Non-Newtonian fluid-structure interaction: Flow of a viscoelastic Oldroyd-B fluid in a deformable channel

We analyze the steady non-Newtonian fluid-structure interaction between the flow of an Oldroyd-B fluid and a deformable channel. Specifically, we provide a theoretical framework for calculating the leading-order effect of the fluid's viscoelasticity on the flow rate-pressure drop relation and on the deformation of the channel's elastic wall. We first identify the characteristic scales and dimensionless parameters governing the fluid-structure interaction in slender and shallow channels. Applying the lubrication approximation for the flow and employing a perturbation expansion in powers of the Deborah number $De$, we derive a closed-form expression for the pressure as a function of the non-uniform shape of the channel in the weakly viscoelastic limit up to $\mathrm{O}(De)$. Coupling the hydrodynamic pressure to the elastic deformation, we provide the leading-order effect of the interplay between the viscoelasticity of the fluid and the compliance of the channel on the pressure and deformation fields, as well as on the flow rate-pressure drop relation. For the flow-rate-controlled regime and in the weakly viscoelastic limit, we show analytically that both the compliance of the deforming top wall and the viscoelasticity of the fluid decrease the pressure drop. Furthermore, we reveal a trade-off between the influence of compliance of the channel and the fluid's viscoelasticity on the deformation. While the channel's compliance increases the deformation, the fluid's viscoelasticity decreases it.Comment: 5 figure

A note about convected time derivatives for flows of complex fluids

We present a direct derivation of the typical time derivatives used in a continuum description of complex fluid flows, harnessing the principles of the kinematics of line elements. The evolution of the microstructural conformation tensor in a flow and the physical interpretation of different derivatives then follow naturally.Comment: 1 figur

Mass Transfer Limitations of Porous Silicon-Based Biosensors for Protein Detection

Porous silicon (PSi) thin films have been widely studied for biosensing applications, enabling label-free optical detection of numerous targets. The large surface area of these biosensors has been commonly recognized as one of the main advantages of the PSi nanostructure. However, in practice, without application of signal amplification strategies, PSi-based biosensors suffer from limited sensitivity, compared to planar counterparts. Using a theoretical model, which describes the complex mass transport phenomena and reaction kinetics in these porous nanomaterials, we reveal that the interrelated effect of bulk and hindered diffusion is the main limiting factor of PSi-based biosensors. Thus, without significantly accelerating the mass transport to and within the nanostructure, the target capture performance of these biosensors would be comparable, regardless of the nature of the capture probe-target pair. We use our model to investigate the effect of various structural and biosensor characteristics on the capture performance of such biosensors and suggest rules of thumb for their optimization.

Shaping liquid films by dielectrophoresis

We present a theoretical model and experimental demonstration of thin liquid film deformations due to a dielectric force distribution established by surface electrodes. We model the spatial electric field produced by a pair of parallel electrodes and use it to evaluate the stress on the interface through Maxwell stresses. By coupling this force with the Young-Laplace equation, we obtain the deformation of the interface. To validate our theory, we design an experimental setup which uses microfabricated electrodes to achieve spatial dielectrophoretic actuation of a thin liquid film, while providing measurements of microscale deformations through digital holographic microscopy. We characterize the deformation as a function of the electrode-pair geometry and film thickness, showing very good agreement with the model. Based on the insights from the characterization of the system, we pattern conductive lines of electrode pairs on the surface of a microfluidic chamber and demonstrate the ability to produce complex two-dimensional deformations. The films can remain in liquid form and be dynamically modulated between different configurations or polymerized to create solid structures with high surface quality.Comment: 14 pages and 7 figures in the main manuscript. 4 sections and 5 figures in the S

REACTIVITY OF RAT ERYTHROCYTES UNDER CONDITIONS OF PHYSICAL STRESS OF DIFFERENT INTENSITY

Цель. Изучить морфофункциональное состояние красной крови крыс в зависимости от физической нагрузки «до отказа».Материалы и методы. В исследовании использовали практически здоровых половозрелых крыс-самцов Wistar: контроль, нахождение в воде без движения, свободное плавание и плавание с грузом 2%, 6%, 8%, 10% и 15% от массы тела «до отказа» согласно протоколу по биоэтике утвержденному ИФ Коми НЦ УрО РАН. В крови определяли уровень гемоглобина, гематокрита, количество эритроцитов и ретикулоцитов. Измеряли диаметр 100 эритроцитов окрашенных по Романовскому-Гимза и бриллиантовым крезиловым синим. Статистическую значимость различий реакций исследованных животных оценивали с помощью непараметрического критерия Крускала-Уоллиса.Результаты. Физическая нагрузка «до отказа» характеризовалась повышением величины гематокрита, гемоглобина, количеством эритроцитов в крови крыс и физиологическим ретикулоцитозом, наряду со снижением концентрации гемоглобина в эритроците. Отмечено, что распределение ретикулоцитов по диаметру в пределах 6.4–6.7 мкм у плавающих крыс с грузом 8% от массы тела соответствует кривой Прайс-Джонса интактных животных.Заключение. У крыс в зависимости от характера и интенсивности физической нагрузки проявились разнонаправленные сдвиги в клеточном составе красной крови. Практическое использование только тестового плавания крыс с грузом 8 % от массы тела, судя по реакции эритроцитов, является физиологически обоснованным.Background. To study the morphofunctional state of red blood in rats depending on the physical load “to failure”.Materials and methods. The study used practically healthy adult male rats of the Wistar: control, staying in water without movement, free swimming and swimming with a load of 2%, 6%, 8%, 10% and 15% of body weight “to failure” according to the protocol on bioethics approved by the IPhys Komi SC UB RAS. In the blood, the level of hemoglobin, hematocrit, the number of erythrocytes and reticulocytes was determined. The diameter of 100 erythrocytes stained according to Romanovsky-Giemsa and diamond cresyl blue was measured. The statistical significance of the differences was assessed using the nonparametric Kruskal-Wallis criterion.Results. The physical load «to failure» was characterized by an increase in hematocrit, hemoglobin, and the number of erythrocytes in the blood of rats and physiological reticulocytosis, along with a decrease in the concentration of hemoglobin in the erythrocyte. It was noted that the distribution of reticulocytes of diameters within 6.4–6.7 μm in floating rats with a weight of 8% of body weight corresponds to the Price-Jones curve of intact animals.Conclusion. In rats, depending on the intensity of exercise, multidirectional shifts in the cellular composition of red blood appeared. Practical use only of the test swimming of rats with a weight of 8% of body weight, judging by the nature of the reaction of erythrocytes, is physiologically justified

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements