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Understanding properties of small particles, such as their size is important in many applications, for example in diagnostics and imaging methods. Often, the particles are dispersed in another solution, think for example of blood cells in serum. A widespread approach uses fluorescent labelling to visualize these particles, by means of chemically attaching a compound to the original particle. This causes differences in structure and how the particle moves. Here, we present a label-free, optical imaging technique, able to detect and characterize single particles in solution. By using microscopy and computational analysis, we image the particles and try to determine their size, down to the single-particle level. In addition, an electric field was applied to a single emulsion particle and its optical response was recorded. The size of the prepared nanoemulsion droplets was estimated to be between 100 and 150 nm. The results from the microscope show that the presented optical detection system is capable of detecting very small particles and that our method produces small and stable emulsions. The optical signal obtained from the application of the electric field was converted into a frequency response, using Fourier transformation. The results showed an expected signal at the starting frequency, but also some higher-order modes. These results serve as a first step into investigating single-particle dynamics, using straightforward experimental methods. From this point forward, other particle properties, such as their shape and response to more external perturbations could be investigated.

Electrochemistry plays a role in green technologies such as catalysts, batteries, hydrogen production and photovoltaics. It is greatly influenced by local effects at the solid-liquid region, importantly electric double layers. Past research shows that atomic force microscopy is a suitable method for studying these inhomogeneous local effects. Our goal is to develop an experimental and data analysis method for studying these systems. This thesis demonstrates EC-AFM measurements in PFT-mode, with a custom electrochemical cell filled with H2SO4 electrolyte, on a normal pulse voltammetry potential-controlled gold substrate. Data analysis of the force spectroscopy shows a clear dependency of the forces on the potential. Average force curves highlight the numerical values, and force curve maps show reproducible spatially inhomogeneous potential dependence of the adhesion. Deviations in the adhesion appear at potentials that are in literature associated with sulfate and oxide adsorption.

In physics, the optical trap has a wide range of uses. It can for example be used trap a cloud of particles. ”Low” density clouds have been used to create more sensitive interferometers, while ”high” density clouds are used to create Bose-Einstein condensates. In this project however, the optical trap isn’t used to trap a cloud of particles, but instead it’s used to trap a singular nanoparticle. Once such a nanoparticle has been trapped, I want to subject it to a high frequency alternating electric field and study the particles response to this field in time. Before I can start applying a high frequency alternating electric field, I first want to check for the expected behavior to different electric fields of which one knows what the response should be. These electric fields lie within the low and middle frequency regime. When both symmetric and asymmetric electric fields within the low frequency regime (f < 10000 Hz) are applied, the results for the measured ingoing electric field and the response of the nanoparticles are similar. However, when the mid frequency regime (10000 Hz < f < 250000 Hz) is reached, the measured ingoing electric field and the response of the nanoparticles start showing different results.

Measuring charges of single molecules is a difficult task. Especially small molecules inan aqueous environment are hard to characterize. One method to do this uses nano-oscillators, which are gold particles attached to a PEG molecule via a biotin-streptavidincomplex. In this thesis, these nano-oscillators will be constructed and optimized, afterwhich a potential will be applied to the system, which causes the particles to move.The observation of this movement will be carried out with a total internal reflectionmicroscope. We find that to construct the optimal slides, the ratio of biotin to non-biotin PEG molecules should be 1:1000, and the salt concentration of the gold particlesolution should be 20 mM. Further research can be done on the effects of the appliedvoltage, and the software used for analysis can be further refined.

When a mixture of colloidal particles and a high salt concentration flow through a microchannel at one side via an electro-osmotic micropump, and a low salt concentration flows into the channel from the other side. A clog of particles might grow inside the channel at the low salt side. I try to determine how far such a clog grows into the channel for different salt concentrations and potentials. Using Python, I will also determine the flow velocity of the particles and electrical current through the channel. Under the conditions I probe, I did not observe clogging systematically but laid a good foundation for further research on this topic.

