In vivo measurement of skin heat capacity: advantages of the scanning calorimetric sensor

Measurement of the heat capacity of human tissues is mainly performed by differential scanning calorimetry. In vivo measurement of this property is an underexplored field. There are few instruments capable of measuring skin heat capacity in vivo. In this work, we present a sensor developed to determine the heat capacity of a 4 cm² skin area. The sensor consists of a thermopile equipped with a programmable thermostat. The principle of operation consists of a linear variation of the temperature of the sensor thermostat, while the device is applied to the skin. To relate the heat capacity of the skin with the signals provided by the sensor, a two-body RC model is considered. The heat capacity of skin varies between 4.1 and 6.6 JK^?1 for a 2?×?2 cm² area. This magnitude is different in each zone and depends on several factors. The most determining factor is the water content of the tissue. This sensor can be a versatile and useful tool in the field of physiology.

     

Optimization of calorimetric systems: Continuous control of baseline stability by monitoring thermostat temperatures

Despite the employment of modern twin calorimeters, the evaluation of baseline stability remained one of the outstanding methodological problems in biological microcalorimeters operated near the limit of detection. Baseline instabilities are mainly caused by thermal disturbances from environmental and thermostat fluctuations. Therefore continuously monitoring the temperatures in the thermostat and heat sink provides a good indication of the reliability of calorimetric measurements. In addition, numerical methods are available to correct the heat flow curves from temperature disturbances. Such temperature correction procedures can improve the accuracy of various types of microcalorimeters.     

Integrated circuit thermopile as a new type of temperature modulated calorimeter

Thermopiles, integrated into a thin silicon membrane, are used in some calorimetric applications (e.g. Setaram SETline120 portable thermal analyzer) that utilize temperature ramps of the thermostat to obtain the caloric response. A new calorimetric method is proposed, which uses an integrated circuit thermopile (ICT) in an oscillating mode. A heater integrated into the membrane drives temperature oscillations and the thermopile senses the temperature gradient across the membrane. ac calorimetry and the 3-omega method do not measure total heat losses and have an inherent quasi-adiabatic low frequency limit. On the other hand, the thermopile setup can measure total heat losses that permits one to monitor enthalpy changes in a sample, e.g. during crystallization. Low frequency measurements are limited only by sensor sensitivity. Preliminary results show that the same experimental setup can be used to make dynamic heat capacity measurements over a frequency range from 1 mHz to 100 Hz. At high frequencies (1 Hz and higher), heat capacity of nanogram samples can be measured.     

New observations regarding deterministic, time-reversible thermostats and Gauss's principle of least constraint

Deterministic thermostats are frequently employed in nonequilibrium molecular dynamics simulations in order to remove the heat produced irreversibly over the course of such simulations. The simplest thermostat is the Gaussian thermostat, which satisfies Gauss's principle of least constraint and fixes the peculiar kinetic energy. There are of course infinitely many ways to thermostat systems, e.g., by fixing sigma(i)/p(i)/mu+l. In the present paper we provide, for the first time, convincing arguments as to why the conventional Gaussian isokinetic thermostat (mu = 1) is unique in this class. We show that this thermostat minimizes the phase space compression and is the only thermostat for which the conjugate pairing rule holds. Moreover, it is shown that for finite sized systems in the absence of an applied dissipative field, all other thermostats (mu not = 1) perform work on the system in the same manner as a dissipative field while simultaneously removing the dissipative heat so generated. All other thermostats (mu not = 1) are thus autodissipative. Among all mu, thermostats, only the mu = 1 Gaussian thermostat permits an equilibrium state.     

