Peter Rasmussen

Optimised cantilever biosensor with piezoresistive read-out

We present a cantilever-based biochemical sensor with piezoresistive read-out which has been optimised for measuring surface stress. The resistors and the electrical wiring on the chip are encapsulated in low-pressure chemical vapor deposition (LPCVD) silicon nitride, so that the chip is well suited for operation in liquids. The wiring is titanium silicide which-in contrast to conventional metal wiring-is compatible with the high-temperature LPCVD coating process.

Microfabricated photoplastic cantilever with integrated photoplastic/carbon based piezoresistive strain sensor

We present an SU-8 micrometer sized cantilever strain sensor with an integrated piezoresistor made of a conductive composite of SU-8 polymer and carbon black particles. The composite has been developed using ultrasonic mixing. Cleanroom processing of the polymer composite has been investigated and it has been shown that it is possible to pattern the composite by standard UV photolithography. The composite material has been integrated into an SU-8 microcantilever and the polymer composite has been demonstrated to be piezoresistive with gauge factors around 15–20. Since SU-8 is much softer than silicon and the gauge factor of the composite material is relatively high, this polymer based strain sensor is more sensitive than a similar silicon based cantilever sensor.

On the Temperature Dependence of the UNIQUAC/UNIFAC Models

Abstract Local composition models for the description of the properties of liquid mixtures do not in general give an accurate representation of excess Gibbs energy and excess enthalpy simultaneously. The introduction of temperature dependent interaction parameters leads to considerable improvements of the simultaneous correlation. The temperature dependent parameters have, however, little physical meaning and very odd results are frequently obtained when the interaction parameters obtained from excess enthalpy information alone are used for the prediction of vapor-liquid equilibria. The UNIQUAC/UNIFAC models are modified in this work by the introduction of a general temperature dependence of the coordination number. The modified UNIQUAC/UNIFAC models are especially suited for the representation of mixtures containing non-associating components. The modified models contain the same number of interaction parameters as the original ones, namely two per binary pair of molecules/groups. The resulting simultaneous representation of excess Gibbs energy and excess enthalpy by means of one unique set of parameters is remarkably successful. One may with very good results predict excess Gibbs energy information from UNIQUAC parameters based on excess enthalpy data, and the prediction of excess enthalpy information from only one isothermal set of vapor-liquid equilibrium data is qualitatively acceptable. A parameter table for the modified UNIFAC model is given for the five main groups: CH 2 , C  C, ACH, ACCH 2 and CH 2 O.

Cantilever surface stress sensors with single-crystalline silicon piezoresistors

We present a cantilever with piezoresistive readout optimized for measuring the static deflection due to isotropic surface stress on the surface of the cantilever [Sens. Actuators B 79(2–3), 115 (2001)]. To our knowledge nobody has addressed the difference in physical regimes, and its influence on cantilever sensors with integrated piezoresistive readout, that one finds between typical atomic force microscopy measurements and the surface stress sensors used in, e.g., biochemical measurements. We have simulated the response from piezoresistive cantilevers as a function of resistor type and placement for the two different regimes, i.e., surface stress measurements and force measurements. The model thus provides the means to specifically design piezoresistive cantilevers for surface stress measurements.

Polymeric mechanical sensors with integrated readout in a microfluidic system

Micrometer sized cantilevers can be used as highly sensitive bio/chemical sensors and are usually fabricated in silicon. Here, we demonstrate the integration of an array of polymeric cantilever sensors into a microfluidic system designed for bio/chemical detection. The cantilever-sensors as well as the microfluidic system are fabricated in the polymer SU-8. Gold strain gauge resistors are incorporated in the cantilevers in order to make them sensitive to changes in surface stress. The cantilever device is fabricated as two separate parts, which are subsequently bonded together using SU-8 as glue. The sensitivity and noise response of the polymeric cantilevers are measured. The measured noise level indicates that the device is suitable for molecular recognition measurements.

Analysis of Infinite Dilution Activity Coefficients of Solutes in Hydrocarbons from UNIFAC

Molecular structural effects on infinite dilution activity coefficients of solutes in n-alkanes and other hydrocarbons are studied within the UNIFAC model. Characteristic chain-length dependencies and other structural relationships imbedded in the model are discussed with emphasis to the consequences this has for model development. The cases treated have subtle but major implications for the correlation of activity coefficients and derivatives since these imply that combinatorial terms may not be small and they can be essential to the success of correlations based on UNIFAC. We have examined a number of infinite dilution properties and find that current expressions do not adequately describe these and other cases. The analysis is described and comparisons of the expressions with data for important systems are presented. New models are not presented, but improvements with either modified group definitions or revised relationships are discussed. The importance of such adjustments, for adding new terms to the existing equations, is stressed.

SOLID-LIQUID EQUILIBRIA FOR THE BINARY MIXTURES 1,4-XYLENE+ETHYLBENZENE AND 1,4-XYLENE+TOLUENE

Solid-liquid equilibrium (SLE) data for the binary mixtures 1,4–xylene + ethylbenzene and 1,4-xylene + toluene were measured using differential scanning calorimetry (DSC) in the temperature range from 133.15 to 293.15 K.