arXiv subjects

H. Bruus

Publications and source records attributed to H. Bruus.

11 recordsLinked to original sources

arXiv2012

Current-induced membrane discharge

Possible mechanisms for over-limiting current (OLC) through aqueous ion-exchange membranes (exceeding diffusion limitation) have been debated for half a century. Flows consistent with electro-osmotic instability (EOI) have recently been observed in microfluidic experiments, but the existing theory neglects chemical effects and remains to be quantitatively tested. Here, we show that charge regulation and water self-ionization can lead to OLC by "current-induced membrane discharge" (CIMD), even in the absence of fluid flow. Salt depletion leads to a large electric field which expels water co-ions, causing the membrane to discharge and lose its selectivity. Since salt co-ions and water ions contribute to OLC, CIMD interferes with electrodialysis (salt counter-ion removal) but could be exploited for current-assisted ion exchange and pH control. CIMD also suppresses the extended space charge that leads to EOI, so it should be reconsidered in both models and experiments on OLC.

physics.chem-ph↗

arXiv2006

Mass and charge transport in micro and nano-fluidic channels

We consider laminar flow of incompressible electrolytes in long, straight channels driven by pressure and electro-osmosis. We use a Hilbert space eigenfunction expansion to address the general problem of an arbitrary cross section and obtain general results in linear-response theory for the mass and charge transport coefficients which satisfy Onsager relations. In the limit of non-overlapping Debye layers the transport coefficients are simply expressed in terms of parameters of the electrolyte as well as the hydraulic radius R=2A/P with A and P being the cross-sectional area and perimeter, respectively. In articular, we consider the limits of thin non-overlapping as well as strongly overlapping Debye layers, respectively, and calculate the corrections to the hydraulic resistance due to electro-hydrodynamic interactions.

physics.flu-dyn↗

arXiv2006

Frequency response in surface-potential driven electro-hydrodynamics

Using a Fourier approach we offer a general solution to calculations of slip velocity within the circuit description of the electro-hydrodynamics in a binary electrolyte confined by a plane surface with a modulated surface potential. We consider the case with a spatially constant intrinsic surface capacitance where the net flow rate is in general zero while harmonic rolls as well as time-averaged vortex-like components may exist depending on the spatial symmetry and extension of the surface potential. In general the system displays a resonance behavior at a frequency corresponding to the inverse RC time of the system. Different surface potentials share the common feature that the resonance frequency is inversely proportional to the characteristic length scale of the surface potential. For the asymptotic frequency dependence above resonance we find a 1/omega^2 power law for surface potentials with either an even or an odd symmetry. Below resonance we also find a power law omega^alpha with alpha being positive and dependent of the properties of the surface potential. Comparing a tanh potential and a sech potential we qualitatively find the same slip velocity, but for the below-resonance frequency response the two potentials display different power law asymptotics with alpha=1 and alpha~2, respectively.

cond-mat.soft↗

arXiv2006

Transport coefficients for electrolytes in arbitrarily shaped nano and micro-fluidic channels

We consider laminar flow of incompressible electrolytes in long, straight channels driven by pressure and electro-osmosis. We use a Hilbert space eigenfunction expansion to address the general problem of an arbitrary cross section and obtain general results in linear-response theory for the hydraulic and electrical transport coefficients which satisfy Onsager relations. In the limit of non-overlapping Debye layers the transport coefficients are simply expressed in terms of parameters of the electrolyte as well as the geometrical correction factor for the Hagen-Poiseuille part of the problem. In particular, we consider the limits of thin non-overlapping as well as strongly overlapping Debye layers, respectively, and calculate the corrections to the hydraulic resistance due to electro-hydrodynamic interactions.

physics.flu-dyn↗

arXiv2005

Universality in edge-source diffusion dynamics

We show that in edge-source diffusion dynamics the integrated concentration N(t) has a universal dependence with a characteristic time-scale tau=(A/P)^2 pi/(4D), where D is the diffusion constant while A and P are the cross-sectional area and perimeter of the domain, respectively. For the short-time dynamics we find a universal square-root asymptotic dependence N(t)=N0 sqrt(t/tau) while in the long-time dynamics N(t) saturates exponentially at N0. The exponential saturation is a general feature while the associated coefficients are weakly geometry dependent.

cond-mat.other↗

arXiv2005

Electro-hydrodynamics of binary electrolytes driven by modulated surface potentials

