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Conclusion and Perspectives

6.1 General conclusion. . . 129

6.2 Perspectives. . . 131

6.1 General conclusion

The present work regards the particle manipulation in biological samples of water treatment using digital holography microscopy (DHM). The challenge to be tackle in this nalapplication was the observation and analysis of a large sample and the relative small size and small content of micro-organisms of interest. Among others, the main issues to face consist in to prevent in the uidic system the sample from clogging and sedimentation, phenomena that are unavoidable when dealing with biological samples, and to enhance the particles visualization by focusing them in the ow channel into the eld and depth of view of the microscope. Belonging to the class of quantitative phase imaging system, DHM enable the observation of transparent and semi-transparent object without any prelim-inary treatment. Taking the advantage of the interferometry techniques, all the information is recorded on a single image - an hologram - using a single camera, it allows a high ow through rate compared to other imaging systems. Moreover, this system could provide the quantitative information of the thickness of the crossed object and its corresponding 3D position in the experimental channel thanks to a plane-by-plane reconstruction process. In the frame of this thesis, the post processing of the recorded images during the experiments enables the analysis of the focusing eciency by calculated a complete set of the (X,Y,Z) coordinates of each particle.

particle focusing approach. The former was investigated through the development of a sheath ow focusing device while the latter has taken advantage of an acoustic eld force to manipulate directly the particles.

The relative focusing capability of these techniques has been investigated through the development, manufacturing and test of an experimental setup as well as through the development of a numerical model that would give a repre-sentation of the physical phenomenon and support the design of future experiments. The emphasis was put on the development of uidic devices where a full 3D focusing of particles could be achieved. Manufacturing of 3D devices is challenging and thus expensive. The goal of the present work was to provide an alternative cost-eective solution that enables a 3D particle focusing while keeping the cost for instrumentation and materiels, lower at the best. In the frame of this project, synthetic particles have been used to mimic these biological objects in the experimental work.

The rst focusing device developed in the frame of this work exploits the hydrodynamic eect of sheath ow warping. Several devices have been investigated - from commercially available ow cells for 2D focusing to custom manufactured devices for 3D focusing - to perform a ow warping of the sample, thus by control-ling its spreading in the cross-section. Assembcontrol-ling a cost-eective 3D hydrofocusing device is then a challenge, and building several successive prototypes was necessary to achieve a robust device able to produce reliable results.

The nal setup consists into the interlocked assembly of two disposable glass tubes respectively used as a sample nozzle and main channel. The eciency of this system has been intensively investigated by evaluating the spreading of the focused streamline as a function of sample and sheath ow rate in order to validate the prototype design. The nal device showed an ability to contract the sample stream section up to 4.4% using this hydrodynamic focusing technique. Further investigation on the focusing subsistence also showed that an increase of the ratio between the sheath ow and the sample ow velocity above a certain threshold - ≈ 10 − 20% of the sheath ow rate - is detrimental for the focusing eciency, i.e. there is an optimal ratio in the ow rates of the sheath and samples ows. A numerical model using FluentT M _{software have been developed to describe the ow}

patterns. They also provided a tool able to foresee the position of the suspended particles in the sample ow.

subsystems was often reported to be the crucial point. Successive attempts of prototypes have been developed for testing the dierent conguration of subsystems and the rationale for the assembling of the nal setup - uidic square channel of 2mm side - has been extensively discussed.

Post-processing of the experimental results show somehow surprisingly that particles were distributed in the channel cross section orthogonal to the main ow axis into matrix-like structures, sometimes collapsing in ribbons-like tracks. A tentative hypothesis was formulated to explain that such patterns have not been reported previously in the literature, at the best our knowledge. It relies on the specicity of the axi-symmetrical geometry of channel used in the present work, that enables strong side wall reections of the acoustic waves, contrary to most devices that present a Hele Shaw-like conguration.These side-reections could eventually be considered as non-planar waves These matrix-like patterns could be further used as a particle separation technique or to enhance reactions, such as anti-gen/antibody binding in immuno-assays. A statistical analysis of the experimental data has been performed and outlined the optimal focusing conditions according to the energy of the acoustic actuation, the sampling ow rate and the size of particles. A numerical model using FluentT M _{software have been developed to tackle}

the experimental observations and propose a realistic model for the determination of the acoustic forces and acoustic potential that lead to the distribution of particles. The emphasis was put on the generation and propagation of the acoustic wave in the numerical model that was achieved by imposing walls motion whose velocity was implemented through a User Dened velocity boundary condition. Contrary to most of the numerical models that deals with acoustouidic that could be found in the literature, the acoustic pressure eld is calculated - using an equation of state of the uid implemented via a User Dened Function - and not imposed as an initial condition. The main achievement of such a model over the latter is the ability to include the sidewall reections in the calculation of the acoustic potential. A good agreement was found between the experimental and the numerical results using asymmetrical amplitude for the transverse acoustic wave - that stands for the piezo actuation - and the orthogonal acoustic wave - that stand for the reection -.

6.2 Perspectives

would be of major interest as a prototype validation step in the frame of further development.

In particular, a simple hypothesis has been made concerning the matrix-like structures that have been observed. A more systematic study of the multiple wall reection would conrm this assumption as it is the main rationale of the patterns formation.

The developed devices present a good focusing eciency. But the acoustic device has also shown an ability to manipulate particles according to their size, that could provide a reliable technique for particle separation. This topic is surely an important perspective work to be done. The broad range of applications based on piezo actuation is also a great indicator of the interest of this technology for sensing (like in accelerometers), swichting (like in control panels), actuating (like in micropumps), speaking, transforming voltage... Piezo material itself is under a great research interest as it could be integrated directly in the system (like in MEMS applications), improving drastically the coupling quality.

The numerical model developed has demonstrated to be a realistic tool able to reproduce the observed phenomena. The models presented in the present thesis were two dimensional cross sections of a ow through channel. A 3D model was developed to investigate the 3D joint eect of the ASW generation in a ow through channel. With a total number of 1713064 cells, the main disadvantage of this model is the signicant increase in calculation time. If, on one side the hardware conguration plays for sure a role - strong parallelization is needed - also an optimization of the numerical scheme is mandatory. Nevertheless, despite its relative time-consuming calculations, this model would be an asset for a complete understanding of the acoustic radiation phenomenon.

Further modeling works would include the calculation of the acoustic radia-tion force in the User Dened Funcradia-tion so that to enhance the accuracy of experimental and numerical comparisons. Then add implementing the acoustic force in the net force balance using a discrete phase model will give access to the particles distribution under an acoustic standing wave. Besides, by implementing a particle-tracking velocimetry post processing, such a model would also provide a representation of the acoustic streaming phenomenon. Also a complete parametric study would be fruitful.

(a) Acoustic potential eld with asymmetrical stimulation amplitude Aortho = 1

2Atransin x-y (t = 3µs)

(b) Acoustic potential eld with asymmetrical stimulation amplitude Aortho = 1

2Atransin x-z plan (t = 3µs)