arXiv subjects

Henrik Bruus

Publications and source records attributed to Henrik Bruus.

At least 73 records · Page 4Linked to original sources

arXiv2007

Acoustic resonances in microfluidic chips: full-image micro-PIV experiments and numerical simulations

We show that full-image micro-PIV analysis in combination with images of transient particle motion is a powerful tool for experimental studies of acoustic radiation forces and acoustic streaming in microfluidic chambers under piezo-actuation in the MHz range. The measured steady-state motion of both large 5 um and small 1 um particles can be understood in terms of the acoustic eigenmodes or standing ultra-sound waves in the given experimental microsystems. This interpretation is supported by numerical solutions of the corresponding acoustic wave equation.

physics.flu-dyn↗

arXiv2007

Reversed flow at low frequencies in a microfabricated AC electrokinetic pump

Microfluidic chips have been fabricated to study electrokinetic pumping generated by a low voltage AC signal applied to an asymmetric electrode array. A measurement procedure has been established and followed carefully resulting in a high degree of reproducibility of the measurements. Depending on the ionic concentration as well as the amplitude of the applied voltage, the observed direction of the DC flow component is either forward or reverse. The impedance spectrum has been thoroughly measured and analyzed in terms of an equivalent circuit diagram. Our observations agree qualitatively, but not quantitatively, with theoretical models published in the literature.

physics.flu-dyn↗

arXiv2006

Scaling behavior of optimally structured catalytic microfluidic reactors

In this study of catalytic microfluidic reactors we show that, when optimally structured, these reactors share underlying scaling properties. The scaling is predicted theoretically and verified numerically. Furthermore, we show how to increase the reaction rate significantly by distributing the active porous material within the reactor using a high-level implementation of topology optimization.

physics.flu-dyn↗

arXiv2006

Universal dynamics in the onset of a Hagen-Poiseuille flow

The dynamics in the onset of a Hagen-Poiseuille flow of an incompressible liquid in a channel of circular cross section is well-studied theoretically. We use an eigenfunction expansion in a Hilbert space formalism to generalize the results to channels of arbitrary cross section. We find that the steady state is reached after a characteristic time scale tau = (A/P)^2 (1/nu) where A and P are the cross-sectional area and perimeter, respectively, and $ν$ is the kinematic viscosity of the liquid. For the initial dynamics of the flow rate Q for t<<tau we find a universal linear dependence, Q(t)= Q_oo(alpha/C)(t/tau), where Q_oo is the asymptotic steady-state flow rate, alpha is the geometrical correction factor, and C=P^2/A is the compactness parameter. For the long-time dynamics Q(t) approaches Q_oo exponentially on the timescale $τ$, but with a weakly geometry-dependent prefactor of order unity, determined by the lowest eigenvalue of the Helmholz equation.

physics.flu-dyn↗

arXiv2006

A high-level programming-language implementation of topology optimization applied to steady-state Navier-Stokes flow

We present a versatile high-level programming-language implementation of nonlinear topology optimization. Our implementation is based on the commercial software package Femlab, and it allows a wide range of optimization objectives to be dealt with easily. We exemplify our method by studies of steady-state Navier-Stokes flow problems, thus extending the work by Borrvall and Petersson on topology optimization of fluids in Stokes flow [Int. J. Num. Meth. Fluids 2003; 41:77--107]. We analyze the physical aspects of the solutions and how they are affected by different parameters of the optimization algorithm. A complete example of our implementation is included as Femlab code in an appendix.

physics.flu-dyn↗

arXiv2005

Particle motion in microfluidics simulated using a Femlab implementation of the level set method

We implement the level set method for numerical simulation of the motion of a suspended particle convected by the fluid flow in a microchannel. The method automatically cope with the interactions between the particle and the channel walls. We apply the method in a study of particles moving in a channel with obstacles of different shapes. The generality of the method also makes it applicable for simulations of motion of particles under influence of external forces.

physics.flu-dyn↗

arXiv2005

Microfluidic capturing-dynamics of paramagnetic bead suspensions

We study theoretically the capturing of paramagnetic beads by a magnetic field gradient in a microfluidic channel treating the beads as a continuum. Bead motion is affected by both fluidic and magnetic forces. The transfer of momentum from beads to the fluid creates an effective bead-bead interaction that greatly aids capturing. We demonstrate that for a given inlet flow speed a critical density of beads exists above which complete capturing takes place.

