arXiv subjects

Alois Würger

Publications and source records attributed to Alois Würger.

34 records · Page 2Linked to original sources

arXiv2014

Thermally driven Marangoni surfers

We study auto-propulsion of a interface particle, which is driven by the Marangoni stress arising from a self-generated asymmetric temperature or concentration field. We calculate separately the long-range Marangoni flow v^{I} due to the stress discontinuity at the interface and the short-range velocity field v^{P} imposed by the no-slip condition on the particle surface; both contributions are evaluated for a spherical floater with temperature monopole and dipole moments. We find that the self-propulsion velocity is given by the amplitude of the "source doublet" which belongs to short-range contribution v^{P}. Hydrodynamic interactions, on the other hand, are determined by the long-range Marangoni flow v^{I}; its dipolar part results in an asymmetric advection pattern of neighbor particles, which in turn may perturb the known hexatic lattice or even favor disordered states.

cond-mat.soft↗

arXiv2014

Electric-field induced capillary interaction of charged particles at a polar interface

We study the electric-field induced capillary interaction of charged particles at a polar interface. The algebraic tails of the electrostatic pressure of each charge results in a deformation of the interface $u\sim ρ^{-4}$. The resulting capillary interaction is repulsive and varies as $ρ^{-6}$ with the particle distance. As a consequence, electric-field induced capillary forces cannot be at the origin of the secondary minimum observed recently for charged PMMA particles at on oil-water interface.

cond-mat.soft↗

arXiv2014

Dephasing of Atomic Tunneling by Nuclear Quadrupoles

Recent experiments revealed a most surprising magnetic-field dependence of coherent echoes in amorphous solids. We show that a novel dephasing mechanism involving nuclear quadrupole moments is the origin of the observed magnetic-field dependence.

cond-mat.dis-nn↗

arXiv2014

Polarization of active Janus particles

We study the collective motion of Janus particles in a temperature or concentration gradient. Because of the torque exerted by an external or self-generated field, the particles align their axis on this gradient. In a swarm of self-driven particles, this polarization enhances the interactiondriven confinement. Self-polarization in a non-uniform laser beam could be used for guiding hot particles along a given trajectory.

cond-mat.soft↗

arXiv2014

Trapping energy of a spherical particle on a curved liquid interface

We derive the trapping energy of a colloidal particle at a liquid interface with contact angle h and principal curvatures c1 and c2. The boundary conditions at the particle surface are significantly simplified by introducing the shift e of its vertical position. We discuss the undulating contact line and the curvatureinduced lateral forces for a single particle and a pair of nearby particles. The single-particle trapping energy is found to decrease with the square of both the total curvature c1 + c2 and the anisotropy c1 ? c2. In the case of non-uniform curvatures, the resulting lateral force pushes particles toward more strongly curved regions.

cond-mat.soft↗

arXiv2014

Is Soret equilibrium a non-equilibrium effect?

Recent thermophoretic experiments on colloidal suspensions revived an old debate, namely whether the Soret effect is properly described by thermostatics, or necessarily requires non-equilibrium thermodynamics. Based on colloidal transport theory and the entropy production of the related viscous flow, our analysis leads to the conclusion that the equilibrium approach may work for small ions, yet fails for colloidal particles and polymers. Regarding binary molecular mixtures, our results shed some doubt on the validity of thermostatic approaches that derive the Soret coefficient from equilibrium potentials.

cond-mat.soft↗

arXiv2014

Thermodiffusion in binary liquids: the role of irreversibility

We study thermal diffusion in binary mixtures in the framework of non-equilibrium thermodynamics. Our formal result displays the role of partial enthalpies and Onsager's generalized mobilities. The mobility ratio provides a measure for the irreversible character of thermal diffusion. Comparison with experimental data on benzene, cyclohexane, toluene and alkanes shows that irreversibility is essential for thermal diffusion, and in particular for the isotope effect.

cond-mat.soft↗

arXiv2014

Collective thermoelectrophoresis of charged colloids

Thermally driven colloidal transport is, to a large extent, due to the thermoelectric or Seebeck effect of the charged solution.We show that, contrary to the generally adopted single-particle picture, the transport coefficient depends on the colloidal concentration. For solutions that are dilute in the hydrodynamic sense, collective effects may significantly affect the thermophoretic mobility. Our results provide an explanation for recent experimental observations on polyelectrolytes and charged particles and suggest that for charged colloids collective behavior is the rule rather than the exception.

