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The use of nematics in microfluidic devices offers methods of flow control that are inaccessible to isotropic fluids. While nematics have been used in channel flows found in lab on chip devices, their flow and microstructure formation still needs to be fully understood in geometries with a cylindrical cross-section. This paper investigates liquid crystal flows through capillaries via 1) the Beris-Edwards (BE) model and 2) the Leslie-Ericksen (LE) model, which emerges as a limiting case (uniaxial, constant order parameter) of the BE model. We show that the microstructure distribution is controlled by the Ericksen (Er) and Deborah (De) numbers in the Beris-Edwards model. Defects at low Ericksen numbers arise when the elastic energy dominates the thermotropic energy. Defect destruction provides a new shear-thinning mechanism, which cannot be observed in the Leslie- Ericksen theory. [ABSTRACT FROM AUTHOR]

Thin films consisting of polymer solutions are typically produced through a combination of extrusion and shearing processes, where the anisotropic, non-Newtonian solution is deformed and subjected to thermal treatment. This paper investigates the shearing of polymeric thin films by studying the channel flow rheology of polymer solutions that experience yield stress. The material rheology is described by the transversely isotropic fluid (TIF) model, which contains a yield behavior term related to microstructure distortion. Our results show that this distortional stress is able to resist the pressure gradient, and non-trivial stress distributions can exist in the absence of a flow. This represents a significant improvement over existing viscosity-based yield stress models (e.g., the Heschel–Bulkley model). The unyielded state is achieved as the end result of a transient process, where a pressure gradient produces a short-lived flow that ceases when opposing stresses from microstructure distortion are produced. Predictions of the TIF model are compared with the phenomenological Saramito model. Both models are found to predict yielding when a threshold stress is exceeded. In both cases, the velocity profile is Newtonian near the wall, while plug flows are encountered close to the centerline. [ABSTRACT FROM AUTHOR]

Mitochondrial DNA mutations transmitted maternally within the oocyte cytoplasm often cause life-threatening disorders. Here we explore the use of nuclear genome transfer between unfertilized oocytes of two donors to prevent the transmission of mitochondrial mutations. Nuclear genome transfer did not reduce developmental efficiency to the blastocyst stage, and genome integrity was maintained provided that spontaneous oocyte activation was avoided through the transfer of incompletely assembled spindle-chromosome complexes. Mitochondrial DNA transferred with the nuclear genome was initially detected at levels below 1%, decreasing in blastocysts and stem-cell lines to undetectable levels, and remained undetectable after passaging for more than one year, clonal expansion, differentiation into neurons, cardiomyocytes or β-cells, and after cellular reprogramming. Stem cells and differentiated cells had mitochondrial respiratory chain enzyme activities and oxygen consumption rates indistinguishable from controls. These results demonstrate the potential of nuclear genome transfer to prevent the transmission of mitochondrial disorders in humans., A crucial determinant of the phenotypic severity in most maternally inherited mitochondrial diseases is heteroplasmy, that is, the proportion of mutant, relative to total, mitochondrial DNA (mtDNA) in a cell. [...]

The exchange of the oocyte's genome with the genome of a somatic cell, followed by the derivation of pluripotent stem cells, could enable the generation of specific cells affected in degenerative human diseases. Such cells, carrying the patient's genome, might be useful for cell replacement. Here we report that the development of human oocytes after genome exchange arrests at late cleavage stages in association with transcriptional abnormalities. In contrast, if the oocyte genome is not removed and the somatic cell genome is merely added, the resultant triploid cells develop to the blastocyst stage. Stem cell lines derived from these blastocysts differentiate into cell types of all three germ layers, and a pluripotent gene expression program is established on the genome derived from the somatic cell. This result demonstrates the feasibility of reprogramming human cells using oocytes and identifies removal of the oocyte genome as the primary cause of developmental failure after genome exchange., The generation of animals by transfer of the genome from an adult cell into an unfertilized oocyte (1), and the isolation of pluripotent stem cells from human blastocysts (2), raised [...]

