Recently, a novel way of driving rapid microcentrifugation was discovered using ionic wind via ionization of the atmosphere around a singular electrode tip, driving liquid recirculation in a small cylindrical cavity due to interfacial shear. In the original work, the primary azimuthal surface recirculation was speculated to drive a secondary flow in the bulk of the liquid which resembles a helical swirling flow that tapers toward a pseudo-stagnation point at the cavity floor, analogous to Batchelor flows between co-axially placed stationary and rotating disks. Here, we employ microParticle Image Velocimetry (microPIV) together with numerical simulations to verify this speculation. Good qualitative and reasonable quantitative agreements were obtained between the experiments and numerical simulations. In both, we were able to capture salient features of the three-dimensional flow; for the experiments, this was achieved by the reconstruction of the three-dimensional flow field from the planar two-dimensional velocity fields obtained in a confocal-like manner. In addition, we formally quantify the micromixing enhancement first demonstrated, but not quantified, in the original experiments. Our results show a mixing enhancement close to two orders of magnitude approaching vigorous mixing intensities as the surface vortices suffer from various instabilities leading towards their breakdown into subvortices at large applied voltages and AC frequencies, reminiscent of that in the original work.
There has been some amount of confusion over the origin of electrohydrodynamic phenomena responsible for the actuation of dielectric fluids in the presence of an electric field. Previous studies have accounted for the possibility of conduction pumping, ion drag pumping and induction pumping as driving mechanisms but have ignored the possibility of Maxwell (electric) pressure driven flow. Until recently, this mechanism has been poorly understood and as a result has often been overlooked. This paper demonstrates how a Maxwell pressure gradient can induce flow in dielectric liquids in the presence of a non-uniform field. We derive, from first principles using lubrication theory, an expression for the flow velocity which exhibits a quadratic dependence on the applied voltage and also proportionality to the ratio of the permittivity and viscosity. The theoretical predictions are supported by experimental results. Although we have examined the phenomenon for a particular class of dielectric liquids, it is believed that this mechanism could well be responsible for the actuation of other low conductivity dielectric fluids previously attributed to conduction or ion drag pumping. In any case, we discuss ways to identify the dominant mechanism by comparing the salient features for a given type of flow.
There is increasing recognition of the value of four-dimensional flow cardiovascular magnetic resonance (4D-flow MRI) as a potential means to detect and measure abnormal flow behaviour that occurs during early left ventricular (LV) dysfunction. We performed a systematic review of current literature on the role of 4D-flow MRI-derived flow parameters in quantification of LV function with a focus on potential clinical applicability. A comprehensive literature search was performed in March 2022 on available databases. A total of 1186 articles were identified, and 30 articles were included in the final analysis. All the included studies were ranked as “highly clinically applicable”. There was considerable variability in the reporting of methodologies and analyses. All the studies were small-scale feasibility or pilot studies investigating a diverse range of flow parameters. The most common primary topics of investigation were energy-related flow parameters, flow components and vortex analysis which demonstrated potentials for quantifying early diastolic dysfunction, whilst other parameters including haemodynamic forces, residence time distribution and turbulent kinetic energy remain in need of further evaluation. Systematic quantitative comparison of study findings was not possible due to this heterogeneity, therefore limiting the collective power of the studies in evaluating clinical applicability of the flow parameters. To achieve broader clinical application of 4D-flow MRI, larger scale investigations are required, together with standardisation of methodologies and analytical approach.
PurposeCurrent intervention guidelines for bicuspid aortic valve (BAV) associated ascending aorta (AAo) dilatation are suboptimal predictors of clinical outcome. There is growing interest in identifying better biomarkers such as wall shear stress (WSS) to help risk stratify BAV aortopathy. The aim of the systematic review is to synthesize existing evidence of the relationship between WSS and aortopathy in the BAV population.MethodsA comprehensive literature search of available major databases was performed in May 2022 to include studies that used four-dimensional flow cardiac magnetic resonance (4D-flow) MRI to quantify WSS in the AAo in adult BAV populations. Summary results and statistical analysis were provided for key numerical results. A narrative summary was provided to assess similarities between studies.ResultsA total of 26 studies that satisfied selection criteria and quality assessment were included in the review. The presence of BAV resulted in significantly elevated WSS magnitude and circumferential WSS, but not axial WSS. The presence of aortic stenosis had additional impact on WSS and flow alterations. BAV phenotypes were associated with different WSS distributions and flow profiles. Altered protein expression in the AAo wall associated with WSS supported the contribution of altered hemodynamics to aortopathy in addition to genetic factors.ConclusionWSS has the potential to be a valid biomarker for BAV aortopathy. Future work would benefit from larger study cohorts with longitudinal evaluations to further characterize WSS association with aortopathy, mortality, and morbidities.Systematic review registrationhttps://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42022337077, identifier CRD42022337077.
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