-
Chaos (Woodbury, N.Y.) 12/2010; 20(4):041102. · 1.80 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This work delineates the mechanism by which directional nanowire growth occurs in the directed electrochemical nanowire assembly (DENA) technique for growing nanowires on micro-electrode arrays. Indium, polythiophene, and polypyrrole nanowires are the subjects of this study. This technique allows the user to specify the growth path without the use of a mechanical template. Nanowire growth from a user-selected electrode to within +/- 3 microm of the straight line path to a second electrode lying within a approximately 140 degrees angular range and a approximately 100 microm radius of the selected electrode is demonstrated. Theory for one-dimensional electrochemical diffusion in the inter-electrode region reveals that screening of the applied voltage is incomplete, allowing a long range voltage component to extend from the biased to the grounded electrode. Numerical analysis of two-dimensional multi-electrode arrays shows that a linear ridge of electric field maxima bridges the gap between selected electrodes but decays in all other directions. The presence of this anisotropic, long range voltage defines the wire growth path and suppresses the inherent tip splitting tendency of amorphous polymeric materials. This technology allows polythiophene and polypyrrole to be grown as wires rather than fractal aggregates or films, establishing DENA as an on-chip approach to both crystalline metallic and amorphous polymeric nanowire growth.
Nanotechnology 07/2009; 20(23):235307. · 3.98 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The sedimentation of hard spheres in a Newtonian solvent is studied as a function of Péclet number in the low-concentration limit. Two functional forms for the sedimentation velocity as a function of particle concentration are realized in the limit of high and low Péclet numbers. We argue that a more ordered phase occurs for large Péclet numbers. Measurements of settling in sheared suspensions support these contentions. Recent explanations of sedimentation in suspensions are examined in light of these results.
Physical Review E 12/2007; 76(5 Pt 2):056302. · 2.26 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A dual-beam dynamic light-scattering arrangement is devised to measure the time-dependent mean squared relative displacement of a pair of tracer particles with a small separation of micrometers. The technique is tested by the measurement of the relative diffusion of polymer latex spheres suspended in a simple viscous fluid. The experiment verifies the theory and demonstrates its applications. The dual-beam dynamic light-scattering technique, when combined with an optical microscope, provides a powerful tool for the study of two-particle microrheology of soft materials. The advantages of the new technique are its high statistical accuracy, faster temporal response, and ease of use.
Applied Optics 07/2004; 43(17):3382-90. · 1.41 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A novel fiber-optic technique is developed to measure one component of
the time and space resolved vorticity vector ω(r,t) = nabla
× v(r,t), where v(r,t) is the local velocity. In the technique,
two single-mode, polarization-maintaining fibers and a fiber-optic
coupler are used to measure an instantaneous velocity difference. The
two input fibers collect the scattered light by seed particles with the
same polarization and scattering wavevector but from two spatially
separated regions in a flow. The obtained signals interfere when
combined via the fiber-optic coupler and the resultant light contains a
beat frequency proportional to the velocity difference between the two
measuring points. With the new technique, a compact vorticity probe
consisting of two sets of optical fibers and couplers can be built,
which would simultaneously measure two velocity gradient components at
four closely spaced locations. The new vorticity probe is capable of
measuring ω(r,t) with a spatial resolution down to 50 μ m. The
operation of the probe is similar to the standard LDV and can be used
widely in the general area of fluid dynamics.
10/2000; -1.
-
[show abstract]
[hide abstract]
ABSTRACT: Small angle neutron scattering(SANS) is used to investigate the local order in aqueous, charge stabilized suspensions of 103 Å diameter polystyrene latexspheres at ∼14 wt. % solids. These samples which evidence a close packed close packed structure in equilibrium are observed to undergo a transition to an amorphous order when sustaining a sufficiently large and steady shear. The shear melting phenomonology is different from that observed by light scattering for very dilute (0.16 wt. % solids) suspensions of polystyrene particles which form bcc lattices in equilibrium. Furthermore, neutron scattering reveals more detail than light scattering for concentrated suspensions and indicates a larger degree of three‐dimensional ordering under steady shear flow than is implied by sliding layer models for similar systems.
The Journal of Chemical Physics 02/1986; 84(4):2344-2349. · 3.33 Impact Factor
-
-
[show abstract]
[hide abstract]
ABSTRACT: The structure of aqueous suspensions of electrostatically interacting submicron polymer spheres is studied as a function of shear. These model colloidal suspensions exhibit a variety of equilibrium and nonequilibrium structures and phase transitions. Both shear induced melting of solid-like structures and shear induced distortion of liquid-like structures are observed. Several models are presented to explain the observed effects. The analogy between these colloidal suspensions and pure fluid systems is discussed.
Physica A: Statistical Mechanics and its Applications.
