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Feedback control of inertial microfluidics using axial control forces
Full text linkDieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Inertial microfluidics is a promising tool for many lab-on-a-chip applications. Particles in channel flows with Reynolds numbers above one undergo cross-streamline migration to a discrete set of equilibrium positions in square and rectangular channel cross sections. This effect has been used extensively for particle sorting and the analysis of particle properties. Using the lattice Boltzmann method, we determined the equilibrium positions in square and rectangular cross sections and classify their types of stability for different Reynolds numbers, particle sizes, and channel aspect ratios. Our findings thereby help to design microfluidic channels for particle sorting. Furthermore, we demonstrated how an axial control force, which slows down the particles and shifts the stable equilibrium position towards the channel center. Ultimately, the particles then stay on the centerline for forces exceeding the threshold value. This effect is sensitive to the particle size and channel Reynolds number and therefore suggests an efficient method for particle separation. In combination with a hysteretic feedback scheme, we can even increase the particle throughput.DFG, SFB 910, Kontrolle selbstorganisierender nichtlinearer Systeme: Theoretische Methoden und Anwendungskonzept
Kontrolle inertialer Mikrofluidik
No full textDas junge Feld der inertialen Mikrofluidik untersucht die Trägheit von Flüssigkeiten, um damit die Effizienz mikrofluidischer Apparate zu erhöhen. Der Anwendungsbereich der inertialen Mikrofluidik ist enorm gewachsen, nachdem die ersten Apparate in den frühen 2000ern demonstriert wurden, und umfasst heute unterschiedlichste Aufgaben. Sie reichen von dem Zählen bestimmter Zelltypen, dem Testen mechanischer Eigenschaften von Zellen bis zu dem Trennen von Teilchen nach ihrer Größe. Hierbei zeigen Teilchen im Fluss eine selbstorganisierte Ordnung vermittelt von inertialen Auftriebskräften. Diese Arbeit untersucht wie die inertiale Mikrofluidik durch die Anwendung externer Kontrolle profitieren kann. Dabei betrachten wir insbesondere Rückkopplungskontrolle und die Methode der optimalen Kontrolle.
Zuerst untersuchen wir die Eigenschaften inertialer Fokussierung mithilfe zweier mesoskopischer Simulationsmethoden, der Vielteilchenstoßdynamik und der Gitter-Boltzmann-Methode. Wir konzentrieren uns hierbei auf das Wechselspiel zwischen Kanalgeometrie und den Profilen inertialer Auftriebskräfte. In Kanälen mit rundem Querschnitt sammeln sich die Teilchen auf dem Segré-Silberberg Ring.
Die verringerte Symmetrie rechteckiger Querschnitte zwingt die Teilchen auf diskrete Gleichgewichtspositionen. Diese werden stark in Lage und Stabilität beeinflusst von Teilchengröße und Flussgeschwindigkeit. Für rechteckige Kanäle mit stark nicht quadratischem Querschnitt beobachten wir, wie das System effektiv zweidimensional wird.
Des Weiteren untersuchen wir die Kontrolle inertialer Kraftprofile mittels elektromagnetischer Felder. Zuerst bremsen wir die Teilchen durch ein elektrisches Feld, das gegen den Fluss gerichtet ist. Dieses Feld reduziert die inertiale Auftriebskraft und schiebt die Gleichgewichtsposition näher zu der Kanalmitte. Damit sind wir in der Lage die Teilchen zu kontrollieren und können in Kombination mit einen hysteretischen Rückkopplungsgesetz den Teilchendurchsatz erhöhen.
Zweitens erlauben magnetische Felder es zusätzliches Drehmoment auszuüben und die Winkelgeschwindigkeit der Teilchen zu beeinflussen.
Dadurch verändern sie ebenfalls die inertialen Auftriebskräften. Wenn wir die Winkelgeschwindigkeit der Teilchen kontrollieren, können wir sie auf eine wohldefinierte Position fokussieren, deren Lage die gewählte Winkelgeschwindigkeit bestimmt.
