Drag Theory Research Articles

Hydrodynamic Coulomb drag of strongly correlated electron liquids

We develop a theory of Coulomb drag in ultraclean double layers with strongly correlated carriers. In the regime where the equilibration length of the electron liquid is shorter than the interlayer spacing the main contribution to the Coulomb drag arises from hydrodynamic density fluctuations. The latter consist of plasmons driven by fluctuating longitudinal stresses, and diffusive modes caused by temperature fluctuations and thermal expansion of the electron liquid. We express the drag resistivity in terms of the kinetic coefficients of the electron fluid. Our results are nonperturbative in interaction strength and do not assume Fermi-liquid behavior of the electron liquid.

Mechanisms of hydrogen-enhanced localized plasticity: An atomistic study using α-Fe as a model system

Atomistic simulations of the effects of H on edge dislocation mobility and pile-ups are performed to investigate possible nanoscale mechanisms for hydrogen-enhanced localized plasticity (HELP). α-Fe is used as a model system because H diffusion is fast enough to capture kinetics within the time scales of molecular dynamics and because edge dislocation glide in α-Fe is similar to glide in face-centered cubic metals. Results over a wide range of H concentrations sufficient to generate a range of sizes in the Cottrell atmospheres show that the Cottrell atmospheres follow the moving dislocations, leading to a resistance to dislocation motion that is consistent with solute drag theory; thus, H reduces the dislocation mobility. Furthermore, once motion stops and a pile-up is established, the H Cottrell atmospheres do not affect the equilibrium spacing of dislocations in the pile-up; thus, the H atmosphere provides no “shielding” of dislocation–dislocation interactions. This result is consistent with conclusions from previous continuum calculations. Two oft-proposed mechanisms for HELP (H-induced increase in true dislocation mobility and decrease in dislocation–dislocation pile-up interactions) are therefore not supported by the present simulations. A mechanistic understanding of HELP phenomena observed in various experiments thus requires evaluation of more complex H–dislocation interactions.

Transient-regime grain growth in nanocrystalline yttria-stabilized zirconia annealed without and with a DC electric field

Grain size exponents considerably larger than normally reported occurred for grain growth in nanoscale 3 mol.% yttria-stabilized zirconia annealed without and with a DC electric field at 1300 and 1400 °C. It is shown that the large exponents correspond to the transition regime predicted by solute drag theory. The observed smaller grain growth rate with an electric field compared to without is in keeping with a reduction in the capillary driving force.

A New Approach to Solute Effect on Grain Boundary Migration

Solute drag theory is critically revisited and an alternative approach is presented to account for the effect of solute elements on grain boundary migration during annealing. A fundamental new concept is introduced in the model that, in the linear range of irreversible thermodynamics, solute atoms segregated in a grain boundary will not lag behind when the boundary migrates. While lagging behind is the very essential assumption for the solute drag theory. Instead of blaming the lagging behind, the mobility drop due to solute addition is attributed to the decrease in boundary energy as a result of boundary segregation. According to this model, grain boundary mobility is dependent on solute concentration rather than migration rate. The predictions of the model are compared with experimental results, with a good agreement.

Round-tip dielectrophoresis-based tweezers for single micro-object manipulation

In this paper, we present an efficient methodology to manipulate a single micro-object using round-tip positive dielectrophoresis-based tweezers. The tweezers consist of a glass needle with a round-tip and a pair of thin gold-film electrodes. The round-tip, which has a radius of 3µm, is formed by melting a finely pulled glass needle and concentrates the electric field at the tip of the tweezers, which allows the individual manipulation of single micro-objects. The tweezers successfully captured, conveyed, and positioned single cell-sized liposomes with diameters of 5–23µm, which are difficult to manipulate with conventional manipulation methodologies, such as optical tweezers or glass micropipettes, due to the similarities between their optical properties and those of the media, as well as the ease with which they are deformed or broken. We used Stokes’ drag theory to experimentally evaluate the positive dielectrophoresis (pDEP) force generated by the tweezers as a function of the liposome size, the content of the surrounding media, and the applied AC voltage and frequency. The results agreed with the theoretically deduced pDEP force. Finally, we demonstrated the separation of labeled single cells from non-labeled cells with the tweezers. This device can be used as an efficient tool for precisely and individually manipulating biological micro-objects that are typically transparent and flexible.

