6,918 research outputs found
Modelling crystal aggregation and deposition\ud in the catheterised lower urinary tract
Urethral catheters often become encrusted with crystals of magnesium struvite and calcium phosphate. The encrustation can block the catheter, which can cause urine retention in the bladder and reflux into the kidneys. We develop a mathematical model to investigate crystal deposition on the catheter surface, modelling the bladder as a reservoir of fluid and the urethral catheter as a rigid channel. At a constant rate, fluid containing crystal particles of unit size enters the reservoir, and flows from the reservoir through the channel and out of the system. The crystal particles aggregate, which we model using BeckerâDöring coagulation theory, and are advected through the channel, where they continue to aggregate and are deposited on the channelâs walls. Inhibitor particles also enter the reservoir, and can bind to the crystals, preventing further aggregation and deposition. The crystal concentrations are spatially homogeneous in the reservoir, whereas the channel concentrations vary spatially as a result of advection, diffusion and deposition. We investigate the effect of inhibitor particles on the amount of deposition. For all parameter values, we find that crystals deposit along the full length of the channel, with maximum deposition close to the channelâs entrance
Many-body effects on adiabatic passage through Feshbach resonances
We theoretically study the dynamics of an adiabatic sweep through a Feshbach
resonance, thereby converting a degenerate quantum gas of fermionic atoms into
a degenerate quantum gas of bosonic dimers. Our analysis relies on a zero
temperature mean-field theory which accurately accounts for initial molecular
quantum fluctuations, triggering the association process. The structure of the
resulting semiclassical phase space is investigated, highlighting the dynamical
instability of the system towards association, for sufficiently small detuning
from resonance. It is shown that this instability significantly modifies the
finite-rate efficiency of the sweep, transforming the single-pair exponential
Landau-Zener behavior of the remnant fraction of atoms Gamma on sweep rate
alpha, into a power-law dependence as the number of atoms increases. The
obtained nonadiabaticity is determined from the interplay of characteristic
time scales for the motion of adiabatic eigenstates and for fast periodic
motion around them. Critical slowing-down of these precessions near the
instability leads to the power-law dependence. A linear power law is obtained when the initial molecular fraction is smaller than the 1/N
quantum fluctuations, and a cubic-root power law is
attained when it is larger. Our mean-field analysis is confirmed by exact
calculations, using Fock-space expansions. Finally, we fit experimental low
temperature Feshbach sweep data with a power-law dependence. While the
agreement with the experimental data is well within experimental error bars,
similar accuracy can be obtained with an exponential fit, making additional
data highly desirable.Comment: 9 pages, 9 figure
Sensitivity of a highâelevation rocky mountain watershed to altered climate and CO2
We explored the hydrologic and ecological responses of a headwater mountain catchment, Loch Vale watershed, to climate change and doubling of atmospheric CO2 scenarios using the Regional HydroâEcological Simulation System (RHESSys). A slight (2°C) cooling, comparable to conditions observed over the past 40 years, led to greater snowpack and slightly less runoff, evaporation, transpiration, and plant productivity. An increase of 2°C yielded the opposite response, but model output for an increase of 4°C showed dramatic changes in timing of hydrologic responses. The snowpack was reduced by 50%, and runoff and soil water increased and occurred 4â5 weeks earlier with 4°C warming. Alpine tundra photosynthetic rates responded more to warmer and wetter conditions than subalpine forest, but subalpine forest showed a greater response to doubling of atmospheric CO2 than tundra. Even though water use efficiency increased with the double CO2 scenario, this had little effect on basinâwide runoff because the catchment is largely unvegetated. Changes in winter and spring climate conditions were more important to hydrologic and vegetation dynamics than changes that occurred during summer
On the conversion efficiency of ultracold fermionic atoms to bosonic molecules via Feshbach resonances
We explain why the experimental efficiency observed in the conversion of
ultracold Fermi gases of K and Li atoms into diatomic Bose gases
is limited to 0.5 when the Feshbach resonance sweep rate is sufficiently slow
to pass adiabatically through the Landau Zener transition but faster than ``the
collision rate'' in the gas, and increases beyond 0.5 when it is slower. The
0.5 efficiency limit is due to the preparation of a statistical mixture of two
spin-states, required to enable s-wave scattering. By constructing the
many-body state of the system we show that this preparation yields a mixture of
even and odd parity pair-states, where only even parity can produce molecules.
The odd parity spin-symmetric states must decorrelate before the constituent
atoms can further Feshbach scatter thereby increasing the conversion
efficiency; ``the collision rate'' is the pair decorrelation rate.Comment: 4 pages, 3 figures, final version accepted to Phys. Rev. Let
Nonlinear adiabatic passage from fermion atoms to boson molecules
We study the dynamics of an adiabatic sweep through a Feshbach resonance in a
quantum gas of fermionic atoms. Analysis of the dynamical equations, supported
by mean-field and many-body numerical results, shows that the dependence of the
remaining atomic fraction on the sweep rate varies from
exponential Landau-Zener behavior for a single pair of particles to a power-law
dependence for large particle number . The power-law is linear, , when the initial molecular fraction is smaller than the 1/N
quantum fluctuations, and when it is larger.
Experimental data agree better with a linear dependence than with an
exponential Landau-Zener fit, indicating that many-body effects are significant
in the atom-molecule conversion process.Comment: 5 pages, 4 figure
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