We introduce PyNTA, a modular instrumentation software for live particle tracking. By using the multiprocessing library of Python and the distributed messaging library pyZMQ, PyNTA allows users to acquire images from a camera at close to maximum readout bandwidth while simultaneously performing computations on each image on a separate processing unit. This publisher/subscriber pattern generates a small overhead and leverages the multi-core capabilities of modern computers. We demonstrate capabilities of the PyNTA package on the featured application of nanoparticle tracking analysis. Real-time particle tracking on megapixel images at a rate of 50 Hz is presented. Reliable live tracking reduces the required storage capacity for particle tracking measurements by a factor of approximately 103, as compared with raw data storage, allowing for a virtually unlimited duration of measurements

Viscosity is a way to measure the internal resistance of a fluid to deformation and it can be seen as the friction between the different layers of the fluid. Contact with a surface can therefore make it harder to measure the viscosity. Because of this we will use acoustic levitation, so that we can remotely measure the viscosity of a liquid droplet without it being in contact with a surface. During our experiments, we will change different variables that have influence on this viscosity. We will change the temperature, so that we can test the accuracy of our method. We will also change the volume fraction of ethylene glycol in demi-water, once again to test the accuracy. In our third experiment, we will look at the viscosity of water droplets containing nano particles because we also want to find out if we are able to see the colloidal glass transition.

In this Thesis we analyze, both in theory and experiment, the phenomenon of stochastic resonance in an optical microcavity with a non-instantaneous nonlinearity. Such a non-instantaneous nonlinearity is found, for example, in media that heat and cool in finite time under constant illumination. Starting from a driven-dissipative Kerr model, we develop a new theory modeling the field in such media by introducing a memory kernel that assigns a timescale to the nonlinearity. The memory kernel makes the state of system explicitly depend on its complete history, and not only on its immediate past. The deviation from a Markovian approximation is characterized by the residence time distribution, which we find is no longer an exponential on timescales comparable to the characteristic timescale of the nonlinearity. This is an indication of level crossings which are correlated in time, a signature of non-Markovian dynamics. The model successfully reproduces experimental results demonstrating dynamical hysteresis in an optical microcavity with a thermal nonlinear medium inside. We furthermore explore the concept of stochastic resonance in this new model. This effect manifests itself in a resonance-like peak in the signal-to-noise ratio as a function of the noise intensity. The role of the thermal relaxation time is also studied and is found to shift the peak in the signal-to-noise ratio to larger noise strengths. We finally report the first experimental observation of stochastic resonance in a thermal nonlinear medium. Comparing experimental results with simulations, we find a good agreement between theory and experiment.

Single particle tracking of diffusing nanoparticles can be used to measure their size distribution. In this project gold nanoparticles and products from disaggregating fibrils both ranging from 30 nm to 100 nm were sideways illuminated by a laser. The emitted Rayleigh scattering from the particles was recorded with camera and used to localise the particles. The same technique was used to measure the electrophoretic mobility of 100 nm particles. It was found that, although it is difficult to find the exact size distribution, general differences between different size distributions can be measured. The measurement of the electrophoretic mobility by single particle tracking using this technique induces electro-osmotic effects that are highly dependent on the electric charge, shape and size of the measurement device itself.

Charged solid structures in the presence of an electrolyte form a electric double layer (EDL). The EDL determines the electrochemical activity and influences the electrical and optical polarizability of these structures. Building a microscope capable of detecting the influence of the EDL on the optical polarizability of particles would assist in research on the EDL and potentiodynamic reactions. I report on the capabilities and workings of our home-build total internal reflection microscope with enhanced stability and conductive flowcell samples. Results include the experimentally demonstration that with this custom total internal reflection microscope we can detect restructuring of the EDL at the nanoscale. The temporal and spatial characterization of the scattering signal demonstrate that the potentiodynamic optical contrast (PDOC) is proportional to the accumulated charge of polarizable ions at the interface and its time derivative represents the nanoscale ionic current.

In this project, Imaging through scattering media, we create an intensity-enhanced focus and scan it around by using the memory effect. Limitations and problems found during the process are described in this thesis. We describe how two techniques could be implemented to check if imaging through wavefront shaped focus works in reflection too.

Elastic scattering signal from optically active molecules below the Rayleigh regime can be detected by probing its absorption band. Within the absorption band, the optical scattering cross section (SCS) of the molecule is a rapidly varying function of light frequency. Knowing the absolute SCS of a single molecule has many applications like learning about the internal molecular levels (orbitals), or identifying molecules or some chemical processes. For this reason, we proposed an experimental scheme for measuring the absolute SCS of nanoparticles and single molecules. We developed a setup for this measurement, from where, currently, we obtained relative SCSs of particles with low extinction. Further improving the setup and samples will lead us towards absolute SCSs of small particles (like single fluorophores) and quantitative description of resonance light scattering.