Zuletzt untersuchen wie die Anwendungen der Theorie der optimalen Kontrolle, um damit Profile von Kontrollkräften in optimaler Weise zu gestalten. Wir entwickeln den notwendigen Formalismus und beschreiben die numerische Methode um Profile optimaler Kontrolle zu bestimmen. Wir zeigen, dass die Kontrollprofile in der Lage sind Teilchen zu gewünschten Positionen am Kanalende zu steuern. Darüber hinaus demonstrieren wir die Trennung verschiedener Teilchen anhand ihrer Größe. Dabei erhöht die zusätzliche Kontrolle die Effizienz verglichen mit passiven Systemen und ist extrem robust gegenüber Störungen.The young field of inertial microfluidics investigates fluid inertia to enhance the performance of microfluidic devices. The scope of inertial microfluidics expanded rapidly to cover a diverse set of applications, after the first devices were proposed in the early 2000s. Recently, devices have been demonstrated for counting, cells, testing the mechanical properties of cells, and separating particles by size. Devices in inertial microfluidics rely on a self-organized order between particles induced by inertial lift forces.
In this thesis, we investigate how inertial microfluidics benefits from the application of external control. Here, we place a special emphasis on feedback control and optimal control theory. In contrast to open-loop control, feedback control does not use a fixed control law, but adapts it to the current system state. Therefore, feedback control is typically less sensitive to noise. Optimal control theory offers a mathematical way to design (feedback) control laws that are optimal with respect to a given performance measure.
First, we use mesoscopic simulations methods, specifically multi-particle collision dynamics and the lattice Boltzmann method, to investigate inertial focusing. In particular, we concentrate on how the channel geometry influences the inertial lift force profiles. In channels with circular cross sections, particles collect on the Segré-Silberberg annulus. In channels with rectangular cross sections, the reduced symmetry restricts the particles to a discrete set of equilibrium positions. Particle size and flow speed strongly influence the position and stability of the equilibrium positions. The system becomes effectively two-dimensional if the height of the channel is large compared to its width.
We further investigate the control of the inertial lift forces profiles using external electromagnetic fields. First, we slow particles down by an external electric field directed against the flow velocity. The field reduces the inertial lift forces and moves the equilibrium positions closer to the channel center. Thereby, we are able to control the particle motion. In combination with an hysteretic feedback law, axial control forces increase the particle throughput. Second, magnetic fields exert additional torques on the particles and change their angular velocity. As a result, they too change the inertial lift force profiles. When we control the angular velocity of the particles, we are able to focus them onto a single equilibrium position controlled by the chosen angular velocity.
Finally, we investigate optimal control theory for designing optimal control force profiles for particle steering and separation. We develop the necessary formalism and describe the numerical solution technique to determine the optimal control force. We show that the designed control force profiles are able to steer single particles into a target interval at the channel outlet. Furthermore, we demonstrate how to separate particles by size. Here, the external control force strongly increases the sensitivity compared to similar passive devices. Importantly, the optimal control force profiles are very robust against imperfections
Optimal control of particle separation in inertial microfluidics
Full text linkRecently, inertial mircofluidics has emerged as a promising tool to manipulate complex liquids with possible biomedical applications, for example, to particle separation. Indeed, in experiments different particle types were separated based on their sizes (A.J. Mach, D. Di Carlo, Biotechnol. Bioeng. 107, 302 (2010)). In this article we use a theoretical study to demonstrate how concepts from optimal control theory help to design optimized profiles of control forces that allow to steer particles to almost any position at the outlet of a microfluidic channel. We also show that one specific control force profile is sufficient to guide two types of particles to different locations at the channel outlet, where they can be separated from each other. The particles just differ by their size which determines the strength of the inertial lift forces they experience. Our approach greatly enhances the efficiency of particle separation in the inertial regime
Controlling inertial focussing using rotational motion
Full text linkIn inertial microfluidics lift forces cause a particle to migrate across streamlines to specific positions in the cross section of a microchannel. We control the rotational motion of a particle and demonstrate that this allows to manipulate the lift-force profile and thereby the particle's equilibrium positions. We perform two-dimensional simulation studies using the method of multi-particle collision dynamics. Particles with unconstrained rotational motion occupy stable equilibrium positions in both halfs of the channel while the center is unstable. When an external torque is applied to the particle, two equilibrium positions annihilate by passing a saddle-node bifurcation and only one stable fixpoint remains so that all particles move to one side of the channel. In contrast, non-rotating particles accumulate in the center and are pushed into one half of the channel when the angular velocity is fixed to a non-zero value
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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