Colloids in external electric and magnetic fields

Kolloide spielen eine wichtige Rolle in der Forschung. In vielen Anwendungsgebieten, beispielsweise Optik, Mikro- und Nanoelektronik, Mikrofluidik, Automobilindustrie, Mineralölindustrie, dienen sie sowohl als Testsubstanzen als auch "Werkzeug", um grundlegende Vorgänge zu erforschen. In der Grundlagenforschung dienen kolloidale Suspensionen als Modelle für atomare und molekulare Systeme. In der vorliegenden Arbeit habe ich Suspensionen aus festen Kolloiden (auch als Partikel bezeichnet) in externen elektrischen und magnetischen Feldern studiert. Externe Felder modifizieren die Wechselwirkungen der Kolloide untereinander sowie zwischen Kolloiden und Lösungsmittel und sind daher eine klassische Methode für Untersuchungen an dem Verhalten im Nichtgleichgewicht. In dieser Arbeit habe ich die Bewegung von Kolloiden aus Polymethylmethacrylat (PMMA) in einem dichteangepassten Lösungsmittel in dielektrophoretischer Flasche (DEP-Flasche, engl. "electric bottle") mittels hoher Gradienten der elektrischen Felder im Nichtgleichgewicht untersucht. Aufgrund der inhomogenen Verteilung der dielektrophoretischen Kraft (DEP-Kraft) bildete sich kurz nach dem Anschalten des elektrischen Feldes am Rand der Elektroden eine an Kolloiden verarmte Zone (Verarmungszone) aus. Später entwickelte die Front der wandernden Kolloide eine wellige Form. Diese Instabilität ähnelt Rayleigh-Taylor-Instabilität. Sie hängt nicht von der Feldfrequenz sondern von der Feldstärke ab. Während dieses Wechselspiel zwischen DEP-Kraft und Hydrodynamik auf einer Längenskala von etwa 100 µm stattfindet, eignet sich die DEP-Flasche auch dazu, die lokale Kolloidkonzentration zu erhöhen, bis Kristallisation eintritt. In den resultierenden koexistierenden flüssigen und kristallinen Phasen von kolloidalen Suspensionen habe ich lokale rheologische und dynamische Eigenschaften untersucht. Magnetische Kolloide wurden dafür als Sonden mittels magnetischer Felder durch die jeweiligen Phasen gezogen. Um die magnetischen Kolloide in Bewegung zu setzen, ist in der kristallinen Phase eine minimale Kraft ("Pinning-Kraft") erforderlich. Nach meinen Ergebnissen hängt diese von den Wechselwirkungen zwischen Kolloiden also auch von der Kolloidkonzentration ab. Aufgrund ihrer starken magnetischen Wechselwirkung können magnetische Kolloide aggregieren und auch in verdünnten Lösungen Ketten (Züge) bilden. Magnetische Kolloidzüge verhalten sich in kolloidalen Suspensionen im Wesentlichen wie zylinderförmige Objekte. Durch wiederholte Drehung magnetischer Kolloidzüge in der Horizontalen habe ich die dynamischen Eigenschaften von Kristallen aus PMMA-Kolloiden auch in zwei Dimensionen studiert. Während der Drehung verformten sich die Kristallgitter weit entfernt von den Kolloidzügen nur elastisch. Eine Analyse des Lindemann-Parameters ergab, dass der Einfluss der Kolloidzüge auf die Bewegung der umgebenden Kolloide bei zunehmendem Abstand zum Drehzentrum exponentiell abfällt. Durchziehen eines Kolloidzuges durch eine kristalline Matrix induzierte lokales Schmelzen des Kristalls. Zeit- und Ortsaufgelöste konfokale Mikroskopie zeigte, dass die PMMA-Kolloide weniger als 2 Minuten benötigten, um zu rekristallisieren. Die beschriebenen Ergebnisse zeigen, dass kolloidale Sonden von komplexer Form (Kolloidzüge) nicht nur für die Untersuchung der mikrorheologischen Eigenschaften kolloidaler Suspensionen, sondern auch für Studium der Defektdynamik besonders geeignet sind. In mikrofluidischen Anwendungen kolloidaler Suspensionen gewinnen Wandeffekte und räumliche Einschränkung der Bewegung an Einfluss. Um diesen Aspekt zu untersuchen, habe ich die Bewegung und Verteilung der Kolloide in elektrischen Gleichspannungsfeldern in Mikrokanälen verfolgt. Mikrokanäle wurden mittels optischer Lithographie hergestellt und der Prozess optimiert. Sowohl die Geschwindigkeit als auch die Packungsdichte hängen von der Kanalform ab. Wie sich herausstellte, wurde nach Anschalten des elektrischen Feldes die Kolloide vorwiegend durch Elektroosmose statt Elektrophorese angetrieben. Im Vergleich dazu hängt die durch Gravitation verursachte Kolloidbewegung von der lokalen Packungsdichte und