Probing the nanohydrodynamics at liquid-solid interfaces using thermal motion

We report on a new method to characterize nanohydrodynamic properties at the liquid-solid interface relying solely on the measurement of the thermal motion of confined colloids. This equilibrium measurement of surface properties--equivalent in spirit to the passive microrheology technique used for bulk properties--is able to achieve nanometric resolution on the slip length measurement. Exploring the "zero shear rate" limit, it rules out shear rate threshold to slip effects and extends the range over which slip lengths are shown to be flow independent. Avoiding the nucleation of gas pockets (nanobubbles) through external forcing, it validates the theoretical picture for intrinsic liquid-solid interfaces, reporting nanometric slip lengths (b=18+/-5 nm) only in nonwetting situations, opening the route to quantitative study on more complex surfaces with combined effects of nonwettability and roughness.     

Spin in a Closed‐Shell Organic Molecule on a Metal Substrate Generated by a Sigmatropic Reaction

Inert metal surfaces present more chances of hosting organic intact radicals than other substrates, but large amounts of delocalized electronic states favor charge transfer and thus spin quenching. Lowering the molecule–substrate interaction is a usual strategy to stabilize radicals on surfaces. In some works, thin insulating layers were introduced to provide a controllable degree of electronic decoupling. Recently, retinoid molecules adsorbed on gold have been manipulated with a scanning tunneling microscope (STM) to exhibit a localized spin, but calculations failed to find a radical derivative of the molecule on the surface. Now the formation of a neutral radical spatially localized in a tilted and lifted cyclic end of the molecule is presented. An allene moiety provokes a perpendicular tilt of the cyclic end relative to the rest of the conjugated chain, thus localizing the spin of the dehydrogenated allene in its lifted subpart. DFT calculations and STM manipulations give support to the proposed mechanism.     

Nanofluidics in the Debye layer at hydrophilic and hydrophobic surfaces

By using evanescent waves, we study equilibrium and dynamical properties of liquid-solid interfaces in the Debye layer for hydrophilic and hydrophobic surfaces. We measure velocity profiles and nanotracer concentration and diffusion profiles between 20 and 300 nm from the walls in pressure-driven and electro-osmotic flows. We extract electrostatic and zeta potentials and determine hydrodynamic slip lengths with 10 nm accuracy. The spectacular amplification of the zeta potential resulting from hydrodynamic slippage allows us to clarify for the first time the dynamic origin of the zeta potential.     

Massive amplification of surface-induced transport at superhydrophobic surfaces

We study electro- and diffusio-osmosis of aqueous electrolytes at superhydrophobic surfaces by means of computer simulation and hydrodynamic theory. We demonstrate that the diffusio-osmotic flow at superhydrophobic surfaces can be amplified by more than 3 orders of magnitude relative to flow in channels with a zero interfacial slip. By contrast, little enhancement is observed at these surfaces for electro-osmotic flow. This amplification for diffusio-osmosis is due to the combined effects of enhanced slip and ion surface depletion or excess at the air-water interfaces on superhydrophobic surfaces. This effect is interpreted in terms of capillary driven Marangoni motion. A practical microfluidic pumping device is sketched on the basis of the slip-enhanced diffusio-osmosis at a superhydrophobic surface.     

A moving microfoil conversion electron detector for selective excitation double Mössbauer experiments

A lightweight microfoil conversion electron detector has been built for use as a detector/analyser for selective excitation double Mössbauer experiments. The detector weighs approximately 40 g; it can be moved with constant acceleration by a conventional electromechanical drive, with peak velocities up to ±15 mm/s. Conversion electrons from resonant absorption events are collected from both sides of two 0.3 μm thick⁵⁷Fe-enriched stainless steel foils inside the detector. The detector's performance (signal to noise per unit time) has been compared with a potassium ferrocyanide absorber and proportional counter, and it is found to be about ten times better for a given source strength and sampling angle. The overall detector linewidth is 0.42 (2) mm/s, compared with 0.37 (1) mm/s for the PFC absorber.     

Experimental beta-decay energies of short-lived light fission products

The beta-decay energy of 20 nuclides in the mass region 88≦A≦102 has been measured using the on-line mass separator LOHENGRIN. The experimental values obtained are compared with theoretical predictions of different mass formulae.     

Gamma-ray spectroscopy of short-lived fission products using the recoil separator “Lohengrin”

γ-spectra and half-lives of⁹⁰Br,⁹⁵Rb,⁹⁶Rb,⁹⁶Sr,⁹⁶Y,¹³²Sb and¹³²Sn have been measured using the unslowed fission fragment separator “Lohengrin”. Preliminary decay schemes for⁹⁰Kr,⁹⁵Sr,⁹⁶Sr and⁹⁶Y are given.     

Charge distributions in low-energy nuclear fission and their relevance to fission dynamics

Radiochemical and energy loss techniques have been used to measure fragment charge distributions in the systems ²²⁹Th(n_th, f), ²³⁵U(n_{3 MeV}, f) and ²⁵²Cf(s, f). Together with those previously obtained [^{233, 235}U(n_th, f) and ^{239, 241}Pu(n_th, f] the results show that these charge distributions depend very sensitively on both the fissioning species and the initial excitation energy. Very strong even-odd effects are present, for example in the charge distribution of ²²⁹Th(n_th, f) while they almost disappear in ²³⁵U(n_{3 MeV}, f). In the ²²⁹Th(n_th f) and ²⁵²Cf(s, f) cases, kinetic energy distributions of the fragments have been obtained. Fragments with even charges are more energetic, on average, than fragments with odd charges. The results are discussed in terms of a rather small probability for pair breaking during the transition from saddle-point to scission. The relationship between even-odd effects in charge distributions and fine structures in mass distributions is also investigated.     

Measurement of charge distributions for 229Th(nth,f) and 232U(nth,f)

A gas ΔE ? E_R telescope has been used to measure charge yields and their correlations with kinetic energies for ²²⁹Th and ²³²U. Even-charge yields are enhanced compared with odd-charge yields for both fissioning systems; this enhancement increases for events with higher kinetic energy. The mean odd-even effect $\text{δ}{}_{\mathrm{<mtext>p</mtext>}}\text{is}\text{= (40±4)\%}\text{for}{}^{229}\text{Th}$; it is $\text{(21 ± 3)\%}\text{for}{}^{232}\text{U-the same as for}{}^{233}\text{U}\text{((22.1±2.1)\%)}\text{and}{}^{235}\text{U}\text{((23.7±0.7)\%)}$. The energy-integrated δ_p and δ_p for different energy windows, vary strongly as a function of charge (Z) due to the underlying shells. The δ_p averaged over Z increases fast with kinetic energy, contrary to the existing results for ²³³U and ²³⁵U, where δ_p flattens off at low energies. For both systems, the most probable kinetic energy ē shows a strong odd-even stagger; the mean odd-even effect on energy, $\text{δE}{}_{\mathrm{<mtext>K</mtext>}}{}^{\mathrm{<mtext>o?e</mtext>}}\text{,}\text{is}\text{1.4 ± 0.3}\text{MeV for}{}^{229}\text{Th, and}\text{1.7±0.4}\text{MeV for}{}^{232}\text{U}\text{—}$ the latter is about twice the value for ${}^{233}\text{U}\text{(0.95 ± 0.09}\text{MeV}\text{)}\text{and}{}^{235}\text{U}\text{(0.7}\text{MeV}\text{)}$. These results are discussed in terms of the existing models.