Effect of vertical temperature variation on the oscillatory wetting instability in a fluid Ga-Pb alloy

Oscillatory wetting instabilities driven by capillary-gravitation forces have been explored very recently in the binary fluid Ga-Pb alloy [A. Turchanin, R. Tsekov and W. Freyland, J. Chem. Phys., 2004, 120, 11 171]. This system is characterized by a complete wetting transition at liquid-liquid coexistence. Due to its metallic nature the bulk and interfacial instabilities are strongly coupled via variation of the respective emissivities. In our previous work we have investigated these phenomena at different cooling cycles and at constant temperature inside the miscibility gap. In this study we present for the first time the observations of the oscillatory wetting instabilities also in heating cycles. The interfacial properties of a Ga0.95Pb0.05 alloy at conditions inside the miscibility gap have been investigated by following the second harmonic generation (SHG) intensity changes. Corresponding model calculations of the Pb-rich wetting film instabilities have been performed taking into account the effect of a temperature variation vertical to the bulk sample. The influence of this temperature variation on the occurrence of the oscillations is discussed.     

Temperature (over)compensation in an oscillatory surface reaction

Biological rhythms are regulated by homeostatic mechanisms that assure that physiological clocks function reliably independent of temperature changes in the environment. Temperature compensation, the independence of the oscillatory period on temperature, is known to play a central role in many biological rhythms, but it is rather rare in chemical oscillators. We study the influence of temperature on the oscillatory dynamics during the catalytic oxidation of formic acid on a polycrystalline platinum electrode. The experiments are performed at five temperatures from 5 to 25 °C, and the oscillations are studied under galvanostatic control. Under oscillatory conditions, only non-Arrhenius behavior is observed. Overcompensation with temperature coefficient (q(10), defined as the ratio between the rate constants at temperature T + 10 °C and at T) < 1 is found in most cases, except that temperature compensation with q(10) ≈ 1 predominates at high applied currents. The behavior of the period and the amplitude result from a complex interplay between temperature and applied current or, equivalently, the distance from thermodynamic equilibrium. High, positive apparent activation energies were obtained under voltammetric, nonoscillatory conditions, which implies that the non-Arrhenius behavior observed under oscillatory conditions results from the interplay among reaction steps rather than from a weak temperature dependence of the individual steps.     

Effect of buoyancy on appearance and characteristics of surface tension repeated auto-oscillations

The effect of buoyancy on spontaneous repeated nonlinear oscillations of surface tension, which appear at the free liquid interface by dissolution of a surfactant droplet under the interface, is considered on the basis of direct numerical simulation of the model system behavior. The oscillations are the result of periodically rising and fading Marangoni instability. The buoyancy force per se cannot lead to the oscillatory behavior in the considered system, but it influences strongly both the onset and decay of the instability and therefore, affects appearance and characteristics of the oscillations. If the surfactant solution density is smaller than the density of the pure liquid, then the buoyancy force leads to a considerable decrease of the induction period and the period of oscillations. The buoyancy force affects also the dependence of the oscillation characteristics on the system dimensions. The results of the simulations are compared with the available experimental data.     

界面不稳定现象与相间迁移

包含有流动界面的多相体系中的溶质相间迁移在一定条件下可导致表现为界面变形、界面流和多种形式界面运动的不稳定现象．该现象的直接驱动力是界面局部区域出现的张力梯度，表面活性物质在该现象中起重要作用[1，2]．溶质相间迁移通常可考虑为三步：（1）溶质在原所在相内部向     

界面不稳定现象与相间迁移

The interfacial instability due to the transfer of the 1-butanol from water to oil in two systems of 1-butanol oil/water has been studied. The dynamic characteristics of the phenomenon are investigated by analysing the motion of an oil lens deposited at the surface of the aqueous solution of 1-butanol. The interfacial adsorption kinetics based on diffusion-controlled transfer is analysed. It is found that the linear relation between the frequence of the oscillatory motion of the lens and t1/2 is a characteristic relation. The type of the solute transfer in the interfacial instability is discussed by comparing experimental results with the linear characteristics.     

