Individual particle handling in a microfluidic system based on parallel laser trapping

We present an optical trapping system combining individually addressable multiple laser traps with fluorescence spectroscopy. An in-line set of 64 near-IR laser diodes is used to create a line of individually addressable traps inside a microfluidic chip. This system is completed by an excitation/detection line for spectrally resolved fluorescence imaging of trapped particles. Highly parallel trapping in a constant flow (up to a few millimeters per second), fast particle handling rates (up to a few particles per second), and the possibility of recording fluorescence spectra of trapped objects lead to a performing bioanalytical platform, e. g., for highly parallel analysis and sorting. (C) 2011 Optical Society of America

Three-dimensional electrodes for dielectrophoretic applications

Kevin Keim

This thesis reports the use of metal-coated three-dimensional SU-8 electrodes for dielectrophoretic bio sensing and particle manipulation applications. Placing free standing three-dimensional electrodes in microfluidic channels, electric fields can be applied homogeneously over the complete height of the device. Due to this at any vertical position in the channel an equal dielectrophoretic force is created. These electrodes have been used to parallelize individual single-cell analysis by the mean of electrorotation and to tune the sorting size of deterministic lateral displacement devices by adding dielectrophoretic forces. Electrorotation uses rotating electric fields to make cells spin and determine their dielectric properties from their speed of rotation at different electric excitation frequencies. Using a double array of individual connected three-dimensional electrodes placed in a wide microfluidic channel, individual cells can be selectively trapped and released in single-cell dielectrophoretic micro cages and analyzed in parallel by electrorotation. The parallel analysis of cells in individual electrorotation cages is demonstrated for the first time and the membrane capacitance of Henrietta Lacks, human embryonic kidney 293, and human T lymphocytes are found in agreement with literature. The membrane capacitance of M17 neuroblastoma is investigated and found to be 7.49 ± 0.39 mF/m2. Additionally membrane capacitance changes within a human embryonic kidney 293 cell population are observed with the system based on an osmolarity study and the viability is assured by monitoring the membrane functionality using an erythrosin B dye. Failure of the membrane sealing function is observed in real-time by a shift of the electrorotation spectrum and a drop in the membrane capacitance. Deterministic lateral displacement uses shifted rows of posts in a microfluidic labyrinth to sort particles based on their size. The principle uses the deterministic behavior of particles in the device, particles below a certain size move straight through the device, while larger particles are displaced to the side. Replacing the passive posts by three-dimensional electrodes allows the creation of local dielectrophoretic forces. Applying voltages of up to 15 Vpp dynamically reduces the sorting size of the device from 6 µm to 250 nm, enabling a device tunable for a specific sorting size. Nanometer sized particles can be separated in within micrometer structures reducing the problem of clogging. Additionally, this device could enable sorting of equal sized nanometer particles based on their dielectric properties. These two examples show the applicability of metal-coated three-dimensional SU-8 electrodes for dielectrophoretic applications. They first enable the design of new devices, which could not be realized previously, as shown by the electrorotation device, and second traditional passive structures can be replaced by electrodes, which can help to improve their performance or enable new functionalities.

EPFL2020

Bootstrap current compensation with electron-cyclotron waves in 3D reactor-size configurations

