Peter Woias

Characterization of different beam shapes for piezoelectric energy harvesting

This paper deals with the analysis of different beam shapes for piezoelectric energy harvesters. The theory is based on the well-established Rayleigh–Ritz method for piezoelectric compound structures. It is validated by experiments with triangular-shaped and rectangular-shaped beams. It turns out that triangular-shaped beams are more effective than rectangular-shaped ones in terms of curvature homogeneity independent of the proof mass. This effect is opposed by the adverse mass distribution and the increased stiffness of triangular-shaped beams. Therefore, the overall efficiency is only weakly influenced by the beam shape. Nevertheless triangular-shaped beams drastically outperform rectangular ones in terms of tolerable excitation amplitude and maximum output power.

Robust design of gas and liquid micropumps

Due to the small stroke of micro actuators, the compression ratio of micropumps is frequently small. This means that serious constraints exist for most micropump designs concerning self-priming capability and bubble tolerance. To access this problem in this study, criteria for the minimum compression ratio of a micropump are derived depending on the medium transported. It turns out that under real operation and handling conditions (outgassing, incomplete priming) design rules applicable for gas pumps also have to be used for liquid pumps. Next, if the liquid pump is to be able to prime itself, a critical pressure has to be achieved, which leads to a further increase of the necessary compression ratio. To determine this critical pressure, the surface and interfacial energies of the liquid have to be taken into account.

Protein detection with a novel ISFET-based zeta potential analyzer

This publication presents a novel ISFET-based measurement concept for the determination of the zeta potential, which is known to be an efficient method for the detection of protein accumulations onto surfaces. The basic set-up consists of two monolithically integrated ISFET sensors arranged in a serial flow configuration together with a precoated fused silica capillary, which provides the reactive surface for the protein detection. In comparison with the state of the art, this novel biosensor system is characterized by a small size, an extremely low reagent consumption, a simple fluidic concept, a short analysis time, and a very effective noise suppression due to the differential ISFET set-up. In the following, an overview is given over the theoretical background of the measurement principle. In order to get deeper insight into the theoretical background of the measurement principle, a simulation model was developed which is based on the site-binding theory and takes into account the different proton dissociation equilibria of the surface groups as well as the influence of monovalent electrolyte ions. A quasi-Newton iteration after Broyden was used for the numerical solution of the formulated equation system. For an experimental confirmation of the simulation results, the calculated zeta potential vs. pH curves were compared with measured data for various modifications of the fused silica capillaries (in untreated state, after a hydrothermal activation, and after the deposition of several silanes) and it could be shown, that the chosen physical model represents a satifactory theoretical basis for the description of the occuring surface effects. Measurements before and after a covalent coupling of the model analyte lysozyme were performed in order to demonstrate the feasibility of an immunosensor based on the measurement of the streaming potential and showed a significant shift of the zeta potential vs. pH curves.

A quartz crystal biosensor for measurement in liquids

The detection of anti-human immunodeficiency virus (HIV) antibodies by means of synthetic HIV peptide immobilized on a piezoelectric quartz sensor is demonstrated. The measurement set-up consists of an oscillator circuit, a suitably modified AT-cut thickness-shear-mode quartz crystal with gold electrodes, which is housed in a special reaction vessel, and a computer-controlled frequency counter for the registration of the measured frequency values. The quartz crystal is adapted for a steady operation in liquids at a frequency of 20 MHz. In phosphate-buffered saline solution the oscillator reaches a stability of about 0.5 Hz within a few seconds, of about 2 Hz within 10 min and about 30 Hz within 1 h. The frequency shift due to the adsorption of various proteins to the uncoated sensor surface has been investigated. It can be shown that a stable adsorptive binding of proteins to an oscillating gold surface is feasible and can be used for the immobilization of a receptor layer (e.g. HIV peptide). Specific binding of the anti-HIV monoclonal antibody to the HIV peptide immobilized on the quartz sensor is demonstrated. Control experiments show, however, additional unspecific binding. According to the experiments, the Sauerbrey formula gives a sufficiently accurate value for the decrease of the resonant frequency due to adsorption or binding of macromolecular proteins on the quartz crystal surface.

Slow pH response effects of silicon nitride ISFET sensors

Abstract To examine the slow pH response of ion sensitive field effect transistors (ISFETs) with a silicon nitride gate-insulator, measurements were carried out in a thermostated measurement set-up by generating a stepwise increase and decrease of the pH in a range between pH 5 and 8. The slow response curves obtained were fitted to a multiexponential model to extract time constants and amplitudes of the various response effects. As a result, two exponential effects with different behaviour were found for the slow pH response. Moreover, a dependence of the response time from the direction of the pH-step was found. Based on these results, a buried layer beneath the gate-insulator surface with a heterogeneous site distribution is proposed as a phenomenological explanation for the slow response.

