Development of a modified standard apparatus for heat transfer measurements in highly viscous binary fluids

A modified so-called standard apparatus for heat transfer measurements in highly viscous binary fluids has been developed. It covers a viscosity range from 0.2 mPa∙s up to 1000 Pa∙s and is designed for measurements over a temperature range from 273 to 423 K. Concept and setup of the apparatus are described in detail, and uncertainties of the different elements of the measurement chain are discussed. An estimation for the total uncertainty of the heat transfer coefficient is ∆α/α = 16% (k = 2). Model mixtures consisting of polydimethylsiloxane of three different viscosities and n-pentane were chosen. The composition of the fluid is calculated with the refractive index and the temperature; a corresponding correlation was developed. N-pentane evaporates on a heated test tube, which is exchangeable and thus enables the investigation of the influence of the surface structure and of the material on the heat transfer coefficient. The design of the test tubes has been adapted to very high temperatures on the tube surface. The effects of the mandatory remixing of the two components on the heat transfer coefficient have been investigated. The modified standard apparatus was validated with pure n-pentane and the data was compared to published data in literature.

Efficient molecular dynamics using geodesic integration and solvent–solute splitting

We present an approach to Langevin dynamics in the presence of holonomic constraints based on decomposition of the system into components representing geodesic flow, constrained impulse and constrained diffusion. We show that a particular ordering of the components results in an integrator that is an order of magnitude more accurate for configurational averages than existing alternatives. Moreover, by combining the geodesic integration method with a solvent–solute force splitting, we demonstrate that stepsizes of at least 8 fs can be used for solvated biomolecules with high sampling accuracy and without substantially altering diffusion rates, approximately increasing by a factor of two the efficiency of molecular dynamics sampling for such systems. The methods described in this article are easily implemented using the standard apparatus of modern simulation codes.