2,556 research outputs found
Dynamics and efficiency of a self-propelled, diffusiophoretic swimmer
Active diffusiophoresis - swimming through interaction with a self-generated,
neutral, solute gradient - is a paradigm for autonomous motion at the
micrometer scale. We study this propulsion mechanism within a linear response
theory. Firstly, we consider several aspects relating to the dynamics of the
swimming particle. We extend established analytical formulae to describe small
swimmers, which interact with their environment on a finite lengthscale. Solute
convection is also taken into account. Modeling of the chemical reaction
reveals a coupling between the angular distribution of reactivity on the
swimmer and the concentration field. This effect, which we term "reaction
induced concentration distortion", strongly influences the particle speed.
Building on these insights, we employ irreversible, linear thermodynamics to
formulate an energy balance. This approach highlights the importance of solute
convection for a consistent treatment of the energetics. The efficiency of
swimming is calculated numerically and approximated analytically. Finally, we
define an efficiency of transport for swimmers which are moving in random
directions. It is shown that this efficiency scales as the inverse of the
macroscopic distance over which transport is to occur.Comment: 16 pages, 11 figure
A clinical evaluation of the Hydrocurve and Naturvue hydrogel contact lenses
Seven patients were fit with two types of hydrogel lenses. Right eyes were fit with Naturvue lenses and left eyes were fit with Hydrocurve II lenses. Each patient was followed for approximately two months to determine which lens was performing better based on several criteria. At the end of the study it was found that Hydrocurve II performed better for three patients, Naturvue performed better for two, and both lenses performed equally well on the remaining two
The Rapidly Rotating, Hydrogen Deficient, Hot Post-Asymptotic Giant Branch Star ZNG 1 in the Globular Cluster M5
We report observations of the hot post-asymptotic giant branch star ZNG 1 in
the globular cluster M5 (NGC 5904) with the Far Ultraviolet Spectroscopic
Explorer (FUSE). From the resulting spectrum, we derive an effective
temperature T_eff = 44300 +/- 300 K, a surface gravity log g = 4.3 +/- 0.1, a
rotational velocity v sin i = 170 +/- 20 km/s, and a luminosity log (L/L_sun) =
3.52 +/- 0.04. The atmosphere is helium-rich (Y = 0.93), with enhanced carbon
(2.6% by mass), nitrogen (0.51%) and oxygen (0.37%) abundances. The spectrum
shows evidence for a wind with terminal velocity near 1000 km/s and an
expanding shell of carbon- and nitrogen-rich material around the star. The
abundance pattern of ZNG 1 is suggestive of the ``born-again'' scenario,
whereby a star on the white-dwarf cooling curve undergoes a very late shell
flash and returns to the AGB, but the star's rapid rotation is more easily
explained by a previous interaction with a binary companion.Comment: 8 pages, 2 PostScript figures, Latex with emulateapj5. Accepted for
publication in ApJ Letter
Motion of vortices in type II superconductors
The methods of formal asymptotics are used to examine the behaviour of a system of curvilinear vortices in a type II superconductor as the thickness of the vortex cores tends to zero. The vortices then appear as singularities in the field equation and are analagous to line vortices in inviscid hydrodynamics. A local analysis near each vortex core gives an equation of motion governing the evolution of these singularities
Droplet Coalescence and Freezing on Hydrophilic, Hydrophobic, and Biphilic Surfaces
Frost and ice formation can have severe negative consequences, such as aircraft safety and reliability. At atmospheric pressure, water heterogeneously condenses and then freezes at low temperatures. To alter this freezing process, this research examines the effects of biphilic surfaces (surfaces which combine hydrophilic and hydrophobic regions) on heterogeneous water nucleation, growth, and freezing. Silicon wafers were coated with a self-assembled monolayer and patterned to create biphilic surfaces. Samples were placed on a freezing stage in an environmental chamber at atmospheric pressure, at a temperature of 295 K, and relative humidities of 30%, 60%, and 75%. Biphilic surfaces had a significant effect on droplet dynamics and freezing behavior. The addition of biphilic patterns decreased the temperature required for freezing by 6 K. Biphilic surfaces also changed the size and number of droplets on a surface at freezing and delayed the time required for a surface to freeze. The main mechanism affecting freezing characteristics was the coalescence behavior.Citation: A. Van Dyke, D. Collard, M. M. Derby and A. R. Betz, "Droplet Coalescence and Freezing on Hydrophilic, Hydrophobic, and Biphilic Surfaces," Applied Physics Letters, 107, Issue 14, 201
- …