During the processing of nematic soft solids through process flow elements (pipe bends, elbows, etc.), the constitutive behavior makes its presence felt via processing (with rheology driven effects increasing pressure drop) and the final product microstructure. This paper explores the flow and microstructure configurations of nematic liquid crystals in a pressure driven flow through 90° pipe bends with different types of wall anchoring. The governing equations of the Leslie–Ericksen theory are solved numerically in a newly developed OpenFOAM solver. We show that the bend curvature deforms the nematic axis distribution; the distortion can be driven either by elastic or hydrodynamic effects. The interaction between the nematic microstructure and flow field generates non-zero normal stresses (in the radial, azimuthal, and streamwise directions), which produce a secondary flow and increase pressure losses. The strength of the secondary flow depends on the type of wall anchoring and Ericksen number; in configurations with homeotropic anchoring, decreasing the Ericksen number increases the relative strength of the secondary flow (with respect to the mean flow velocity). Conversely, homogeneous (planar) anchoring reduces normal stresses, thus weakening the secondary flow strength. We show that as the fluid enters/leaves the bend, there is a perturbation in the transverse velocity caused by streamwise stress gradients. The perturbation magnitude depends on material properties and can be of different values at the bend exit and entrance. Finally, we show that the spatial development of the nematic field downstream of the bend exit is controlled by both material properties and the Ericksen number. [ABSTRACT FROM AUTHOR]

The assumption of homogeneous isotropic turbulence when modeling drop breakage in industrially relevant geometries is questionable. We describe the development of an anisotropic breakage model, where the anisotropy is introduced via the inclusion of a perturbed turbulence spectrum. The selection of the perturbed spectrum is itself motivated by our previous large‐eddy simulations of high‐pressure homogenizers. The model redistributes energy from small to large scales, and assumes that the anisotropic part of the Reynolds stresses is confined to the energy‐containing range. The second‐order structure function arising from the perturbed spectrum is used in the standard framework of Coulaloglou and Tavlarides to calculate breakage frequency. While the base model exhibits non‐monotonic behavior (by predicting a maximum value for a certain drop size), the effect of anisotropy is shown to increase breakage frequency in length scales larger than this peak, thereby reducing non‐monotonicity. This effect is more pronounced for small turbulence Reynolds numbers. [ABSTRACT FROM AUTHOR]

We explore the use of an Elastic Perfectly Plastic (EPP) constitutive equation for the modelling of yield stress fluids. Contrary to many other models, stresses in the EPP model arise from elastic deformation rather than as a viscous effect. In this paper, the EPP model is coupled to a standard viscous treatment of the post-yield flow stresses to produce Bingham-like behaviour, and the timescale associated with the yielding mechanism is linked to material parameters. We also show that when the yield stress is much smaller than the elastic modulus, EPP and Bingham models can produce very similar flow fields in channel and contraction geometries. The EPP model is found to be significantly cheaper computationally in both geometries. Additionally, in the case of channel flow where analytical solutions exist, the EPP model is associated with a much smaller error than the regularised Bingham model. • The Elastic Perfectly Plastic (EPP) model for yield stress fluids is proposed. • Convected stress derivatives are discussed in the context of yield stress fluids. • Predictions of the new model are discussed in basic rheometric flows. • EPP and Bingham models are compared in channel and contraction flows. [ABSTRACT FROM AUTHOR]

The simulation of dilute emulsions in a model ACIP2 Sonolator is investigated using computational fluid dynamics and population balance methods. Two breakage frequency models are used that differ in the expression of the drop breakage time. Drop breakage modeling based on homogenous isotropic turbulence (HIT) shows poor agreement of the Sauter mean diameter when compared to the experiments; simulations with the empirical model from Alopaeus et al. yield better agreement. By perturbing the classical HIT spectrum, it is shown that the breakage time in the empirical model corresponds to a non‐isotropic energy spectrum. Such spectra have been observed in the non‐isotropic near field in a model A Sonolator, which provides a plausible explanation of why the empirical model performs better than the HIT‐based model. [ABSTRACT FROM AUTHOR]