cond-mat.soft↗

arXiv2014

Stokes-Einstein diffusion of colloids in nematics

We report the experimental observation of anisotropic diffusion of polystyrene particles immersed in a lyotropic liquid crystal with two different anchoring conditions. Diffusion is shown to obey the Stokes-Einstein law for particle diameters ranging from 190 nm up to 2 micron. In the case of prolate micelles, the beads diffuse four times faster along the director than in perpendicular directions. In the theory part we present a perturbative approach to the Leslie-Ericksen equations and relate the diffusion coefficients to the Miesovicz viscosity parameters.We provide explicit formulas for the cases of uniform director field and planar anchoring conditions which are then discussed in view of the data. As a general rule, we find that the inequalities ηb < ηa < ηc, satisfied by various liquid crystals of rodlike molecules, imply Dpar > Dperp.

cond-mat.soft↗

arXiv2014

Transport in Charged Colloids Driven by Thermoelectricity

We study the thermal diffusion coefficient DT of a charged colloid in a temperature gradient, and find that it is to a large extent determined by the thermoelectric response of the electrolyte solution. The thermally induced salinity gradient leads in general to a strong increase with temperature. The difference of the heat of transport of coions and counterions gives rise to a thermoelectric field that drives the colloid to the cold or to the warm, depending on the sign of its charge. Our results provide an explanation for recent experimental findings on thermophoresis in colloidal suspensions.

cond-mat.soft↗

arXiv2014

Leidenfrost Gas Ratchets Driven by Thermal Creep

We show that thermal creep is at the origin of the recently discovered Leidenfrost ratchet, where liquid droplets float on a vapor layer along a heated sawtooth surface and accelerate to velocities of up to 40 cm=s. As the active element, the asymmetric temperature profile at each ratchet summit rectifies the vapor flow in the boundary layer. This mechanism works at low Reynolds number and provides a novel tool for controlling gas flow at nanostructured surfaces.

cond-mat.soft↗

arXiv2014

Charging of Heated Colloidal Particles Using the Electrolyte Seebeck Effect

We propose a novel actuation mechanism for colloids, which is based on the Seebeck effect of the electrolyte solution: Laser heating of a nonionic particle accumulates in its vicinity a net charge Q, which is proportional to the excess temperature at the particle surface. The corresponding long-range thermoelectric field provides a tool for controlled interactions with nearby beads or with additional molecular solutes. An external field Eext drags the thermocharged particle at a velocity that depends on its size and absorption properties; the latter point could be particularly relevant for separating carbon nanotubes according to their electronic band structure.

cond-mat.soft↗

arXiv2014

Specific Salt Effects on Thermophoresis of Charged Colloids

We study the Soret effect of charged polystyrene particles as a function of temperature and electrolyte composition. As a main result we find that the Soret coefficient is determined by charge effects, and that non-ionic contributions are small. In view of the well-kown electric-double layer interactions, our thermal field-flow fractionation data lead us to the conclusion that the Soret effect originates to a large extent from diffusiophoresis in the salt gradient and from the electrolyte Seebeck effect, both of which show strong specific-ion effects. Moreover, we find that thermophoresis of polystyrene beads is fundamentally different from proteins and aqueous polymer solutions, which show a strong non-ionic contribution.

cond-mat.soft↗

arXiv2014

Flow pattern in the vicinity of self-propelling hot Janus particles

We study the temperature field and the resulting flow pattern in the vicinity of a heated metal-capped Janus particle. If its thickness exceeds about 10 nm, the cap forms an isotherm and the flow pattern comprises a quadrupolar term that decays with the square of the inverse distance 1/r^2. For much thinner caps the velocity varies as 1/r^3. These findings could be relevant for collective effects in dense suspensions and for the circular tracer motion observed recently in the vicinity of a tethered Janus particle.

cond-mat.soft↗

arXiv2000

Collective Phenomena in Defect Crystals

We investigate effects of interactions between substitutional defects on the properties of defect crystals at low temperatures, where defect motion is governed by quantum effects. Both, thermal and dynamical properties are considered. The influence of interactions on defect motion is described via a collective effect. Our treatment is semiclassical in the sense that we analyze collective effects in a classical setting, and analyze the influence on quantized defect motion only thereafter. Our theory describes a crossover to glassy behavior at sufficiently high defect concentration. Our approach is meant to be general. For the sake of definiteness, we evaluate most of our results with parameters appropriate for Li-doped KCl crystals.

cond-mat.stat-mech↗

arXiv1994

Relaxation due to Incoherent Tunnelling in Dielectric Glasses

A new relaxation mechanism is shown to arise from overdamped two-level systems above a critical temperature $T^*\approx 5$ K, thus yielding an explanation for experimental observations in dielectric glasses in the temperature range between $T^*$ and the relaxation peak at 50 K. Using the distribution function of the tunnelling model for the parameters of the two-level systems, both the linear decrease of the sound velocity and the linear increase of the absorptionup to the relaxation maximum, are quantitatively accounted for by our theory.

cond-mat↗