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In the framework of the classical equations for the magnetization dynamics and the collective variables approximation, the dynamics of magnetic vortices in two-dimensional ferromagnets with the easy-plane anisotropy and spatially inhomogeneous exchange interaction is considered. In the case of a wide straight line of the interface between magnetic media with different but slightly different exchange interaction, the dependence of the magnetic vortex velocity on its distance to the structure defect and position inside the interface domain is obtained.

A detailed analysis of solutions to the Landau–Lifshitz equation describing solitons in a physically selected domain structure of a biaxial ferromagnet is presented. The structure and properties of new types of solitons that are strongly bound to the domain structure are investigated. Soliton behavior near the boundaries of their region of existence is considered. A comparative analysis of soliton cores in the domain structures of easy-axis and biaxial ferromagnets is performed, and their connection with solitons against a uniformly magnetized background is established.

The AC magnetic susceptibility (ACS) of Fe3O4 nanoparticles was measured between 10 kHz and 4 MHz at different temperatures and in applied DC fields. In this frequency range, magnetostatic interactions impact magnetization dynamics even for dilute assemblies. The ACS spectrum of relaxation frequencies changes both with temperature and the addition of a small DC field. Because both the relaxation peak frequency and amplitude can be tuned with the DC field, these results could be applied to magnetic hyperthermia applications to optimize heat delivery.

Motivated by the chromosomes enclosed in a cell nucleus, we study a spherically confined system of a small number of long unknotted and nonconcatenated polymer rings in a melt and systematically compare it with the bulk results. We find that universal scaling exponents of the bulk system also apply in the confined case; however, certain important differences arise. First, due to confinement effects, the static and threading properties of the rings depend on their radial position within the confining sphere. Second, the rings’ dynamics is overall subdiffusive, but anisotropic along the directions parallel and perpendicular to the sphere’s radius. The radial center of mass displacements of the rings are in general much smaller than the angular ones, which is caused by the confinement-induced inhomogeneous radial distribution of the whole rings within the sphere. Finally, we find enhanced contact times between rings as compared to the bulk, which indicates slow and predominantly coordinated pathways of the relaxation of the system.

We propose a novel formulation of capillarity, which geometrically extends the Bosanquet equation to irregular geometries, taking the effect of inertia and the dynamic contact angle into account. The governing equation is an integrodifferential equation that is solved numerically and compared with computer simulations, experimental data, and other cases available in the literature. The numerical examples investigated in this work show that contrary to flat channels and tubes, inertial effects decay much slower in corrugated channels and tubes due to the walls' geometrical fluctuations. We also draw the paramount conclusion that the true solution for Jurin's height in irregular capillaries is path-dependent and highly sensitive to the initial conditions, and no single static-equilibrium solution can necessarily be attributed to the eventual position of the meniscus. Resulting from the non-linear dynamics, the multiple equilibria in the presence of gravity for irregular capillaries can only be analyzed if the effect of inertia is considered, which has largely been neglected in the literature thus far.

We study the motion of radially driven fluid-immersed particles in a novel Hele-Shaw cell with open boundaries. The initially uniform suspension forms a striped pattern within a specific range of horizontal oscillation frequencies and for sufficiently large amplitudes. We observe that the initial coarsening dynamics of the stripes gradually slows down and the pattern reaches a steady state after a few minutes. The distance between the stripes in the steady state exhibits an exponentially saturating increase with increased oscillation amplitude or frequency. The width of the stripes decreases as a power-law with the frequency while its amplitude dependence follows a logistic function. We propose a mechanism -- based on the interplay between shear stress, hydrodynamic interactions, and frictional forces -- to link the structural characteristics of the stripes to the properties of the oscillatory external drive., Comment: 7 pages, 4 figures

We investigate the dynamics, interactions, and decay of immiscible viscous fingers in two and three dimensions over time in a high-aspect ratio (up to 100:1) system. The behavior is related to the viscosity ratio and a macroscopic capillary number. The same four fingering regimes are observed as in miscible displacements (spreading of the interface between wetting and non-wetting fluid but no fingers, the growth of many fingers that can be described by perturbation analysis, non-linear interactions between fingers and decay to a single finger) for low viscosity ratio and high capillary to viscous ratios. At higher viscosity ratios and lower capillary to viscous ratios, periodic tip-splitting and decay results in a fluctuation between one and two fingers at late time. This has not been seen in miscible displacements. We provide a stability plot that can be used to identify when this will occur. Similar behaviors were seen in both two and three dimensions, suggesting that learnings from two-dimensional (2D) linear displacements can be applied to similar three-dimensional (3D) flows. In particular, the square root of the number of fingers seen in the 3D simulations and their decay with time was almost identical to 2D.

