Biomedical subjects

Heinrich M Jaeger

Publications and source records attributed to Heinrich M Jaeger.

9 recordsLinked to original sources

PubMed2006

Kinetically driven self assembly of highly ordered nanoparticle monolayers.

When a drop of a colloidal solution of nanoparticles dries on a surface, it leaves behind coffee-stain-like rings of material with lace-like patterns or clumps of particles in the interior. These non-uniform mass distributions are manifestations of far-from-equilibrium effects, such as fluid flows and solvent fluctuations during late-stage drying. However, recently a strikingly different drying regime promising highly uniform, long-range-ordered nanocrystal monolayers has been found. Here we make direct, real-time and real-space observations of nanocrystal self-assembly to reveal the mechanism. We show how the morphology of drop-deposited nanoparticle films is controlled by evaporation kinetics and particle interactions with the liquid-air interface. In the presence of an attractive particle-interface interaction, rapid early-stage evaporation dynamically produces a two-dimensional solution of nanoparticles at the liquid-air interface, from which nanoparticle islands nucleate and grow. This self-assembly mechanism produces monolayers with exceptional long-range ordering that are compact over macroscopic areas, despite the far-from-equilibrium evaporation process. This new drop-drying regime is simple, robust and scalable, is insensitive to the substrate material and topography, and has a strong preference for forming monolayer films. As such, it stands out as an excellent candidate for the fabrication of technologically important ultra thin film materials for sensors, optical devices and magnetic storage media.

Colloids↗

PubMed2006

Three-dimensional shear in granular flow.

The evolution of granular shear flow is investigated as a function of height in a split-bottom Couette cell. Using particle tracking, magnetic-resonance imaging, and large-scale simulations, we find a transition in the nature of the shear as a characteristic height H* is exceeded. Below H* there is a central stationary core; above H* we observe the onset of additional axial shear associated with torsional failure. Radial and axial shear profiles are qualitatively different: the radial extent is wide and increases with height, while the axial width remains narrow and fixed.

Colloids↗

PubMed2005

Effect of air on granular size separation in a vibrated granular bed.

Using high-speed video and magnetic resonance imaging (MRI) we study the motion of a large sphere in a vertically vibrated bed of smaller grains. As previously reported we find a nonmonotonic density dependence of the rise and sink time of the large sphere. We show that air drag causes relative motion between the intruder and the bed during the shaking cycle and is ultimately responsible for the observed density dependence of the risetime. We investigate in detail how the motion of the intruder sphere is influenced by size of the background particles, initial vertical position in the bed, ambient pressure, and convection. We explain our results in the framework of a simple model and find quantitative agreement in key aspects with numerical simulations to the model equations.

Journal Article↗

PubMed2005

Scattered light interference from a single metal nanoparticle and its mirror image.

The spatial distribution of surface plasmon scattering from a single nanoparticle changes dramatically near a metal surface as a result of interference from the direct scattered light and indirect scattered light from the mirror reflection. The unique interference patterns have been reproduced by simulations based on Huygens-Fresnel wave propagation theory. The large spectral width of the surface plasmon scattering enables a vertical distance measurement with 10 nm resolution through this nonintrusive far field interferometry.

Letter↗

PubMed2005

Structural signature of jamming in granular media.

Glasses are rigid, but flow when the temperature is increased. Similarly, granular materials are rigid, but become unjammed and flow if sufficient shear stress is applied. The rigid and flowing phases are strikingly different, yet measurements reveal that the structures of glass and liquid are virtually indistinguishable. It is therefore natural to ask whether there is a structural signature of the jammed granular state that distinguishes it from its flowing counterpart. Here we find evidence for such a signature, by measuring the contact-force distribution between particles during shearing. Because the forces are sensitive to minute variations in particle position, the distribution of forces can serve as a microscope with which to observe correlations in the positions of nearest neighbours. We find a qualitative change in the force distribution at the onset of jamming. If, as has been proposed, the jamming and glass transitions are related, our observation of a structural signature associated with jamming hints at the existence of a similar structural difference at the glass transition--presumably too subtle for conventional scattering techniques to uncover. Our measurements also provide a determination of a granular temperature that is the counterpart in granular systems to the glass-transition temperature in liquids.

Journal Article↗

PubMed2004

Intruders in the dust: air-driven granular size separation.

Using MRI and high-speed video we investigate the motion of a large intruder particle inside a vertically shaken bed of smaller particles. We find a pronounced, nonmonotonic density dependence, with both light and heavy intruders moving faster than those whose density is approximately that of the granular bed. For light intruders, we furthermore observe either rising or sinking behavior, depending on intruder starting height, boundary condition, and interstitial gas pressure. We map out the phase boundary delineating the rising and sinking regimes. A simple model can account for much of the observed behavior and shows how the two regimes are connected by considering pressure gradients across the granular bed during a shaking cycle.

Journal Article↗

PubMed2004

Percolating through networks of random thresholds: finite temperature electron tunneling in metal nanocrystal arrays.

We investigate how temperature affects transport through large networks of nonlinear conductances with distributed thresholds. In monolayers of weakly coupled gold nanocrystals, quenched charge disorder produces a range of local thresholds for the onset of electron tunneling. Our measurements delineate two regimes separated by a crossover temperature T*. Up to T* the nonlinear zero-temperature shape of the current-voltage curves survives, but with a threshold voltage for conduction that decreases linearly with temperature. Above T* the threshold vanishes and the low-bias conductance increases rapidly with temperature. We develop a model that accounts for these findings and predicts T*.

Journal Article↗

PubMed2002

Force distributions in three-dimensional compressible granular packs.

We present an experimental investigation of the probability distribution of normal contact forces, P(F), at the bottom boundary of static three-dimensional packings of compressible granular materials. We find that the degree of deformation of individual grains plays a large role in determining the form of this distribution. For small amounts of deformation we find a small peak in P(F) below the mean force with an exponential tail for forces larger than the mean force. As the degree of deformation is increased the peak at the mean force grows in height and the slope of the exponential tail increases.

Journal Article↗

PubMed2002

Stress transmission through three-dimensional ordered granular arrays.

We measure the local contact forces at both the top and bottom boundaries of three-dimensional face-centered-cubic and hexagonal-close-packed granular crystals in response to an external force applied to a small area at the top surface. Depending on the crystal structure, we find markedly different results which can be understood in terms of force balance considerations in the specific geometry of the crystal. Small amounts of disorder are found to create additional structure at both the top and bottom surfaces.

Journal Article↗