Binary mixtures of ultrasoft colloids and linear polymer chains were investigated by small-angle neutron scattering and liquid state theory. We show that experimental data can be described by employing recently developed effective interactions between the colloid and the polymer chains, in which both components are modeled as point particles in a coarse-grained approach, in which the monomers have been traced out. Quantitative, parameter-free agreement between experiment and theory for the pair correlations, the phase behavior and the concentration dependence of the interaction length is achieved. DOI: 10.1103/PhysRevLett.106.228301 PACS numbers: 82.70.Ày, 61.20.Gy, 64.70.km, 82.70.Dd Hard spheres have been established in the past as model systems to investigate on a fundamental level the effective interactions and phase behavior of soft matter [1]. Following in complexity, colloid-polymer mixtures are usually described by the Asakura-Oosawa (AO) model [2]. Building on these simple models, great advances have been made in the study of gel and glass formation in colloidal systems [3,4]. More recently, the interest of colloid scientists has shifted towards the study of soft particles, among which star polymers have emerged as a model for a wide class of soft spheres, including blockcopolymer micelles [5,6] and microgel particles [7,8]. For a star polymer, softness can be controlled by varying its number of arms (or functionality f), allowing to bridge the gap between linear polymer chains (f ¼ 2) and hard spheres (f ! 1) [9]. Therefore, star polymers feature tunable softness, which is responsible for the observation of anomalous structural behavior [9] and for the formation of several crystal structures [5,10]. Mixtures of soft particles offer a much higher versatility with respect to their hard counterparts, both in terms of structural and rheological properties [11][12][13] and of effective interactions [14]. In particular, mixtures of star polymers of different sizes and functionalities have been recently investigated in a joint theoretical and experimental effort, revealing the existence of multiple glassy states [15]. On the other hand, the paradigmatic case of a mixture of star polymers and linear chains (the direct soft counterpart of the AO model) has been investigated theoretically [14,16] and experimentally by (mainly) macroscopic rheology [11,12,17]; however, detailed structural information is still missing. Recently a microscopic theory [14,16], capable to appropriately coarse-grain stars and chains, has been developed, but an accurate comparison between theoretical predictions and experimental results for the structural correlations for starchains mixtures has not been attempted so far.Recently, we introduced kinetically frozen starlike micelles [6,18] formed by the amphiphilic block copolymer poly(ethylene-alt-propylene)-poly(ethylene oxide), PEP-PEO, as a tunable model system for ultrasoft colloids [9]. In this Letter we study mixtures of starlike micelles and linear (PEO) chains and provide a s...

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We present a comprehensive experimental and theoretical study covering micro-, meso- and macroscopic length and time scales, which enables us to establish a generalized view in terms of structure-property relationship and equilibrium dynamics of soft colloids. We introduce a new, tunable block copolymer model system, which allows us to vary the aggregation number, and consequently its softness, by changing the solvophobic-to-solvophilic block ratio (m : n) over two orders of magnitude. Based on a simple and general coarse-grained model of the colloidal interaction potential, we verify the significance of interaction length σint governing both structural and dynamic properties. We put forward a quantitative comparison between theory and experiment without adjustable parameters, covering a broad range of experimental polymer volume fractions (0.001 ≤ϕ≤ 0.5) and regimes from ultra-soft star-like to hard sphere-like particles, that finally results in the dynamic phase diagram of soft colloids. In particular, we find throughout the concentration domain a strong correlation between mesoscopic diffusion and macroscopic viscosity, irrespective of softness, manifested in data collapse on master curves using the interaction length σint as the only relevant parameter. A clear reentrance in the glass transition at high aggregation numbers is found, recovering the predicted hard-sphere (HS) value in the hard-sphere like limit. Finally, the excellent agreement between our new experimental systems with different but already established model systems shows the relevance of block copolymer micelles as a versatile realization of soft colloids and the general validity of a coarse-grained approach for the description of the structure and dynamics of soft colloids.