physics.flu-dyn↗

arXiv2005

Reexamination of Hagen-Poiseuille flow: shape-dependence of the hydraulic resistance in microchannels

We consider pressure-driven, steady state Poiseuille flow in straight channels with various cross-sectional shapes: elliptic, rectangular, triangular, and harmonic-perturbed circles. A given shape is characterized by its perimeter P and area A which are combined into the dimensionless compactness number C = P^2/A, while the hydraulic resistance is characterized by the well-known dimensionless geometrical correction factor alpha. We find that alpha depends linearly on C, which points out C as a single dimensionless measure characterizing flow properties as well as the strength and effectiveness of surface-related phenomena central to lab-on-a-chip applications. This measure also provides a simple way to evaluate the hydraulic resistance for the various shapes.

physics.flu-dyn↗

arXiv2002

Checkerboard local density of states in striped domains pinned by vortices

Within a Green's function formalism we calculate the electronic structure around static extended magnetic and non-magnetic perturbations in a d-wave superconductor. In partucular, we discuss recent elastic neutron scattering and scanning tunneling experiments on High-T_c cuprates exposed to an applied magnetic field. A physical picture consisting of antiferromagnetic vortex cores operating as pinning centers for surrounding stripes is qualitatively consistent with the neutron data provided the stripes have the usual antiphase modulation. The low energy electronic structure in such a region reveals a checkerboard interference pattern consistent with recent scanning tunneling experiments.

cond-mat.supr-con↗

arXiv2001

Bias dependent subband edges and the 0.7 conductance anomaly

The 0.7 2e^2/h conductance anomaly is studied in strongly confined, etched GaAs/GaAlAs quantum point contacts by measuring the differential conductance G as a function of source-drain bias V_sd and gate-source bias V_gs as well as a function of temperature. In the V_gs - V_sd plane we use a grayscale plot of the transconductance dG/dV_gs to map out the bias dependent transitions between the normal and anomalous conductance plateaus. Any given transition is interpreted as arising when the bias controlled chemical potential mu_d (mu_s) of the drain (source) reservoir crosses a subband edge e_a in the point contact. From the grayscale plot we extract the constant normal subband edges e_0, e_1, ... and most notably the bias dependent anomalous subband edge e'_0(mu_d) split off from e_0. We show by applying a finite-bias version of the recently proposed BCF model, how the bias dependence of the anomalous subband edge is the key to analyze various experimental observations related to the 0.7 anomaly.

cond-mat.mes-hall↗

arXiv2001

The anomalous 0.5 and 0.7 conductance plateaus in quantum point contacts

The anomalous 0.5 and 0.7 conductance plateaus in quantum point contacts in zero magnetic field are analyzed within a phenomenological model. The model utilizes the Landauer-Buttiker formalism and involves enhanced spin correlations and thermal depopulation of spin subbands. In particular we can account for the plateau values 0.5 and 0.7, as well as the unusual temperature and magnetic field dependences of the 0.7 plateau. Finally, the model predicts the possibility of coexisting 0.5 and 0.7 plateaus.

cond-mat.mes-hall↗

arXiv2001

A single-particle path integral for composite fermions and the renormalization of the effective mass

To study composite fermions around an even denominator fraction, we adapt the phase space single-particle path integral technique for interacting electrons in zero magnetic field developed recently by D.S. Golubev and A.D. Zaikin, Phys. Rev. B {\bf 59}, 9195 (1999). This path integral description gives an intuitive picture of composite fermion propagation very similar to the Caldeira-Leggett treatment of a particle interacting with an external environment. We use the new description to explain the origin of the famous cancellation between the self-energy and the vertex corrections in semi-classical transport measurements. The effective range of the cancellation is given by the size of the propagating particle, which for the Coulomb interaction scales with the temperature T as T^{-1/4} |log T|^{-1} in the semi-classical limit. Using this scheme we find that the effective mass in the semi-classical limit for composite fermions in GaAs is approximately 6 times the bare mass.

cond-mat.mes-hall↗

arXiv2000

The magnetic neutron scattering resonance of high-T_c superconductors in external magnetic fields: an SO(5) study