We use high-speed imaging to investigate the movement of a floating sphere pulled horizontally along a water surface. The model sphere is 10 cm in diameter and has half of the water density resulting in a half-submerged static sphere. By varying the pulling force, we investigate the flow dynamics in the subcritical Reynolds number range, of Re ≈ 2 × 104 to 2 × 105. We characterize three hydrodynamic regimes with the increase in the pulling force, to which we refer to as: low Froude number, Fr 1.2 regimes. In the low Fr regime, the sphere moves with little disturbance of the water surface and the drag is close to half of the drag on a fully submerged sphere. In the intermediate Fr regime, a pronounced wave pattern is developed which together with the dipping of the sphere below the water level leads to an increase in the drag force. Based on a potential flow approximation for the downward force on the sphere moving along the surface, we derive a semiempirical relation for the sphere dipping as a function of the Froude number. Finally, in the high Fr regime, the sphere movement switches to a mode of periodic dipping below and surfacing above the water surface. The periodic vertical motion portrays a decrease in the average drag force.

We study the fully-coupled dynamics between a fully-developed turbulent flow and an ensemble of immersed flexible fibers. We vary the concentration of the suspension, the mechanical properties and the length of the fibers in a vast parametric range. For all configurations, the fiber dynamics falls in only two possible dynamical states: (i) the fiber manifests its natural response to the flow forcing or (ii) its motion fully synchronizes to the hydrodynamic timescales of the turbulent flow. This scenario holds for both a dilute condition, where the carrier flow is not affected by the fluid-structure interaction, as well as in the case where the flow is substantially altered by the presence of the immersed objects. Such backreaction effect can be macroscopically modelled in terms of the mass fraction of the suspension. Our results can be readily extended to any elastic objects interacting with fluid turbulence.

Droplet impact on a spinning surface has been observed in different industries and plays an important role in the performance of industrial systems. In the current study, the dynamics of water droplet impact on a hydrophilic spinning disk is investigated. An experimental setup is designed in a way that droplet diameter, impact velocity, disk rotational speed, and location of impact are precisely controlled. While the droplet diameter is fixed in the present study, other mentioned parameters are changed and their effects on the droplet behavior are discussed. High-speed imaging is used to record the droplet dynamics under various operating conditions. It is demonstrated that after impact, droplet spreads on the surface due to a high adhesion between water and the hydrophilic substrate. It is indicated that the wetted area is a function of time, impact velocity, disk rotational speed, and centrifugal acceleration. Furthermore, depending on the mentioned parameters, different phenomena such as rivulet formation, fingering, and detachment of secondary droplet(s) are observed. In the angular direction, in general, the wetted length increases as time passes. However, in the radial direction, the droplet first spreads on the surface and reaches a maximum value, and then recedes until a plateau is attained. At this instant, a bulk of liquid, which is called wave in this study, moves radially outward from the inner boundary of the droplet toward its outer boundary due to the effect of centrifugal force. Once the wave reaches the outer boundary, depending on its size and momentum, fingers or rivulets are formed, and small droplet(s) may detach. The process is analyzed comprehensively, and different empirical correlations for wetted lengths in radial and angular directions, secondary droplet formation, number of fingers, the onset of fingering, and wave velocity are developed.