dem Neigungswinkel des Kanals ab. Bei hinreichender Kolloidkonzentration wurde eine Separation der Kolloide von der Wasserströmung beobachtet. Die quantitativen Resultate zu diesen Phänomenen ermöglichen uns, das kolloidale Verhalten in den komplexen Situationen in Mikrofluidik zu verstehen und mikrofluidische Anwendungen zu optimieren.Colloids play an important role in research. They are used both as test objects and as investigation tools in many different fields, such as optics, micro- and nanoelectronics, microfluidics, automobile and petrol industry etc. In fundamental research, colloidal suspensions serve as models for atomic and molecular systems. In the current work, I investigated suspensions of solid colloids (also called particles) in external electric and magnetic fields. External fields modify the interactions of colloids among themselves and with the solvent and are therefore a classical approach to study non-equilibrium behavior. In the present thesis, I studied non-equilibrium effects of high electric field gradients on the motion of PMMA particles in a density-matched solvent in an electric bottle. Due to the inhomogeneous distribution of the dielectrophoretic force (DEP-force) a zone depleted of colloids formed in the region of the electrode edge shortly after switching on the electric voltage. Later the front of migrating colloids developed into a wave-like form. This instability resembles the Rayleigh-Taylor-Instability. It does not depend on frequency but on field strength. Whereas the interplay between dielectric force and hydrodynamics described above happens on the scale of 100 µm, the electric bottle also enables to gradually increase the concentration of colloids to a value sufficient for crystallization. I investigated the local rheological and dynamic propertied of coexisting fluid and crystalline phases of PMMA colloid suspensions by dragging magnetic colloids through these two phases using magnetic fields. In the crystalline phase a minimum force ("pinning-force") is necessary to induce motion of magnetic colloids which depends on the interaction between colloids within the crystal. According to my experiments, the pinning-force depends on colloid concentration and colloidal interactions. Due to their strong interactions, magnetic particles may aggregate and form chains even in dilute solutions (later called trains). Magnetic colloid trains in colloidal suspensions rheologically behave as cylinders. I also studied the dynamic properties of crystals of PMMA colloids in two dimensions by repeatedly rotating magnetic colloid trains. Crystal lattices far from the colloid trains only elastically deformed while the magnetic colloid trains were oscillating. The influence of the colloid trains on the motion of surrounding colloids exponentially decreases with distance according to the Lindemann parameter. The crystal melted when a colloid train was pulled through a crystalline matrix. According to time- and space resolved laser scanning confocal microscopy, the colloids took less than 2 minutes to recrystallize. These results show that colloidal probe of complex shape not only give information on microrheological properties, but are also suitable for studying defects dynamics. Wall effects and confinement become increasingly important in colloidal suspensions in microfluidic applications. To address the influence of these aspects, I studied colloid motion and distribution in micro-channels under the influence of electric DC fields. Micro-channels were fabricated using optical lithography and the process was optimized. Both the migration velocity and the packing density of colloids depend on channel shape. It turned out that in electric fields colloids in microchannels are driven by electroosmosis rather than electrophoresis. In contrast, gravity driven colloid motion depends on the local packing density of the colloids and on the slope (tilting angle) of the channel. At sufficient packing density I also observed separation of colloid and solvent streams. All of these phenomena may be relevant in processes carried out in microfluidic channels, and the quantitative results presented here may contribute to optimization