The magnetic resonance at 41 meV observed in neutron scattering studies of Y Ba_2 Cu_3 O_7 holds a key position in the understanding of high-T_c superconductivity. Within the SO(5) model for superconductivity and antiferromagnetism, we have calculated the effect of an applied magnetic field on the neutron scattering cross-section of the magnetic resonance. In the presence of vortices, the neutron scattering cross-section shows clear signatures of not only the fluctuations in the superconducting order parameter ψ, but also the modulation of the phase of ψdue to vortices. In reciprocal space we find that i) the scattering amplitude is zero at (pi/a,pi/a), ii) the resonance peak is split into a ring with radius pi/d centered at (pi/a,pi/a), d being the vortex lattice constant, and consequently, iii) the splitting pi/d scales with the magnetic field as sqrt{B}.

cond-mat.supr-con↗

arXiv2000

From mesoscopic magnetism to the anomalous 0.7 conductance plateau

We present a simple phenomenological model which offers a unifying interpretation of the experimental observations on the 0.7 conductance anomaly of quantum point contacts. The model utilizes the Landauer-Buttiker formalism and involves enhanced spin correlations and thermal depopulation of spin subbands. In particular our model can account for the plateau value 0.7 and the unusual temperature and magnetic field dependence. Furthermore it predicts an anomalous suppression of shot noise at the 0.7 plateau.

cond-mat.mes-hall↗

arXiv1999

SO(5) theory of insulating vortex cores in high-$T_c$ materials

We study the fermionic states of the antiferromagnetically ordered vortex cores predicted to exist in the superconducting phase of the newly proposed SO(5) model of strongly correlated electrons. Our model calculation gives a natural explanation of the recent STM measurements on BSCCO, which in surprising contrast to YBCO revealed completely insulating vortex cores.

cond-mat.supr-con↗

arXiv1998

Localized plasmons in point contacts

Using a hydrodynamic model of the electron fluid in a point contact geometry we show that localized plasmons are likely to exist near the constriction. We attempt to relate these plasmons with the recent experimental observation of deviations of the quantum point contact conductance from ideal integer quantization. As a function of temperature this deviation exhibits an activated behavior, exp(-T_a/T), with a density dependent activation temperature T_a of the order of 2 K. We suggest that T_a can be identified with the energy needed to excite localized plasmons, and we discuss the conductance deviations in terms of a simple theoretical model involving quasiparticle lifetime broadening due to coupling to the localized plasmons.

cond-mat.mes-hall↗

arXiv1998

Periodic magnetoconductance fluctuations in triangular quantum dots in the absence of selective probing

We have studied the magnetoconductance of quantum dots with triangular symmetry and areas down to 0.2 square microns, made in a high mobility two-dimensional electron gas embedded in a GaAs-AlGaAs heterostructure. Semiclassical simulations show that the gross features in the measured magnetoconductance are caused by ballistic effects. Below 1 K we observe a strong periodic oscillation, which may be explained in terms of the Aharanov-Bohm flux quantization through the area of a single classical periodic orbit. From a numerical and analytical analysis of possible trajectories in hard- and soft-walled potentials, we identify this periodic orbit as the enscribed triangle. Contrary to other recent experiments, this orbit is not accessible by classical processes for the incoming collimated beam.

cond-mat.mes-hall↗

arXiv1998

Excitations in antiferromagnetic cores of superconducting vortices

We study excitations of the predicted antiferromagnetically ordered vortex cores in the superconducting phase of the newly proposed SO(5) model of strongly correlated electrons. Using experimental data from the literature we show that the susceptibilities in the spin sector and the charge sector are nearly equal, and likewise for the stiffnesses. In the case of strict equality SO(5) symmetry is possible, and we find that if present the vortices give rise to an enhanced neutron scattering cross section near the so called pi resonance at 41 meV. In the case of broken SO(5) symmetry two effects are predicted. Bound excitations can exist in the vortex cores with ``high'' excitation energies slightly below 41 meV, and the massless Goldstone modes corresponding to the antiferromagnetic ordering of the core can acquire a mass and show up as core excitation with ``low'' excitation energies around 2 meV.

cond-mat.str-el↗