Christian Cierpka

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.

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.

Flow field around a vibrating cantilever: coherent structure eduction by continuous wavelet transform and proper orthogonal decomposition

The velocity field around a vibrating cantilever plate was experimentally investigated using phase-locked particle image velocimetry. Experiments were performed at Reynolds numbers of Re h = 101, 126 and 146 based on the tip amplitude and the speed of the cantilever. The averaged vector fields indicate a pseudo-jet flow, which is dominated by vortical structures. These vortical structures are identified and characterized using the continuous wavelet transform. Three-dimensional flow features are also clearly revealed by this technique. Furthermore, proper orthogonal decomposition was used to investigate regions of vortex production and breakdown. The results show clearly that the investigation of phase-averaged data hides several key flow features. Careful data post-processing is therefore necessary to investigate the flow around the vibrating cantilever and similar highly transient periodic flows.

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.

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.

Investigation of scaling laws in a turbulent boundary layer flow with adverse pressure gradient using PIV

We present an experimental investigation and data analysis of a turbulent boundary layer flow at a significant adverse pressure gradient at Reynolds number up to Reθ = 10, 000. We combine large-scale particle image velocimetry (PIV) with microscopic PIV for measuring the near wall region including the viscous sublayer. We investigate scaling laws for the mean velocity and for the total shear stress in the inner part of the boundary layer. In the inner part the mean velocity can be fitted by a log-law. In the outer part of the inner layer the log-law ceases to be valid. Instead, a modified log-law provides a good fit, which is given in terms of the pressure gradient parameter and a parameter for the mean inertial effects. Finally we describe and assess a simple quantitative model for the total shear stress distribution which is local in wall-normal direction without streamwise history effects.

Clarifying the mechanism of reverse structuring during electrodeposition in magnetic gradient fields.

Deviating from the common expectation, magnetoelectrochemical structuring during deposition of diamagnetic ions was demonstrated, very recently. To achieve this, electrochemically inert paramagnetic ions have to be added to the electrolyte and the deposition has to be performed in a magnetic gradient field. A reverse structuring occurs, yielding thinner deposits near high gradient regions. In this paper we aim to clarify the mechanism of this reverse structuring. Potentiodynamic and potentiostatic investigations were performed, including measurements of the deposited mass with an electrochemical quartz crystal microbalance (EQCM). The convection of the electrolyte was studied in situ by astigmatism particle tracking velocimetry (APTV). It was revealed that during the reverse structuring a convection is induced in the electrolyte, which is directed away from the working electrode in regions of high magnetic gradients. Due to this additional convection, the overall deposition rate is increased, whereby it is locally reduced in regions of high magnetic gradients. The mechanism for reverse structuring is discussed in detail. Also, the influence of all relevant magnetic forces is addressed.

Volumetric reconstruction of the 3D boundary of stream tubes with general topology using tracer particles

A general method is proposed to reconstruct the volumetric interface between two fluid flows using tracer particles and 3D particle tracking techniques. The method relies on the fact that a homogeneous dispersion of tracer particles introduced in a stream tube remains confined in that tube so that the cloud of particles can be used to reconstruct the boundary of the flow covered by the stream tube. Thus it becomes possible to quantitatively determine the interface between laminar and turbulent flow regions in boundary or shear layers as well as the interface between two mixing fluids. Tracer particles, as opposed to dye tracers, have negligible diffusion and their position in the measurement volume can be precisely localized by means of 3D particle tracking methods. On the other hand, they provide a discrete representation of a continuous volume and the reconstruction of the interface cannot be implemented in a straight forward fashion. In this work, the problem of interface reconstruction, from a randomly scattered particle cloud, is addressed and two different reconstruction algorithms are proposed: one based on numeric diffusion and one based on Delaunay triangulation. The two methods are qualified and compared by means of numerical simulations using the Monte Carlo method. The simulations are used to estimate the accuracy of the method and to provide guidelines for the choice of parameter settings. Finally, results on the interface between two mixing fluids in a microfluidic mixer are shown. A resolution of 2.5 µm in the optical-axis direction, with a maximum estimated error of 5.5 µm in the three directions, was obtained.

Synchronized force and particle image velocimetry measurements on a NACA 0015 in poststall under control of time periodic electromagnetic forcing

The paper shows the results of an experimental flow control investigation using periodic actuation by streamwise Lorentz forces at the leading edge of a NACA 0015 profile. Synchronized measurements of force and time resolved particle image velocimetry were performed in a sodium hydroxide channel for Reynolds numbers of Re=0.5×105 and 105. A wavelet algorithm was used to detect vortical structures and reveals the relation between these large scale structures and the observed lift and drag forces. Complementary information on vortical structures was gained by vorticity based proper orthogonal decomposition of the data. In order to minimize the energetic effort of the flow actuation, the investigation focuses on small momentum coefficients. For small angles of attack, a complete reattachment of the flow can be established by the actuation, whereas for large angles of attack, the interaction of vortical structures becomes more prominent. This paper discusses the effects of excitation frequency and wave form f...