From molecular dynamics to hydrodynamics: a novel Galilean invariant thermostat

This paper proposes a novel thermostat applicable to any particle-based dynamic simulation. Each pair of particles is thermostated either (with probability P) with a pairwise Lowe-Andersen thermostat [C. P. Lowe, Europhys. Lett. 47, 145 (1999)] or (with probability 1-P) with a thermostat that is introduced here, which is based on a pairwise interaction similar to the Nosé-Hoover thermostat. When the pairwise Nosé-Hoover thermostat dominates (low P), the liquid has a high diffusion coefficient and low viscosity, but when the Lowe-Andersen thermostat dominates, the diffusion coefficient is low and viscosity is high. This novel Nosé-Hoover-Lowe-Andersen thermostat is Galilean invariant and preserves both total linear and angular momentum of the system, due to the fact that the thermostatic forces between each pair of the particles are pairwise additive and central. We show by simulation that this thermostat also preserves hydrodynamics. For the (noninteracting) ideal gas at P = 0, the diffusion coefficient diverges and viscosity is zero, while for P > 0 it has a finite value. By adjusting probability P, the Schmidt number can be varied by orders of magnitude. The temperature deviation from the required value is at least an order of magnitude smaller than in dissipative particle dynamics (DPD), while the equilibrium properties of the system are very well reproduced. The thermostat is easy to implement and offers a computational efficiency better than (DPD), with better temperature control and greater flexibility in terms of adjusting the diffusion coefficient and viscosity of the simulated system. Applications of this thermostat include all standard molecular dynamic simulations of dense liquids and solids with any type of force field, as well as hydrodynamic simulation of multiphase systems with largely different bulk viscosities, including surface viscosity, and of dilute gases and plasmas.     

Configurational temperature control for atomic and molecular systems

A new configurational temperature thermostat suitable for molecules with holonomic constraints is derived. This thermostat has a simple set of motion equations, can generate the canonical ensemble in both position and momentum space, acts homogeneously through the spatial coordinates, and does not intrinsically violate the constraints. Our new configurational thermostat is closely related to the kinetic temperature Nosé-Hoover thermostat with feedback coupled to the position variables via a term proportional to the net molecular force. We validate the thermostat by comparing equilibrium static and dynamic quantities for a fluid of n-decane molecules under configurational and kinetic temperature control. Practical aspects concerning the implementation of the new thermostat in a MOLECULAR DYNAMICS code and the potential applications are discussed.     

Multibaric-multithermal ensemble molecular dynamics simulations

We present new generalized-ensemble molecular dynamics simulation algorithms, which we refer to as the multibaric-multithermal molecular dynamics. We describe three algorithms based on (1) the Nosé thermostat and the Andersen barostat, (2) the Nosé-Poincaré thermostat and the Andersen barostat, and (3) the Gaussian thermostat and the Andersen barostat. The multibaric-multithermal simulations perform random walks widely both in the potential-energy space and in the volume space. Therefore, one can calculate isobaric-isothermal ensemble averages in wide ranges of temperature and pressure from only one simulation run. We test the effectiveness of the multibaric-multithermal algorithm by applying it to a Lennard-Jones 12-6 potential system.     

On statistics of the ensemble of oscillators under excitation. II. Parametric excitation of linear quantum oscillators

The Wigner function of the parametrically excited linear oscillator in a thermostat (heat bath) is found. It is shown that quantum effects promote a parametric excitation of oscillations. Increase of the thermostat temperature T also promotes the excitation if the relaxation time of the oscillator does not depend on T. If this time is increasing while T is decreasing, the present model shows that the rate of chemical reactions induced by laser radiation may also increase, in spite of the decreasing thermal excitation. The case of simultaneous parametric and ordinary (force-induced) resonance is also considered.     

煤焦分形维数及其对比热容的影响研究

比热容是煤炭热物理性质之一,在煤矿的矿井防火、防止煤与瓦斯突出、井下降温设计及煤炭加工利用(如煤炭的燃烧、气化、焦化、液化等)等方面是关键参数之一,对提高煤炭热能利用率、提高经济效益、减少环境污染等,具有非常重要的意义。比热容的影响因素很多,如煤化程度、水分质量分数、热解温度等,煤焦微观结构的影响也是其中很重要的一方面。分形几何由Mandelbrot 1982年创立,是定量描述自相似或自相关等不规则形体的工具。研究表明,煤焦微观结构具有分形特征。在煤焦分形的研究中,常用的实验技术方法为吸附法、小角度X射线散射法和孔度法,采用扫描电子显微镜和数字图象处理方法研究煤焦的分形结构,能更加深入地理解其分形维数与性能的关系。     