Sergi Ferrando I Margalet

In magnetic confinement devices, the inhomogeneity of the confining magnetic field along a magnetic field line generates the trapping of particles (with low ratio of parallel to perpendicular velocities) within local magnetic wells. One of the consequences of the trapped particles is the generation of a current, known as the bootstrap current (BC), whose direction depends on the nature of the magnetic trapping. The BC provides an extra contribution to the poloidal component of the confining magnetic field. The variation of the poloidal component produces the alteration of the winding of the magnetic field lines around the flux surfaces quantified by the rotational transform ι. When ι reaches low rational values, it can trigger the generation of ideal MHD instabilities. Therefore, the BC may be responsible for the destabilisation of the configuration. This thesis is divided into two parts. In the first part, we present a self-consistent method to calculate the BC and assess its effect on equilibrium and stability in general 3D configurations. This procedure is applied to two reactor-size prototypes (both with plasma volumes ∼ 1000m3): a quasi-axisymmetric (QAS) system and a quasi helically symmetric (QHS) system with magnetic structures that develop BC in opposite directions. The BC increases with the plasma pressure, therefore its relevance is enhanced when dealing with reactor-level scenarios. The behaviour of both prototypes at reactor level values of β ≡ (kinetic plasma pressure)/(magnetic pressure) is assessed, as well as its alteration of the equilibrium and stability. In the QAS prototype, BC-consistent equilibria have been computed up to β = 6.7% and the configuration is shown to be stable up to β = 6.4%. Convergence of self-consistent BC calculations for the QHS case is achieved only up to β = 3.5%, but the configuration is unstable for β ≥ 0.6%. The relevance of symmetry breaking modes of the Fourier expansion of the confining magnetic field on the generation of BC is studied for each prototype. This proves the close relationship between magnetic structure and BC. Having established the potentially dangerous implication of the BC, principally, in reactor prototypes, a method to compensate its harmful effects is proposed in the second part of the thesis. It consists of the modelling of the current driven by externally launched ECWs within the plasma to compensate the effects of the BC. This method is flexible enough to allow the identification of the appropriate scenarios in which to generate the required CD depending on the nature of the confining magnetic field and the specific plasma parameters of the configuration. Both the BC and the CD calculations are included in a self-consistent scheme which leads to the computation of a stable BC+CD-consistent MHD equilibrium. This procedure is applied in this thesis to simulate the required CD to stabilise the QAS and QHS prototypes introduced in the first part. The estimation of the input power required and the effect of the driven current on the final equilibrium of the system is performed for several relevant scenarios and wave polarisations providing various options of stabilising driven currents. Several scenarios have been devised for each prototype in order to drive current at the appropriate location and with the desired direction. Different polarisations and launching conditions have been employed to this purpose. In particular, a HFS launched X2 ECW with an input power of 1.5MW has been shown to drive sufficient current to maintain the rotational transform below the critical value 2/3 at β = 6.4% for the QAS reactor. Correspondingly, in the QHS reactor, an X3-mode ECW of 100KW was sufficient to drive the current required to push the rotational transform below unity near the magnetic axis at β = 3%. Thus, stabilisation of BC-driven instabilities with externally launched ECWs has been achieved for both contrasting configurations. The method proposed in this thesis allows also the utilisation of EBW in the generation of CD. The possible advantages of EBCD for compensation studies are described as well as their possible application to the two prototypes under consideration. The BC+CD procedure is particularly interesting to investigate new magnetic geometries as potential candidates for fusion reactors. With this numerical tool, it is possible to assess the implications of their consistent BC when operating at reactor level. It also allows to quantify how much power would be required to maintain the system MHD stable in these circumstances. Nevertheless, this method is flexible enough to be applicable to any configuration.

EPFL2006

Local excitations in amorphous solids

Wencheng Ji

Amorphous solids are structurally disordered. They are very common and include glasses, colloids, and granular materials, but are far less understood than crystalline solids. Key aspects of these materials are controlled by the presence of excitations in which a group of particles rearranges. This motion can be triggered by (a) quantum fluctuations associated with two-level systems (TLS), which dominate the low temperature properties of conventional glasses and have practical importance on superconducting qubits; by (b) thermal fluctuations associated with activations, which are related to the famous and challenging

glass transition'' problem; or by (c) exerting an external stress or strain associated with shear transformations, which control the plasticity. Hence, it is important to understand how temperature and system preparation determines the density and geometry of these excitations. The possible unification of these excitations into a common description is also a fundamental problem.  These local excitations are thought to have a close relationship with

Quasi-localised modes (QLMs)'' which are present in the low-frequency vibrational spectrum in amorphous solids. Understanding the properties of QLMs and clarifying the relation between QLMs and these local excitations are important to the study of the latter. In this thesis: (1) we provide a theory for the QLMs, D_L(omega) ~ omega^alpha, that establishes the link between QLMs and shear transformations for systems under quasi-static loading. It predicts two regimes depending on the density of shear transformations P(x)~ x^theta (with x the additional stress needed to trigger a shear transformation). If theta>1/4, alpha=4 and a finite fraction of quasi-localised modes form shear transformations, whose amplitudes vanish at low frequencies. If theta