During the last decade, large and costly instruments are being replaced by system based on microfluidic devices. Microfluidic devices hold the promise of combining a small analytical laboratory onto a chip-sized substrate to identify, immobilize, separate, and purify cells, bio-molecules, toxins, and other chemical and biological materials. Compared to conventional instruments, microfluidic devices would perform these tasks faster with higher sensitivity and efficiency, and greater affordability. Dielectrophoresis is one of the enabling technologies for these devices. It exploits the differences in particle dielectric properties to allow manipulation and characterization of particles suspended in a fluidic medium. Particles can be trapped or moved between regions of high or low electric fields due to the polarization effects in non-uniform electric fields. By varying the applied electric field frequency, the magnitude and direction of the dielectrophoretic force on the particle can be controlled. Dielectrophoresis has been successfully demonstrated in the separation, transportation, trapping, and sorting of various biological particles. / Singapore-MIT Alliance (SMA)

Microfluidics is a field of research that enables the manipulation of fluids in the submillimetre length scale. The technology allows the development of lab-on-a-chip devices, which are miniaturized systems for chemical and biological analysis. Currently, the conventional manufacturing methods for these systems require multiple time-consuming steps. Therefore, focus has shifted towards laser micromachining as an alternative method. Direct laser writing would circumvent many of the steps required for the conventional methods, drastically reducing the process time. In this Master thesis project, it was shown that microfluidic chips can be manufactured using a Nd:YVO4 (532 nm) nanosecond laser system. The process was optimized for silicon and borosilicate glass substrates. Acoustic focusing of polystyrene beads was demonstrated for a system etched in silicon. The optimized process used a power of 50%, a frequency of 10 kHz, a scan speed of 60 mm/s with triple lines as fill type and it had an etch rate of 4.3 μm/pass. Processed wafers were cleaned in buffered HF and bonded using anodic bonding as well as adhesive bonding. Processing of glass proved unpredictable, resulting in cracks and chippings. However, in- and outlets were successfully etched through thin glass wafers. It was found that crucial factors for the process were to control the focus, positioning of structures, structure orientation and the pulse separation for a uniform distribution of pulses. Based on the results, it is estimated that the manufacturing process could be done in two to three days using the laser micromachining process.