Testing a Missing Spectral Link in Turbulence

Although the cardinal attribute of turbulence is the velocity fluctuations, these fluctuations have been ignored in theories of the frictional drag of turbulent flows. Our goal is to test a new theory that links the frictional drag to the spectral exponent α, a property of the velocity fluctuations in a flow. We use a soap-film channel wherein for the first time the value of α can be switched between 3 and 5/3, the two theoretically possible values in soap-film flows. To induce turbulence with α = 5/3, we make one of the edges of the soap-film channel serrated. Remarkably, the new theory of the frictional drag holds in both soap-film flows (for either value of the spectral exponent α) and ordinary pipe flows (where α = 5/3), even though these types of flow are governed by different equations.

Kinetic theory of Coulomb drag in two monolayers of graphene: From the Dirac point to the Fermi liquid regime

We theoretically investigate Coulomb drag in a system of two parallel monolayers of graphene. Using a Boltzmann equation approach we study a variety of limits ranging from the non-degenerate interaction dominated limit close to charge neutrality all the way to the Fermi liquid regime. In the non-degenerate limit we find that the presence of the passive layer can largely influence the conductivity of the active layer despite the absence of drag. This induces a non-trivial temperature behavior of the single layer conductivity and furthermore suggests a promising strategy towards increasing the role of inelastic scattering in future experiments. For small but finite chemical potential we find that the drag resistivity varies substantially as a function of the ratio of inelastic and elastic scattering. We find that an extrapolation from finite chemical potential to zero chemical potential and to the clean system is delicate and the order of limits matters. In the Fermi liquid regime we analyze drag as a function of temperature $T$ and the distance $d$ between the layers and compare our results to existing theoretical and experimental results. In addition to the conventional $1/d^4$-dependence with an associated $T^2$-behavior we find there is another regime of $1/d^5$-dependence where drag varies in linear-in-$T$ fashion. The relevant parameter separating these two regimes is given by $\bar{d}=T d/v_F$ ($v_F$ is the Fermi velocity), where $\bar{d} \ll1$ corresponds to $T^2$-behavior, while $\bar{d}\gg1$ corresponds to $T$-behavior.

Phase field crystal model of solute drag

The atomic scale interaction of solute with a migrating grain boundary has been studied using a binary phase field crystal (PFC) model. This model bridges between atomistic and continuum simulation techniques as it operates on diffusive timescales but at atomistic length scales. For this study, a two-dimensional channel containing two grains separated by a flat grain boundary has been constructed that allows for a channel length on the order of one micrometer. A new formalism has been developed to allow for the application of an external driving pressure for the growth of one grain. These simulations account for solute/grain boundary interactions, resulting in a solute drag effect on the grain boundary motion. The PFC simulations show good agreement with classical solute drag theory, though deviations due to the atomic scale nature of the interface exist.

Characterisation and airborne deployment of a new counterflow virtual impactor inlet

Abstract. A new counterflow virtual impactor (CVI) inlet is introduced with details of its design, laboratory characterisation tests and deployment on an aircraft during the 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE). The CVI inlet addresses three key issues in previous designs; in particular, the inlet operates with: (i) negligible organic contamination; (ii) a significant sample flow rate to downstream instruments (∼15 l min−1) that reduces the need for dilution; and (iii) a high level of accessibility to the probe interior for cleaning. Wind tunnel experiments characterised the cut size of sampled droplets and the particle size-dependent transmission efficiency in various parts of the probe. For a range of counter-flow rates and air velocities, the measured cut size was between 8.7–13.1 μm. The mean percentage error between cut size measurements and predictions from aerodynamic drag theory is 1.7%. The CVI was deployed on the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter for thirty flights during E-PEACE to study aerosol-cloud-radiation interactions off the central coast of California in July and August 2011. Results are reported to assess the performance of the inlet including comparisons of particle number concentration downstream of the CVI and cloud drop number concentration measured by two independent aircraft probes. Measurements downstream of the CVI are also examined from one representative case flight coordinated with shipboard-emitted smoke that was intercepted in cloud by the Twin Otter.

