(Dissertation) A Novel Model for Precipitation Reactions in Microfluidic Devices,
[PDF] [bibtex citation]
P.S. Eastham
FSU Dissertation Publishing
(2020)
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Abstract:
Chemical processes within flows are ubiquitous. There exists an important class of reactions that
result in a phase change from liquid to solid: precipitation reactions. Inspired by recent microfluidic experiments, this dissertation develops a unified mathematical framework for handling such
reactions occurring within a slow-moving fluid flow. A key challenge for precipitate reactions is
that, in general, the location of the developed solid is unknown a priori. To model this situation,
we use a multiphase framework with fluid and solid phases; the aqueous chemicals exist as scalar
fields that react within the fluid to induce phase change. We conduct several analytic and numerical validation studies to verify that the model exhibits desired fluid–structure behaviors without
requiring interface boundary conditions. To demonstrate the functionality of this framework, we
conduct full-scale simulations of two scientific applications: a microfluidic reaction experiment as
well as a geophysical study of sinkhole formation. The results of this dissertation are (1) a rigorous
derivation of a model framework that conserves mass and incorporates fluid-structure interaction,
and (2) numerical methods for solving the full PDE system, which have been implemented in
open-source software. The framework can be applied to precipitate reactions where the precipitate
greatly affects the surrounding flow, a situation appearing in many laboratory and geophysical
contexts including the hydrothermal vent theory for the origin of life. More generally, this model
can be used to address low Reynolds number fluid–structure interaction problems that feature the
dynamic generation of structures.
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Axisymmetric squirmers in Stokes fluid with nonuniform viscosity,
[PRF link]
P.S. Eastham, and K. Shoele
Physical Review Fluids
(2020)
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Abstract:
The ciliary locomotion and feeding of an axisymmetric microswimmer in a complex fluid whose viscosity depends on a surrounding nutrient field are investigated numerically in order to extend previous asymptotic results for weak nutrient-viscosity coupling. Numerical simulations capture nonlinearities inherent in the full system that are missed using perturbation-based linearization methods. The microswimmer's ciliary beating is modeled by a slip velocity, i.e., the squirmer model, and body geometry is modeled by spheroids. It is found that swimming speed and feeding are most affected by a nonuniform viscosity environment when the ratio of advection forces to diffusion transport, characterized by the nondimensional Peclét number, is moderate, i.e., Pe=O(5). These changes are correlated to significant increases in the pressure force on the surface of the squirmer, as well as differences in power expenditure and hydrodynamic efficiency compared to the constant-viscosity case. Additionally, the swimming and feeding changes are found to be more significant in oblate spheroids than prolate spheroids, although the shape has a smaller effect on performance than Peclét number or surface stroke. These results suggest that nonlocal effects from viscosity variation are caused by a modification to the pressure force, as opposed to the strain rate. These results should be useful in interpreting experiments where a microswimmer affects a fluid's local rheology.
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Multiphase modelling of precipitation-induced membrane formation,
[read-only PDF] [JFM link]
P.S. Eastham, M.N.J. Moore, N.G. Cogan, Q. Wang, and O. Steinbock
Journal of Fluid Mechanics
(2020)
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Abstract:
We formulate a model for the dynamic growth of a membrane developing in a flow as the result of a precipitation reaction, a situation inspired by recent microfluidic experiments. The precipitating solid introduces additional forces on the fluid and eventually forms a membrane that is fixed in the flow due to adhesion with a substrate. A key challenge is that, in general, the location of the immobile membrane is unknown a priori. To model this situation, we use a multiphase framework with fluid and membrane phases; the aqueous chemicals exist as scalar fields that react within the fluid to induce phase change. To verify that the model exhibits desired fluid–structure behaviours, we make simplifying assumptions to obtain a reduced form of the equations that is amenable to exact solution. This analysis demonstrates no-slip behaviour on the developing membrane without requiring fluid–membrane interface boundary conditions. The model has applications towards precipitate reactions where the precipitate greatly affects the surrounding flow, a situation appearing in many laboratory and geophysical contexts including the hydrothermal vent theory for the origin of life. More generally, this model can be used to address fluid–structure interaction problems that feature the dynamic generation of structures.
