As a result of recent advances in micro and nanotechnology, the tiny movements of nanoscale active and passive objects in fluids can be probed with ultrahigh sensitivity and time resolution. The overarching theme of this dissertation is to harness these movements in fluids in order to study fundamental fluid dynamics and develop novel biomedical devices. First, we use the oscillatory movements of nanocantilevers to investigate the scaling behavior of unsteady fluid flow. Here, our expansive experimental data and rigorous theoretical analysis suggest that a generalized scaling parameter combining the length and time scales of the flow governs the scaling. Second, we turn our attention to nanoscale movements of bacteria in a buffer. We develop a simple, robust and sensitive experimental method to detect and track the random movements of bacteria. Using this method, we show evidence that these random movements of bacteria correlate with their antibiotic susceptibility. In the first part of this thesis, we explore, through experimental and theoretical work, the breakdown of the Navier-Stokes equations in oscillatory fluid flows. The Navier-Stokes equations of hydrodynamics are based on two crucial assumptions. First, the fluid is approximated as a continuum, with a well-defined ``fluid particle." Second, the stress in the fluid is assumed to be a linear function of the rate-of-strain, resulting in a so-called Newtonian fluid. If a fluid such as an ideal gas is gradually rarefied, the Navier-Stokes equations begin to fail and a kinetic description of the flow becomes appropriate. The failure of the Navier-Stokes equations can be thought to take place via two different physical mechanisms: either the continuum hypothesis breaks down as a result of a finite size effect; or the local equilibrium is violated due to the high rate of strain. Our experimental approach is to create an unsteady flow by oscillating a finite-sized body in a gas and to measure the dissipation (or the drag force) acting on the body. By using micro and nanofabrication techniques, we independently tune the relevant linear dimensions and the frequencies of the oscillating bodies. We then measure the pressure-dependent dissipation of these micro/nano oscillators in three different gases, Helium, Nitrogen, and Argon. We observe that the scaling of the fluidic dissipation is governed by a subtle interplay between the length scale and the frequency, embodied respectively in the dimensionless Knudsen (Kn) and the Weissenberg ( Wi) numbers. We collapse all the experimental data using a single scaling parameter: Wi + Kn. This new dimensionless parameter, which can be regarded as a generalized Knudsen number, combines the relevant linear dimension and the frequency of the body; it is rooted in Galilean invariance and can be obtained rigorously from the Chapman-Enskog expansion of the Boltzmann equation. In the second part of the thesis, we turn to the movements of active micro-swimmers in a buffer. This portion of the work is motivated by a serious global public health problem: the rise of multi-drug resistant bacteria. One way to prevent this threat from growing is to treat bacterial infections with effective antibiotics using the minimum dosage. However, currently-used antibiotic susceptibility tests (ASTs), which determine whether or not bacterial isolates from a patient are susceptible to administered antibiotics, take too long. Here, we aim to develop a robust and rapid AST by exploiting a recently-observed microbiological phenomenon: various nanomechanical movements of bacteria subside promptly (within minutes) when the bacteria are exposed to an effective antibiotic. Our approach is to transduce bacterial movements into electrical voltage fluctuations in a microchannel filled with a buffer solution. When a small but constant current is driven through the microchannel, bacterial movements are converted into strong voltage fluctuations due to the fact that they modulate the effective microchannel diameter. Our experiments with E. coli show that the proposed detection method can provide antibiotic susceptibility results in ~1 hour, making it a promising rapid AST. Because this approach is based on a simple electrical measurement and does not require delicate process steps and instrumentation, it may eventually be used at the point of care. / 2019-03-09T00:00:00Z

Several applications, including those in the energy sector that require high thermal efficiency, such as those in the solar energy industry, require a careful thermal analysis of heat exchange components. In this regard, thermal resistance is a major cause of exergy destruction and must be minimised as much as possible, but also adequately designed. In the past, a number of correlations have been developed to predict heat transfer coefficients in compact heat exchangers. The designers of such heat exchangers often exploit the development of thermal boundary layers to achieve higher overall efficiency due to increases in local heat transfer coefficients. However, most of the correlations that have been developed for heat exchangers neglect the specific effect of the thermal boundary layer development in the inlet region, and instead only offer effective average heat transfer coefficients, which most users assume to be constant throughout the heat exchanger. This is often an over-simplification and leads to over-designed heat exchangers. In this study, focus is placed on annular flow passages with uniform heating on the inner wall. This geometry has many applications. This study aims to collect experimental heat transfer data for water at various flow rates and inlet geometries, to process the data and determine local and overall heat transfer coefficients, and to develop an improved local heat transfer coefficient correlation. Experimental tests were performed on a horizontal concentric tube-in-tube heat exchanger with a length of 1.05 m and a diameter ratio of 0.648. The surface of the inner tube was treated with thermochromic liquid crystals (TLCs), which allowed for high-resolution temperature mapping of the heated surface when combined with an automated camera position system in order to determine local heat transfer coefficients. Conventional in-line and out-of-line annular inlet configurations were evaluated for Reynolds numbers from 2 000 to 7 500, as well as the transition from laminar to turbulent flow for a single in-line inlet configuration. It was found that the local heat transfer coefficients were significantly higher at the inlets, and decreased as the boundary layers developed. With the high resolution of the results, the local heat transfer coefficients were investigated in detail. Local maximum and minimum heat transfer coefficients were identified where the thermal boundary layers merged for high turbulent flow cases. The annular inlet geometries only influenced the heat transfer for Reynolds numbers larger than 4 000, for which larger inlets are favoured. Out-of-line inlet geometries are not favoured for heat transfer. A new heat transfer correlation was developed from the experimental data, based on an existing heat transfer correlation for turbulent flow in an annular flow passage, considering the boundary layer development. The new correlation estimated the area-weighted heat transfer coefficients within 10% of the experimental data and closely followed trends for local heat transfer coefficients. / Dissertation (MEng)--University of Pretoria, 2017. / Mechanical and Aeronautical Engineering / MEng / Unrestricted