Theory of Coulomb drag for massless Dirac fermions

Coulomb drag between two unhybridized graphene sheets separated by a dielectric spacer has recently attracted considerable theoretical interest. We first review, for the sake of completeness, the main analytical results which have been obtained by other authors. We then illustrate pedagogically the minimal theory of Coulomb drag between two spatially separated two-dimensional systems of massless Dirac fermions which are both away from the charge-neutrality point. This relies on second-order perturbation theory in the screened interlayer interaction and on Boltzmann-transport theory. In this theoretical framework and in the low-temperature limit, we demonstrate that, to leading (i.e. quadratic) order in temperature, the drag transresistivity is completely insensitive to the precise intralayer momentum-relaxation mechanism (i.e. to the functional dependence of the transport scattering time on energy). We also provide analytical results for the low-temperature drag transresistivity for both cases of ‘thick’ and ‘thin’ spacers and for arbitrary values of the dielectric constants of the media surrounding the two Dirac-fermion layers. Finally, we present numerical results for the low-temperature drag transresistivity for the case when one of the media surrounding the Dirac-fermion layers has a frequency-dependent dielectric constant. We conclude by suggesting an experiment that can potentially allow for the observation of departures from the canonical quadratic-in-temperature behavior of the transresistivity.

The drag on a flattened bubble moving across a plane substrate

AbstractThe equilibrium shape of an axisymmetric liquid drop or gas bubble in an immiscible supporting liquid held under gravity against a horizontal plane rigid surface is derived. A thin film of supporting liquid remains between the drop/bubble surface and the rigid substrate at equilibrium, of thickness ${h}_{0} $ determined by the balance of the disjoining pressure $\Pi (h)$ between the drop and the substrate and the internal Laplace pressure of the drop/bubble. The interface is macroscopically flat around the drop axis out to a radius $a$ but matches smoothly into the outer shape of radius $A$ through a boundary layer region of width $a{ H}_{0}^{1/ 2} $ where ${H}_{0} = 2{h}_{0} A/ {a}^{2} $ is a small parameter. The outer drop shape is determined by a balance of buoyancy forces and local Laplace pressure and is roughly spherical if $A/ \lambda \ll 1$, where $\lambda = \sqrt{\gamma / \mrm{\Delta} \rho g} $ is the capillary length in the interface with a logarithmic correction due to the action of the disjoining pressure across the flattened region. With the shape determined, we calculate the drag force on this flattened bubble to lowest order in the velocity as it moves across the rigid substrate using a lubrication approximation valid to terms of $O( \mathop{ ({h}_{0} / a)}\nolimits ^{2} )$ as an integral over the flattened bubble surface of the hydrodynamic pressure. The lubrication theory of itself is not sufficient to determine the drag due to the divergence of that integral if the outer flow field properties are neglected. By using the known exact result for the drag force on an undistorted bubble, the drag on the flattened bubble can be computed as an integral over the lubrication region alone. We derive the drag as a series expansion in the small parameter ${H}_{0} $ by means of a fairly intricate boundary layer analysis. The logarithmic divergence of the translational drag with film thickness ${h}_{0} $ for the undistorted bubble is replaced by the stronger divergence ${ h}_{0}^{\ensuremath{-} 1/ 2} $ to leading order for the flattened bubble case. We present explicit numerical results for the first few terms in the expansion for the case of an exponentially repulsive disjoining pressure, and analytic expressions for these terms in the limit of very short-range disjoining pressure forces. The results of this calculation are compared with recent work (Hodges, Jensen & Rallison, J. Fluid Mech., vol. 512, 2004, p. 95) where disjoining pressure is neglected and hydrodynamic pressure is balanced against buoyancy and Laplace forces. The limits of validity of this linear drag theory are also presented.

