One of the challenges in the process industry, such as the food and pharmaceutical industry, is the quantitative measurement of various quantities in fluid flows. Examples are the relative volume fractions of the different components of mixtures, the particle size of suspended or emulsified particles and the flow speed. The use of optical sensors to measure these quantities has many advantages: it is fast, non-invasive, and the application is generally straightforward. This thesis describes how (spectral) interferometry and heterodyne dynamic light scattering (DLS) can be used to measure the aforementioned quantities. Special attention is paid to the combination of the two techniques, and the combination of the multiple physical quantities that can be measured.

In Chapter 2, a transmission spectral Mach-Zehnder interferometer is described. From the interference spectrum, both the transmission spectrum and the path length difference due to the sample can be measured. These can be measured simultaneously, due to the application of Fourier filtering techniques on the interference spectrum. The measured path length difference depends on the refractive properties of the material of interest. These are the group index and the group velocity dispersion (GVD). With these two parameters, the volume fraction of the mixture can be determined. This is applied to water and glycerol mixtures, water and ethanol mixtures, and turbid samples of water and Intralipid. Broadband interferometric sensing allows for a more precise measurement of the GVD than traditional wavelength swept Abbe refractometry. In addition, the single shot measurement of the transmission spectrum and the interference spectrum allows this method to be used for in-line optical sensing.

Chapter 3 explores in more depth the nonlinear volume fraction dependence of both the transmission and the refractive index of colloidal suspensions. The transmission and the refractive index can be described by a complex refractive index: the real part is related to the phase delay of the light wave, and the imaginary part to the attenuation of the light due to scattering. The complex refractive index is determined for samples of various volume fractions of 100 nm sodium silicate particles with the same interferometer as described in Chapter 2. The measured attenuation is well described with far field interference, and the group index has a linear relationship with the volume fraction as expected for independent scattering. However, an interesting new non-linear effect was found in the GVD showing that the increase of the GVD with the volume fraction is lower than what is expected from independent scattering. We believe that this work is the first experimental demonstration of concentration-dependent scattering in the real part of the effective refractive index of colloidal media. With a dipole model very similar to the Lorentz-Lorenz model, the particle size and polydispersity of the sample are determined. This method is particularly useful for determining the refractive index of porous particles, as a conventional index matching experiment cannot be used reliably.

For industrial applications, real-time sensing is often necessary. Chapter 4 describes how a transmission interferometer is augmented with the addition of dynamic light scattering (DLS). By means of DLS the speed and size of nano and micro particles can be measured from the time correlations in the scattered light. In heterodyne DLS this backscattered light is amplified with a strong reference signal. With the combined optical transmission and DLS measurements, various process relevant parameters were simultaneously measured such as the volume fraction, mean particle size, size polydispersity, and flow speed. This is not possible with conventional DLS and spectral interferometry separately. This method is applied to sodium silicate particles to test the method. Furthermore, the applicability for industrial sensing is shown with a real-time measurement of the dissolution and aggregation of an Intralipid emulsion mixed with hydrochloric acid.

Organ-on-chip (OoC) systems are microfluidic devices for maintaining live tissue under physiologically relevant (flow) conditions. Imaging of structure and flow is important for the characterization of OoC device design and visualizing tissue/fluid interaction. Here, we present 3D tissue and flow imaging in an OoC device with multi-modal optical coherence tomography (OCT) using a combination of OCT structural imaging and flow imaging with Doppler OCT, number fluctuation dynamic light scattering OCT, and particle image velocimetry OCT. We demonstrate the feasibility of combined imaging of OoC tissue culture morphology and high flow velocities. We also measure low velocities in the OoC tissue well showing good agreement with computational fluid dynamics simulations. Our results open up the way for studying the effect of flow on living tissue in OoC devices.

The complex refractive index is analyzed by measuring its scattering attenuation µ_s, group index n_g, and group velocity dispersion (GVD) for 100 nm diameter silica nanoparticles dispersed in water. The experiments were performed for wavelengths between 410 nm and 930 nm. The experimental results were compared with different mixing models for the complex refractive index of colloidal suspensions. The group index linearly scaled with the volume fraction both in experiment and for all tested models. It was found that the GVD has a nonlinear dependence on volume fraction in agreement with the coupled dipole model of Parola et al. [J. Chem. Phys. 141, 124902 (2014)] The scattering attenuation is in good agreement with both the coupled dipole model and the low frequency quasi-crystalline approximation [J. Electromagn. Waves Appl. 2, 757 (1988)] that take particle correlations into account. With an iterative fitting procedure of all the data based on both the coupled dipole model and the quasi-crystalline approximation, the refractive index, porosity, and size of the nanoparticles were determined. We determined that the coupled dipole model is in best agreement with the data.

Organ-on-chip (OoC) systems are novel microfluidic microsystems that combine the advantages of well-characterised human cells with the benefits of engineered, physiological-like microenvironments manufactured in the system. The extracellular matrix (ECM) is the natural microenvironment of cells in the human body responsible for providing the appropriate stimuli to cells to control cell processes such as proliferation, migration, and apoptosis. OoCs can mimic the ECM, via channels and porous membranes, by providing the cells with physiological-like mechanical stimuli governed by the fluid dynamics in the system [1]. Understanding the fluid dynamics in OOC can aid in fine-tuning the stimuli sensed by the cultured cells, understanding cell behavior and cell fate. The current state of the art methods for characterizing fluid dynamics in the OoC systems are simulations, theoretical calculations, and empirical observations, therefore a quantitative characterization technique is lacking. Optical coherence tomography (OCT) has been used in previous studies to measure omnidirectional flow velocities in flow systems [2]. In this study, we measured the flow in a cuvette using a Thorlabs GANYMEDE II HR series (high axial resolution of 3 mm in air) spectral domain OCT system. We made quantitative 2D flow measurements using the phase-resolved Doppler method. This work was then extended to extract flow dynamics, in the Bi/ond inCHIPit using titania scattering nanoparticles, which would be a novel way of flow characterization in the field of OOC. The results are compared to the theoretical Hagen-Poiseuille equations and COMSOL simulations and found to be in good agreement. The results of the study were further extended to determine the shear stress experienced by the cells in the culture well of the OoC.

The group index, n_g , group velocity dispersion (GVD), and scattering attenuation coefficient, µ_s, were measured for dilutions of glycerol, ethanol, and Intralipid 20% with water. Experiments were performed with a supercontinuum laser based Mach–Zehnder spectroscopic interferometry setup for wavelengths between 400 and 930 nm. All optical properties could be retrieved from a single calibrated measurement of the interference spectrum. Scattering attenuation was determined from the envelope of the interference. The group index and GVD were retrieved from the unwrapped spectral phase. It was found that the group indices of glycerol and ethanol dilutions are in accordance with the Lorentz–Lorenz mixing formula. The scattering attenuation matches well to a semi-empirical model based on the Twerksy effective packing fraction.