Understanding the dynamics of micro-organisms will help in developing artificial swimmers for applications like drug delivery. In the present study, a two-dimensional one-hinge swimmer resembling a scallop in Newtonian fluid is explored. To model the one-hinge swimmer, we use bead-spring model and the fluid is simulated using multi-particle collision dynamics with Anderson thermostat. We consider a non-uniform distribution of the bending rigidity along the arms of the swimmer, where we reduce the bending rigidity progressively from the hinge to the end of the arms. The non-uniform arms show higher swimming speed for the same average bending rigidity, thereby enhancing the efficiency of the swimmer. It was observed that the bending rigidity variation along the arm of the swimmer following a geometric sequence was more efficient than linear or quadratic for the same average bending rigidity. We also study the maneuverability of the one-hinge swimmer having asymmetrical bending rigidity for the arms, thereby the swimmer undergoes curved path. We find that depending upon the stiffness of the arm, the swimmer undergoes clockwise or anticlockwise rotation. We also find that the angular and transnational velocities of the swimmer are maximum at approximately the same sperm number ∼ 1.8. The angular velocity of the swimmer scaled linearly with the amplitude of actuation as predicted by resistive force theory. Finally, we show that in the case of a two-dimensional one-hinge swimmer angular velocity, curvature and the direction of rotation can be controlled by just changing the relative bending rigidity of the arms.

The dynamic of an impacting drop with a sessile drop of the same liquid on different solid surfaces is investigated experimentally and theoretically. By controlling the surface wettability and the impact velocity, different regimes were observed, such as complete rebounding, direct coalescence, and coalescence during the retraction phase or the spreading phase. It is observed that the complete rebounding phenomenon has widely occurred during the impact on hydrophilic surfaces. In addition, both the maximum and minimum limits of coalescence/complete rebounding thresholds were determined. During the complete rebounding phenomena, and based on the energy balance, the maximum spreading diameter of the falling drop diameter on the sessile drop was proposed. In addition, the restitution coefficient and the contact time between the falling and sessile drops were studied experimentally, and theoretically, the models show a good agreement with the experimental work.

Despite growing interest in the focusing and manipulation of particles in non-Newtonian fluids in confined flows, the combined effect of viscoelastic and shear-thinning effects on particle dynamics is not well understood. Herein, we report the dynamics of rigid microparticles in confined flows of strongly shear-thinning viscoelastic (STVE) fluids at very low Reynolds numbers. Our experiments with different STVE fluids reveal five different regimes: original streamline, bimodal, center migration, defocusing, and wall migration (WM), depending upon the fluid properties and flow rates. It is found that the occurrence of the different regimes depends on the STVE parameter ( ψ ) and average strain rate ( γ ¯). We find that the dynamics of particles in the different regimes is underpinned by the synergy between viscoelastic lift force ( F VE) and shear-thinning lift force ( F ST). Numerical simulation results of strain rate and viscosity profiles at different ψ and γ ¯ enable estimation of the forces and explaining the dynamics observed. We expect that our study will find relevance in applications involving positioning and manipulation of particles in confined flows of STVE fluids.

In this study, the coalescence dynamics of two unequal sized vertically inline bubbles rising in a liquid column have been investigated using the coupled level-set and volume-of-fluid (CLSVOF) method. A wide range of bubble radius ratios of trailing bubble and leading bubble ( 0.25 ≤ R ≤ 2.0) and separation distances between the bubbles ( 2.5 ≤ S ≤ 3.5) have been deployed to investigate the evolution of the bubble wakes and bubble shapes. It is discovered that the coalescence time increases with R, the maxima being around 0.75 ≤ R ≤ 1, and then it decreases. With the increase in S, the coalescence time gradually increases. The existence of a pair of counter-rotating vortex rings has been observed between the bubbles, which are seen to accelerate the bubble coalescence process. For the present range of R and S, we show a regime map with four distinct coalescence pathways: coalescence with liquid entrapment, coalescence without liquid entrapment, penetration of the leading bubble, and premature splitting of the trailing bubble.