Effects of Solute and Second-Phase Particles on the Texture of Nd-Containing Mg Alloys

Hot-rolled, binary Mg-Nd alloys with compositions ≥0.095 at. pct undergo the texture weakening phenomenon that has been reported in a number of Mg–rare earth (RE) alloys. However, alloys with compositions ≤0.01 at. pct retain a strong basal texture typical of pure Mg and other Mg alloys. Measurements of intragranular misorientation axes obtained using electron backscatter diffraction (EBSD) show that more dilute alloys contain predominantly basal $$ $$ dislocations, while richer alloys contain primarily prismatic $$ $$ dislocations. It is suggested that this change in dislocation content is related to a change in the dynamic recrystallization (DRX) mechanism. Metastable second-phase Mg x Nd1–x intermetallic particles are present within the alloys, and an annealing study indicates that the alloys undergoing texture weakening have grain sizes well predicted by classical Zener drag theory. Even though the more dilute alloys also contain second-phase particles, they are not sufficient to induce pinning. The promotion of nonbasal slip and the reduction in grain boundary mobility due to Zener drag are suggested as controlling mechanisms that promote the observed texture weakening phenomena.

Translational and rotational diffusion of micrometer-sized solid domains in lipid membranes

We use simultaneous observation of translational and rotational Brownian motion of domains in lipid membranes to test the hydrodynamics-based theory for the viscous drag on the membrane inclusion. We find that translational and rotational diffusion coefficients of micrometer-sized solid (gel-phase) domains in giant unilamellar vesicles showing fluid–gel phase coexistence are in excellent agreement with the theoretical predictions.

Analytical and Semi‐Analytical Treatment of the Satellite Motion in a Resisting Medium

The orbital dynamics of an artificial satellite in the Earth′s atmosphere is considered. An analytic first‐order atmospheric drag theory is developed using Lagrange′s planetary equations. The short periodic perturbations due to the geopotential of all orbital elements are evaluated. And to construct a second‐order analytical theory, the equations of motion become very complicated to be integrated analytically; thus we are forced to integrate them numerically using the method of Runge‐Kutta of fourth order. The validity of the theory is checked on the already decayed Indian satellite ROHINI where its data are available.

Coulomb drag in graphene single layers separated by a thin spacer

Motivated by very recent studies of Coulomb drag in a graphene-BN-graphene system we develop a theory of Coulomb drag for the Fermi-liquid regime, for the case when the ratio of spacer thickness $d$ to the Fermi wavelength of electrons is arbitrary. The concentration ($n$) and thickness dependence of the drag resistivity is changed from ${n}^{\ensuremath{-}3}{d}^{\ensuremath{-}4}$ for the thick spacer to ${n}^{\ensuremath{-}1}|\mathrm{ln}({\mathit{nd}}^{2})|$ for the thin one.

Are There Rings Around the Sun?

The Theory of Alfven drag (Drell et al. in J Geophys Res 70: 3131–3145 1965; Anselmo and Farinella in Icarus, 58, 182–185 1983) is applied here to show that the existence of a possible solar ring structure at a radial distance of 0.02 AU (~4R ⊙ , R ⊙ = radius of the sun) predicted by earlier authors (Brecher et al. in Nature 282, 50–52 1979; Rawal in Bull. Astr. Soc. India 6, 92–95 1978, Moon Planets 24, 407–414 1981, Moon Planets 31, 175–182 1984, J Astrophys Astr 10, 257–259 1989) may not survive Alfven drag produced during even moderate solar magnetic storms which take place from time to time through the age of the sun, but a possible solar ring structure at a radial distance of 0.13 AU (~27R ⊙ ) (Brecher et al. in Nature 282, 50–52 1979; Rawal in Bull. Astr. Soc. India 6, 92–95 1978, Moon Planets 24, 407–414 1981, Moon Planets 31, 175–182 1984, J Astrophys Astr 10, 257–259 1989) may survive intense Alfven drag produced during even strong magnetic storms of magnetic field value up to 1,000 G.

