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Using fluorescence microscopy we study the adsorption of single latex microparticles at a water/water interface between demixing aqueous solutions of polymers, generally known as a water-in-water emulsion. Similar microparticles at the interface between molecular liquids have exhibited an extremely slow relaxation preventing the observation of expected equilibrium states. This phenomenon has been attributed to "long-lived" metastable states caused by significant energy barriers $\Delta{\cal F}\sim \gamma A_d\gg k_B T$ induced by high interfacial tension ($\gamma \sim 10^{-2}$ N/m) and nanoscale surface defects with characteristic areas $A_d \simeq$ 10--30 nm$^2$. For the studied water/water interface with ultra-low surface tension ($\gamma \sim 10^{-4}$ N/m) we are able to characterize the entire adsorption process and observe equilibrium states prescribed by a single equilibrium contact angle independent of the particle size. Notably, we observe crossovers from fast initial dynamics to slower kinetic regimes analytically predicted for large surface defects ($A_d \simeq$ 500 nm$^2$). Moreover, particle trajectories reveal a position-independent damping coefficient that is unexpected given the large viscosity contrast between phases. These observations are attributed to the remarkably diffuse nature of the water/water interface and the adsorption and entanglement of polymer chains in the semidilute solutions. This work offers some first insights on the adsorption dynamics/kinetics of microparticles at water/water interfaces in bio-colloidal systems., Comment: Supplemental Material includes analysis of damping coefficients and experimental measurements of polymer adsorption on latex particles

We present results of a coherent x-ray diffractive imaging experiment performed on a single colloidal crystal grain. The full three-dimensional (3D) reciprocal space map measured by an azimuthal rotational scan contained several orders of Bragg reflections together with the coherent interference signal between them. Applying the iterative phase retrieval approach, the 3D structure of the crystal grain was reconstructed and positions of individual colloidal particles were resolved. As a result, an exact stacking sequence of hexagonal close-packed layers including planar and linear defects were identified., Comment: 8 pages, 5 figures

Polymeric nanoparticles (NPs) have a great application potential in science and technology. Their functionality strongly depends on their size. We present a theory for the size of NPs formed by precipitation of polymers into a bad solvent in the presence of a stabilizing surfactant. The analytical theory is based upon diffusion-limited coalescence kinetics of the polymers. Two relevant time scales, a mixing and a coalescence time, are identified and their ratio is shown to determine the final NP diameter. The size is found to scale in a universal manner and is predominantly sensitive to the mixing time and the polymer concentration if the surfactant concentration is sufficiently high. The model predictions are in good agreement with experimental data. Hence the theory provides a solid framework for tailoring nanoparticles with a priori determined size., 4 pages, 3 figures

Physical review letters 109, 246001(2012). doi:10.1103/PhysRevLett.109.246001, Dispersions of platelets in the nematic phase are submitted to large amplitude oscillatory shear flow and probed by high temporal resolution small angle x-ray scattering. The response displays rich dynamic and structural behavior. Under small amplitude deformations we observe an elastic response, while structurally symmetry is broken: a preferential direction of deformation is selected which induces off-plane orientation of the platelets. We associate the elastic responses with the tilting director of the platelets towards the flow direction at all strain amplitudes. At large strain amplitudes there is a yielding transition between elastic and plastic deformation, accompanied by a flipping of the director. At intermediate strain amplitudes the director has a rich dynamic behavior, illustrating the complex motion of platelets in shear flow. These observations are confirmed by steady-shear flow reversal experiments, which underline the unique character of sheared nematic platelet dispersions., Published by APS, College Park, Md.

The 3D distribution of nanocrystals at the liquid-air interface is imaged for the first time on a single-particle level by cryogenic electron tomography, revealing the equilibrium concentration profile from the interface to the bulk of the liquid. When the surface tension of the liquid is decreased, the interaction of the nanocrystals with the liquid-air interface shifts from adsorption to desorption. Macroscopic surface tension measurements do not detect this transition, due to the presence of surface-active molecular species.

We report on detailed measurements of the core structure of Shockley partial dislocations in colloidal crystals. In crystalline arrays of micrometer sized thermosensitive particles, the interactions between the colloidal building blocks were tuned by changing the temperature. Individual dislocation cores were observed in a confocal microscope and their behavior as a function of temperature was studied. The obtained results qualitatively agree with the Peierls theory and are promising for further studies in which both Peierls stress and dislocation core width are measured simultaneously. For over a century, since the early experiments on colloids by Perrin [1], colloidal particles have been employed as a model system for atoms. The merits of using colloids as model atoms are reflected in a process such as crystal nucleation. Nucleation of an atomic crystal occurs very quickly and on a length scale of angstroms or nanometers, precluding a real space study of crystal nucleation. The same process in a colloidal system takes minutes and can be observed directly with a microscope [2], while the physics behind the process remains essentially the same. In this way, homogeneous nucleation [2], as well as nucleation at impurity dopants [3], has been studied in great detail. In many ways, this example illustrates one of the keyaspects of colloid science: to generate slow and seeable model systems. Another area of research that could benefit from the colloidal approach is dislocation theory. The historic Peierls theory for dislocations was put forward shortly before the outbreak of World War II [4]. Over a decade later, the core structure of dislocations was observed experimentally for the first time, as a result of the advent of high resolution electron microscopy [5‐7]. Despite these advances a direct comparison between core structure and macroscopic material properties has not been made. Here, we will discuss a system of colloidal thermosensitive spheres that readily crystallize into a random hexagonal close packed lattice. Through analysis of confocal microscopic images, Shockley partial dislocations can be identified and characterized. The width of the dislocations is shown to be strongly dependent on temperature, which is discussed within the light of the generalized Peierls theory [8] and the properties of the thermosensitive colloids. The results hold promise for experiments in which dislocation width and crystal shear strength are measured simultaneously.

Multiple energy scales contribute to the radiative properties of colloidal quantum dots, including magnetic interactions, crystal field splitting, Pauli exclusion, and phonons. Identification of the exact physical mechanism which couples first to the dark ground state of colloidal quantum dots, inducing a significant reduction in the radiative lifetime at low temperatures, has thus been under significant debate. Here we present measurements of this phenomenon on a variety of materials as well as on colloidal heterostructures. These show unambiguously that the dominant mechanism is coupling of the ground state to a confined acoustic phonon, and that this mechanism is universal.