87 research outputs found
Electrostatic Cancellation of Gravity Effects in Liquid Mixtures
We point out that a spatially-varying electric field can be used to cancel
the effect of gravity in liquid mixtures by coupling to the different
components' permittivities. Cancellation occurs if the system under
consideration is small enough. For a simple ``wedge'' electrode geometry we
show that the required system size and voltage are practical, easily realizable
in the Lab. Thus this setup might be a simple alternative to more expensive or
hazardous options such as the space-shuttle, drop-tower, or magnetic levitation
experiments.Comment: 1.5 pages, one figure. Accepted to PRE brief report
Thermodynamics of gas–liquid colloidal equilibrium states: hetero-phase fluctuations
Following on from two previous JETC (Joint European Thermodynamics Conference) presentations, we present a preliminary report of further advances towards the thermodynamic description of critical behavior and a supercritical gas-liquid coexistence with a supercritical fluid mesophase defined by percolation loci. The experimental data along supercritical constant temperature isotherms (T >= T-c) are consistent with the existence of a two-state mesophase, with constant change in pressure with density, rigidity, (dp/d rho) (T), and linear thermodynamic state-functions of density. The supercritical mesophase is bounded by 3rd-order phase transitions at percolation thresholds. Here we present the evidence that these percolation transitions of both gaseous and liquid states along any isotherm are preceded by pre-percolation hetero-phase fluctuations that can explain the thermodynamic properties in the mesophase and its vicinity. Hetero-phase fluctuations give rise to one-component colloidal-dispersion states; a single Gibbs phase retaining 2 degrees of freedom in which both gas and liquid states with different densities percolate the phase volume. In order to describe the thermodynamic properties of two-state critical and supercritical coexistence, we introduce the concept of a hypothetical homo-phase of both gas and liquid, defined as extrapolated equilibrium states in the pre-percolation vicinity, with the hetero-phase fractions subtracted. We observe that there can be no difference in chemical potential between homo-phase liquid and gaseous states along the critical isotherm in mid-critical isochoric experiments when the meniscus disappears at T = T-c. For T > T-c, thermodynamic states comprise equal mole fractions of the homo-phase gas and liquid, both percolating the total phase volume, at the same temperature, pressure, and with a uniform chemical potential, stabilised by a positive finite interfacial surface tension.info:eu-repo/semantics/publishedVersio
Thermodynamic fluid equations-of-state
As experimental measurements of thermodynamic properties have improved in accuracy, to five or six figures, over the decades, cubic equations that are widely used for modern thermodynamic fluid property data banks require ever-increasing numbers of terms with more fitted parameters. Functional forms with continuity for Gibbs density surface (p,T) which accommodate a critical-point singularity are fundamentally inappropriate in the vicinity of the critical temperature (T-c) and pressure (p(c)) and in the supercritical density mid-range between gas- and liquid-like states. A mesophase, confined within percolation transition loci that bound the gas- and liquid-state by third-order discontinuities in derivatives of the Gibbs energy, has been identified. There is no critical-point singularity at T-c on Gibbs density surface and no continuity of gas and liquid. When appropriate functional forms are used for each state separately, we find that the mesophase pressure functions are linear. The negative and positive deviations, for both gas and liquid states, on either side of the mesophase, are accurately represented by three or four-term virial expansions. All gaseous states require only known virial coefficients, and physical constants belonging to the fluid, i.e., Boyle temperature (T-B), critical temperature (T-c), critical pressure (p(c)) and coexisting densities of gas ((cG)) and liquid ((cL)) along the critical isotherm. A notable finding for simple fluids is that for all gaseous states below T-B, the contribution of the fourth virial term is negligible within experimental uncertainty. Use may be made of a symmetry between gas and liquid states in the state function rigidity (dp/d)(T) to specify lower-order liquid-state coefficients. Preliminary results for selected isotherms and isochores are presented for the exemplary fluids, CO2, argon, water and SF6, with focus on the supercritical mesophase and critical region.info:eu-repo/semantics/publishedVersio
The Nature of Asymmetry in Fluid Criticality
By combining accurate liquid-vapor coexistence and heat-capacity data, we
have unambiguously separated two non-analytical contributions of liquid-gas
asymmetry in fluid criticality and proved the validity of "complete scaling"
[Fisher et al., Phys. Rev. Lett. 85, 696 (2000); Phys. Rev. E, 67, 061506,
(2003)]. We have also developed a method to obtain two scaling-field
coefficients, responsible for the two sources of the asymmetry, from mean-field
equations of state. Since the asymmetry effects are completely determined by
Ising critical exponents, there is no need for a special renormalization-group
theoretical treatment of asymmetric fluid criticality.Comment: 4 pages, 3 figure
Relationship between Diffusion, Selfdiffusion and Viscosity
We investigate the experimental limits of validity of the Stokes-Einstein
equation. There is an important difference between diffusion and
self-diffusion. There are experimental evidences, that in the case of
self-diffusion the product D /T is constant and the equation is still valid.
