26 research outputs found
The Casimir effect: from quantum to critical fluctuations
The Casimir effect in quantum electrodynamics (QED) is perhaps the best-known
example of fluctuation-induced long-ranged force acting on objects (conducting
plates) immersed in a fluctuating medium (quantum electromagnetic field in
vacuum). A similar effect emerges in statistical physics, where the force
acting, e.g., on colloidal particles immersed in a binary liquid mixture is
affected by the classical thermal fluctuations occurring in the surrounding
medium. The resulting Casimir-like force acquires universal features upon
approaching a critical point of the medium and becomes long-ranged at
criticality. In turn, this universality allows one to investigate theoretically
the temperature dependence of the force via representative models and to
stringently test the corresponding predictions in experiments. In contrast to
QED, the Casimir force resulting from critical fluctuations can be easily tuned
with respect to strength and sign by surface treatments and temperature
control. We present some recent advances in the theoretical study of the
universal properties of the critical Casimir force arising in thin films. The
corresponding predictions compare very well with the experimental results
obtained for wetting layers of various fluids. We discuss how the Casimir force
between a colloidal particle and a planar wall immersed in a binary liquid
mixture has been measured with femto-Newton accuracy, comparing these
experimental results with the corresponding theoretical predictions.Comment: Talk delivered at the International Workshop "60 Years of Casimir
Effect", Brasilia, 23-27 June 2008 (17 pages, 7 figures
Critical Casimir forces between homogeneous and chemically striped surfaces
Recent experiments have measured the critical Casimir force acting on a
colloid immersed in a binary liquid mixture near its continuous demixing phase
transition and exposed to a chemically structured substrate. Motivated by these
experiments, we study the critical behavior of a system, which belongs to the
Ising universality class, for the film geometry with one planar wall chemically
striped, such that there is a laterally alternating adsorption preference for
the two species of the binary liquid mixture, which is implemented by surface
fields. For the opposite wall we employ alternatively a homogeneous adsorption
preference or homogeneous Dirichlet boundary conditions, which within a lattice
model are realized by open boundary conditions. By means of mean-field theory,
Monte Carlo simulations, and finite-size scaling analysis we determine the
critical Casimir force acting on the two parallel walls and its corresponding
universal scaling function. We show that in the limit of stripe widths small
compared with the film thickness, on the striped surface the system effectively
realizes Dirichlet boundary conditions, which generically do not hold for
actual fluids. Moreover, the critical Casimir force is found to be attractive
or repulsive, depending on the width of the stripes of the chemically patterned
surface and on the boundary condition applied to the opposing surface.Comment: 29 pages, 29 figures; v2: 29 pages, 31 figures, two new figures,
added comparison with chemical-step estimate
Inter-cycle variability of ignition delay in an ethanol fumigated common rail diesel engine
An experimental study has been performed to investigate the ignition delay of a modern heavy-duty common-rail diesel engine run with fumigated ethanol substitutions up to 40% on an energy basis. The ignition delay was determined through the use of statistical modelling in a Bayesian framework this framework allows for the accurate determination of the start of combustion from single consecutive cycles and does not require any differentiation of the in-cylinder pressure signal. At full load the ignition delay has been shown to decrease with increasing ethanol substitutions and evidence of combustion with high ethanol substitutions prior to diesel injection have also been shown experimentally and by modelling. Whereas, at half load increasing ethanol substitutions have increased the ignition delay. A threshold absolute air to fuel ratio (mole basis) of above ~110 for consistent operation has been determined from the inter-cycle variability of the ignition delay, a result that agrees well with previous research of other in-cylinder parameters and further highlights the correlation between the air to fuel ratio and inter-cycle variability. Numerical modelling to investigate the sensitivity of ethanol combustion has also been performed. It has been shown that ethanol combustion is sensitive to the initial air temperature around the feasible operating conditions of the engine. Moreover, a negative temperature coefficient region of approximately 900{1050 K (the approximate temperature at fuel injection) has been shown with for n-heptane and n-heptane/ethanol blends in the numerical modelling. A consequence of this is that the dominate effect influencing the ignition delay under increasing ethanol substitutions may rather be from an increase in chemical reactions and not from in-cylinder temperature. Further investigation revealed that the chemical reactions at low ethanol substitutions are different compared to the high (> 20%) ethanol substitutions