Precursor phenomena in frustrated systems

To understand the origin of the dynamical transition, between high-temperature exponential relaxation and low-temperature nonexponential relaxation, that occurs well above the static transition in glassy systems, a frustrated spin model, with and without disorder, is considered. The model has two phase transitions, the lower being a standard spin glass transition (in the presence of disorder) or fully frustrated Ising (in the absence of disorder), and the higher being a Potts transition. Monte Carlo results clarify that in the model with (or without) disorder the precursor phenomena are related to the Griffiths (or Potts) transition. The Griffiths transition is a vanishing transition which occurs above the Potts transition and is present only when disorder is present, while the Potts transition which signals the effect due to frustration is always present. These results suggest that precursor phenomena in frustrated systems are due either to disorder and/or to frustration, giving a consistent interpretation also for the limiting cases of Ising spin glass and of Ising fully frustrated model, where also the Potts transition is vanishing. This interpretation could play a relevant role in glassy systems beyond the spin systems case.     

Nonlocality and short-range wetting phenomena

We propose a nonlocal interfacial model for 3D short-range wetting at planar and nonplanar walls. The model is characterized by a binding-potential functional depending only on the bulk Ornstein-Zernike correlation function, which arises from different classes of tubelike fluctuations that connect the interface and the substrate. The theory provides a physical explanation for the origin of the effective position-dependent stiffness and binding potential in approximate local theories and also obeys the necessary classical wedge covariance relationship between wetting and wedge filling. Renormalization group and computer simulation studies reveal the strong nonperturbative influence of nonlocality at critical wetting, throwing light on long-standing theoretical problems regarding the order of the phase transition.     

Precursor film controlled wetting of Pb on Cu

Wetting in a system where the kinetics of drop spreading are controlled by the rate of formation of a precursor film is modeled for the first time at the atomistic scale. Molecular dynamics simulations of Pb(l) wetting Cu(111) and Cu(100) show that a precursor film of atomic thickness evolves and spreads diffusively. This precursor film spreads significantly faster on Cu(111) than on Cu(100). For Cu(100), the kinetics of drop spreading are dramatically decreased by slow advancement of the precursor film. Slow precursor film kinetics on Cu(100) are partly due to the formation of a surface alloy at the solid-liquid interface which does not occur on Cu(111).     

润湿现象和毛细现象的热力学描述

应用多组分、多界面系统的热力学基本公式,对沾湿、浸湿、铺展、毛细现象等进行讨论,所得结论与传统方法一致。     

Phonon contribution to wetting phenomena: macroscopic theory

We present a macroscopic treatment that clarifies the role of phonons in the wetting of a solid by an ideal liquid, such as liquid ⁴He. We show that in the equation for the wetting temperature the phonon contribution cancels exactly. Therefore, a pure phonon mechanism always results in wetting, and other mechanisms determine whether wetting or non-wetting occurs. The importance of ripplon excitations for the wetting phenomenon is discussed.     

Precursor diamagnetism in superconducting Al films

The precursor diamagnetism is studied in a stack of Al films whose thickness is much smaller than the coherence length. The magnetization shows the (T ? T_c)^?1 behavior in a finite range of temperature above T_c. At T_c, in low magnetic fields constant magnetization independent of the field strength is observed. The results are in agreement with the theories of the two-dimensional superconductor.     

X-ray study of growing wetting films

X-ray scattering experiments of liquid films on top of solid substrates were performed. With a short pulse disturbance, caused by a temperature difference between the substrate and the vapour in the X-ray cell, the wetting film thickness is reduced. Afterwards the time dependence of the growing film is monitored by X-ray reflectivity measurements in the region of total external reflection. We have examined CCl₄- and CCl₃Br-films on top of silicon wafers and CCl₃Br on glass/gold and glass/silver substrates. The film thickness as function of time is explained by the Kolmogorov growth model. From the data we obtain rather long time constants and the dimensiond=2 of the growing process     

Aging phenomena in polystyrene thin films

The aging behavior is investigated for thin films of atactic polystyrene through measurements of complex electric capacitance. During isothermal aging process the real part of the electric capacitance increases with aging time, while the imaginary part decreases with aging time. This result suggests that the aging time dependence of the real and imaginary parts are mainly associated with change in thickness and dielectric permittivity, respectively. In thin films, the thickness depends on thermal history of aging even above the glass transition. Memory and `rejuvenation' effects are also observed in the thin films.     

Heterogeneous hole nucleation in electron-charged 4He wetting films

4He wetting layers on Cs exhibit an unusually long-lived metastable state upon undercooling to temperatures well below the wetting temperature T(w) approximately 1.9 K. The decay of this state by homogeneous thermal nucleation of holes is disfavored by the incipient divergence of the free energy barrier separating the metastable thick film from the stable thin-film state. We propose that interface deformations ("dimples") created by electrons bound at the 4He liquid-vapor interface can be used as nuclei for the heterogeneous nucleation of holes. The size and excess free energy of the dimple can be tuned by an applied electric field E which allows the lifetime of the metastable film to be controlled.     

Dynamic fluctuation phenomena in double membrane films

Dynamics of double membrane films is investigated in the long-wavelength limit qh≪1 (q is the wave vector, and h is the thickness of the film) including the overdamped squeezing mode. We demonstrate that thermal fluctuations essentially modify the character of the mode due to its nonlinear coupling to the transverse shear hydrodynamic mode. The renormalization can be analyzed if the condition g≪1 is satisfied (where g∼T/κ, T is the temperature, and κ is the bending modulus). The corresponding Green’s function acquires as a function of the frequency ω a cut along the imaginary semiaxis. At the effective length of the cut is ∼Tq ³/η (where η is the shear viscosity of the liquid). At the fluctuations lead to an increase in the attenuation of the squeezing mode: it is larger than the ‘bare’ value by a factor . We also present the analysis of the elastic modes. Zh. éksp. Teor. Fiz. 113, 2096–2108 (June 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Precursor of a plasma wave due to nonlocal kinetic phenomena

The propagation of a finite amplitude quasi-monochromatic wave packet in a plasma gives rise to a a particular incoherent precursor of the wave due to nonlinear kinetic effects if the velocity of resonant electrons is greater than the wave group velocity.     

Near-critical fluid boiling: Overheating and wetting films

The heating of coexisting gas and liquid phases of pure fluid through its critical point makes the fluid extremely compressible, expandable, slows the diffusive transport, and decreases the contact angle to zero (perfect wetting by the liquid phase). We have performed experiments on near-critical fluids in a variable volume cell in the weightlessness of an orbiting space vehicle, to suppress buoyancy-driven flows and gravitational constraints on the liquid-gas interface. The high compressibility, high thermal expansion, and low thermal diffusivity lead to a pronounced adiabatic heating called the piston effect. We have directly visualized the near-critical fluid’s boundary layer response to a volume quench when the external temperature is held constant. We have found that when the system’s temperature T is increased at a constant rate past the critical temperature T _c , the interior of the fluid gains a higher temperature than the hot wall (overheating). This extends previous results in temperature quenching experiments in a similarly prepared system when the gas is clearly isolated from the wall. Large elliptical wetting film distortions are also seen during these ramps. By ray tracing through the elliptically shaped wetting film, we find very thick wetting film on the walls. This wetting film is at least one order of magnitude thicker than films that form in the Earth’s gravity. The thick wetting film isolates the gas bubble from the wall allowing gas overheating to occur due to the difference in the piston effect response between gas and liquid. Remarkably, this overheating continues and actually increases when the fluid is ramped into the single-phase supercritical phase.