Bernhard Weiss

Miniaturized sensors for the viscosity and density of liquids-performance and issues

This paper reviews our recent work on vibrating sensors for the physical properties of fluids, particularly viscosity and density. Several device designs and the associated properties, specifically with respect to the sensed rheological domain and the onset of non-Newtonian behavior, are discussed.

Particle manipulation using 3D ac electro-osmotic micropumps

We present a novel mechanism of particle manipulation in alternating-current (ac)- driven electro-osmotic micropumps, utilizing the tunable ratio of viscous drag, inertial and dielectrophoretic forces. The latter are induced by three-dimensional (3D) stepped electrode arrays in the channel, which are driven by ac voltages enabling so-called 3D-ac-electro-osmotic pumping, as has been proposed recently. Due to size- and density-dependent differences in polarizability, targeted particles in the fluid stream can be slowed down or even pinned above the electrode structures solely by adjusting the operation parameters of the pump. Hence the presented device, fabricated in SU-8/glass technology, enables simultaneous pumping and manipulation of particles in suspension.

Dielectrophoretic particle dynamics in alternating-current electro-osmotic micropumps

This letter deals with the dynamics of dielectric microparticles in alternating-current electro-osmotic micropumps, which is based on the competitive interplay of inertial, dielectrophoretic, and viscous drag forces. The electric field and the electro-osmotic flow are modeled and computed by means of finite elements and particle trajectories are evaluated taking into account the forces above. Thereby, mean traveling height and velocity depend on driving voltage and frequency, which allows for an effective type of particle manipulation and separation. The obtained particle velocities show good agreement with measured velocities of hollow glass beads in a device realized in SU8/glass technology.

Buying Online: An Analysis of Shopbot Visitors

Abstract We conduct an empirical study on the search and purchasing behavior of buyers on an Austrian price comparison site. On such a market a consumer typically searches for the cheapest price of a given product. Reliability and service of the supplier, however, are other important characteristics of an offer. We find robust evidence of consumer behavior that can be described as a two-stage procedure: shoppers first select a group of candidate offers based on the price only; then, in the actual buying decision consumers tradeoff a lower price with higher reliability of the retailer.

Buying Online: Sequential Decision Making by Shopbot Visitors

In this article we propose a two stage procedure to model demand decisions by customers who are balancing several dimensions of a product. We then test our procedure by analyzing the behavior of buyers from an Austrian price comparison site. Although in such a market a consumer will typically search for the cheapest price for a given product, reliability and service of the supplier are other important characteristics of a retailer. In our data, consumers follow such a two stage procedure: they select a shortlist of suppliers by using the price variable only; finally, they trade off reliability and price among these shortlisted suppliers.

Density sensitive driving mode of a double membrane viscometer

This contribution shows the applicability of the double membrane sensor presented earlier for high sensitivity mass density sensing. The sensor is based on two opposed membranes vibrating in parallel in a sample liquid. Excitation and read-out of the membrane vibration are based on Lorentz forces induced in a static magnetic field. Each membrane carries three conductive paths for excitation which can be separately connected to the excitation currents. This possibility allows for switching between viscosity or mass density sensitive driving mode, which is analyzed in this contribution. Measurements with various test liquids show, that the frequency of the fundamental mode strongly decreases with increasing mass density. Comparing the results with measurements achieved with earlier designs indicates an increased sensitivity on density featuring a reasonably sustained quality factor for increasing viscosities.

Simultaneous pumping and density-selective particle focusing using AC-driven electro-osmotic micropumps

In this contribution we present a novel concept of density-selective particle focusing in microchannels, based on the tunable ratio of viscous drag, inertial and dielectrophoretic forces. The latter are induced by arrowhead-shaped, stepped electrode arrays in the channel, which, driven by AC voltages, enable so-called 3D-AC-electro-osmotic pumping (3D-ACEO). By adjusting the operation parameters of the pump, particle mixtures can be pumped and separated at the same time.

Droplet and particle manipulation in emulsions and suspensions using 3D electro-osmotic micropumps

This paper presents investigations on droplet and particle manipulation in microfiuidic channels realized in SU-8/glass technology, which is based on a competitive interplay of viscous drag, gravitational, buoyancy and dielectrophoretic forces. The latter are induced by alternating current (AC) driven three-dimensional (3D) stepped electrode arrays in the channel, enabling so called 3D AC electro-osmotic pumping, as it has been proposed recently. Due to size- and density-dependent differences in polarizability, targeted particles resp. droplets in the fluid stream can be slowed down or even pinned above the electrode structures by adjusting the operation parameters of the pump. Hence the presented device can be utilized for simultaneous pumping and manipulation of droplets and particles in emulsions and suspensions.

Determining Liquid Properties Using Mechanically Vibrating Sensors

Miniaturized sensors for physical liquid parameters can be utilized in applications where liquids in industrial processes are monitored in order to maintain the quality of a process or the associated product. Due to the adverse properties commonly associated with chemical interfaces(lacking reversibility, drift, etc.), sensing physical parameters as indicators for the state of the liquid, in particularly density, viscosity(or more general rheological properties), and infrared absorption is an attractive alternative to conventional chemical sensors. The miniaturization of suitable sensor principles on the one hand facilitates the implementation of these devices online. On the other hand, scaling effect have to be taken into account, which, e.g., in case of viscosity sensors, lead to issues when it comes to applications in complex liquids such as suspensions. In our paper we provide an overview on our recent work discussing the device design, the associated modeling, and the application of the devices

Messung physikalischer Flüssigkeitseigenschaften mit beidseitig eingespannten Balkenstrukturen (Measurement of Liquid Properties Using a Vibrating Clamped–Clamped Beam Structure)

In diesem Beitrag wird ein Sensorkonzept zur Messung von Viskosität und Dichte von Flüssigkeiten untersucht. Dabei wird das Resonanzverhalten eines beidseitig eingespannten Balkens ausgewertet. Die durch die Flüssigkeit zustande kommende Widerstandskraft pro Längenheit wird aus einer numerischen Berechnung des Strömungsfeldes abgeleitet. Messungen wurden an einem Sensorelement, basierend auf kupferbeschichteten Polyesterfolien, durchgeführt. Die Balkenschwingung wird elektromagnetisch angeregt. Als Messsignal dient die durch Bewegungsinduktion zustande kommende Spannung. Zusätzlich wurde das Frequenzverhalten mit einer einfachen optischen Methode verifiziert. In this contribution the concept of a viscosity and mass density sensor for the liquid phase based on a vibrating clamped-clamped beam structure is investigated. The liquid´s influence on the frequency response is modelled by calculating the resistance force acting on the structure by numerically solving the two-dimensional flow and pressure fields in the cross sectional plane. Besides presenting a micromachined prototype in silicon technology, measurements have been carried out with a PMMA-based sensor element featuring an electromagnetic actuation and an inductive readout. A simple optical method is used to verify the measured frequency response.