Modeling of water droplets response in a time- and space-dependent light-induced electric field

It has been observed that droplets under electric fields change shape and this phenomenon plays an important role in tailoring the droplet size. This aspect is one of primary importance, allowing the droplet to become a resonator cavity for the light therein coupled. Light amplification as well as fluorescence enhancement induced by efficient light confinement in a droplet have attracted great attention in medical and biological applications, where luminescent molecules are often attached to the biounit under investigation for sorting, targeting and dynamical responses investigations. Bio activity, as well as efficiencies in medical treatments, require online monitoring and are eager to find systems in which it is possible to isolate the bio unit to be investigated and measure its response. Microfluidics has been addressed as a perfect tool for handling small quantities of bio units at a time but requires the implementation of a suitable physical tool to probe and analyze the physical response. The crucial factor is the synergic combination of the functionalities of integrated optics with microfluidics: this is achieved integrating on the same lithium niobate (LN) substrate a microfluidic stage and an optical one, i. e., an array of waveguides in Mach-Zehnder interferometer (MZI) configuration. Such a device is able to illuminate and detect the transmitted light of droplets, measuring their speed, refractive index and size. This thesis is focusing on a new theoretical modeling oriented to the phenomenology of the response of microfluidic water droplets to a time-varying, spatially non-uniform electric field, identifying the key parameters that control the droplet deformation. The photoinduced electric field acts in the microchannel of the platform by exploiting the photo-inducing properties of lithium niobate, a widely employed material in the photonic and integrated optics industry thanks to its excellent properties. In addition to the electrohydrodynamic problem, this thesis proposes a novel approach to describe an observed novel interaction between the electric field and the water droplets. The research is relying on a wide-ranging scientific project that has already developed several new methods of real-time detection and monitoring of micro and sub-micrometric objects dispersed in fluid media.