Rodrigo Segura

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.

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.

Formation of a Polymer Surface with a Gradient of Pore Size Using a Microfluidic Chip

Here we demonstrate the generation of polymer monolithic surfaces possessing a gradient of pore and polymer globule sizes from ~0.1 to ~0.5 μm defined by the composition of two polymerization mixtures injected into a microfluidic chip. To generate the gradient, we used a PDMS microfluidic chip with a cascade micromixer with a subsequent reaction chamber for the formation of a continuous gradient film. The micromixer has zigzag channels of 400 × 680 μm(2) cross section and six cascades. The chip was used with a reversible bonding connection, realized by curing agent coating. After polymerization in the microfluidic chip the reversible bond was opened, resulting in a 450 μm thick polymer film possessing the pore size gradient. The gradient formation in the microfluidic reaction chamber was studied using microscopic laser-induced fluorescence (μLIF) and different model fluids. Formation of linear gradients was shown using the fluids of the same density by both diffusive mixing at flow rates of 0.001 mL/min and in a convective mixing regime at flow rates of 20 mL/min. By using different density fluids, formation of a two-dimensional wedge-like gradient controlled by the density difference and orientation of the microfluidic chip was observed.

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.

Zeitaufgelöste 3D3K Geschwindigkeitsfeldmessungen mit der fernmikroskopischen Astigmatismus PTV zur Analyse der elektrochemischen Kupferabscheidung

Zusammenfassung Der Einsatz der fernmikroskopischen 3D Astigmatismus PTV Methode ermöglicht die zeitlich und räumlich hochaufgelöste Messung des kompletten Geschwindigkeitsfeldes in einem Volumen. Die Methode kommt dabei ohne komplexe Kalibrierung mit nur einem optischen Zugang aus und arbeitet sehr zuverlässig. Es gelang hiermit erstmals die sehr komplexe Strömung, welche durch die Überlagerung der Lorentzkraft und der Magnetfeldgradientenkraft bei der elektrochemischen Kupferabscheidung entsteht, experimentell zu analysieren. Abstract Using the astigmatism particle tracking method in connection with a long-range microscope enables highly temporal and spatial resolved measurements of the three dimensional velocity field in a fluid volume by a single camera. Thus, the complex interplay of magnetic field gradient force and the Lorentz force induced convective effects was analyzed experimentally for the first time.