1,398 research outputs found
Brownian Dynamics of a Sphere Between Parallel Walls
We describe direct imaging measurements of a colloidal sphere's diffusion
between two parallel surfaces. The dynamics of this deceptively simple
hydrodynamically coupled system have proved difficult to analyze. Comparison
with approximate formulations of a confined sphere's hydrodynamic mobility
reveals good agreement with both a leading-order superposition approximation as
well as a more general all-images stokeslet analysis.Comment: 4 pages, 3 figures, REVTeX with PostScript figure
Strain-dependent solid surface stress and the stiffness of soft contacts
Surface stresses have recently emerged as a key player in the mechanics of
highly compliant solids. The classic theories of contact mechanics describe
adhesion with a compliant substrate as a competition between surface energies
driving deformation to establish contact and bulk elasticity resisting this.
However, it has recently been shown that surface stresses provide an additional
restoring force that can compete with and even dominate over elasticity in
highly compliant materials, especially when length scales are small compared to
the ratio of the surface stress to the elastic modulus, . Here, we
investigate experimentally the contribution of surface stresses to the force of
adhesion. We find that the elastic and capillary contributions to the adhesive
force are of similar magnitude, and that both are required to account for
measured adhesive forces between rigid silica spheres and compliant, silicone
gels. Notably, the strain-dependence of the solid surface stress contributes
significantly to the stiffness of soft solid contacts.Comment: 6 pages, 3 figure
Many-Body Electrostatic Forces Between Colloidal Particles at Vanishing Ionic Strength
Electrostatic forces between small groups of colloidal particles are measured
using blinking optical tweezers. When the electrostatic screening length is
significantly larger than the particle radius, forces are found to be
non-pairwise additive. Both pair and multi-particle forces are well described
by the linearized Poisson-Boltzmann equation with constant potential boundary
conditions. These findings may play an important role in understanding the
structure and stability of a wide variety of systems, from micron-sized
particles in oil to aqueous nanocolloids.Comment: 5 pages 2 figure
The Deformation of an Elastic Substrate by a Three-Phase Contact Line
Young's classic analysis of the equilibrium of a three-phase contact line
ignores the out-of-plane component of the liquid-vapor surface tension. While
it has long been appreciated that this unresolved force must be balanced by
elastic deformation of the solid substrate, a definitive analysis has remained
elusive because conventional idealizations of the substrate imply a divergence
of stress at the contact line. While a number of theories of have been
presented to cut off the divergence, none of them have provided reasonable
agreement with experimental data. We measure surface and bulk deformation of a
thin elastic film near a three-phase contact line using fluorescence confocal
microscopy. The out-of-plane deformation is well fit by a linear elastic theory
incorporating an out-of-plane restoring force due to the surface tension of the
gel. This theory predicts that the deformation profile near the contact line is
scale-free and independent of the substrate elastic modulus.Comment: 4 pages, 3 figure
A minimal model for kinetic arrest
To elucidate slow dynamics in glassy materials, we introduce the {\it
Figure-8 model} in which hard blocks undergo Brownian motion around a
circuit in the shape of a figure-8. This system undergoes kinetic arrest at a
critical packing fraction , and for
long-time diffusion is controlled by rare, cooperative `junction-crossing'
particle rearrangements. We find that the average time between junction
crossings , and hence the structural relaxation time, does not
simply scale with the configurational volume \OmegaLow of transition states,
because also depends on the time to complete a junction crossing.
The importance of these results in understanding cage-breaking dynamics in
glassy systems is discussed.Comment: 4 pages, 4 figure
Parabolic lithium refractive optics for x rays
Excellent x-ray optics for photons at around 10 keV can be expected with lithium metal. One of the best compound refractive lens designs [Lengeler et al., J. Appl. Phys. 84, 5855 (1998)] is now produced routinely in aluminum, and more recently has been demonstrated using beryllium [M. Kuhlmann et al. (unpublished)]. Here, we report a similar refractive lens made from lithium. At 10.87 keV, this lens has a ≃2 m focal length, more than 90% peak transmission, and an average transmission of 49%. The lens shows a very useful gain of up to 40. The full widths at half maximum (FWHM) of the focus are blurred by roughly 20 μm, resulting in a horizontal and vertical FWHM of 33 and 17 μm for an image distance of 2.13 m. The lens produces speckle on the x-ray beam, which is likely due to the inhomogeneities of the lens surface: Coherent x-ray scattering is useful in understanding imperfections in x-ray optics, such as mirrors and lenses. Better molding techniques should result in improved performance and enable microbeam techniques with this type of Li lens. © 2004 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70176/2/RSINAK-75-1-37-1.pd
An Imaging System for Focusing Tests of Li Multiprism X‐ray Refractive Lenses
For rapid and efficient tests of novel X‐rays optics, such as lithium‐based compound refractive lenses, we have built a fast X‐ray sensitive CCD imaging system. We report on the linearity, response and resolution of the microscope‐based imaging system. For the low magnifications used here (X2‐X10), we find that a thinly doped YAG screen has a poorer resolution than a thick YAG screen. We provide an example of its use in testing a new 2D focusing multiprism X‐ray lens. © 2004 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87661/2/780_1.pd
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