Molecular Dynamic Simulation on the Absorbing Process of Isolating and Coating of α-olefin Drag Reducing Polymer

The absorbing process in isolating and coating process of α-olefin drag reducing polymer was studied by molecular dynamic simulation method, on basis of coating theory of α-olefin drag reducing polymer particles with polyurethane as coating material. The distributions of sodium laurate, sodium dodecyl sulfate, and sodium dodecyl benzene sulfonate on the surface of α-olefin drag reducing polymer particles were almost the same, but the bending degrees of them were obviously different. The bending degree of SLA molecules was greater than those of the other two surfactant molecules. Simulation results of absorbing and accumulating structure showed that, though hydrophobic properties of surfactant molecules were almost the same, water density around long chain sulfonate sodium was bigger than that around alkyl sulfate sodium. This property goes against useful absorbing and accumulating on the surface of α-olefin drag reducing polymer particles; simulation results of interactions of different surfactant and multiple hydroxyl compounds on surface of particles showed that, interactions of different surfactant and one kind of multiple hydroxyl compound were similar to those of one kind of surfactant and different multiple hydroxyl compounds. These two contrast types of interactions also exhibited the differences of absorbing distribution and closing degrees to surface of particles. The sequence of closing degrees was derived from simulation; control step of addition polymerization interaction in coating process was absorbing mass transfer process, so the more closed to surface of particle the multiple hydroxyl compounds were, the easier interactions with isocyanate were. Simulation results represented the compatibility relationship between surfactant and multiple hydroxyl compounds. The isolating and coating processes of α-olefin drag reducing polymer were further understood on molecule and atom level through above simulation research, and based on the simulation, a referenced theoretical basis was provided for practical optimal selection and experimental preparation of α-olefin drag reducing polymer particles suspension isolation agent.

A model for interphase precipitation based on finite interface solute drag theory

A model for interphase precipitation with the ledge mechanism, based on a eutectoid reaction, has been developed and combined with the finite interface solute drag model and a numerical solution of the diffusion equations inside the migrating phase interface. In the model, niobium flows in two directions, i.e. perpendicular to the direction of the ledge migration by eutectoid-like reaction and simultaneously parallel to the direction of the ledge migration inside the ledge interface. The difference between ledge transformation and typical phase transformation is compared using this model and the effects of row spacing, temperature and segregation energy are discussed. The calculation results using the model are compared with experimental results and the critical driving force for interphase precipitation is evaluated. The estimations of the niobium carbide precipitation using this model are in good agreement with experimental results.

Theory of resonant photon drag in monolayer graphene

Photon drag current in monolayer graphene with degenerate electron gas is studied under interband excitation near the threshold of fundamental transitions. Two main mechanisms generate an emergence of electron current. Nonresonant drag effect results from direct transfer of in-plane photon momentum $\mathbf{q}$ to electron and dependence of matrix elements of transitions on $\mathbf{q}$. Resonant drag effect (RDE) originates from $\mathbf{q}$-dependent selection of transitions due to a sharp form of the Fermi distribution in energy. The drag current essentially depends on the polarization of radiation and, in general, is not parallel to $\mathbf{q}$. The perpendicular current component appears if the in-plain electric field is tilted toward $\mathbf{q}$. The RDE has no smallness connected with $q$ and exists in a narrow region of photon frequency $\ensuremath{\omega}$, $|\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{-}2{ϵ}_{F}|<\ensuremath{\hbar}sq$, where $s$ is the electron velocity.

Dislocation motion in magnesium: a study by molecular statics and molecular dynamics

The motion of dislocations with Burgers' vector lying on the basal, prismatic and pyramidal slip planes in pure magnesium was investigated numerically under static and dynamic loading conditions. The analysis of the dislocation core structures revealed that the basal slip system was the most favorable energetically, and therefore a dislocation loop cannot extend on the pyramidal slip plane, because screw dislocations were not stable in this slip plane. In agreement with experimental data, a strong anisotropy between slip systems was observed. In both the basal and the prismatic slip planes, the dislocation velocity is consistent with phonon drag theory. In addition, the edge dislocation velocity was always larger than the screw dislocation velocity independent of the slip system, while the dislocation velocity on the prismatic slip plane was always lower than the dislocation velocity on the basal plane regardless of the dislocation character.