We study the electro-hydrodynamics of the Debye screening layer that arises in an aqueous binary solution near a planar insulating wall when applying a spatially modulated AC-voltage. Combining this with first order perturbation theory we establish the governing equations for the full non-equilibrium problem and obtain analytic solutions in the bulk for the pressure and velocity fields of the electrolyte and for the electric potential. We find good agreement between the numerics of the full problem and the analytics of the linear theory. Our work provides the theoretical foundations of circuit models discussed in the literature. The non-equilibrium approach also reveals unexpected high-frequency dynamics not predicted by circuit models.

cond-mat.soft↗

arXiv2002

Temperature induced shift in the pinch-off voltage of mesoscopic devices

Detailed experimental studies of the conductance of mesoscopic GaAs devices in the few-mode regime reveal a novel thermal effect: for temperatures up to at least 10 K the measured gate characteristics, i.e. conductance $G$ versus gate voltage $V_g$, exhibit a systematic downward shift in gate voltage with increasing temperature. The effect is 'universal', in the sense that it is observed in different modulation doped GaAs/GaAlAs heterostructures, in different device geometries, and using different measurement setups. Our observations indicate that the effect originates in the surrounding 2D electron gas and not in the mesoscopic devices themselves.

cond-mat.mes-hall↗

arXiv2002

Temperature dependent deviations from ideal quantization of plateau conductances in GaAs quantum point contacts

We present detailed experimental studies of the temperature dependence of the plateau conductance of GaAs quantum point contacts in the temperature range from 0.3 K to 10 K. Due to a strong lateral confinement produced by a shallow-etching technique we are able to observe the following unexpected feature: a linear temperature dependence of the measured mid-plateau conductance. We discuss an interpretation in terms of a temperature dependent, intrinsic series resistance, due to non-ballistic effects in the 2D-1D transition region. These results have been reproduced in several samples from different GaAs/GaAlAs heterostructures and observed in different experimental set-ups.

cond-mat.mes-hall↗

arXiv2000

Bias and temperature dependence of the 0.7 conductance anomaly in Quantum Point Contacts

The 0.7 (2e^2/h) conductance anomaly is studied in strongly confined, etched GaAs/GaAlAs quantum point contacts, by measuring the differential conductance as a function of source-drain and gate bias as well as a function of temperature. We investigate in detail how, for a given gate voltage, the differential conductance depends on the finite bias voltage and find a so-called self-gating effect, which we correct for. The 0.7 anomaly at zero bias is found to evolve smoothly into a conductance plateau at 0.85 (2e^2/h) at finite bias. Varying the gate voltage the transition between the 1.0 and the 0.85 (2e^2/h) plateaus occurs for definite bias voltages, which defines a gate voltage dependent energy difference $Δ$. This energy difference is compared with the activation temperature T_a extracted from the experimentally observed activated behavior of the 0.7 anomaly at low bias. We find Δ= k_B T_a which lends support to the idea that the conductance anomaly is due to transmission through two conduction channels, of which the one with its subband edge Δbelow the chemical potential becomes thermally depopulated as the temperature is increased.

cond-mat.mes-hall↗

arXiv1998

Activated Behavior of the 0.7 2(e^2)/h Conductance Anomaly in Quantum Point Contacts

The 0.7 conductance anomaly in the quantized conductance of trench etched GaAs quantum point contacts is studied experimentally. The temperature dependence of the anomaly measured with vanishing source-drain bias reveals the same activated behavior as reported earlier for top-gated structures. Our main result is that the zero bias, high temperature 0.7 anomaly found in activation measurements and the finite bias, low temperature 0.9 anomaly found in transport spectroscopy have the same origin: a density dependent excitation gap.

cond-mat.mes-hall↗

arXiv1998

Temperature dependence of the ``0.7'' 2(e^2)/h quasi plateau in strongly confined quantum point contacts

We present new results of the ``0.7'' 2(e^2)/h structure or quasi plateau in some of the most strongly confined point contacts so far reported. This strong confinement is obtained by a combination of shallow etching and metal gate deposition on modulation doped GaAs/GaAlAs heterostructures. The resulting subband separations are up to 20 meV, and as a consequence the quantized conductance can be followed at temperatures up to 30 K, an order of magnitude higher than in conventional split gate devices. We observe pronounced quasi plateaus at several of the lowest conductance steps all the way from their formation around 1 K to 30 K, where the entire conductance quantization is smeared out thermally. We study the deviation of the conductance from ideal integer quantization as a function of temperature, and we find an activated behavior, exp(-T_a/T), with a density dependent activation temperature T_a of the order of 2 K. We analyze our results in terms of a simple theoretical model involving scattering against plasmons in the constriction.

cond-mat.mes-hall↗