Session Y18: Fluids XIII

Author not Attending

Nonlinear interaction and breaking of internal ocean waves are responsible for much of the interior ocean mixing, affecting ocean carbon storage and the global overturning circulation. These interactions are also believed to dictate the observed Garrett-Munk wave energy spectrum, which is still unexplained after 50 years of studies. According to the resonance wave interaction approximation used to derive the kinetic equation for the energy spectrum, the dominant interactions are between wave triads whose wavevectors satisfy k=p+q and their frequencies satisfy ω_k=|ω_p-ω_q| or ω_k=|ω_p+ω_q|. In order to test the validity of the resonance wave interaction approximation, we examine several analytical derivations of the theory. The assumptions underlying each derivation are tested using direct 2d numerical simulations representing near-observed energy levels of the internal wave field. We show that the assumptions underlying the derivations are not consistent with the simulated dynamics. In addition, most of the triads satisfying the resonant conditions do not contribute significantly to nonlinear wave energy transfer, while some interactions that are dominant in nonlinear energy transfers do not satisfy the resonance conditions. We also point to possible self-consistency issues with some derivations found in the literature.   

Modeling turbidity currents from a moving source: vehicle path optimization and polydispersity effects

We model turbidity currents generated by a moving source as applicable to particle clouds released by underwater vehicles such as those used in deep-sea dredging and mining. We use both the shallow-water approximation and box models to capture the dominant dynamics at play. Extending results obtained for a vehicle moving in a straight line, we consider more complex paths and determine how to minimize the impact of the moving vehicle on the surrounding environment. We also investigate the impact of polydispersity, considering both continuous and discrete particle size distributions.   

Simulations of settling marine aggregates in a stratified fluid

Settling marine aggregates are essential in transporting dissolved carbon dioxide from the ocean surface to the deep sea. While sinking, they accumulate in thin layers where density stratifications are present, becoming nutrient hotspots for bacterial and animal activity. Here, we simulate settling aggregates in a density stratified fluid. We assemble fractal aggregates as a collection of cubes to model a marine aggregate. In the absence of stratification, the flow around the aggregate is computed in the limit of zero Reynolds number using a boundary integral method. A term involving a volume integral is added to the boundary integral formulation to allow variable density in the ambient fluid. We couple the velocity with the advection-diffusion equation to track the density over time. We use this method to quantify how the presence of stratification affects the aggregate settling speed and residence time in a sharp stratification.   

Cerebrospinal Fluid Pressure Distribution in Head During External Impacts

Mild Traumatic Brain Injury (mTBI) has a significant contribution to injury-related disabilities that impact a substantial number of patients in many age groups per year. The brain as soft tissue is located inside skull. The subarachnoid space between brain and skull contains Cerebrospinal Fluid (CFS). During a sudden impact, this fluid which is surrounding the brain experiences stress variations that can affect brain tissue depends on the dynamic of impact, including rotational and translational acceleration.  In this study the interaction between skull, brain, and CFS during mTBI incident is investigated to better. Cerebrospinal Fluid pressure distribution, brain displacement and motions, and acceleration during the impact are studied by taking into account the interaction of CFS as fluid, and brain tissue as solid.

    

Graphene-based thermal pump to enable continuous water flow through slit nanochannels: The Role of Flexural Phonons

In this study, by performing all-atom Non-Equilibrium Molecular Dynamics simulations, we propose a graphene-based thermal nanopump which produces controlled and continuous water flow in nanoslit channels. Upon systematically imposing thermal gradients, a significant net fluid flow towards the low-temperature zone is measured in all cases. The present nanopump produces water flow with average velocities up to 3 m/s. We find that the water flow rates increase due to an enhancement of the wall thermal rippling as higher temperature gradients are imposed. Moreover, vibrational and phonon Density-of-States analyses on graphene walls reveal that the out-of-plane flexural phonons are responsible for the water flow production. We notice that lower frequency phonon branches are activated with higher imposed temperature gradients. This study provides the basis for developing a functional thermal pumping system based on sheets of graphene, which is capable to produce continuous water flow in nanofluidic conduits, opening the door to practical exploitation of thermal gradients for fluid transport in integrated nanofluidic devices.

    

High-Resolution Simulations of H2/O2 Detonations

The accurate simulation of gaseous combustion is difficult due to the multi-scale nature and geometrical complexity of detonation and deflagration waves. To address this difficulty, coarse-grained simulations were performed which take advantage of accelerated compute architectures. High-resolution, three-dimensional, large eddy simulations of the detonation of pre-mixed H₂/O₂ were performed with FURY, a GPU capable, compressible flow solver. The purpose of these simulations is to validate the FURY code for reactive flows, to demonstrate and quantify the code performance and scaling characteristics on GPU clusters, and to generate high-resolution reference solutions for the development of reaction and interface models. For these simulations, detonation waves were initiated by prescribing a point-like hot spot within a volume of pre-mixed H₂ and O₂. A single-step, global reaction mechanism is used to model the combustion of hydrogen with oxygen (2H₂+O₂->2H₂0). Additionally, the effects of an artificial viscosity model and noise filter are evaluated as a mechanism to stabilize the ignition discontinuity and to eliminate spurious oscillations. The performance and scaling characteristics of FURY are presented and detonation wave structure and propagation speeds are compared to experimental data for pre-mixed H₂/O₂ detonations.   

Electrohydrodynamic stability of a two-layer channel flow in the presence of interfacial surfactant

The electrohydrodynamic stability of a two-layer Poiseuille flow has been considered under the influence of an electric field acting normally to the interface.