Wavelet Monte Carlo dynamics simulations are used to study the dynamics of passive particles in the presence of microswimmers, taking account of the often-omitted thermal motion alongside the hydrodynamic flows generated by the swimmers. Although the P\'{e}clet numbers considered are large, we find the thermal motion to have a significant effect on the dynamics of our passive particles, and can be included as a decorrelation factor in the velocity autocorrelation with a decay time proportional to the P\'{e}clet number. Similar decorrelation factors come from swimmer rotations, e.g.~run and tumble motion, and apply to both entrainment and far field loop contributions. These decorrelation factors lead to active diffusivity having a weak apparent power law close to $Pe^{0.2}$ for small tracer-like particles at P\'{e}clet numbers appropriate for E. coli swimmers at room temperature. Meanwhile, the reduced hydrodynamic response of large particles to nearby forces has a corresponding reduction in active diffusivity in that regime. Together, they lead to a non-monotonic dependence of active diffusivity on particle size that can shed light on similar behaviour observed in experiments by Patteson et al., Comment: 33 pages, 8 figures. The following article has been submitted to the journal: Physics of Fluids. If/after it is published, it will be found at https://aip.scitation.org/journal/phf

The interaction and evolution dynamics of two counterpropagating shockwaves generated by two collinear laser induced air plasmas of equal and unequal energies are presented. The formation of a stagnation layer during the interaction of the shockwaves from the two plasma sources, namely, S1 and S2 separated by a distance of 4 mm, were studied over a duration of 30 μs, using two-dimensional FLASH radiation hydrodynamic code. The stagnation layer formed is observed to be “hard” in the case of interaction of equal energy sources, S1 = S2. While in the case of unequal energy sources with S2 > S1, the stagnation layer is observed to allow the propagation of energy. For both the cases of equal and unequal energy sources, a maximum increase in temperature at the stagnation layer by ∼1.5 and 1.9 times, respectively, is observed due to the interaction of the expanding plasmas around the stagnation layer. The density and pressure in the unequal interaction case at the stagnation layer is observed to be more than that of the ambient conditions by 4.9 and 19 times, respectively. A clear visualization of shock front of high energy plasma source traversing through that of a lower energy source leads to a channel formation, followed by a jet-let along the laser axis due to density drag. The evolution of interacting plasma is compared with that of the evolution of individual sources. The simulations were validated by the experimental observations giving a one-on-one mapping of the spatiotemporal evolution of plasma and shock front.

We study the magnetization dynamics of nanomagnetic beads with a diameter ranging from 80 to 250 nm in frequency and time domains using a high-resolution elliptical planar Hall effect (EPHE) sensor integrated with a fluidic channel. We find that the dynamics are well described with Cole–Cole and exponential decay models for frequency and time domains, respectively. We discuss the potential use of EPHE sensors in lab-on-a-chip applications.

Collisionless particle dynamics in a helical magnetic mirror is investigated by methods of the Hamilton dynamics. In the limit of small Larmor radius, the Hamiltonian is fully integrable and particle trajectories can be divided into three types, differing in the direction of the drift velocity. The structure of the trajectories of resonant particles and the width of the resonance are studied. The influence of weak axi-symmetrical corrugation on the particle dynamics and efficiency of plasma flow suppression through a helical mirror are discussed.

Low-frequency simulations of a one-layer model with lateral buoyancy variations (i.e., thermodynamically active) have revealed circulatory motions resembling quite closely submesoscale observations in the surface ocean rather than indefinitely growing in the absence of a high-wavenumber instability cutoff. In this note it is shown that the existence of a convex pseudoenergy--momentum integral of motion for the inviscid, unforced dynamics provides a mechanism for the nonlinear saturation of such thermal instabilities in the zonally symmetric case. The result is an application of \citet{Arnold-66} and \citet{Shepherd-88a} methods., Comment: Comments welcomed!

Basic physics of drift-wave turbulence and zonal flows has long been studied within the framework of wave-kinetic theory. Recently, this framework has been re-examined from first principles, which has led to more accurate yet still tractable "improved" wave-kinetic equations. In particular, these equations reveal an important effect of the zonal-flow "curvature" (the second radial derivative of the flow velocity) on dynamics and stability of drift waves and zonal flows. We overview these recent findings and present a consolidated high-level picture of (mostly quasilinear) zonal-flow physics within reduced models of drift-wave turbulence., Paper BI2 6, Bull. Am. Phys. Soc. 65 (2020)