De Bortoli, Lorenzo (2017) Study of light driven phenomena in integrated opto-microfluidic lithium niobate platforms. [Magistrali biennali]
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In last decades microfluidics has gained an increasing interest by the scientific community due to its capability of control liquids on the microscale. Especially, droplet microfluidics technology provides a high manipulation level on very small volumes of fluids. This feature makes it a promising candidate for biological, medical and chemistry applications. With a compact and simple microfluidic device, droplets can be coalesced, mixed, sorted and employed either as micro chemical reactors or as carriers of biological sample. From the experimental point of view there are many ways to produce and see micro droplets. The device used for this thesis consists of an opto-microfluidic chip made of lithium niobate, in which are integrated a microfluidic circuit and an optical waveguide, normal to the channel and produced by titanium diffused in lithium niobate. Droplets flowing in the channel are detected using a laser light transmitted in the optical waveguide. After having illuminated the microfluidic channel, where a ”train” of water droplets in oil pass through, the light is collected by another waveguide facing on the other side of the channel: the intensity transmitted is recorded by way of a tailored electronic circuit. The water droplet in oil makes the signal of the light transmitted beam change in intensity, from a fixed value, relative to oil, to another basically due to the different refractive index of the droplet respect the surrounding flux. The signal variation is not sharp, but is preceded by a peak of intensity. Thus, the acquisition profile of the transmitted light intensity identifies the phenomenon of droplet passage, but up to now no studies were reported from a theoretical point of view. In this thesis, we aimed to clarify this aspect and provide a fine comprehension of it. Therefore, we correlated the curves obtained to some physical parameters of the droplets and to give a theoretical explanation to the shape of such a profile. Since the length and the velocity of the droplets are proportional to the ratio of the liquids’ fluxes flowing in the microfluidic channel, it is possible to measure and analyse the effect of the droplet passage and find new physical observables to identify the droplet, its shape and relative velocity. In particular, since the optical transmitted light across the droplet presents intensity modulation, great care was devoted to identifying their origin and their systematic behaviour. The aim was that of relating the modulation intensity peaks to the meniscus of the droplet and then to investigate all the other zones in its middle with an accurate phenomenological analysis. The idea that features in the light transmitted intensity signal could be related with the meniscus of the droplet, as well as intuitive, has never proved in literature and has confirmed by a simulation carried out in the frame of this thesis. In fact, a theoretical analysis of this phenomena is made: assuming ray optic, two-dimensional version of the problem and after having chosen the best shape for the source, a program that simulate the passage of a droplet in the microfluidic channel was written. After having simulates the physical interaction between the droplet that advances in the channel and the light exiting the waveguide, the software computes the transmitted intensity in order to reproduce the acquisition profile. The numerical profiles thus obtained are reported and commented, also by varying the parameters of the simulation. Finally, comparisons between the results of the simulations and the experimental profiles enable us to understand better the shape of the droplet. Based on these results we analysed also the shape that the interface water-oil should be in order to reproduce the low transmitted intensity typical of the droplet's centre. Performing an accurate analysis over a wide range of the fluxes ratio ϕ=Q_oil/Q_water also the role of the so called "secondary peaks" is investigated and fingerprints for a detailed description of the transmitted intensity profile are provided. This, up to our knowledge, has not been investigated yet. Finally, we can identify the droplet passage, so that to estimate its length with perspectives of setting up an automatized and fast data analysis procedure. This feature exploits an integrated opto-microfluidic device, independent from the standard imagining processes and with the possibility to perform a feedback loop with manipulation stages. Moreover, the same measures, with a one-to-one comparison with synchronous acquisitions of microscope images of the droplets, let us investigate also the sensibility of the optical transmitted light from the overall geometrical shape of the droplet. New physical observables in the acquisition profile allow us to describe and identify the transition from different production regimes of the droplets, also in different geometrical configuration of the microchannels, in a more accurate way than other imaging technique presented in literature.
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