Wolfgang Hilber

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.

A Magnetic Membrane Actuator in Composite Technology Utilizing Diamagnetic Levitation

We present the application of diamagnetic levitation of a small floater magnet for the realization of a magnetically actuated membrane actuator in polymer composite technology. Following this approach the neutral position of the actuating membrane can be adjusted by diamagnetically stabilized levitation and the actuation of the membrane is induced by an integrated coil which is driven by an ac-signal. This setup holds the advantage that the neutral position of the flexible membrane can be stabilized with no external energy input, and the separation of the off-position from the driving mechanism allows for many possible applications of the device in, for instance, microfluidic systems.

A Viscosity and Density Sensor Based on Diamagnetically Stabilized Levitation

We investigate the feasibility of viscosity and density measurements using diamagnetically stabilized levitation of a floater magnet on pyrolytic graphite. This principle avoids any clamping structures in the measurement chamber and is, therefore, not suffering under unknown mounting conditions and is furthermore easy to integrate into microfluidic systems. The only part that has to be in contact with the liquid is the floater magnet. Immersing it in a liquid, buoyancy forces will come into play. Keeping the levitation height of the floater magnet constant in different liquid surroundings by accordingly adjusting the lifter magnet, the buoyancy force and, therefore, the density of the fluid can be determined from these adjustments. For more accurate results, a magnetic field modeling was used to determine the levitation height of the floater magnet out of the superposed magnetic fields of both magnets. For viscosity measurements, we add an additional ac-driven coil to the setup, which yields a superposed alternating force on the floater magnet causing periodic vibrations of the floater magnet. The vibrations are damped according to the viscosity of the surrounding fluid. By performing a frequency sweep, the frequency response of the damped spring mass resonator can be obtained where the resonance frequency for our setup is around 6 Hz. Furthermore, the influence of the levitation height on the resonance characteristics was examined by studying the resonance frequency and quality factor for different lifter magnet positions.

Viscosity and density sensor principle based on diamagnetic levitation using pyrolytic graphite

We investigate the feasibility of viscosity and density measurements using diamagnetically stabilized levitation of a floater magnet on pyrolytic graphite. Immersing the floater magnet in a liquid, buoyancy forces will come into play. Keeping the levitation height of the floater magnet constant in different liquid surroundings by accordingly adjusting the lifter magnet, the buoyancy force and therefore the density of the fluid can be determined from these adjustments. For viscosity measurements, we add an additional AC-driven coil to the setup, which yields a superposed alternating force on the floater magnet causing periodic vibrations of the floater magnet. The vibrations are damped according to the viscosity of the surrounding fluid. By performing a frequency sweep, the frequency response of the damped spring mass resonator can be obtained where the resonance frequency for our setup is around 6 Hz.

Hot‐electron power loss in a doped GaAs/AlGaAs superlattice at intermediate temperature studied by infrared differential spectroscopy

The power loss of electrons in a strongly coupled n‐type GaAs/AlxGa1−xAs superlattice is studied by analyzing the temperature and electric‐field dependence of the interminiband absorption. Electrons are heated by an electric‐field pulse and the resulting change of the infrared absorption spectrum is monitored by a step‐scan Fourier transform spectrometer operated in a time‐resolved, gated mode. The measured power loss is higher than predicted by a simple three‐dimensional calculation including acoustic (deformation potential and piezoelectric) and polar‐optical phonon emission. Possible explanations for this, such as relaxation via folded acoustic phonons or coupled plasmon–phonon modes, are discussed. The energy relaxation time, which can be extracted from the power balance, decreases from 300 ps at 15 K to 20 ps at 48 K.

Temporal change in the electromechanical properties of dielectric elastomer minimum energy structures

Dielectric elastomer minimum energy structures (DEMES) are soft electronic transducers and energy harvesters with potential for consumer goods. The temporal change in their electromechanical properties is of major importance for engineering tasks. Therefore, we study acrylic DEMES by impedance spectroscopy and by optical methods for a total time period of approx. 4.5 months. We apply either compliant electrodes from carbon black particles only or fluid electrodes from a mixture of carbon black particles and silicone oil. From the measurement data, the equivalent series capacitances and resistances as well as the bending angles of the transducers are obtained. We find that the equivalent series capacitances change in average between −12 %/1000 h and −4.0 %/1000 h, while the bending angles decrease linearly with slopes ranging from −15 %/1000 h to −7 %/1000 h. Transducers with high initial bending angles and electrodes from carbon black particles show the smallest changes of the electromechanical characteri...

Dynamic capacitive extensometry setup for in-situ monitoring of dielectric elastomer actuators

Dielectric elastomer actuators (DEAs) and their subcategory energy minimum structures (DEMES) are promising candidates for biomimetic robotic elements. Especially interesting for their characterization is the transient strain or stretch-response of the deforming elastomer part to high voltage driving signals, and the state of the applied compliant electrodes. However, this information can hardly be obtained by optical measurement techniques only. Especially DEMES with their complex three dimensional shapes are difficult to study. To this end, we present a setup of dynamic capacitive extensometry which is able to monitor DEAs and DEMES in situ; both the stretch dependent capacitance of the actuated elastomer part and the stretch dependent resistance of the compliant electrodes are recorded in situ during dynamic high voltage actuation. With the help of this measurement data the transient stretch response is studied with regard to the frequency, magnitude and shape of the actuation signal. Furthermore, the stretch-dependent resistance of the compliant electrodes and their percolation limit are investigated. The developed measurement technique can be applied also for actuator control with feedback loops.

A magnetic membrane actuator utilizing diamagnetic levitation

We present the application of diamagnetic levitation of a small neodymium (NdFeB) floater magnet for the realization of a scalable magnetically actuated membrane actuator in polymer technology. Following this approach the neutral position of the actuating membrane can be adjusted and stabilized by diamagnetic levitation. The actuation of the membrane which encloses the small floater magnet is induced by an integrated coil, driven by an AC-signal. The proposed setup holds the advantage that the neutral position is stabilized with no external energy input, and the separation of the off-position from the driving mechanism allows for versatile possible applications of the device in, for instance, microfluidic systems.

Fast thermo-pneumatic actuation of a thin PDMS membrane using a micro Peltier-element for microfluidic applications

ZusammenfassungIn diesem Beitrag wird ein Konzept für eine schnelle thermopneumatische Aktuation einer dünnen, flexiblen PDMS-Membran vorgestellt. Dabei wird die Temperatur einer Arbeitsflüssigkeit in einem kolbenähnlichen Flüssigkeitsreservoir mittels eines Mikro-Peltier-Elements gesteuert. Durch Variation der Temperatur der Arbeitsflüssigkeit, in diesem Fall Ethanol, wird über die volumetrische thermische Ausdehnung direkt eine Auslenkung der Membran erzielt. Das Abschnüren und anschließende Öffnen eines mikrofluidischen Kanals oberhalb der Membran benötigt nur wenige Sekunden. Das vorgestellte Setup kann für die Realisierung von schnellen und robusten mikrofluidischen Ventilen und Pumpen benutzt werden.SummaryIn this paper we present a concept for fast thermo-pneumatic actuation of a thin flexible PDMS membrane using a micro Peltier-element for controlling the temperature of the working fluid in a piston-like liquid reservoir. By adjusting the temperature of the working fluid, in our case ethanol, the volumetric thermal expansion is controlled, which is in turn directly coupled to membrane deflection. Pinch-off and subsequent release of a microfluidic channel above the membrane is achieved in a few seconds. The proposed setup can be potentially utilized for the realization of fast and robust microfluidic valves and pumps.