Theo S. J. Lammerink

Micro mixer with fast diffusion

A concept for micromixing of liquid is introduced, and its feasibility is demonstrated. The mixer allows fast mixing of small amounts of two liquids and is applicable to microliquid handling systems. The mixer has a channel for the liquid, an inlet port for the reagent, a 2.2-mm*2-mm*330- mu m mixing area, and 400 micronozzles (15 mu m*15 mu m) through with a reagent is injected into the sample liquid. The resulting microplumes greatly increase the contact surface between the two liquids and hasten the speed of the mixing by diffusion. The fabrication process is extremely simple. The mixing is complete within a few seconds; a homogeneous state of mixing is reached in 1.2 s when the total volume injected is 0.5 mu l and the injection flow rate is 0.75 mu l/s.<<ETX>>

Characterization method for a new diffusion mixer applicable in micro flow injection analysis systems

A new mixer is designed for mixing a phenolic solution into water. The mixer design is such that it can be easily adjusted for the controlled mixing of a specific compound within a certain time. This paper describes the working principle of the mixer as well as a suitable characterization method for the mixer. Measurement results are presented which show the correct working of the mixer. A quantitative measure is introduced to express the extent of mixing performed by the mixer. The characterization method allows the measurement of the flow rate, pressure drop and extent of mixing.

The μ-flown: a novel device for measuring acoustic flows

An acoustic wave consists of two elements, the acoustic pressure and the acoustic flow. Up to now one has to measure the pressure and calculate the flow to determine the acoustic flow, so it would be convenient to have a sensor that is able to measure acoustic flows. At the University of Twente a novel device has been developed which fulfils this need. In this paper a short introduction to the governing principles of this dynamic flow sensor, the fabrication process, the electronics and some of its interesting applications will be presented. This micromachined device measuring acoustic flows is called the microflown or μ-flown.

A micromachined pressure/flow-sensor

The micromechanical equivalent of a differential pressure flow-sensor, well known in macro mechanics, is discussed. Two separate pressure sensors are used for the device, enabling to measure both, pressure as well as volume flow-rate. An integrated sensor with capacitive read-out as well as a hybrid, piezo-resistive variant is made. The fabrication processes are described, using silicon and glass processing techniques. Based on the sensor layout, equations are derived to describe the sensor behavior both statically as well as dynamically. With the derived equations, the working range of the sensor and the thermal and time stability is estimated. The computed results of the stationary behavior are verified with the measured data. A good similarity in linearity of the pressure/flow relation is found. The computed hydraulic resistance, however, differs from the measured value for water with 21%. This difference can be explained by the high sensitivity of the resistance to the resistor channel cross-section parameter in combination with the difference between the rounded etched shape and the rectangular approximation. From fluid dynamics simulations, a working range bandwidth of about 1 kHz is expected. Thermal influences on the sensor signal due to viscosity changes are in the order of 2% flow signal variation per Kelvin. From these results, it can be concluded that the sensor can be used as a low cost, low power consuming flow and pressure-sensing device, for clean fluids without particles and without the tendency to coat the channel walls. If a high accuracy is wanted, an accurate temperature sensing or controlling system is needed.

Advancements in Technology and Design of Biomimetic Flow-Sensor Arrays

This paper reports on recent developments to increase the performance of biomimetic flow-sensor arrays by means of several technological advancements in the fabrication procedures and corresponding sensor design optimizations. Advancements include fabrication procedures with higher process latitude and geometrical modifications of several parts of the flow sensor. The conclusive measurements in this paper support our sensor-model predictions for a 100-fold increase in acoustic sensitivity (down to oscillating flow amplitudes in the order of 1 mm·s-1) translating to substantially higher capacitive outputs in comparison to our first-generation biomimetic flow-sensor arrays.

Modeling, design and testing of the electrostatic shuffle motor

The shuffle motor is a linear electrostatic stepper motor employing a mechanical transformation to obtain large forces and small steps. A model has been made to calculate the step size and the driving voltage as a function of the load force and the motor geometry. The motor consists of three polysilicon layers and has been fabricated using surface micromachining. Tests show an effective step size of about 85 nm and a produced force of 43 μN at 40 V driving voltage.

Modeling, design, fabrication and characterization of a micro Coriolis mass flow sensor

This paper discusses the modeling, design and realization of micromachined Coriolis mass flow sensors. A lumped element model is used to analyze and predict the sensor performance. The model is used to design a sensor for a flow range of 0–1.2 g h−1 with a maximum pressure drop of 1 bar. The sensor was realized using semi-circular channels just beneath the surface of a silicon wafer. The channels have thin silicon nitride walls to minimize the channel mass with respect to the mass of the moving fluid. Special comb-shaped electrodes are integrated on the channels for capacitive readout of the extremely small Coriolis displacements. The comb-shaped electrode design eliminates the need for multiple metal layers and sacrificial layer etching methods. Furthermore, it prevents squeezed film damping due to a thin layer of air between the capacitor electrodes. As a result, the sensor operates at atmospheric pressure with a quality factor in the order of 40 and does not require vacuum packaging like other micro Coriolis flow sensors. Measurement results using water, ethanol, white gas and argon are presented, showing that the sensor measures true mass flow. The measurement error is currently in the order of 1% of the full scale of 1.2 g h−1.

Intelligent gas-mixture flow sensor

A simple way to realize a gas-mixture flow sensor is presented. The sensor is capable of measuring two parameters from a gas flow. Both the flow rate and the helium content of a helium-nitrogen gas mixture are measured. The sensor exploits two measurement principles in combination with (local) information handling in an artificial neural network. An analysis of the measurement principles is given and the IC-compatible realization process is described. The sensor is simple to integrate with other micro gas-handling components such as valves, pressure sensors, etc. The sensing elements are combined with a small electronic circuit in which the artificial neural network is implemented. The experimental results are very promising.

Selective Wafer Bonding by Surface Roughness Control

Selective wafer bonding is presented as a technique for fabrication of microelectromechanical systems (MEMS) devices with movable, contacting elements, e.g., micromachined valves. The selectivity of the wafer bonding is obtained by tailoring the wafer surface microroughness. The adhesion parameter is used as the design rule for the wafer bonding technique. The technique is demonstrated with bulk micromachined check valves and a pressure actuated normally closed valve, but can be used for fabricating MEMS devices using surface micromachining processes as well. For these valves the selective fusion bonding technique turned out to be a convenient way to bond different wafer layers and a promising fabrication step with a high, reliable product yield.

Biomimetic micromechanical adaptive flow-sensor arrays

We report current developments in biomimetic flow-sensors based on flow sensitive mechano-sensors of crickets. Crickets have one form of acoustic sensing evolved in the form of mechanoreceptive sensory hairs. These filiform hairs are highly perceptive to low-frequency sound with energy sensitivities close to thermal threshold. In this work we describe hair-sensors fabricated by a combination of sacrificial poly-silicon technology, to form silicon-nitride suspended membranes, and SU8 polymer processing for fabrication of hairs with diameters of about 50 &mgr;m and up to 1 mm length. The membranes have thin chromium electrodes on top forming variable capacitors with the substrate that allow for capacitive read-out. Previously these sensors have been shown to exhibit acoustic sensitivity. Like for the crickets, the MEMS hair-sensors are positioned on elongated structures, resembling the cercus of crickets. In this work we present optical measurements on acoustically and electrostatically excited hair-sensors. We present adaptive control of flow-sensitivity and resonance frequency by electrostatic spring stiffness softening. Experimental data and simple analytical models derived from transduction theory are shown to exhibit good correspondence, both confirming theory and the applicability of the presented approach towards adaptation.