The Carrier of Shame and Glory-An Interpretation of the Letter A in The Scarlet Letter from the Perspective of Symbolism

Hawthorne, an American romantic writer in the 19th century, has a talent for using symbolism as a literary expression. This kind of artistic feature can be specifically seen in his masterpiece The Scarlet Letter. From the initial derogatory meaning of “adultery” and “awful” to the later “able” and “adorable”, the letter A in The Scarlet Letter has different meanings in various periods. This article starts with the letter A and provides a comprehensive and concise analysis of the use of symbolism, including the symbolic implications and the artistic effects produced by these symbolic means, aiming to help understand the ideological content of the work

A Textual Study of Imagery Representing Female Characters in Little Women under the Theory of Signifier and Signified

As a symbol, feminine imagery carries the character traits and conveys the author’s own ideology. There are many distinctive female characters in Little Women, but related previous studies mostly focus on the superficial analysis of female images in the book. Guided by the theory of signifier and signified in Saussurean linguistics, this paper starts with the traditional and transformed female images in Little Women and the reasons for their use, explores how the use of traditional and transformed feminine imagery and their signifier and signified in Little Women by American female writer Louisa May Alcott helps construct the novel’s main characters and thus expresses the author’s own feminist views. This could help readers have a deeper understanding of the text and a more generalized view of the protagonist

Conditional Teacher-Student Learning

The teacher-student (T/S) learning has been shown to be effective for a variety of problems such as domain adaptation and model compression. One shortcoming of the T/S learning is that a teacher model, not always perfect, sporadically produces wrong guidance in form of posterior probabilities that misleads the student model towards a suboptimal performance. To overcome this problem, we propose a conditional T/S learning scheme, in which a "smart" student model selectively chooses to learn from either the teacher model or the ground truth labels conditioned on whether the teacher can correctly predict the ground truth. Unlike a naive linear combination of the two knowledge sources, the conditional learning is exclusively engaged with the teacher model when the teacher model's prediction is correct, and otherwise backs off to the ground truth. Thus, the student model is able to learn effectively from the teacher and even potentially surpass the teacher. We examine the proposed learning scheme on two tasks: domain adaptation on CHiME-3 dataset and speaker adaptation on Microsoft short message dictation dataset. The proposed method achieves 9.8% and 12.8% relative word error rate reductions, respectively, over T/S learning for environment adaptation and speaker-independent model for speaker adaptation.Comment: 5 pages, 1 figure, ICASSP 201

Frequency dependent deformation of liquid crystal droplets in an external electric field

Nematic drops suspended in the isotropic phase of the same substance were subjected to alternating electrical fields of varying frequency. The system was carefully kept in the isotropic-nematic coexistance region, which was broadened due to small amounts of non-mesogenic additives. Whereas the droplets remained spherical at low (order of 10 Hz) and high frequencies (in the kHz range), at intermediate frequencies, we observed a marked flattening of the droplet in the plane perpendicular to the applied field. The deformation of the liquid crystal (LC) droplets occurred both in substances with positive and negative dielectric anisotropy. The experimental data can be quantitatively modelled with a combination of the leaky dielectric model and screening of the applied electric field due to the finite conductivity.Comment: minor change

Improved training for online end-to-end speech recognition systems

Achieving high accuracy with end-to-end speech recognizers requires careful parameter initialization prior to training. Otherwise, the networks may fail to find a good local optimum. This is particularly true for online networks, such as unidirectional LSTMs. Currently, the best strategy to train such systems is to bootstrap the training from a tied-triphone system. However, this is time consuming, and more importantly, is impossible for languages without a high-quality pronunciation lexicon. In this work, we propose an initialization strategy that uses teacher-student learning to transfer knowledge from a large, well-trained, offline end-to-end speech recognition model to an online end-to-end model, eliminating the need for a lexicon or any other linguistic resources. We also explore curriculum learning and label smoothing and show how they can be combined with the proposed teacher-student learning for further improvements. We evaluate our methods on a Microsoft Cortana personal assistant task and show that the proposed method results in a 19 % relative improvement in word error rate compared to a randomly-initialized baseline system.Comment: Interspeech 201