Oscillatory surface tension due to finite-size effects

The simulation results of surface tension at the liquid-vapor interface are presented for fluids interacting with Lennard Jones and square-well potentials. From the simulation of liquids we have reported [M. González-Melchor et al., J. Chem. Phys. 122, 4503 (2005)] that the components of pressure tensor in parallelepiped boxes are not the same when periodic boundary conditions and small transversal areas are used. This fact creates an artificial oscillatory stress anisotropy in the system with even negative values. By doing direct simulations of interfaces we show in this work that surface tension has also an oscillatory decay at small surface areas; this behavior is opposite to the monotonic decay reported previously for the Lennard Jones fluid. It is shown that for small surface areas, the surface tension of the square-well potential artificially takes negative values and even increases with temperature. The calculated surface tension using a direct simulation of interfaces might have two contributions: one from finite-size effects of interfacial areas due to box geometry and another from the interface. Thus, it is difficult to evaluate the true surface tension of an interface when small surface areas are used. Care has to be taken to use the direct simulation method of interfaces to evaluate the predicted surface tension as a function of interfacial area from capillary-wave theory. The oscillations of surface tension decay faster at temperatures close to the critical point. It is also discussed that a surface area does not show any important effect on coexisting densities, making this method reliable to calculate bulk coexisting properties using small systems.     

Experimental observation of oscillatory effects on the relaxation of a sheared liquid drop

This work deals with the experimental observation of the shape oscillations followed by a viscous liquid drop immersed in another viscous fluid matrix when retracting from a deformed state into the spherical shape under the action of interfacial forces. The droplet is firstly deformed into an ellipsoidal shape by a shear flow and later allowed to recover the equilibrium shape after cessation of the flow. It is observed that such an oscillatory process occurs for a wide range of viscosity ratios and it may be described by a dampened oscillation. Viscous components dominate the drop retraction, just allowing few oscillations. The dampening factor, frequency and amplitude of the oscillations are affected by the drop viscosity. Frequencies and amplitudes are also influenced by the initial drop deformation.     

Dynamic complexity in the electrochemical oxidation of thiourea

We explored the temperature-dependent dynamics of the electrochemical oxidation of thiourea under potential-control mode and found complex oscillations with one large peak and one small peak per period. Adjusting the temperature caused the relative amplitudes and positions of the two peaks to vary. Experiments showed that there were two distinct oscillatory regimes as a function of the external current, and at some temperatures, three-frequency quasiperiodic oscillations occurred for current densities between the two oscillatory windows. We examined the species present and the component reactions by employing cyclic voltammetry and electrolysis-HPLC-MS in the two oscillatory regimes. Adsorption and desorption of multiple species, including water and chloride, contribute to the rich dynamical phenomena that were observed.     

Marangoni instability and spontaneous non-linear oscillations produced at liquid interfaces by surfactant transfer

The systems producing non-linear spontaneous oscillations of the interfacial tension and electric potential are considered and the available criteria for development of convective instability by the surfactant transfer through a liquid interface are discussed. The non-linear oscillations are observed by the surfactant transfer from a point-like source situated in the bulk of liquid, by the transfer of two ionic solutes through a liquid interface in two opposite directions, and by the transfer of ionic solutes through a liquid membrane. All these systems are governed by more complicated mechanisms than merely arising oscillatory convective instability. The main experimental results obtained for these three systems as well as theoretical models proposed for their explanation are discussed.     

Temperature-induced route to chaos in the H2O2-HSO3(-)-S2O3(2-) flow reaction system

Low-frequency, high-amplitude pH-oscillations observed experimentally in the H2O2-HSO3(-)-S2O3(-) flow reaction system at 21.0 degrees C undergo period-doubling cascades to chemical chaos upon decreasing the temperature to 19.0 degrees C in small steps. Period-4 oscillations are observed at 20.0 degrees C and can be calculated on the basis of a simple model. A reverse transition from chaos to high-frequency limit cycle oscillations is also observable in the reaction system upon decreasing further the temperature step by step to 15.0 degrees C. Period-2 oscillations are measured at 18.0 degrees C. Such a temperature-change-induced transition between periodic and chaotic oscillatory states can be understood by taking into account the different effects of temperature on the rates of composite reactions in the oscillatory system. Small differences in the activation energies of the composite reactions are responsible for the observed transitions. Temperature-change-induced period doubling is suggested as a simple tool for determining whether an experimentally observed random behavior in chemical systems is of deterministic origin or due to experimental noise.     

Multiple types of spatio-temporal oscillations induced by differential diffusion in the Landolt reaction

The acid autoactivated iodate-sulfite redox reaction (Landolt reaction) exhibits bistability but no oscillatory dynamics when operated in a continuous stirred tank reactor (CSTR). However, it has been previously found experimentally that this reaction can exhibit both spatial bistability and oscillations when carried out in a one side diffusely fed spatial reactor. The precise origin of the oscillatory instability remained mainly elusive. We unambiguously show, in numerical simulations based of a kinetic model recently proposed by Csek?et al., J. Phys. Chem., 2008, 112, 5954), that the observed oscillations are due to the faster diffusion of the proton relative to the other feed species (long range activation instability). Furthermore, our calculations account for the previous experimental observation of two different oscillatory modes. The first one is associated to localized front oscillations, as already reported in another reaction. The other one is a periodic switch between the two states of the spatial bistability and affects the system as a whole. This oscillatory mode was undocumented in the previous studies of long range activation instabilities. More complex dynamical behaviors that mix these two types of oscillations are also reported.     

