Modelling microgels with controlled structure across the volume phase transition

Thermoresponsive microgels are soft colloids that find widespread use as model systems for soft matter physics. Their complex internal architecture, made of a disordered and heterogeneous polymer network, has been so far a major challenge for computer simulations. In this work we put forward a coarse-grained model of microgels whose structural properties are in quantitative agreement with results obtained with small-angle X-ray scattering experiments across a wide range of temperatures, encompassing the volume phase transition. These results bridge the gap between experiments and simulations of individual microgel particles, paving the way to theoretically address open questions about their bulk properties with unprecedented nano and microscale resolution

Effective interactions between soft-repulsive colloids: Experiments, theory, and simulations

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

Fluid-solid transitions in soft-repulsive colloids

We use monodisperse poly(N-isopropylacrylamide) (PNIPAM) microgels as a model system for soft repulsive colloids and study their density dependent structural ordering and phase behaviour using confocal laser scanning microscopy (CLSM). The experiments are carried out at low temperatures, where the particles are in the swollen state and interact via a Hertzian potential, evidenced through a quantitative comparison of the pair correlation functions g(r) obtained with CLSM and computer simulations. We worked over a broad range of effective volume fractions phi(eff) below and above close packing (phi(cp)). CLSM allows us to identify a fluid-glass and a fluid-crystal transition by looking at the structure and dynamics of the suspensions. The density dependent g(r) values exhibit clearly visible anomalies at high phi(eff) > phi(cp) which we interpret as a structural signature of the glass transition related to the particle softness. These results are discussed in light of the previously studied phase behaviour of colloidal systems interacting with hard and soft repulsive interaction potentials