Deconstructing classical water models at interfaces and in bulk

Using concepts from perturbation and local molecular field theories of liquids we divide the potential of the SPC/E water model into short and long ranged parts. The short ranged parts define a minimal reference network model that captures very well the structure of the local hydrogen bond network in bulk water while ignoring effects of the remaining long ranged interactions. This deconstruction can provide insight into the different roles that the local hydrogen bond network, dispersion forces, and long ranged dipolar interactions play in determining a variety of properties of SPC/E and related classical models of water. Here we focus on the anomalous behavior of the internal pressure and the temperature dependence of the density of bulk water. We further utilize these short ranged models along with local molecular field theory to quantify the influence of these interactions on the structure of hydrophobic interfaces and the crossover from small to large scale hydration behavior. The implications of our findings for theories of hydrophobicity and possible refinements of classical water models are also discussed

The SBRC-190 a cryogenic multiplexer for moderate-background FIR astronomy

The SBRC 190, a cryogenic multiplexer developed for far-infrared (FIR) photoconductor detectors operating at moderate backgrounds, is described. The circuit is based on the 32-channel CRC 696 CMOS device used on SIRTF. For applications such as encountered on SOFIA or Herschel, the new device permits higher backgrounds, a wider range of backgrounds, faster sampling, and enhanced synchronization of sampling with chopping. A relationship between sampling efficiency and noise requirements needed to achieve background-limited instrument (BLIP) performance is derived. Major design differences relative to the CRC 696 which have been incorporated in the SBRC 190 are: (a) an AC coupled, capacitive feedback transimpedance unit cell, to minimize input offset effects, thereby enabling low detector biases, (b) selectable feedback capacitors to enable operation over a wide range of backgrounds, and (c) clamp and sample & hold output circuits to improve sampling efficiency, which is a concern at the relatively high readout rates required. The paper emphasizes requirements for use on SOFIA, and touches on the design, expected performance, and fabrication of the new multiplexer