Advanced Optical Techniques for micro-Fluid Dynamics

The impact of microfluidic technologies in the field of life sciences has dramatically increased during the last years, due to the need to develop new products for real applications. The work reported in the present dissertation is focused on the development of optical systems for the study of fluid dynamic phenomena at the micro and nano-scale. The research activities were carried out in the frame of two main topics and, consequently, the dissertation is divided into two main parts. The first part of the dissertation is focused on the experimental study of cavitation in liquids, in which a bubble is generated by focusing a laser beam under extremely controlled conditions. The aim is addressing all different phases of the process. Cavitation is studied using a combined system of a fast camera, for the complete reconstruction of the plasma shape and bubbles dynamics, and a fiber optical hydrophone (FOHP), for the detection of the pressure shock waves in proximity of the bubble. The breakdown phenomenon and the bubble dynamics have been characterized when the optical arrangement of the focusing system is modified, e.g. using different expansion factors of the beam expander to change the focusing angle of the laser beam, or when the laser pulse energy is tuned. Data analysis shows a strong correlation between the number of plasma sites and the number of shock waves detected by the fiber hydrophone. The second part of the dissertation is focused on the study of a particular class of surface electromagnetic waves and on their use for sensing applications. Such waves are sustained at the surface of finite one-dimensional photonic crystals (1DPC) and are generally named Bloch surface waves (BSW). The robustness of optical sensors based on BSW has been investigated experimentally and numerically. The distributions of sensor characteristics caused by the fabrication uncertainties in dielectric layer thicknesses have been analysed. It is demonstrated that the performance of the surface wave sensors is sufficiently robust with respect to the changes of the photonic crystal layer thicknesses. Layer thickness optimization of the photonic crystal, carried out to achieve low limit of detection, leads to an improvement of the robustness of the surface wave sensors that is attributed to Bloch states lying deeper in the photonic band gap. The work reported in the dissertation demonstrates the use of an optical sensing platform based on BSW as a novel optical tool to probe in real time the fluid flow at a boundary wall of a microfluidic channel under dynamic conditions. Exploiting the properties of the BSW, we can put into evidence the temporal evolution of the interface during the injection of liquids with different refractive index (RI). We introduce ξ defined as the distance between the interface of the liquids of different RI and the 1DPC surface. Reconstructed experimental maps of ξ(z,t), as a function of time and of the position across the μ-fluidic channel, allow us to recovery the temporal evolution of the fluids interface in proximity of the wall. From the data analysis, the diffusion coefficient of a solute in water is measured and found in good agreement with the literature value. Moreover, biosensors based on BSWs have been studied and their practical application was demonstrated by detecting a specific glycoprotein, Angiopoietin 2, that is involved in angiogenesis and inflammation processes, and to detect clinically relevant concentrations of the breast cancer biomarker ERBB2 in cell lysates. The protocol used for the label-free detection of Angiopoietin 2 is described and the results of an exemplary assay are given, confirming that an efficient detection can be achieved. The limit of detection of the biochips for Angiopoietin 2, based on the protocol used, is 1.5 pg/mm^2 in buffer solution. To detect soluble ERBB2, we develoed an optical set-up which operates in both label-free and fluorescence modes. The resolution obtained in both modes meet international guidelines and recommendations (15 ng/mL ) for ERBB2 quantification assays, providing an alternative tool to phenotype and diagnose molecular cancer subtypes. During the last part of the doctorate period, the two research directions pursued during the first and the second part, devoted to bubble cavitation and to BSW based sensors, merged in a new innovative setup. Starting from the BSW concept, a new type of optical hydrophone based on BSW for the detection of pressure shock waves with a larger sensitivity than FOHP has been proposed and setup. Some preliminary and encouraging results are presented and discussed.