843 research outputs found
Rate Dependence and Role of Disorder in Linearly Sheared Two-Dimensional Foams
The shear flow of two dimensional foams is probed as a function of shear rate
and disorder. Disordered foams exhibit strongly rate dependent velocity
profiles, whereas ordered foams show rate independence. Both behaviors are
captured quantitatively in a simple model based on the balance of the
time-averaged drag forces in the foam, which are found to exhibit power-law
scaling with the foam velocity and strain rate. Disorder modifies the scaling
of the averaged inter-bubble drag forces, which in turn causes the observed
rate dependence in disordered foams.Comment: 4 Figures, 4 page
Detection of longitudinal waves in resonance with capillary waves at the air-water interface by energy transfer.
Capillary (transverse) ripples are generated at a monolayer-covered air-water interface. Compression of the monolayer changes the surface dilatational modulus Δ and the reduced elasticity Δ/Ï of the monolayer. When the reduced elasticity reaches a particular value, resonance between the capillary wave and the longitudinal wave should occur. Longitudinal waves (lateral oscillations, modulation of the average distances of the molecules) are detected by using energy transfer (Förster type) between dye molecules incorporated in the monolayer. Capillary ripples are detected by specular reflection of a laser beam from the water surface. The dependence of the longitudinal and the transverse wave characteristics on the area per molecule was investigated, and the resonance phenomenon was observed near the theoretical value of Δ/Ï for the resonance condition
Photoinduced electron transfer in Langmuir-Blodgett films.
Photoinduced electron transfer from an excited donor molecule (oxacyanine) to an acceptor molecule (viologen) located at the same interface in monolayer assemblies has been studied by measuring steady state fluorescence quenching and the excited state decay of the donor molecule. Donor and acceptor molecules are incorporated in matrix monolayers of arachidic acid and methyl arachidate, with a molar ratio 9:1. It is seen that the electron transfer efficiency increases with increasing donor density (Ïd=0.005â0.425 nmâ2). The fluorescence decay functions of the donor are described by assuming at least two fluorescent species and the results show that the contribution of long-lived species increases with increasing donor density. The average rate constant for the excited state electron transfer depends on the donor density and was found to be 2Ă108sâ1 (Ïa = 0.025nmâ2; Ïd = 0.005 nmâ2). The results are rationalized in terms of energy delocalization via incoherent exciton hopping
Shearing or Compressing a Soft Glass in 2D: Time-concentration superposition
We report surface shear rheological measurements on dense insoluble
monolayers of micron sized colloidal spheres at the oil/water interface and of
the protein -lactoglobulin at the air/water surface. As expected, the
elastic modulus shows a changing character in the response, from a viscous
liquid towards an elastic solid as the concentration is increased, and a change
from elastic to viscous as the shear frequency is increased. Surprisingly,
above a critical packing fraction, the complex elastic modulus curves measured
at different concentrations can be superposed to form a master curve, by
rescaling the frequency and the magnitude of the modulus. This provides a
powerful tool for the extrapolation of the material response function outside
the experimentally accessible frequency range. The results are discussed in
relation to recent experiments on bulk systems, and indicate that these two
dimensional monolayers should be regarded as being close to a soft glass state.Comment: to appear in PR
Non-ergodic effects in the Coulomb glass: specific heat
We present a numerical method for the investigation of non-ergodic effects in
the Coulomb glass. For that, an almost complete set of low-energy many-particle
states is obtained by a new algorithm. The dynamics of the sample is mapped to
the graph formed by the relevant transitions between these states, that means
by transitions with rates larger than the inverse of the duration of the
measurement. The formation of isolated clusters in the graph indicates
non-ergodicity. We analyze the connectivity of this graph in dependence on
temperature, duration of measurement, degree of disorder, and dimensionality,
studying how non-ergodicity is reflected in the specific heat.Comment: Submited Phys. Rev.
Assessment of detectability of neutral interstellar deuterium by IBEX observations
The abundance of deuterium in the interstellar gas in front of the Sun gives
insight into the processes of filtration of neutral interstellar species
through the heliospheric interface and potentially into the chemical evolution
of the Galactic gas. We investigate the possibility of detection of neutral
interstellar deuterium at 1 AU from the Sun by direct sampling by the
Interstellar Boundary Explorer (IBEX). We simulate the flux of neutral
interstellar D at IBEX for the actual measurement conditions. We assess the
number of interstellar D atom counts expected during the first three years of
IBEX operation. We also simulate observations expected during an epoch of high
solar activity. In addition, we calculate the expected counts of D atoms from
the thin terrestrial water layer, sputtered from the IBEX-Lo conversion surface
by neutral interstellar He atoms. Most D counts registered by IBEX-Lo are
expected to originate from the water layer, exceeding the interstellar signal
by 2 orders of magnitude. However, the sputtering should stop once the Earth
leaves the portion of orbit traversed by interstellar He atoms. We identify
seasons during the year when mostly the genuine interstellar D atoms are
expected in the signal. During the first 3 years of IBEX operations about 2
detectable interstellar D atoms are expected. This number is comparable with
the expected number of sputtered D atoms registered during the same time
intervals. The most favorable conditions for the detection occur during low
solar activity, in an interval including March and April each year. The
detection chances could be improved by extending the instrument duty cycle,
e.g., by making observations in the special deuterium mode of IBEX-Lo.Comment: Accepted for Astronomy & Astrophysic
Interaction of nitrophenols with lipids at the air/water interface.
The uptake of ortho and para nitrophenol to charged and neutral lipid monolayers spread at the air/solution interface was studied by reflection spectroscopy. The adsorption characteristics of the two nitrophenols have been studied by measuring the surface pressure and surface potential as a function of molecular area of the different lipid monolayers in the presence of nitrophenols in the subphase. The results have been interpreted in terms of the electrostatic interaction between the negatively charged dissociated phenolate ions and the positively charged head group of dioctadecyldimethylammonium bromide monolayers
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