However, comparison of existing experimental data on viscosity and diffusion
coefficients D of small, fast moving ions unambiguously show that the product D
/T depends strongly on temperature T. The temperature dependence of diffusion
coefficient declines from that of viscosity. Therefore, the Stokes-Einstein
equation is not valid in this case
Asymmetric Fluid Criticality I: Scaling with Pressure Mixing
The thermodynamic behavior of a fluid near a vapor-liquid and, hence,
asymmetric critical point is discussed within a general ``complete'' scaling
theory incorporating pressure mixing in the nonlinear scaling fields as well as
corrections to scaling. This theory allows for a Yang-Yang anomaly in which
\mu_{\sigma}^{\prime\prime}(T), the second temperature derivative of the
chemical potential along the phase boundary, diverges like the specific heat
when T\to T_{\scriptsize c}; it also generates a leading singular term,
|t|^{2\beta}, in the coexistence curve diameter, where t\equiv
(T-T_{\scriptsize c}) /T_{\scriptsize c}. The behavior of various special loci,
such as the critical isochore, the critical isotherm, the k-inflection loci, on
which \chi^{(k)}\equiv \chi(\rho,T)/\rho^{k} (with \chi = \rho^{2}
k_{\scriptsize B}TK_{T}) and C_{V}^{(k)}\equiv C_{V}(\rho,T)/\rho^{k} are
maximal at fixed T, is carefully elucidated. These results are useful for
analyzing simulations and experiments, since particular, nonuniversal values of
k specify loci that approach the critical density most rapidly and reflect the
pressure-mixing coefficient. Concrete illustrations are presented for the
hard-core square-well fluid and for the restricted primitive model electrolyte.
For comparison, a discussion of the classical (or Landau) theory is presented
briefly and various interesting loci are determined explicitly and illustrated
quantitatively for a van der Waals fluid.Comment: 21 pages in two-column format including 8 figure
Generic mechanism for generating a liquid-liquid phase transition
Recent experimental results indicate that phosphorus, a single-component
system, can have two liquid phases: a high-density liquid (HDL) and a
low-density liquid (LDL) phase. A first-order transition between two liquids of
different densities is consistent with experimental data for a variety of
materials, including single-component systems such as water, silica and carbon.
Molecular dynamics simulations of very specific models for supercooled water,
liquid carbon and supercooled silica, predict a LDL-HDL critical point, but a
coherent and general interpretation of the LDL-HDL transition is lacking. Here
we show that the presence of a LDL and a HDL can be directly related to an
interaction potential with an attractive part and two characteristic
short-range repulsive distances. This kind of interaction is common to other
single-component materials in the liquid state (in particular liquid metals),
and such potentials are often used to decribe systems that exhibit a density
anomaly. However, our results show that the LDL and HDL phases can occur in
systems with no density anomaly. Our results therefore present an experimental
challenge to uncover a liquid-liquid transition in systems like liquid metals,
regardless of the presence of the density anomaly.Comment: 5 pages, 3 ps Fig
Isotropic soft-core potentials with two characteristic length scales and anomalous behaviour
Isotropic soft-core potentials with two characteristic length scales have
been used since 40 years to describe systems with polymorphism. In the recent
years intense research is showing that these potentials also display
polyamorphism and several anomalies, including structural, diffusion and
density anomaly. These anomalies occur in a hierarchy that resembles the
anomalies of water. However, the absence of directional bonding in these
isotropic potentials makes them different from water. Other systems, such as
colloidal suspensions, protein solutions or liquid metals, can be well
described by these family of potentials, opening the possibility of studying
the mechanism generating the polyamorphism and anomalies in these complex
liquids
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