Integrated Opto-Microfluidic Lab-on-a-Chip in Lithium Niobate for Droplet Generation and Sensing

In the last decades microfluidics has gained an increasing interest by the scientific community due to its capability of manipulating liquids on the microscale. In particular droplet microfluidics technology holds great promise due to its precise control on very small volumes of fluid: droplets can be coalesced, mixed and sorted, employed either as micro chemical reactors or as carriers of biological samples. This features, in combination with fast analysis tools, allow for the realization of Lab-on-a- Chips (LOCs), miniaturized and portable devices able to perform chemical, biological, environmental or medical analyses where laboratory facilities lack. Nevertheless, in most cases, sensing inside LOCs is performed by external optical stages somehow added to the microfluidic chip. One of the hurdle towards the effective employment of such systems is indeed the complete integration between the microfluidic stage and the optical one. Often extended systems such as microscope objectives and fast CCD cameras have been used to detect droplets inside microfluidic channels nullifying the efforts spent to decrease the dimensions of LOCs and their related advantages. In this work the first opto-microfluidic Lab-on-a-Chip (LOC) for both generation and detection of droplets, entirely integrated in lithium niobate is presented. The main elements of the LOC are a passive droplet generator, where water in oil droplets are produced by the cross-flow of immiscible phases, and two waveguides on the surface of the crystal able to illuminate droplets perpendicularly to their flow and to collect the transmitted intensity. The realization of single mode channel waveguides at a wavelength of 632.8nm on lithium niobate by titanium in-diffusion is achieved and the obtained waveguides are characterized by Rutherford Backscattering Spectrometrxy (RBS), Secondary Ion Mass Spectrometry (SIMS) and near field measurements. A deep investigation on the applicability of lithium niobate on the field of microfluidics is carried out through a study of its wettability. In addition a functionalization procedure to improve its hydrophobicity is defined. Various techniques to engrave the microfluidic channels directly on the crystal are taken into account. In particular the ablation by a femtosecond pulsed laser at a wavelength of 800nm is widely investigated by optical microscopy and atomic force microscopy (AFM) in order to define the best process parameters to get the lower average roughness of the channel walls (Ra˜50nm). Channels engraved with a dicing saw are also characterized showing to be the best solution for optofluidic applications due to their extremely low average roughness (Ra = (6÷7) nm). New techniques for sealing the channels engraved on the surface of the crystal are described with a particular care to the device durability and its applicability to different purposes. Passive droplet generators with a T-junction geometry obtained by laser ablation in lithium niobate are shown to generate droplets in a wide range of frequencies (10÷1000Hz) and with a very sharp distribution of droplets volumes (sigma < 3%). Their performances are characterized employing an optical microscopy setup and the experimental data are discussed with respect to the theoretical models reported in literature. Discrepancies from the theory at low values of the capillary number (Ca < 3*10ˆ-3) are highlighted and described by means of an empirical law. Finally the coupling of the waveguides to the microfluidic stage is discussed showing how it can be used to count and trigger the droplets during their flow, achieving better performances than the standard optical microscopy setup. The ultimate configuration of the presented LOC prototype is characterized by two crossing channels obtained by mechanical dicing with three inlet branches and one outlet branch. Three phases are flown together (oil, water and a saline solution) and alternating droplets of pure water and saline solution are produced in oil. The transmitted intensity from the waveguide is shown to be sensitive to the refractive index of the solution with a sensitivity of dn = 2*10ˆ{-3} in the range n = [1.339, 1.377]. This is the first example of a Lab-On-a-Chip for real time droplet counting and refractive index sensing, completely integrated in lithium niobate