24 research outputs found
Dense granular flow around a penetrating object: Experiments and hydrodynamic model
We present in this Letter experimental results on the bidimensional flow
field around a cylinder penetrating into dense granular matter together with
drag force measurements. A hydrodynamic model based on extended kinetic theory
for dense granular flow reproduces well the flow localization close to the
cylinder and the corresponding scalings of the drag force, which is found to
not depend on velocity, but linearly on the pressure and on the cylinder
diameter and weakly on the grain size. Such a regime is found to be valid at a
low enough "granular" Reynolds number.Comment: 5 pages, 4 figure
Quantifying the Reversible Association of Thermosensitive Nanoparticles
Under many conditions, biomolecules and nanoparticles associate by means of
attractive bonds, due to hydrophobic attraction. Extracting the microscopic
association or dissociation rates from experimental data is complicated by the
dissociation events and by the sensitivity of the binding force to temperature
(T). Here we introduce a theoretical model that combined with light-scattering
experiments allows us to quantify these rates and the reversible binding energy
as a function of T. We apply this method to the reversible aggregation of
thermoresponsive polystyrene/poly(N-isopropylacrylamide) core-shell
nanoparticles, as a model system for biomolecules. We find that the binding
energy changes sharply with T, and relate this remarkable switchable behavior
to the hydrophobic-hydrophilic transition of the thermosensitive nanoparticles
Anisotropic dynamics and kinetic arrest of dense colloidal ellipsoids in the presence of an external field studied by differential dynamic microscopy
Anisotropic dynamics on the colloidal length scale is ubiquitous in nature. Of particular interest is the dynamics of systems approaching a kinetically arrested state. The failure of classical techniques for investigating the dynamics of highly turbid suspensions has contributed toward the limited experimental information available up until now. Exploiting the recent developments in the technique of differential dynamic microscopy (DDM), we report the first experimental study of the anisotropic collective dynamics of colloidal ellipsoids with a magnetic hematite core over a wide concentration range approaching kinetic arrest. In addition, we have investigated the effect of an external magnetic field on the resulting anisotropic collective diffusion. We combine DDM with small-angle x-ray scattering and rheological measurements to locate the glass transition and to relate the collective short- and long-time diffusion coefficients to the structural correlations and the evolution of the zero shear viscosity as the system approaches an arrested state
Giant hollow fiber formation through self-assembly of oppositely charged polyelectrolyte brushes and gold nanoparticles
We report on the use of binary mixtures of oppositely charged gold nanoparticles (AuNPs) and spherical polyelectrolyte brushes (SPBs), consisting of a polystyrene core onto which long polystyrene sulfonate chains are grafted, as a simple model system to investigate the influence of directional interactions on self-assembly. We demonstrate that the mixing ratio, i.e., the number of AuNPs per SPB, has a profound influence on self-assembly. In particular we report on the formation of giant hollow fibers, and present a thorough characterization of these nanostructures. We speculate that the adsorption of a few AuNPs on the SPBs appears to direct the tubular self-assembly, and discuss the analogy to the case of modified proteins such as tubulin under the action of nucleotides
Colloidal assembly and 3D shaping by dielectrophoretic confinement
For decades, scientists and engineers have strived to design means of assembling colloids into ordered structures. By now, the literature is quite peppered with reports of colloidal assemblies. However, the available methods can assemble only a narrow range of structures or are applicable to specific types of colloids. There are still only few generic methods that would lead to arbitrary colloidal arrays or would shape colloidal assemblies into predesigned structures. Here, we first discuss in detail how to spatially control the assembly and crystallization of colloids through the balance of dielectrophoretic and dipolar forces. Furthermore, we demonstrate how to flexibly program and shape arrays of 3D microstructures that can be permanently affixed by in situ UV polymerization and calcination by using two facing similar or distinct micro-fabricated electrodes
Multiscale directed self-assembly of composite microgels in complex electric fields
This study explored the application of localized electric fields for reversible directed self-assembly of colloidal particles in 3 dimensions. Electric field microgradients, arising from the use of micro-patterned electrodes, were utilized to direct the localization and self-assembly of polarizable (charged) particles resulting from a combination of dielectrophoretic and multipolar forces. Deionized dispersions of spherical and ellipsoidal core-shell microgels were employed for investigating their assembly under an external alternating electric field. We demonstrated that the frequency of the field allowed for an exquisite control over the localization of the particles and their self-assembled structures near the electrodes. We extended this approach to concentrated binary dispersions consisting of polarizable and less polarizable composite microgels. Furthermore, we utilized the thermosensitivity of the microgels to adjust the effective volume fraction and the dynamics of the system, which provided the possibility to dynamically “solidify” the assembly of the field-responsive particles by a temperature quench from their initial fluid state into an arrested crystalline state. Reversible solidification enables us to re-write/reconstruct various 3 dimensional assemblies by varying the applied field frequency
Anisotropic responsive microgels with tuneable shape and interactions
Highly monodisperse polystyrene/poly(N-isopropylmethacrylamide) (PS-PNIPMAM) core-shell composite microgels were synthesized and further nanoengineered in either ellipsoidal, faceted or bowl-shaped particles. Beside their anisotropy in shape, the microgel design enables an exquisite control of the particle conformation, size and interactions from swollen and hydrophilic to collapsed and hydrophobic using temperature as an external control variable. The post-processing procedures and the characterization of the different particles are first presented. Their potential as model systems for the investigation of the effects of anisotropic shape and interactions on the phase behavior is further demonstrated. Finally, the self-assembly of bowl-shaped composite microgel particles is discussed, where the temperature and an external AC electric field are employed to control the interactions from repulsive to attractive and from soft repulsive to dipolar, respectively
Hybrid raspberry microgels with tunable thermoresponsive behavior
We report the synthesis of thermosensitive raspberry-like hybrid microgels. They consist of a poly(N-isopropylacrylamide) core decorated with in situ formed silica particles. A linking silane agent was copolymerized to help the anchoring of silica particles formed by a controlled silica precursor addition onto the outer shell. The thermosensitive behaviour is tuneable by adjusting the size of the silica particles
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