A Kinetic Study on Catechol‐Based Belousov–Zhabotinsky Reaction

The present investigation pertains to the kinetic study on bromate‐driven and manganese ion catalyzed Belousov–Zhabotinsky oscillatory chemical reaction having catechol (1,2‐dihydroxybenzene) as the organic substrate in aqueous acid media at 30 ± 0.1^oC under stirred conditions. Although some polyphenols as chemical oscillators are known in the literature but the role of catechol has been marginalized. The reaction system with catechol shows long‐time series of periodic as well as aperiodic oscillations, which were monitored potentiometrically in the oxidation reduction potential (ORP) mode under batch conditions. The various oscillatory parameters such as induction time (t_in), time period (t_p), amplitude (A), and number of oscillations (N) were obtained by drawing a plot between redox potential (mV) versus time (s). It is found that the oscillatory parameters of the system vary with the concentration of the reacting species. The reaction system was also studied in different aqueous acid media, but the detailed studies with respect to experimental parameters have been made in aqueous sulfuric acid medium as it showed a broad oscillatory window and better oscillatory characteristics. Moreover, the oscillatory parameters in general and induction time in particular vary significantly with temperature (15 to 45 ± 0.1^oC). The activation parameters of the system have also been calculated. The oscillatory behavior of the reaction system was also confirmed spectrophotometrically. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 141–151, 2013     

Oscillatory behavior in the electrochemical oxidation of formic acid on Pt(100): rotation and temperature effects

We investigated the oscillatory behavior in the kinetics of formic acid electrooxidation on Pt(100) in 1 mM HClO₄ solution. We studied the effect of different experimental parameters on the oscillatory behavior, viz. defined HCOOH mass-transport to the electrode surface by using the rotating disk electrode technique, the temperature of the supporting electrolyte, and the nature of anions. We suggest that the interdependence of the reaction steps during HCOOH oxidation, the adsorption of anions and the competition for adsorption sites among the reaction partners and intermediates lead to complex non-linear kinetics. It was evident that once the individual reactions in the dual path mechanism reach steady state the oscillations vanish. These conditions can be reached either by enhanced formic acid reaction rates induced by electrode rotation or by increased temperature. Under specific conditions of anion and formic acid concentration, relaxational oscillations can be transformed into mixed-mode oscillations.     

Kinetic studies on resorcinol-bromate-manganese(II) ion oscillator with and without additives

The present work pertains to the effect of temperature on the oscillatory behavior of bromate driven, Manganese (II) ion catalyzed BZ reaction with aromatic substrate i.e. resorcinol under batch conditions in 1.3 M sulfuric acid as aqueous acid medium. In order to study the effect of methyl ketones as additives (co-substrates), acetone is added to the aforesaid reaction system and the oscillations of the mixed substrate systems were studied at different temperatures. Further the effect of temperature with respect to ternary systems comprising of acetone with other ketones like butanone, pentanone, hexanone and acetyl acetone in 1:1 (v/v) ratio is also studied. It is found that temperature has a marked influence on the reactivity of the reaction systems, with and without methyl ketones and at all concentrations of the additives. Moreover, the rate of enolization of ketones also affects the oscillatory parameters like t _in and t _p. Although the exact values of enolization constants for the methyl ketones are not known to us, but by studying the oscillatory behavior, a trend can be predicted. Further the formation of end products in the ternary systems is influenced due to hydrophobic interactions of the ketones.     

Molecular dynamics study of carbon nanotube oscillators revisited

We performed molecular dynamics simulation of double walled carbon nanotube (DWCNT) oscillators under constant energy and constant temperatures with various commensurations and nanotube lengths. We clarify and resolve questions and differences raised by previous simulation results of similar systems. At constant energy, sustained oscillation is available for a wide range of initial temperatures. But low initial temperature is advantageous for DWCNTs to sustain oscillation under constant energy. We observed sustained oscillation at constant energy for both commensurate and incommensurate DWCNTs. On the other hand, under constant temperatures, both high and low temperatures are disadvantageous to sustain DWCNT oscillations. At constant low temperature, neither commensurate nor incommensurate DWCNTs can maintain oscillation. At appropriate constant temperatures, the oscillatory behavior of incommensurate nanotubes is much more sustained than that of commensurate tubes. The oscillatory frequency of DWCNTs depends significantly on the length of tubes. The initial oscillatory frequency is inversely proportional to the DWCNT lengths. The oscillation frequency of DWCNTs is insensitive to the initial temperatures at constant energy, but slightly dependent on the temperature at constant temperatures.     