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Effects of nonuniform viscosity on ciliary locomotion,
[PRF link]
K. Shoele and P.S. Eastham
Physical Review Fluids
3(4), 043101 (2018)
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Abstract:
The effect of nonuniform viscosity on the swimming velocity of a free swimmer at zero Reynolds number is examined. Using the generalized reciprocal relation for Stokes flow with nonuniform viscosity, we formulate the locomotion problem in a fluid medium with spatially varying viscosity. Assuming the limit of small variation in the viscosity of the fluid as a result of nonuniform distribution of nutrients around a swimmer, we derive a perturbation model to calculate the changes in the swimming performance of a spherical swimmer as a result of position-dependent viscosity. The swimmer is chosen to be a spherical squirmer with a steady tangential motion on its surface modeling ciliary motion. The nutrient concentration around the body is described by an advection-diffusion equation. The roles of the surface stroke pattern, the specific relationship between the nutrient and viscosity, and the Péclet number of the nutrient in the locomotion velocity of the squirmer are investigated. Our results show that for a pure treadmill stroke, the velocity change is maximum at the limit of zero Péclet number and monotonically decreases toward zero at very high Péclet number. When higher surface stroke modes are present, larger modification in swimming velocity is captured at high Péclet number where two mechanisms of thinning the nutrient boundary layer and appearance of new stagnation points along the surface of squirmer are found to be the primary reasons behind the swimming velocity modifications. It is observed that the presence of nonuniform viscosity allows for optimal swimming speed to be achieved with stroke combinations other than pure treadmill.
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Local oxytocin tempers anxiety by activating GABAA receptors in the hypothalamic paraventricular nucleus,
[link]
A. S. Smith, M. Tabbaa, K. Lei, P. S. Eastham, M. J. Butler, L. Linton, R. Altshuler, Y. Liu, Z. Wang
Psychoneuroendocrinology
63, 50-58 (2016)
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Abstract:
Oxytocin (Oxt) is released in various hypothalamic and extrahypothalamic brain areas in response to anxiogenic stimuli to regulate aspects of emotionality and stress coping. We examined the anxiolytic action of Oxt in the hypothalamic paraventricular nucleus (PVN) while appraising if Oxt recruits GABA neurons to inhibit the behavioral, hormonal, and neuronal response to stress in female prairie voles (Microtus ochrogaster). Voles received an injection of Oxt in the PVN either before or after an elevated platform stress to determine a time-course for the effects of Oxt on the hormonal stress response. Subsequently, we evaluated if ante-stress injections of Oxt affected anxiety-like behaviors as well as neuronal activity in the PVN, using real-time in-vivo retrodialysis and immunohistochemistry with c-Fos expression as a biomarker of neural activity. In addition, we exposed voles to Oxt and a GABAA receptor antagonist, concurrently, to evaluate the impact of pharmacological blockade of GABAA receptors on the anxiolytic effects of Oxt. Elevated platform stress amplified anxiety-like behaviors and hypothalamic-pituitary-adrenal (HPA) axis activity-catalyzing corticotrophin-releasing hormone (CRH) neuronal activity and augmenting corticosterone release in circulation. Ante-stress Oxt injections in the PVN blocked these stress effects while promoting PVN GABA activity and release. Post-stress Oxt treatments were ineffective. The anxiolytic effects of Oxt were hindered by concurrent pharmacological blockade of GABAA receptors. Together, our data demonstrate ante-stress treatments of Oxt in the PVN inhibit stress activation of the HPA axis through recruitment of GABAergic neurons, providing insights to the local circuitry and potential therapeutically-relevant mechanisms.
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Last revised on 10 August 2020.
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