Alois Würger

Thermal non-equilibrium transport in colloids

A temperature gradient acts like an external field on colloidal suspensions and drives the solute particles to the cold or to the warm, depending on interfacial and solvent properties. We discuss different transport mechanisms for charged colloids, and how a thermal gradient gives rise to companion fields. Particular emphasis is put on the thermal response of the electrolyte solution: positive and negative ions diffuse along the temperature gradient and thus induce a thermoelectric field which in turn acts on the colloidal charges. Regarding polymers in organic solvents, the physical mechanism changes with decreasing molecular weight: high polymers are described in the framework of macroscopic hydrodynamics; for short chains and molecular mixtures of similar size, the Brownian motion of solute and solvent becomes important.

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 co-ions 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.

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 ~r(-2). For much thinner caps the velocity varies as ~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.

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 saw-tooth 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. PACS numbers: 47.61.-k, 47.15.Rq, 44.20.+b, 68.03.-g

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 tail of the electrostatic pressure of each charge results in a deformation of the interface u approximately r(-4), where r is the lateral distance. The capillary interaction of nearby particles is repulsive and varies as rho(-6) with their distance rho. As a consequence, electric-field induced capillary forces cannot be at the origin of the secondary minimum observed recently for charged poly(methyl methacrylate) particles at an oil-water interface.

Polarization of active Janus particles.

We theoretically study the motion of surface-active Janus particles, driven by an effective slip velocity due to a nonuniform temperature or concentration field ψ. With parameters realized in thermal traps, we find that the torque exerted by the gradient ∇ψ inhibits rotational diffusion and favors alignment of the particle axes. In a swarm of active particles, this polarization adds a novel term to the drift velocity and modifies the collective behavior. Self-polarization in a nonuniform laser beam could be used for guiding hot particles along a given trajectory.

Nuclear spin conversion of methyl groups

A theory is presented for nuclear spin conversion of methyl groups, where the total spin of the three protons changes fromI=1/2 toI=3/2. The transition may be mediated by the magnetic dipolar interaction of the protons among themselves or with a nearby unpaired electron. In general the excess energy, i.e. the tunnelling energy Δ, is transferred from the spin system to the lattice via the rotor-phonon coupling; for the case of an almost free rotor in the magnetic field of an unpaired electron spin, the direct coupling of the electron-proton interaction to the lattice motion may be the more efficient mechanism. AtT=0 the rate is found to be finite, at high temperatures it shows an Arrhenius behaviour. In the intermediate range, two different power laws may govern the temperature dependence, namely 1/τ∝T or 1/τ∝T7; the latter is due to two-phonon scattering.

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.

Rotational tunnelling in (CH3)2SnCl2: a neutron scattering study

The temperature dependence of the rotational tunnelling excitations in (CH3)2SnCl2 is investigated between 5 and 232K. The quasi-elasticEa−Eb-transitions are found to be narrower than the inelasticA−E-transitions. At high temperatures the broadening is linear in the temperature. The results are in qualitative agreement with a recent theory.

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 due to the stress discontinuity at the interface and the short-range velocity field v 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 . Hydrodynamic interactions, on the other hand, are determined by the long-range Marangoni flow v ; 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.