Heat capacity of alcohols in aqueous solutions in the temperature range from 5 to 125°C

Using a precise technique of scanning microcalorimetry the heat capacity differences between water and dilute aqueous solutions of ethanol, n-propanol, n-butanol and n-pentanol were measured from 5 to 125°C and the partial molar heat capacities of these substances in water were determined. It was found that the heat capacity increment for alcohol disolved in water is proportional to the number of the-CH ₂ ^– groups and decrease with a temperature increase. The heat capacity increment of hydration of non-polar groups is shown to be positive and large at room temperature and decreases in magnitude as the temperature increases. In contrast, the heat capacity increment of hydration of polar groups is negative at room tempreature and increases as the temperature increases. From the temperature dependence of the heat capacity increment one can assume that the water molecules solvated by the non-polar groups of the alcohols behave in a non-cooperative manner.     

Temperature control algorithms in dual control volume grand canonical molecular dynamics simulations of hydrogen diffusion in palladium

The effectiveness of five temperature control algorithms for dual control volume grand canonical molecular dynamics is investigated in the study of hydrogen atom diffusion in a palladium bulk. The five algorithms, namely, Gaussian, generalized Gaussian moment thermostat (GGMT), velocity scaling, Nosé-Hoover (NH), and its enhanced version Nosé-Hoover chain (NHC) are examined in both equilibrium and nonequilibrium simulation studies. Our numerical results show that Gaussian yields the most inaccurate solutions for the hydrogen-palladium system due to the high friction coefficient generated from the large velocity fluctuation of hydrogen, while NHC, NH, and GGMT produce the most accurate temperature and density profiles in both equilibrium and nonequilibrium cases with their feedback control mechanisms. However, this feedback control also overestimates the self-diffusion coefficients in equilibrium systems and the diffusion coefficient in nonequilibrium systems. Velocity scaling thermostat produces slight inhomogeneities in the temperature and density profiles, but due to the dissipated heat accumulated in the control volumes it still yields accurate self-diffusion coefficients that are in good agreement with the experimental data at a wide range of temperatures while others tend to deviate.     

Micro-Effects on Continuous-Injection Heat Conduction Calorimetry

A localized-constant model involving two capacities reliably describes two injection calorimeters: a mass-variation calorimeter and a constant-volume calorimeter (TAM 2277 by Thermometric). The model distinguishes the place and the types of dissipation, and its parameters depend on the rates and on the heat capacities of the liquids. In the case of the TAM 2277 calorimeter, the dependence between the detected heat of mixing and the injection rate is revealed. The proposed model permits the inclusion of perturbations on the baseline originating from the temperature variation of the thermostat. This revised version was published online in July 2006 with corrections to the Cover Date.     

Implementations of Nosé-Hoover and Nosé-Poincaré thermostats in mesoscopic dynamic simulations with the united-residue model of a polypeptide chain

Molecular dynamics (MD) simulations generate a canonical ensemble only when integration of the equations of motion is coupled to a thermostat. Three extended phase space thermostats, one version of Nose-Hoover and two versions of Nose-Poincare, are compared with each other and with the Berendsen thermostat and Langevin stochastic dynamics. Implementation of extended phase space thermostats was first tested on a model Lennard-Jones fluid system; subsequently, they were implemented with our physics-based protein united-residue (UNRES) force field MD. The thermostats were also implemented and tested for the multiple-time-step reversible reference system propagator (RESPA). The velocity and temperature distributions were analyzed to confirm that the proper canonical distribution is generated by each simulation. The value of the artificial mass constant, Q, of the thermostat has a large influence on the distribution of the temperatures sampled during UNRES simulations (the velocity distributions were affected only slightly). The numerical stabilities of all three algorithms were compared with each other and with that of microcanonical MD. Both Nose-Poincare thermostats, which are symplectic, were not very stable for both the Lennard-Jones fluid and UNRES MD simulations started from nonequilibrated structures which implies major changes of the potential energy throughout a trajectory. Even though the Nose-Hoover thermostat does not have a canonical symplectic structure, it is the most stable algorithm for UNRES MD simulations. For UNRES with RESPA, the "extended system inside-reference system propagator algorithm" of the RESPA implementation of the Nose-Hoover thermostat was the only stable algorithm, and enabled us to increase the integration time step.     

