Quantitative volumetric microscopy for label-free characterization of emulsion droplets and liquid crystals in microfluidic environments

Interplay between emulsion droplets and liquid crystals in microfluidic environments provides a unique platform for research of unexplored phenomena in soft matter, enabling detection of chemical compounds and biological structures, which is expected to provide solutions to various biological and health related problems. On the one hand, emulsion droplets act as microreactors, offering low sample consumption, enhanced control over mass or heat transport, monodispersity, high-throughput analysis, compartmentalization of chemical reactions, and prevention of sample contamination. On the other hand, liquid crystals exhibit properties of anisotropic fluids with long-range orientational order of the constituent molecules, and represent highly responsive materials that react to external fields and chemical gradients in the surrounding media. Quantitative characterization is key to understanding the underlying physics and chemistry of the phenomena involving emulsion droplets and liquid crystals. Such characterization is currently accomplished by various optical microscopy techniques using polarized light, fluorescence, stimulated emission depletion, and Raman scattering. These techniques lack volumetric information, require fluorescent labeling, and have low time efficiency. Therefore, the introduction of a new microscopy technique that enables time-efficient, label-free, and quantitative volumetric characterization of dynamic processes involving emulsion droplets and liquid crystals in microfluidic environments is of paramount importance. Ideally suited for this task is digital holographic microscopy, which is based on the digital recording of an interference pattern (a hologram) between the reference light wave and the scattered light wave emitted by microscopic objects. The recorded hologram can be reconstructed computationally into quantitative volumetric information including the morphology and three-dimensional refractive index distribution of the microscopic objects. The refractive index distribution reveals general structure of the objects, local variations in molecular concentration and their orientation, quantitative information about mass transport, and even optical birefringence. This information could prove pivotal in the field of emulsions and liquid crystals in microfluidic environments, making it an important research topic. The goal of the proposed research project is to conceive and promote novel quantitative volumetric and label-free methods based on digital holographic microscopy to characterize the morphology and three-dimensional refractive index distribution of microfluidic droplets and liquid crystal emulsions. The main objectives of the proposed research are: 1) development of analytical and numerical models for light scattering and propagation through emulsion droplets, liquid crystals and optical elements, 2) development of inverse models and reconstruction algorithms to determine the morphology and three-dimensional refractive index distribution of emulsion droplets and liquid crystals, 3) experimental realization of several digital holographic microscopy configurations based on on-axis and off-axis light path systems in transmission and reflection modes, 4) establishment of a reference standard for emulsion droplets for critical evaluation and comparison of models and experimental configurations, 5) time-efficient improvement of inverse models, reconstruction algorithms, and digital holographic microscopy configurations, and finally 6) characterization of dynamic processes and phenomena involving emulsion droplets and liquid crystals in microfluidic environments. We anticipate that realization of the proposed methods will not only provide novel quantitative volumetric and label-free technologies for characterization of emulsion droplets and liquid crystals but also open new avenues for use in interdisciplinary sciences and industry, where current detection methods are considered inadequate.