Shear-Driven Mechanism of Temperature Gradient Formation in Microfluidic Nematic Devices: Theory and Numerical Studies

The purpose of this paper is to show some routes in describing the mechanism responsible for the formation of the temperature difference ΔT at the boundaries of the microfluidic hybrid aligned nematic (HAN) channel, initially equal to zero, if one sets up the stationary hydrodynamic flow vst or under the effect of an externally applied shear stress (SS) to the bounding surfaces. Calculations based on the nonlinear extension of the classical Ericksen–Leslie theory, supplemented by thermomechanical correction of the SS σzx and Rayleigh dissipation function while accounting for the entropy balance equation, show that the ΔT is proportional to the heat flux q across the HAN channel and grows by up to several degrees under the influence of the externally applied SS. The role of vst=ust(z)i^ with a sharp triangular profile ust(z) across the HAN in the production of the highest ΔT is also investigated.

Actin filament alignment causes mechanical hysteresis in cross-linked networks

Cells dynamically control their material properties through remodeling of the actin cytoskeleton, an assembly of cross-linked networks and bundles formed from the biopolymer actin. We recently found that cross-linked networks of actin filaments reconstituted in vitro can exhibit adaptive behavior and thus serve as a model system to understand the underlying mechanisms of mechanical adaptation of the cytoskeleton. In these networks, training, in the form of applied shear stress, can induce asymmetry in the nonlinear elasticity. Here, we explore control over this mechanical hysteresis by tuning the concentration and mechanical properties of cross-linking proteins in both experimental and simulated networks. We find that this effect depends on two conditions: the initial network must exhibit nonlinear strain stiffening, and filaments in the network must be able to reorient during training. Hysteresis depends strongly and non-monotonically on cross-linker concentration, with a peak at moderate concentrations. In contrast, at low concentrations, where the network does not strain stiffen, or at high concentrations, where filaments are less able to rearrange, there is little response to training. Additionally, we investigate the effect of changing cross-linker properties and find that longer or more flexible cross-linkers enhance hysteresis. Remarkably plotting hysteresis against alignment after training yields a single curve regardless of the physical properties or concentration of the cross-linkers.

Adhesion of marzipan pastes based on dry demineralized whey

The results of researches of rheological and adhesion characteristics of marzipan pastes with dry demineralized whey (DDW) and glycerin are given. The positive effect of dry demineralized whey and glycerin on the characteristics of model compositions of marzipan pastes has been established. The component compatibility of DDW and almonds has been confirmed. It has been experimentally established that DDW and glycerin lead to changes in the structural state of marzipan pastes, changing the quantitative values ​​of rheological characteristics. It is confirmed that with increasing DDW concentration, the indicators of deformation and plasticity increase. The indicators of elasticity and resilience of marzipan paste decrease, which in general leads to an increase in molding ability. The surface effect on the properties of marzipan pastes with dry demineralized whey and glycerin was carried out. The technological expediency of using glycerin in the composition of marzipan pastes with DDW to increase their plasticity and pliability while maintaining high molding properties is substantiated. The rational content of glycerin is established, which allows regulating the adhesion properties within the set limits for paste-like finishing semi-finished products from marzipan masses. The rational content of DDW and glycerin in the composition of marzipan pastes while ensuring high functional and technological properties in the application of confectionery flour on the surface is substantiated. Mathematical modeling of adhesion behavior of marzipan pastes is carried out. The adhesive properties of the pastes are mathematically substantiated and their ability to have flexibility under the action of the applied shear stress is confirmed within the established limits. Its currents are considered and substantiated. A study of moisture absorption by flour confectionery when covered with marzipan paste is provided. The wetting angle is considered and the work of adhesion is determined. Based on researches the directions of use of the developed marzipan pastes in confectionery production as finishing semi-finished products for a covering and as a layer for flour confectionery products and modeling of figured products are offered.

