Urs Gasser

Crystallization in three-?and two-dimensional colloidal suspensions

Despite progress in the understanding of crystal nucleation and crystal growth since the first theories for nucleation were developed, an exact quantitative prediction of the nucleation rates in most systems has remained an unsolved problem. Colloidal suspensions show a phase behavior that is analogous to atomic or molecular systems and serve accordingly as ideal model systems for studying crystal nucleation with an accuracy and depth on a microscopic scale that is hard to reach for atomic or molecular systems. Due to the mesoscopic size of colloidal particles they can be studied in detail on the single-particle level and their dynamics is strongly slowed down in comparison with atomic or molecular systems, such that the formation of a crystal nucleus can be followed in detail. In this review, recent progress in the study of homogeneous and heterogeneous crystal nucleation in colloids and the controlled growth of crystalline colloidal structures is reviewed. All this work has resulted in unprecedented insights into the early stage of nucleation and it is also relevant for a deeper understanding of soft matter materials in general as well as for possible applications based on colloidal suspensions.

Melting of Crystals in Two Dimensions

While the melting of crystals is in general not understood in detail on a microscopic scale, there is a microscopic theory for a class of two-dimensional crystals, which is based on the formation and unbinding of topological defects. Herein, we review experimental work on a colloidal two-dimensional model system with tunable interactions that has given the first conclusive evidence for the validity of this theory on a microscopic level. Furthermore, we show how the mechanism of melting depends on the particle interaction and that a strong anisotropy of the interaction leads to a changed melting scenario.

Structural changes of poly(N-isopropylacrylamide)-based microgels induced by hydrostatic pressure and temperature studied by small angle neutron scattering

We study the structural properties of microgels made of poly(N-isopropylacrylamide) and acrylic acid as a function of hydrostatic pressure and temperature using small angle neutron scattering. Hydrostatic pressure induces particle deswelling by changing the mixing of the microgel with the solvent, similar to temperature. We extend this analogy to the structural properties of the particles and show that the form factor at a certain temperature is equal to the form factor at a certain hydrostatic pressure. We fit the results with an existent model for the microgel structure and carefully analyze the fitting procedure in order to obtain physically meaningful values of the free parameters in the model.

The role of ions in the self-healing behavior of soft particle suspensions

Significance Understanding when a material crystallizes is of fundamental importance in condensed matter. In many materials, the presence of point defects suppresses crystallization. Surprisingly, colloidal hydrogels can overcome this limitation: A small number of large microgels can spontaneously deswell to fit in the crystal lattice of smaller microgels, thus avoiding the occurrence of point defects. We find that this unique particle deswelling is due to an osmotic pressure difference between the inside and the outside of the microgels resulting from the overlap of counterion clouds of neighboring particles. When this pressure difference exceeds the bulk modulus of the large microgels, these shrink, enabling crystallization without point defects. Impurities in crystals generally cause point defects and can even suppress crystallization. This general rule, however, does not apply to colloidal crystals formed by soft microgel particles [Iyer ASJ, Lyon LA (2009) Angew Chem Int Ed 48:4562–4566], as, in this case, the larger particles are able to shrink and join the crystal formed by a majority of smaller particles. Using small-angle X-ray scattering, we find the limit in large-particle concentration for this spontaneous deswelling to persist. We rationalize our data in the context of those counterions that are bound to the microgel particles as a result of the electrostatic attraction exerted by the fixed charges residing on the particle periphery. These bound counterions do not contribute to the suspension osmotic pressure in dilute conditions, as they can be seen as internal degrees of freedom associated with each microgel particle. In contrast, at sufficiently high particle concentrations, the counterion cloud of each particle overlaps with that of its neighbors, allowing these ions to freely explore the space outside the particles. We confirm this scenario by directly measuring the osmotic pressure of the suspension. Because these counterions are then no longer bound, they create an osmotic pressure difference between the inside and outside of the microgels, which, if larger than the microgel bulk modulus, can cause deswelling, explaining why large, soft microgel particles feel the squeeze when suspended with a majority of smaller particles. We perform small-angle neutron scattering measurements to further confirm this remarkable behavior.

Form factor of pNIPAM microgels in overpacked states

We study the form factor of thermoresponsive microgels based on poly(N-isopropylacrylamide) at high generalized volume fractions, ζ, where the particles must shrink or interpenetrate to fit into the available space. Small-angle neutron scattering with contrast matching techniques is used to determine the particle form factor. We find that the particle size is constant up to a volume fraction roughly between random close packing and space filling. Beyond this point, the particle size decreases with increasing particle concentration; this decrease is found to occur with little interpenetration. Noteworthily, the suspensions remain liquid-like for ζ larger than 1, emphasizing the importance of particle softness in determining suspension behavior.

Heterogeneous nucleation and crystal growth on curved surfaces observed by real-space imaging

We present a real-space imaging study of homogeneous and heterogeneous crystal nucleation and growth in colloidal suspensions of slightly charged and polydisperse particles. Heterogeneous crystallization is observed close to curved surfaces with radii of curvature, R, in the range from 4 to 40 particle diameters, d. Close to a curved surface, we find crystal nucleation and growth to be suppressed for R approximately < 10d. For R approximately > 15d, fast crystal growth is observed similar to that on a flat wall (R = ∞). We use the purely topological method of shortest path rings to determine the orientation of the crystal on the length scale of the nearest neighbor distance. Crystal nuclei forming close to a curved surface are oriented analogous to crystal growth on a flat wall with hexagonal planes parallel to the wall. While the smallest nuclei appear to be unaffected by the surface, larger nuclei are found to be suppressed for radii of curvature R approximately < 10d. The critical nucleus size in the vicinity of a curved surface is found to be about the same as for homogeneous nucleation.

Surfactant adsorption and aggregate structure at silica nanoparticles: Effects of particle size and surface modification

The influence of particle size and a surface modifier on the self-assembly of the nonionic surfactant C12E5 at silica nanoparticles was studied by adsorption measurements and small-angle neutron scattering (SANS). Silica nanoparticles of diameter 13 to 43 nm were synthesized involving the basic amino acid lysine. A strong decrease of the limiting adsorption of C12E5 with decreasing particle diameter was found. To unveil the role of lysine as a surface modifier for the observed size dependence of surfactant adsorption, the morphology of the surfactant aggregates assembled on pure siliceous nanoparticles (Ludox-TMA, 27 nm) and their evolution with increasing lysine concentration at a fixed surfactant-to-silica ratio was studied by SANS. In the absence of lysine, the surfactant forms surface micelles at silica particles. As the concentration of lysine is increased, a gradual transition from the surface micelles to detached wormlike micelles in the bulk solution is observed. The changes in surfactant aggregate morphology cause pronounced changes of the system properties, as is demonstrated by turbidity measurements as a function of temperature. These findings are discussed in terms of particle surface curvature and surfactant binding strength, which present new insight into the delicate balance between the two properties.

Influence of B-site disorder in La0.5Ca0.5Mn1 − xBxO3 (B = Fe, Ru, Al and Ga) manganites

We have investigated the influence of B-site doping on the crystal and magnetic structure in La(0.5)Ca(0.5)Mn(1 - x)B(x)O(3) (B = Fe, Ru, Al and Ga) compounds using neutron diffraction, small angle neutron scattering, magnetization and resistivity techniques. The B-site doped samples are isostructural and possess an orthorhombic structure in the Pnma space group at 300 K. A structural transition from orthorhombic to monoclinic is found to precede the magnetic transition to the CE-type antiferromagnetic state in a few of these samples. On doping with Fe, the charge and orbitally ordered CE-type antiferromagnetic state is suppressed, followed by growth of the ferromagnetic insulating phase in 0.02 ≤ x ≤ 0.06 compounds. At higher Fe doping in x > 0.06, the ferromagnetic state is also suppressed and no evidence of long range magnetic ordering is observed. In Ru doped samples (0.01 ≤ x ≤ 0.05), the ferromagnetic metallic state is favored at T(C)≈200 K and T(MI)≈125 K and no significant change in T(C) and T(MI) as a function of Ru doping is found. In contrast, with non-magnetic Al substitution for 0.01 ≤ x ≤ 0.03, the charge ordered CE-type antiferromagnetic state coexists with the ferromagnetic metallic phase. With further increase in Al doping (0.05 ≤ x ≤ 0.07), both CE-type antiferromagnetic and ferromagnetic phases are gradually suppressed. This behavior is accompanied by the evolution of an A-type antiferromagnetic insulating state. Eventually, at higher Al doping (0.10 ≤ x ≤ 0.13), this phase is also suppressed and the signature of a spin glass like transition is evident in M(T). Likewise, substitution with Ga is observed to induce similar effects to those described for Al doped samples. The presence of short ranged ferromagnetic ordering has been further explored using small angle neutron scattering measurements in a few of the selected samples.

Crystal electric field splitting of R3+-ions in RNi2B2C (R = rare earth)

Abstract The parameters of the crystal electric field (CEF) hamiltonian of the R 3+ -ions in RNi 2 B 2 C (R  Ho, Er, Tm) have been determined from neutron CEF-spectroscopy. These parameters were extrapolated to other lanthanides. The eigen-energies, eigen-states, magnetic single-ion susceptibilities, and magnetic moments in the ordered state were calculated.

The effect of hydrostatic pressure over the swelling of microgel particles

We review our recent results on the use of hydrostatic pressure to change the size and the structure of microgel particles based on poly-(N-isopropylacrylamide). These changes are brought about through changes in the miscibility of the polymer in the solvent. Swelling induced by hydrostatic pressure can thus be thought of as an alternative to swelling induced by temperature, an interesting fact that can be exploited in the study of fundamental problems in soft condensed matter.