Soft particles such as polymeric microgels can form ultra-dense phases, where the average center-to-center distance a s can be smaller than the initial unperturbed particle diameter σ 0, due to their ability to interpenetrate and compress. However, despite of the effort devoted to microgels at ultrahigh densities, we know surprisingly little about their response to their environment at effective volume fractions ϕ eff above close packing (ϕ cp), and the existing information is often contradictory. Here we report direct measurements of the size and shape of poly(N-isopropylacrylamide) microgels at concentrations below and above ϕ cp using the zero average contrast method in small-angle neutron scattering. We complement these experiments with measurements of the average interparticle distances using small-angle x-ray scattering, and a determination of the glass transition using dynamic light scattering. This allows us to unambiguously decouple interaction effects from density-dependent variations of the particle size and shape at all values of ϕ eff. We demonstrate that the microgels used in this study significantly interpenetrate and thus change their size and shape only marginally even for ϕ eff ≫ ϕ cp, a finding that may require changes in the interpretation of a number of previously published studies on the structural and dynamic properties of dense soft particle systems.

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We describe a combined experimental, theoretical, and simulation study of the structural correlations between cross-linked highly monodisperse and swollen Poly(N-isopropylacrylamide) microgel dispersions in the fluid phase in order to obtain the effective pair-interaction potential between the microgels. The density-dependent experimental pair distribution functions g(r)'s are deduced from real space studies using fluorescent confocal microscopy and compared with integral equation theory and molecular dynamics computer simulations. We use a model of Hertzian spheres that is capable to well reproduce the experimental pair distribution functions throughout the fluid phase, having fixed the particle size and the repulsive strength. Theoretically, a monodisperse system is considered whose properties are calculated within the Rogers-Young closure relation, while in the simulations the role of polydispersity is taken into account. We also discuss the various effects arising from the finite resolution of the microscope and from the noise coming from the fast Brownian motion of the particles at low densities, and compare the information content from data taken in 2D and 3D through a comparison with the corresponding simulations. Finally different potential shapes, recently adopted in studies of microgels, are also taken into account to assess which ones could also be used to describe the structure of the microgel fluid.

The nature of solid-solid phase transformations has been a long-standing question spanning the fields of metallurgy and condensed-matter physics, with applications from metallic alloys and ceramics to modern shape-memory materials. In spite of the importance of solid-to-solid transformations in many areas of materials science and condensed-matter physics and the numerous experimental and theoretical studies, a deep understanding of the microstructural changes and the underlying kinetic mechanisms is still missing. In this work, we establish a versatile model system composed of micron-scale ionic microgel colloids, where we not only probe the single-particle kinetics in real space and real time but also tune the phase transition in a multiple-parameter space. In the presence of an imposed electric field, a face-centered cubic (FCC) crystal transforms diffusively into a body-centered tetragonal (BCT) crystal via nucleation and growth. In the reverse direction, however, the BCT phase transforms cooperatively into a long-lived metastable body-centered orthorhombic phase, which only relaxes back to the equilibrium FCC when annealed at higher temperatures. The kinetics is thus either diffusive or martensitic depending on the path, and we believe that these two path-dependent transitions provide the first real-space, particle-level insights of diffusive and martensitic transformations, respectively, in a single system.

Recent theoretical phase diagrams for loosely cross-linked ionic microgels with a low monomer volume fraction (Gottwald; et al. Phys. Rev. Lett. 2004, 92 , 068301 ) have predicted a re-entrant order-disorder transition (i.e., fluid-FCC-BCC-fluid) as a function of concentration and so far there has been no experimental verifications of these theoretical predictions. Here, we present experimental results on phase behavior of loosely cross-linked charged poly(N-isopropylacrylamide co acrylic acid) (PNIPAm-co-AAc) microgesls with a low monomer volume fraction (approximately 0.003) for a wide range of concentrations (0.02-0.6 wt %) using static and dynamic light scattering methods. These microgel dispersions exhibit a short-range liquid order at low concentration (<0.03 wt %), a FCC crystalline order at intermediate concentrations (0.03- 0.3 wt %). In addition, we suggested a possible coexistence of BCC and FCC phases at higher concentration crystalline suspension (approximately 0.34 wt %). These results clearly demonstrate the experimental verification of above theoretical prediction below the overlap concentration and also reveal that the interaction potential between the microgel particles is of screened Coulomb repulsive type within these concentration ranges. At further higher concentration (approximately 0.57 wt %), we once again observed a disordered state and this disordered state from dynamic light scattering was confirmed to be a glass. These initial results are discussed in the light of previously reported results on the phase behavior of ionic microgel colloidal dispersions.

In this work, we discuss the statistical mechanics of many-body systems consisting of electrically charged microgels, and we show that their collective behavior is determined by an interplay between the screened electrostatic and the elastic contributions to their effective interaction potential. The former is derived by means of a statistical-mechanical approach due to Denton [A. R. Denton, Phys. Rev. E 67, 011804 (2003)], and it includes the screened electrostatic potential between penetrable spheres and the counterion entropic contribution. The latter is based on the Hertzian model of the theory of elasticity. Comparisons with experimental results demonstrate the realistic nature of the coarse-graining procedure, which makes it possible to put forward theoretical predictions on the phase diagram of ionic microgels and on the behavior of soft, neutral microgels under confinement in narrow pores.

The swelling of ionic microgel particles is investigated at a wide range of concentrations using a combination of light, X-ray and neutron scattering techniques. We employ a zero-average contrast approach for small-angle neutron scattering experiments, which enables a direct determination of the form factor at high concentrations. The observed particle size initially decreases strongly with the particle concentration in the dilute regime but approaches a constant value at intermediate concentrations. This is followed by a further deswelling at high concentrations above particle overlap. Theory and experiments point at a pivotal contribution of dangling polymer ends to the strong variation in size of ionic microgels, which presents itself mainly through the hydrodynamics properties of the system.

We report observations of stable bound pairs in very dilute deionized aqueous suspensions of highly charged polystyrene colloidal particles, with monovalent counterions, using a confocal laser scanning microscope. Through an analysis of several thousands of time series of confocal images recorded deep inside the bulk suspension, we find that the measured pair-potential, U(r) has a long-range attractive component with well depths larger than the thermal energy. These observations provide a direct and unequivocal evidence for the existence of longrange attraction in U(r) of like-charged colloidal particles.