Stochastic bi-resonance in non-isothermal carbon monoxide catalytic oxidation system

The Pt(110)/CO O2 system subject to reaction heat, heat conduction and radiative heat transfer is non-isothermal and its temperature varies in time and space. In this paper, taking support temperature (ST) as the control parameter, the effect of the ST fluctuations in the oscillatory dynamics of the non-isothermal Pt(110)/CO O2 system is numerically studied. It is found that the ST fluctuations may induce stochastic oscillations and the oscillations exhibit stochastic bi-resonance (SBR) with the change of the strength or correlation time of the fluctuations. This result shows that the temperature fluctuations may enhance the chemical reaction oscillations. Moreover, the system can selectively and repeatedly employ the temperature fluctuations to enhance its reaction oscillations. It is also shown when the distance of the ST temperature to the oscillatory region increases a little, the effect of the temperature fluctuations would obviously weaken.     

Thermal Marangoni Convection of a Fluid Film Coating a Deformable Membrane

The thermal Marangoni instability of a fluid film coating a deformable membrane has been investigated by taking into account the deformation of the fluid free surface. Numerical calculations for different thermal boundary conditions are presented. The prestressed membrane is supposed to be very thin and therefore its behavior is similar to that of an isothermal fluid free surface with a surface tension but with a different mechanical boundary condition; that is, the fluid should stick on its surface and thus the fluid velocity is zero. An important assumption is that the membrane has no temperature dependence and therefore that only one Marangoni number exists for the free surface of the fluid. Numerical results are presented for stationary and oscillatory thermocapillary instability in both the sinuous and the varicose modes. It is shown that membrane deformation has important implications on the Marangoni instability of the fluid layer for positive and negative Marangoni numbers. Copyright 2001 Academic Press.     

Nonstationary Phenomena in the Low-Temperature Gas-Phase Catalytic Oxidation of H2S with Oxygen

The conditions of the existence were determined and reasons were revealed for the appearance of periodic oscillations of the SO₂concentration in the reaction products and warming temperature in the catalyst bed in the oxidation of H₂S with oxygen at temperatures below the dew point of sulfur on the V–Al–Ti oxide catalyst. The formation of polysulfides during the reaction was experimentally found. It was proposed that the adsorption of sulfur and polysulfides on the catalyst surface is responsible for the observed oscillatory processes.     

反应NH~4ClO~4+Mg+K~2Cr~2O~7的非线性化学动力学2: 固相 振荡燃烧的化学模 型

建立了NH~4ClO~4+Mg+K~2Cr~2O~7固相振荡燃烧体系的非吸热三变量立方自催化化学模型,应用非线性数学分析方法,研究了固相振荡燃烧的非线性化学动力学机理,并对此进行了数值模拟,结果反映了这一振荡燃烧体系所具有的非线性化学动力学特性。     

New Experimental and Mechanistic Investigation on the KSCN‐H2O2‐NaOH‐Cu(II)‐Catalyzed Oscillating System (Orbàn–Epstein Reaction): Inhibitory Effects by Diphenols

The KSCN‐H₂O₂‐NaOH‐Cu(II)‐catalyzed system is one of the few reactions in which chemical oscillations can be observed in batch conditions. In the present paper, this oscillating reaction was revisited in a wide range of initial concentrations of all components in batch. A mixture with a long lasting oscillation time (1 h 34 min) and a great number of oscillations (24) was found and used to investigate the effect of temperature. An Arrhenius‐type temperature dependence was observed from which an apparent “average activation energy” E_av = 76 ± 5 kJ for the overall oscillatory reaction was observed. A mechanistic study based on a modified model analyzed by the stoichiometric network analysis approach gave a satisfactory agreement between calculated and experimental oscillating behaviors and temperature dependence. The addition of the three diphenols (catechol, resorcinol, and hydroquinone) causes perturbations similar to those observed in the Briggs‐Rauscher oscillating system, i.e., an inhibition of the oscillatory regime. These inhibitory effects were described in detail, and a reasonable qualitative interpretation is given.