The two viscous immiscible fluids considered in this study are leaky dielectrics.The electric field induced instability is further enhanced or suppressed by the

interfacial surfactant. The interplay between the effects of the electric field and the insoluble surfactant leads to the switching of the dominance of the unstable

modes. The dispersion curves obtained in the present study uncover the most unstable wave number and the corresponding growthrate of perturbation and

this information finds its relevance and application in the field of microfluidics.

    

Author not Attending

A wide array of processes from contaminant transport to membrane defouling involve interactions between deposited colloidal particles and an immiscible fluid interface. Previous works have studied interactions between individual particles and a moving interface. However, in many cases, particle deposits are dense aggregates, giving rise to new complexities that cannot be described by single-particle models. We use confocal microscopy to visualize interactions between these multilayer deposits and moving immiscible fluid droplets in microchannels. As the immiscible fluid interface passes over particles, we observe that they strongly adsorb to it due to the influence of capillary forces exerted by the fluid interface as it impinges on particles. Eventually, the fluid interface becomes saturated with adsorbed particles and reaches its "carrying capacity," after which it continually sloughs off particles. While injection of immiscible fluid interfaces has been explored for its potential to remove deposited particles from solid surfaces, our study reveals that rapid saturation of the fluid interface by particles presents a shortcoming of this process. Our results show that this limitation can be overcome by increasing fluid interfacial area, thus suggesting a new approach to anti-fouling.   

The tunability and influence of wetting on liquid/solid interfaces

Wetting, the study of how a liquid spreads on a solid (or a immiscible liquid) substrate, is crucial for a number of biological and engineering applications, such as wetting of the eye, rise of sap in a plant, and the anti-frost of airplane surfaces. Wetting not only influences the adhesion of nanometer-sized objects but also affects the transport of ions. It is believed that the wetting contact angle between water droplets and solids is increased by adding inorganic salts to water. Previous work suggests that the cosine of the contact angle is proportional to the inverse of liquid/vapor surface tension (Bonn, Daniel; Eggers, Jens; Indekeu, Joseph; Meunier, Jacques; Rolley, Etienne 2009). However, this inverse relation breaks down when the solid/liquid interfacial tension is strongly dependent on the presence of salt. Here, we use molecular dynamics simulations to investigate and understand the effect of liquid composition on the water contact angle on solid surfaces. Our work sheds light on the microscopic mechanisms of wetting and ion transport at solid/liquid interfaces.   

Osmotic swelling of chemically responsive hydrogels induced by non-uniform chemical signals

Experiments have shown that hydrogels that couple external chemical stimuli to their constituent polymers offer great potential for controlled shape transformations. For example, rapid release of a chemical stimulus (e.g., copper) trapped within a polyacrylic acid (PAA) hydrogel thin film upon arrival of a second stimulus (acid) can give rise to transient swelling. In contrast with swelling of most gels caused by the osmotic imbalance due to their polymer composition, we hypothesize that the transient swelling of the PAA gels must be driven by the temporary osmotic pressure accumulation upon rapid release of the trapped copper into the gel's fluid phase. Through an augmented 2-dimensional poroelastic theory, we simulate the mechanical response of the copper-complexed gel film to a non-uniform acid front, the subsequent dynamics of the released copper, and the resulting interstitial fluid flow that drives swelling. Our simulations confirm the emergence of traveling swelling fronts at the gel surface, in agreement with experiments. Overall, our theory elucidates the deformation dynamics of the PAA gel films, paving the way for their rational control by using chemical signals.   

Chiral Co-assembly of Titania Nanorods and Cellulose Nanocrystals

Chiral inorganic nanomaterials are sought after in various fields owing to their unique optical, electric, magnetic properties and could impact various applications in optical materials, catalysis, chiral separation and biomedicine. Here, a novel and cost-effective bottom-up method for fabricating chiral mesomorphic ceramics was achieved via co-assembly between TiO₂ nanorods and cellulose nanocrystals (CNCs). The methods is general, and the colloidal stability offered by TiO₂ and CNC stabilized with tetramethylammonium hydroxide (TMAH) could likely be extended to other inorganic particles. A phase diagram showing the mesophase behavior of co-assembled TiO₂ nanorods and CNCs at various compositions was experimentally determined and provided guidance for film fabrication. Films were fabricated by both evaporation-induced self-assembly (EISA) and vacuum-assisted self-assembly (VASA).  A comparison of the two methods shows that EISA offers higher transparency, and therefore it was selected to prepare chiral mesomorphic ceramic films.  Composite films prepared by EISA were subject to thermal treatment to remove the CNCs, and the inorganic superstructure partially preserved the chirality templated by the CNCs.    

Viscoelastic Properties of Molecularly Thin Liquid Crystal Films by Surface Light Scattering Spectroscopy

We present a surface light scattering spectroscopy study of the surface properties of molecularly thin films of 8CB (4′-Octyl-4-biphenylcarbonitrile) at an air-water interface as a function of increasing temperature, covering the range over which the bulk phase transition from smectic to nematic occurs. The spectrometer analyzes light scattered from thermocapillary waves to determine surface viscoelastic properties, knowing surface tension, which is measured independently with a Wilhelmy plate. Historically, this experiment has been difficult. Enhanced design has improved signal quality, increasing accessible wave vectors and ease of use. With this much-improved signal, we reproduced previous measurements from a monomolecular layer of pentadecanoic acid at an air-water interface. We also compare the visco-elastic properties deduced from the power spectrum of capillary waves of a smectic liquid crystal at an air-water interface with equivalent properties of a free-standing smectic liquid crystal film in air.