The study of acoustic radiation force and torque phenomena has attracted an enormous interest of the scientific community, due to applications of these phenomena in noncontact particles manipulation. In this work, we perform a theoretical analysis of acoustic radiation force and torque exerted on a homogeneous visco elastic particle in the Rayleigh scattering limit (the particle radius is much smaller than the incident wavelength) by a wave with arbitrary geometry. Our study is based on the partial-wave expansion in spherical coordinates of the incident and scattered waves. In this context, the radiation force and torque are obtained analytically in terms of an infinite series which involves the scattering and incident expansion coefficients. We assume that the particle behaves as a linear viscoelastic solid, which obeys the fractional Kelvin-Voigt model. Analytical expressions for the radiation force and torque are obtained considering the low- and high-frequency approximation in the viscoelastic model. The developed theory is used to describe the interaction of acoustic waves (traveling and standing plane waves, and zero and first-order Bessel beams) with a low-and high-density polyethylene particle. Our results show that the axial acoustic radiation force might become negative (i.e. in opposition to the wave propagation direction) when a certain condition involving the physical parameters of the particle is satisfied. Negative acoustic radiation torque due a beam of first-order Bessel may also occur when the same condition of negative radiation force is met. Remarkably, this is the first time that negative radiation force is predicted on a homogeneous particle in the Rayleigh scattering regime. Further more, the stability of the transverse acoustic radiation force generated by a Bessel beam is also investigated. We show a full 3D tractor Bessel vortex beam acting on the high-density polyethylene (HDPE). In the analysis of acoustic radiation force generated on a viscoelastic particle by a standing plane wave, relevant deviations arose in comparison with the solid elastic model for the particles. The magnitude of the radiation force and torque on a HDPE described by the viscoelastic model behaves differently (negative radiation force) compared with other materials (solid elastic and compressible fluid particle) due to traveling plane wave and Bessel beams. Finally, we believe that this study may help further enhance the development of acoustic levitation, particle handling in acoustofluids, and acoustical tweezers devices. / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O estudo dos fenômenos de força e torque de radiação acústico tem atraído um enorme interesse da comunidade científica, devido a aplicações desses fenômenos em manipulação de partículas sem contato. Neste trabalho, realizamos uma análise teórica da força e torque de radiação acústico exercido sobre uma partícula visco elástica homogênea no limite do espalhamento de Rayleigh (o raio da partícula é muito menor que o comprimento de onda incidente) por uma onda com geometria arbitrária. Nosso estudo baseia-se na expansão de ondas parciais em coordenadas esféricas das ondas incidente e espalhada. Nesse contexto, a força e o torque de radiação são obtidos analiticamente em termos de uma série infinita que envolve os coeficientes de expansão das ondas espalhada e incidente. Assumimos que a partícula se comporta como um sólido viscoelástico linear, que obedece o modelo de Kelvin-Voigt fracionário. Fórmulas analíticas para a força e torque de radiação são obtidas considerando uma aproximação de baixa e de alta frequência no modelo viscoelástico. A teoria desenvolvida é usada para descrever a interação de ondas acústicas (onda plana progressiva, onda plana estacionária, feixes de Bessel de ordem zero e de primeira ordem) com partículas de polietileno de baixa e de alta densidade. Os nossos resultados mostram que a força de radiação acústica axial pode ser negativa (isto é, em oposição à direção de propagação da onda) quando uma determinada condição envolvendo os parâmetros físicos da partícula é satisfeita. Torque de radiação acústico negativo devido a um feixe de Bessel de primeira ordem também pode ocorrer quando a mesma condição da força de radiação negativa for atendida. Notavelmente, esta é a primeira vez que a força de radiação negativa é prevista sobre uma partícula homogênea no regime de espalhamento Rayleigh. Além disso, a estabilidade transversal da força de radiação acústica gerada pelo feixe de Bessel também é investigada. Mostramos que um feixe de Bessel trator 3D completo atua sobre uma partícula de polietileno de alta densidade (PEAD). Na análise da força de radiação acústica gerada sobre uma partícula viscoelástica por uma onda plana estacionária, desvios relevantes surgiram em comparação com partícula sólida elástica. A amplitude da força e torque de radiação sobre uma PEAD descrita pelo modelo viscoelástico apresenta um comportamento diferente (força de radiação negativa) comparados com os outros materiais (sólida elástica e fluida com absorção longitudinal) devido a onda plana progressiva e feixes de Bessel. Por fim, acreditamos que este estudo pode ajudar a melhorar ainda mais o desenvolvimento de dispositivos de levitação acústica, manipulação de partículas em acustofluídica e pinças acústicas.

La lévitation acoustique par des ondes stationnaires ultrasonores (USW), permettent la manipulation des objets micrométriques. L'objectif principal de cette thèse est d'explorer les possibilités offertes par la lévitation acoustique pour manipuler des particules, des cellules et même des bactéries. Nous avons conçu et construit tous les résonateurs et nous avons développé les méthodologies que nous allons montrer dans ce travail expérimental. Selon la nature des particules, leur déplacement est donné par son interaction avec la force acoustique primaire. La position où les particules se déplacent est le point dont les forces acoustique et gravitationnel sont équilibrées. Dans le plan de lévitation, les interactions connues comme force secondaire de Bjerknes est la première étape du processus d'agrégation. Nous présentons une méthodologie pour mesurer cette force. Nous avons mesuré cette force en conditions de micropesanteur. Dans nous résonateurs, nous travaillons avec un grand nombre des particules dont les agrégats sont 3D. Nous introduisons le mode acoustique pulsé que nous permet générer des agrégats 2D. Lorsque les particules deviennent plus petites de 1µm, sa manipulation est difficile en raison de l'influence de l'acoustic streaming qui modifie le comportement des particules. Le mode acoustique pulsé permet de réduire ou de contrôler l'acoustic streaming que nous permet manipuler des particules de taille submicronique, des bactéries et des micro-cylindres catalytiques. Une séparation a été faite par un mélange des particules de 7-12µm dans le dispositif s-SPLITT. Néanmoins la combinaison de forces hydrodynamique et acoustique (HACS) a permis améliorer la séparation.