Christian J. Kähler

Particle imaging techniques for volumetric three-component (3D3C) velocity measurements in microfluidics

The reliable measurement of the 3D3C velocity field in microfluidic devices becomes more and more important for future optimization and developments for lab-on-a-chip applications or point-of-care medical diagnosis systems. In the past years, different particle-based imaging methods, such as confocal scanning microscopy, holography, stereoscopic and tomographic imaging or approaches based on defocused particle images or optical aberrations have been developed and applied successfully to measure velocity fields in microfluidic systems. The benefits and drawbacks of these methods will be discussed in detail as the proper understanding of the measurement principle is essential to select the most appropriate technique for a desired measurement application. Once an imaging method is chosen, the velocity can be estimated by correlation-based methods or tracking approaches. The advantages and disadvantages of both methods and the importance of image preprocessing will also be discussed in detail.Graphical abstract

A simple single camera 3C3D velocity measurement technique without errors due to depth of correlation and spatial averaging for microfluidics

A method to determine the three components (3C) of the velocity field in a micro volume (3D) using a single camera is proposed. The technique is based on tracking the motion of individual particles to exclude errors due to depth of correlation (DOC) and spatial averaging as in µPIV (micro particle image velocimetry). The depth position of the particles is coded by optical distortions initiated by a cylindrical lens in the optical setup. To estimate the particle positions, a processing algorithm was developed based on continuous wavelet analysis and autocorrelation. This algorithm works robustly and gives accurate results comparable to multi-camera systems (tomographic PIV, V3V). Particle tracking was applied to determine the full 3C velocity vector in the volume without the error due to spatial averaging and DOC, which are inherent limitations in µPIV due to the interrogation windows size and volume illumination. To prove the applicability, measurements were performed in a straight channel with a cross section of 500 × 500 µm2. The depth of the measurement volume in the viewing direction was chosen to be 90 µm in order to resolve the near-wall gradients. The three-dimensional velocity distribution of the whole channel could be resolved clearly by using wave front deformation particle tracking velocimetry.

Ultrasound-induced acoustophoretic motion of microparticles in three dimensions

We derive analytical expressions for the three-dimensional (3D) acoustophoretic motion of spherical microparticles in rectangular microchannels. The motion is generated by the acoustic radiation force and the acoustic streaming-induced drag force. In contrast to the classical theory of Rayleigh streaming in shallow, infinite, parallel-plate channels, our theory does include the effect of the microchannel side walls. The resulting predictions agree well with numerics and experimental measurements of the acoustophoretic motion of polystyrene spheres with nominal diameters of 0.537 and 5.33 μm. The 3D particle motion was recorded using astigmatism particle tracking velocimetry under controlled thermal and acoustic conditions in a long, straight, rectangular microchannel actuated in one of its transverse standing ultrasound-wave resonance modes with one or two half-wavelengths. The acoustic energy density is calibrated in situ based on measurements of the radiation dominated motion of large 5-μm-diameter particles, allowing for quantitative comparison between theoretical predictions and measurements of the streaming-induced motion of small 0.5-μm-diameter particles.

On the calibration of astigmatism particle tracking velocimetry for microflows

Astigmatism particle tracking velocimetry (APTV) is a method to determine three components (3C) of the velocity field in a volume (3D) using a single camera. The depth position of the particles is coded by optical distortions caused by a cylindrical lens in the optical setup. This technique is particularly suited for microfluidic applications as measurement errors due to spatial averaging and depth of correlation, typically encountered with ?PIV approaches, are eliminated so that the measurement precision is enhanced. Unfortunately, the current state of the technique is limited by the small measurement region achievable with the current calibration procedures as well as by higher order image aberrations (Cierpka et al 2010 Meas. Sci. Technol. 21 045401). In order to extend the size of the measurement volume and to account for all image aberrations, a new intrinsic calibration procedure, based on the imaging function of the particles, is proposed in the paper at hand. It provides an extended measurement depth, taking into account all image aberrations. In this work, the calibration procedure was applied to a ?PIV arrangement but could also be implemented on macroscopic experimental setups. The calibration procedure is qualified with synthetic data as well as Poiseuille flow in a straight rectangular micro-channel with a cross-sectional area of 200 ? ?500??m2. The three-dimensional velocity distribution of the whole channel was resolved via APTV with uncertainties of 0.9% and 3.7% of the centerline velocity, uc, for the in-plane and out-of-plane components, respectively. Further investigations using different cylindrical-lens focal lengths, magnifications and particle sizes provide information about achievable measurement depths and help to design and adapt the optimal system for the desired experiment.

Recent applications of particle image velocimetry in aerodynamic research

Abstract Particle image velocimetry (PIV) is increasingly used to investigate unsteady velocity fields instantaneously. For the first time the PIV technique allows the recording of a complete velocity field in a plane of the flow within a few microseconds. The PIV technique thereby provides information about unsteady flow fields which is difficult to obtain with other experimental techniques. The short acquisition times and fast availability of data reduce the operational time, and hence cost, in large scale wind tunnels and test facilities. At DLR a variety of PIV systems for use in industrial wind tunnels has been developed in the past decade. The flexibility of these portable systems is illustrated by presenting several results of recent PIV applications. More recently the original photographic means of PIV image recording has been partially replaced by high resolution electronic imaging which can provide PIV data nearly on-line. Images recorded by either system use the same multiple-pass, cross-correlation analysis software, whose algorithms are briefly described. Several examples of actual applications are given: the flow issuing from a jet nozzle was imaged by a specially developed high-speed video camera at close proximity. A high resolution dual-frame digital camera was applied in the study of helicopter rotor aerodynamics and wake vortex measurements of an airplane model. Further, large image sequences exceeding 100 PIV recordings provided detailed information on the structure of a turbulent boundary layer.