Heat capacity of superfine oxides of iron under applied magnetic fields

Heat capacity is one of the most characteristic and important properties when the peculiarities of magnetic nanosystems are studied. In these systems the magnetic ordering becomes obvious due to the thermal effects such as heat capacity anomalies. It was considered earlier that heat capacity change under magnetic fields applied is slight and it cannot be taken into account in thermodynamic calculations. However the experimental heat capacity data for ferrofluids under magnetic fields applied show that field and temperature heat capacity dependences have a complicated behavior and in magnetic fields an essential heat capacity change takes place. On the other hand in the literature the contradictory data about heat capacity of nanoparticles appear. According to some papers nanoparticles heat capacity can exceed heat capacity of a bulk material a few times.     

Influence of frictional heating on temperature gradients in ultra-high-pressure liquid chromatography on 2.1mm I.D. columns

The effects of viscous heat dissipation on some important HPLC parameters, such as efficiency (N) and retention factors (k), using 2.1mm columns at pressures up to 1000 bar have been investigated from both a theoretical and experimental point of view. Two distinct experimental set-ups and their respective influences on non-homogenous temperature gradients within the column are described and discussed. In the first instance, a still-air column heater was used. This set-up leads to approximate 'adiabatic' conditions, and a longitudinal temperature gradient is predicted across the length of the column. The magnitude of this gradient is calculated, and its occurrence confirmed with experimental measurements also indicating that no appreciable loss in efficiency occurs. Secondly, when a water bath is used to thermostat the column, a radial temperature gradient is prevalent. The extent of this gradient is estimated, and the loss in efficiency associated with this gradient is predicted and demonstrated experimentally. It is also observed that approximate adiabatic conditions can lead to floating retention factors. The implications of temperature gradients for routine HPLC analysis at ultra-high pressure are discussed.     

A New Theoretical Approach to Steady State of Temperature Modulated Heat Flux DSC

The steady state of temperature modulated heat flux DSC, in which the sample temperature is controlled at a fixed frequency, a fixed amplitude and a constant underlying heating rate, is theoretically investigated for complex heat capacity of the sample, taking accounts of heat capacities of heat paths, heat loss to the environment and mutual heat exchange between the sample and the reference material. Rigorous and general solutions for the temperature difference oscillation are obtained in relation to the sample temperature as a reference oscillation. The results are quite different from those obtained in functions of the heat source temperature as a reference oscillation. From these solutions, application of the technique to heat capacity measurements is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Complex heat capacity measurements by TMDSC Part 1. Influence of non-linear thermal response

To treat data from temperature modulated differential scanning calorimetry (TMDSC) in terms of complex or reversing heat capacity one should know heat transfer and apparatus influences on experimental results. On the other hand one should pay attention that the response is linear because this is a prerequisite for data evaluation. The reason for non-linear thermal response is discussed and its influence on complex heat capacity determination is shown. The criterion for linear response is proposed. This allows to choose correct experimental conditions for any complex heat capacity measurements. In the case when these conditions cannot be fulfilled because of experimental restrictions one can estimate the influence of non-linear response on measured value of complex or reversing heat capacity.     

Estimation of molar heat capacities in solution from gas chromatographic data

The temperature dependence of retention data (retention or capacity factors) is measured for 35 aliphatic ketones and aldehydes as model compounds on a dimethylpolysiloxane stationary phase. A novel model is derived to determine the heat of solution and the solution molar heat capacities from the fits of the log natural of the difference of the retention factor and the column temperature (T) versus 1/T and the temperature arrangement. The convex curvature present in the residual plots of a former defined equation of ours disappears when applying a newly defined model. A detailed statistical analysis clearly shows the superiority of the refined model to the earlier one in a broader temperature range. The validation of this model is made through a comparison of heat capacity values taken from literature determined by different methods. The molar heat capacity of a pure liquid oxo compound is similar to that of when the same compound is solvated in a stationary phase.