelation. We conclude that appropriate selection of the solvent allows for manipulation of the sol-gel transition in such ultrasoft colloids.

Multiarm star polymers were used as model grafted colloidal particles with long hairs, to study their size variation due to osmotic forces arising from added linear homopolymers of smaller size. This is the origin of the depletion phenomenon that has been exploited in the past as a means to melt soft colloidal glasses by adding linear chains and analyzed using dynamic light scattering experiments and an effective interactions analysis yielding the depletion potential. Shrinkage is a generic phenomenon for hairy particles, which affects macroscopic properties and state transitions at high concentrations. In this work we present a small angle neutron scattering study of star/linear polymer mixtures with different size ratios (varying the linear polymer molar mass) and confirm the depletion picture, i.e., osmotic star shrinkage. Moreover, we find that as the linear/star polymer size ratio increases for the same effective linear volume fraction (c/c * with c * the overlapping concentration), the star shrinkage is reduced whereas the onset of shrinkage appears to take place at higher linear polymer volume fractions. A theoretical description of the force balance on a star polymer in solution, accounting for the classic Flory contributions, i.e. elastic and excluded volume, as well as the osmotic force due to the linear chains, accurately predicts the experimental findings of reduced star size as function of linear polymer concentration. This is done in a parameter-free fashion, in which the size of the cavity created by the star, and from which the chains are excluded, is related to the radius of the former from first principles.

Soft colloids -such as polymer-coated particles, star polymers, block-copolymer micelles, microgels -constitute a broad class of materials where microscopic properties such as deformability and penetrability of the particle play a key role in tailoring their macroscopic properties which is of interest in many technological areas. The ability to access these microscopic properties is not yet demonstrated despite its great importance. Here we introduce novel DNA-coated colloids with star-shaped architecture that allows accessing the above local structural information by directly visualizing their intramolecular monomer density profile and arm's free-end locations with Confocal Fluorescent Microscopy. Compression experiments on a two-dimensional hexagonal lattice formed by these macromolecular assemblies reveal an exceptional resistance to mutual interpenetration of their charged corona at pressuresapproaching the MPa range. Furthermore, we find that this lattice, in a close packing configuration, is surprisingly tolerant to particle size variation. We anticipate that these stimuliresponsive materials could aid to get deeper insight in a wide range of problems in soft matter including the study and design of biomimetic lubricated surfaces. 2Charged polymeric layers densely attached by one end to an interface, also known as polyelectrolyte brushes, are systems that exhibit rich and complex behavior [1][2][3][4][5][6][7] with direct relevance to biological systems [8][9][10][11] and with a diverse range of applications such as surface modification technologies [10,12,13] and emerging biotechnologies [12,14,15]. Experimental observations of charged brushes available at present are not so sensitive to the details of the structure but are mainly concerned with more global brush properties, such as brush thickness [16][17][18] or force-distance dependence between brushes [19][20][21]. However, the detailed picture of a polyelectrolyte brush comprises the determination of intramolecular density profiles and freeend arm distributions for the case of a dilute, non-interacting system. In the case of crowded conditions additional parameters have to be quantified, namely how the grafted charged coronas of brushes interact in terms of shrinkage, deformation or penetration. In particular for the crowded case, many of the above mentioned parameters are poorly accessible in a variety of physically distinct soft and penetrable colloidal systems [22][23][24][25]. In this Letter, we show how the above issues can be addressed by grafting very long double-stranded DNA (dsDNA) chains to the surface of a micron-sized spherical super-paramagnetic particle for the construction of strongly charged, ultra-dense, highly monodisperse PolyElectrolyte brushes with a star-shaped architecture (abbreviated by PE-star hereafter, see Supplemental Material (SM) [26] for details on the synthetic strategy, section 1.1-1.2).Direct visualization of these water soluble DNA-based microstructures is achieved by fluorescent staining either the whole ...

Smectic ordering in aqueous solutions of monodisperse stiff double-stranded DNA fragments is known not to occur, despite the fact that these systems exhibit both chiral nematic and columnar mesophases. Here, we show, unambiguously, that a smectic-A type of phase is formed by increasing the DNA's flexibility through the introduction of an unpaired single-stranded DNA spacer in the middle of each duplex. This is unusual for a lyotropic system, where flexibility typically destabilizes the smectic phase. We also report on simulations suggesting that the gapped duplexes (resembling chain-sticks) attain a folded conformation in the smectic layers, and argue that this layer structure, which we designate as smectic-fA phase, is thermodynamically stabilized by both entropic and energetic contributions to the system's free energy. Our results demonstrate that DNA as a building block offers an exquisitely tunable means to engineer a potentially rich assortment of lyotropic liquid crystals.