Microfluidic reactor for continuous cultivation of Saccharomyces cerevisiae

A diffusion‐based microreactor system operated with a reaction volume of 8 μL is presented and characterized to intensify the process understanding in microscale cultivations. Its potential as screening tool for biological processes is evaluated. The advantage of the designed microbioreactor is the use for the continuous cultivation mode by integrating online measurement technique for dissolved oxygen (DO) and optical density (OD). A further advantage is the broaden application for biological systems. The bioreactor geometry was chosen to achieve homogeneous flow during continuous process operation. The device consisted of a microstructured top layer made of poly(dimethylsiloxane) (PDMS), which was designed and fabricated using UV‐depth and soft lithography assembled with a glass bottom. CFD simulation data used for geometry design were verified via microparticle‐image‐velocimetry (μPIV). In the used microreactor geometry no concentration gradients occurred along the entire reaction volume because of rapid diffusive mixing, the homogeneous medium flow inside the growth chamber of the microreactor could be realized. Undesirable bubble formation before and during operation was reduced by using degassed medium as well as moistened and moderate incident air flow above the gas permeable PDMS membrane. Because of this a passive oxygen supply of the culture medium in the device is ensured by diffusion through the PDMS membrane. The oxygen supply itself was monitored online via integrated DO sensors based on a fluorescent dye complex. An adequate overall volumetric oxygen transfer coefficient KLa as well as mechanical stability of the device were accomplished for a membrane thickness of 300 μm. Experimental investigations considering measurements of OD (online) and several metabolite concentrations (offline) in a modified Verduyn medium. The used model organism Saccharomyces cerevisiae DSM 2155 tended to strong reactor wall growth resembling a biofilm.

In situ analysis of three-dimensional electrolyte convection evolving during the electrodeposition of copper in magnetic gradient fields.

A novel three-dimensional particle tracking velocimetry technique was used to examine the flow during electrodeposition of Cu. For the first time electrode-normal, circumferential, and radial velocities were spatially resolved during deposition in superimposed low and high magnetic gradient fields. In this way the complex interaction of magnetic field gradient force and Lorentz force induced convective effects could be measured and analyzed. Magnetic field gradient force induced electrolyte flow was detected only in high gradient magnetic fields, and it was found to be directed toward regions of gradient maxima. Since this electrode-normal flow causes enhanced transport of Cu(2+) ions from the bulk electrolyte to those regions of the working electrode where maxima of magnetic gradients are present, a structured deposit is formed during diffusion-limited electrodeposition. Lorentz force driven convection was observed during deposition in the low and the high magnetic gradient experiments. The overall fluid motion and the convection near the working electrode were determined experimentally and discussed with regard to the acting magnetic forces and numerical simulations.

Leading-Edge Separation Control by Means of Pulsed Vortex Generator Jets

DOI: 10.2514/1.26176 An airfoil was designed to stall with an abrupt turbulent leading-edge separation and equipped with a pneumatic system for active flow control. The system pulses compressed air through 45 deg skewed slots. Depending on the position of the slots, two very different behaviors were found. With the actuators positioned in the region of the separation line, the control system could not prevent separation. However, a very advantageous influence on the separation resulted with even higher normal forces than in prestall condition. The duty cycle was found to be the major control parameter for this kind of control. When the actuation was positioned well in front of the separation line the influence of the actuation system on the stall behavior changed completely. Here the system was able to prevent the leading-edge separation. The influence of the duty cycle becomes much weaker and maximum normal force scaled mainly with the mean ejected momentum, varied either by the duty cycle or the supply pressure.

Parallax correction for precise near-wall flow investigations using particle imaging

For the basic understanding of turbulence generation in wall-bounded flows, precise measurements of the mean velocity profile and the mean velocity fluctuations very close to the wall are essential. Therefore, three techniques are established for high-resolution velocity profile measurements close to solid surfaces: (1) the nanoprobe sensor developed at Princeton University, which is a miniaturization of a classical hot-wire probe [Exp. Fluids 51, 1521 (2011)]; (2) the laser Doppler velocimetry (LDV) profile sensor, which allows measurement of the location of the particles inside the probe volume using a superposition of two fringe systems [Exp. Fluids 40, 473 (2006)]; and (3) the combination of particle image velocimetry and tracking techniques (PIV/PTV), which identify the location and velocity of submicrometer particles within the flow with digital imaging techniques [Exp. Fluids 52, 1641 (2006)]. The last technique is usually considered less accurate and precise than the other two. However, in addition to the measurement precision, the effect of the probe size, the position error, and errors due to vibrations of the model, test facility, or measurement equipment have to be considered. Taking these into account, the overall accuracy of the PTV technique can be superior, as all these effects can be compensated for. However, for very accurate PTV measurements close to walls, it is necessary to compensate the perspective error, which occurs for particles not located on the optical axis. In this paper, we outline a detailed analysis for this bias error and procedures for its compensation. To demonstrate the capability of the approach, we measured a turbulent boundary layer at Re(δ)=0.4×10(6) and applied the proposed methods.

Active Control of Leading-Edge Separation within the German Flow Control Network

The paper describes experiments with a 2D two-element high-lift airfoil with active flow control in a 2.8 m windtunnel. The flow control system is integrated into the nose region of the airfoil to delay the onset of leading-edge stall. It consists of pulsed vortex generator jets that utilize compressed air. The results indicate that active flow control can be applied at a practical high-lift system. The pulsed vortex generator jets effectively delay the onset of separation at the main element and increase the maximum angle of attack and the maximum lift coefficent, respectively. However, the margin that can be gained is very dependent on the overall stall behavior of the airfoil.