MAGNETOPHORETIC CIRCUIT BIOCOMPATIBILITY

Recently, we introduced magnetophoretic circuits, composed of overlaid magnetic and metallic layers, as a novel single-cell analysis (SCA) tool. We showed the ability of these circuits in organizing large single-particle and particle-pair arrays. Assembling the cells in microarrays is performed with the ultimate goal of running temporal phenotypic analyses. However, for long-term studies, a suitable microenvironment for the cells to normally grow and differentiate is needed. Towards this goal, in this study, we run required biocompatibility tests, based on which we make the magnetophoretic-based microchip a suitable home for the cells to grow. The results confirm the ability of these chips in cell handling and show no unwanted cell behavior alteration due to the applied shear stress on them, the magnetic labeling, or the microenvironment. After this achievement, this tool would be ready for running important single-cell studies in oncology, virology, and medicine.

Blood Clot Phenotyping by Rheometry: Platelets and Fibrinogen Chemistry Affect Stress-Softening and -Stiffening at Large Oscillation Amplitude

(1) Background: Together with treatment protocols, viscoelastic tests are widely used for patient care. Measuring at broader ranges of deformation than currently done will add information on a clot’s mechanical phenotype because fibrin networks follow different stretching regimes, and blood flow compels clots into a dynamic non-linear response. (2) Methods: To characterize the influence of platelets on the network level, a stress amplitude sweep test (LAOStress) was applied to clots from native plasma with five platelet concentrations. Five species were used to validate the protocol (human, cow, pig, rat, horse). By Lissajous plots the oscillation cycle for each stress level was analyzed. (3) Results: Cyclic stress loading generates a characteristic strain response that scales with the platelet quantity at low stress, and that is independent from the platelet count at high shear stress. This general behavior is valid in the animal models except cow. Here, the specific fibrinogen chemistry induces a stiffer network and a variant high stress response. (4) Conclusions: The protocol provides several thresholds to connect the softening and stiffening behavior of clots with the applied shear stress. This points to the reversible part of deformation, and thus opens a new route to describe a blood clot’s phenotype.

Shear thinning in non-Brownian suspensions explained by variable friction between particles

We propose to explain shear-thinning behaviour observed in most concentrated non-Brownian suspensions by variable friction between particles. Considering the low magnitude of the forces experienced by the particles of suspensions under shear flow, it is first argued that rough particles come into solid contact through one or a few asperities. In such a few-asperity elastic–plastic contact, the friction coefficient is expected not to be constant but to decrease with increasing normal load. Simulations based on the force coupling method and including such a load-dependent friction coefficient are performed for various particle volume fractions. The results of the numerical simulations are compared to viscosity measurements carried out on suspensions of polystyrene particles ($40~\unicode[STIX]{x03BC}\text{m}$in diameter) dispersed in a Newtonian silicon oil. The agreement is shown to be satisfactory. Furthermore, the comparison between the simulations conducted either with a constant or a load-dependent friction coefficient provides a model for the shear-thinning viscosity. In this model the effective friction coefficient$\unicode[STIX]{x1D707}^{eff}$is specified by the effective normal contact force which is simply proportional to the bulk shear stress. As the shear stress increases,$\unicode[STIX]{x1D707}^{eff}$decreases and the jamming volume fraction increases, leading to the reduction of the viscosity. Finally, using this model, we show that it is possible to evaluate the microscopic friction coefficient for each applied shear stress from the rheometric measurements.

Effect of degree of cure on viscoplastic shear strain development in layers of [45/−45]s glass fibre/ epoxy resin composites

Effect of degree of cure on irreversible (viscoplastic) shear strain development in layers of glass fibre/ epoxy resin (LY5052 epoxy resin) [+45 °/−45 °]s laminate is studied performing a sequence of constant stress creep and viscoelastic strain recovery tests. For fixed values of degree of cure in range from 79.7% to 100%, the viscoplastic strains were measured as dependent on time and stress and Zapa's integral representation was used to characterize the observed behaviour. It is shown that at all degrees of cure the viscoplastic behaviour can be described by Zapa's model with parameters dependent on degree of cure. It is shown that for degree of cure lower than 80% the viscoplastic strains grow much faster and are much more sensitive to the increase of the applied shear stress. These irreversible strains developing in the final phase of the curing can significantly alter the residual stress state in the composite structure.

Real-time quantification of endothelial response to shear stress and vascular modulators

Real-time quantification of endothelial cell morphology and activity under applied shear stress.