We study the response of a fluid in near-critical conditions to a heat pulse, in the absence of gravity effects. The fluid under investigation is CO 2 at critical density. It is enclosed in a thermostated sample cell. We apply a theory that accounts for hydrodynamics and a real equation of state. Comparison with experiments performed under reduced gravity on board the MIR orbital station show quantitative agreement and demonstrate that the dynamics of relaxation is ruled by two typical times, a diffusion time t D and a time t c associated to adiabatic heat transport, the so-called ''Piston effect'' ͑PE͒. Three regions are observed in the fluid. First, a hot boundary layer, developing at the heat source, which shows large coupled density-temperature inhomogeneities. This part relaxes by a diffusive process, whose density and temperature relaxations are slowed down close to the critical point. Second, the bulk fluid, which remains uniform in temperature and density and whose dynamics is accelerated near the critical point and governed by the PE time. At the thermostated walls a slightly cooler boundary layer forms that cools the bulk also by a PE mechanism. The final equilibration in temperature and density of the fluid is governed by the diffusion time t D , which corresponds to the slowest mechanism. Comparison with a one-dimensional model for temperature relaxation is performed showing good agreement with experimental temperature measurements. A brief comparison is given with the situation in the presence of gravity.
We report the study of behenic acid Langmuir monolayers spread over chloride salts solutions of cadmium,
lead, magnesium, or manganese. These monolayers were investigated by means of surface pressure−area
isotherms and grazing incidence X-ray diffraction (GIXD). The effect of the concentration of cations in the
subphase on the structure of the monolayers was probed at room temperature and for three different
subphase pHs (5.5, 7.5, 10.5). A threshold in subphase concentration is detected for the formation of a
superlattice structure corresponding to an inorganic organized layer in addition to the ordered behenic
acid monolayer. This threshold is shown to strongly depend on the cation and the subphase pH. Above
the threshold, the superstructure is independent of both the cation concentration and the pH. Below the
threshold, the ions are disordered but induce a condensing effect on the fatty acid molecules, which is more
or less pronounced depending on the ions. Moreover, the combination of isotherms and GIXD allows us
to show that the existence of superstructures can be predicted from the shape of the isotherms. Indeed,
a good agreement is obtained between the thresholds determined by the two experimental techniques.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.