Characterization of Laminar Transient Flow Regimes and Mixing in T-shaped Micromixers

Convective micromixers create vortices in curved channel elements allowing characteristic mixing times below 1 millisecond for gaseous and liquid media. This contribution gives an overview about the flow regimes of symmetrical 1:1 mixing in T-shaped micromixers with rectangular cross-sections for Reynolds numbers (Re) from 0.01 to 1000 in the mixing channel. First, symmetrical vortices are formed for Re > 10 in the so-called Dean flow. At Re > 140, fluid from one side swaps to the opposite side and creates a double vortex, which enhances mixing. For Re > 240, the flow becomes unsteady, and a kind of wake flow establishes. From 240 < Re < 500, the wake flow is periodic with a Strouhal number of about 0.23. The mixing quality also shows a periodic behavior and reaches its maximum at this point. With further increasing the Re number, the flow becomes chaotic, and the two components are often flowing parallel in the mixing channel, which decreases the mixing quality. Besides detailed CFD simulations, the periodic flow is observed in experimental studies with stroboscopic imaging. The decreasing mixing quality is also reflected in a lower selectivity of parallel chemical test reactions for Re > 500. With the knowledge of the flow regimes in microchannels, design criteria can be formulated for efficient mixing devices.

A self-priming and bubble-tolerant piezoelectric silicon micropump for liquids and gases

In this paper a novel silicon micropump for liquids and gases is presented, which is tolerant towards gas-bubbles and which is able to prime itself. The micropump is based on a piezoelectrically driven diaphragm actuator, which is combined with a valve unit consisting of two cantilever valves. The self-priming and bubble-tolerant operation mode was achieved by maximizing the compression ratio, which was realized by minimizing the dead volume of the valve unit as well as of the actuator unit and by maximizing the stroke volume of the pump diaphragm. The optimization of the actuator is based on simulations and experimental investigations of the pump diaphragm displacement. These studies yield the optimal dimensions of the pump diaphragm and the piezoactuator. The piezoelectrically actuated micropump was characterized by investigating the pump rate in dependence of the actuation frequency and the pressure on the inlet and the outlet port of the micropump. As essential results a maximum pumprate of 1 mY/min and a maximum backpressure of about I bar were measured for water. For gases the pumprate ranges up to 3 ml/min.

Microchannels for applications in liquid dosing and flow-rate measurement

Abstract To investigate the performance of microengineered fluid channels in liquid dosing applications, flow-rate measurements have been performed with various channel geometries in a range from 0.01 to 1000 μl min -1 . An optical flow-measurement technique has been developed to enhance the measurement range into the desired low flow range (10 -3 to 1 μl min -1 ), and is compared to a standard gravimetric method, which is preferably used for flow rates above 1 μl min -1 . In addition, influences of the temperture-dependent viscosity and effects arising from fluidmechanical characteristics are studied. These influences are also calculated from laminar flow theory and semi-emprical models to obtain a theoretical model. It is found that the theoretical model is able to describe the measurement results well in the whole flow range. The model is implemented on a PC-based system, which measures the pressure drop across the microchannel and the fluid temperature and calculates the flow. In a temperature range from 20 to 50°C excellent agreement is found.

Bidirectional frequency tuning of a piezoelectric energy converter based on a cantilever beam

A piezoelectric energy converter is presented, whose resonance frequency can be tuned by applying mechanical stress to its structure. The converter consists of a piezo-polymer cantilever beam with two additional thin arms, which are used to apply an axial preload to the tip of the beam. The compressive or tensile prestress applied through the arms leads to a shift of the beams resonance frequency. Experiments with this structure indicate a high potential: the resonance frequency of a harvester to which a compressive preload was applied could be altered from 380 Hz to 292 Hz. In another experiment, a harvester with stiffened arms was tuned from 440 Hz to 460 Hz by applying a tensile preload. In combination with automatic control of the applied force, this type of structure could be used to enhance the performance of energy harvesters in vibrating environments with occasional shifts of the vibrational frequency.

Modelling the short-time response of ISFET sensors

In this publication, a model for the short-time response of ISFET sensors is presented. In contrast to the static site-binding theory, this approach takes into account the kinetics of the electrochemical reactions occurring on the gate-insulator surface. As a result, a system of coupled non-linear differential equations is formulated, which is able to describe the pH-step response of an ISFET with an Si 3 N 4 gate insulator. In the small-signal range (ΔpH < 0.1), an increase of the response time with pH is found ; time constants can be calculated between 1 ms (pH 2.6) and 600 ms (pH 8). Outside the small-signal regime, the behaviour of the sensor is non-linear and depends on the direction and the starting value of the pH step. For a confirmation of these results, measurements of the pH-step response have been performed in a specially designed ISFET/flow-injection set-up, which are in a good qualitative agreement with the theoretical model presented here.