Elastohydrodynamics of a sliding, spinning and sedimenting cylinder near a soft wall

We consider the motion of a fluid-immersed negatively buoyant particle in the vicinity of a thin compressible elastic wall, a situation that arises in a variety of technological and natural settings. We use scaling arguments to establish different regimes of sliding, and complement these estimates using thin-film lubrication dynamics to determine an asymptotic theory for the sedimentation, sliding, and spinning motions of a cylinder. The resulting theory takes the form of three coupled nonlinear singular-differential equations. Numerical integration of the resulting equations confirms our scaling relations and further yields a range of unexpected behaviours. Despite the low-Reynolds feature of the flow, we demonstrate that the particle can spontaneously oscillate when sliding, can generate lift via a Magnus-like effect, can undergo a spin-induced reversal effect, and also shows an unusual sedimentation singularity. Our description also allows us to address a sedimentation-sliding transition that can lead to the particle coasting over very long distances, similar to certain geophysical phenomena. Finally, we show that a small modification of our theory allows to generalize the results to account for additional effects such as wall poroelasticity

Rotation of an immersed cylinder sliding near a thin elastic coating

It is known that an object translating parallel to a soft wall in a viscous fluid produces hydro- dynamic stresses that deform the wall, which, in turn, results in a lift force on the object. Recent experiments with cylinders sliding under gravity near a soft incline, which confirmed theoretical arguments for the lift force, also reported an unexplained steady-state rotation of the cylinders [Saintyves et al. PNAS 113(21), 2016]. Motivated by these observations, we show, in the lubrication limit, that an infinite cylinder that translates in a viscous fluid parallel to a soft wall at constant speed and separation distance must also rotate in order to remain free of torque. Using the Lorentz reciprocal theorem, we show analytically that for small deformations of the elastic layer, the angular velocity of the cylinder scales with the cube of the sliding speed. These predictions are confirmed numerically. We then apply the theory to the gravity-driven motion of a cylinder near a soft incline and find qualitative agreement with the experimental observations, namely that a softer elastic layer results in a greater angular speed of the cylinder.Comment: 16 pages, 4 figure

Designing arrays of Josephson junctions for specific static responses

We consider the inverse problem of designing an array of superconducting Josephson junctions that has a given maximum static current pattern as function of the applied magnetic field. Such devices are used for magnetometry and as Terahertz oscillators. The model is a 2D semilinear elliptic operator with Neuman boundary conditions so the direct problem is difficult to solve because of the multiplicity of solutions. For an array of small junctions in a passive region, the model can be reduced to a 1D linear partial differential equation with Dirac distribution sine nonlinearities. For small junctions and a symmetric device, the maximum current is the absolute value of a cosine Fourier series whose coefficients (resp. frequencies) are proportional to the areas (resp. the positions) of the junctions. The inverse problem is solved by inverse cosine Fourier transform after choosing the area of the central junction. We show several examples using combinations of simple three junction circuits. These new devices could then be tailored to meet specific applications.Comment: The article was submitted to Inverse Problem

Photoassociative creation of ultracold heteronuclear 6Li40K* molecules

We investigate the formation of weakly bound, electronically excited, heteronuclear 6Li40K* molecules by single-photon photoassociation in a magneto-optical trap. We performed trap loss spectroscopy within a range of 325 GHz below the Li(2S_(1/2))+K(4P_(3/2)) and Li(2S_(1/2))+K(4P_(1/2)) asymptotic states and observed more than 60 resonances, which we identify as rovibrational levels of 7 of 8 attractive long-range molecular potentials. The long-range dispersion coefficients and rotational constants are derived. We find large molecule formation rates of up to ~3.5x10^7s^(-1), which are shown to be comparable to those for homonuclear 40K_2*. Using a theoretical model we infer decay rates to the deeply bound electronic ground-state vibrational level X^1\Sigma^+(v'=3) of ~5x10^4s^(-1). Our results pave the way for the production of ultracold bosonic ground-state 6Li40K molecules which exhibit a large intrinsic permanent electric dipole moment.Comment: 6 pages, 4 figures, submitted to EP

Herschel/HIFI deepens the circumstellar NH3 enigma

Circumstellar envelopes (CSEs) of a variety of evolved stars have been found to contain ammonia (NH3) in amounts that exceed predictions from conventional chemical models by many orders of magnitude. The observations reported here were performed in order to better constrain the NH3 abundance in the CSEs of four, quite diverse, oxygen-rich stars using the NH3 ortho J_K = 1_0 - 0_0 ground-state line. We used the Heterodyne Instrument for the Far Infrared aboard Herschel to observe the NH3 J_K = 1_0 - 0_0 transition near 572.5 GHz, simultaneously with the ortho-H2O J_Ka,Kc = 1_1,0 -1_0,1 transition, toward VY CMa, OH 26.5+0.6, IRC+10420, and IK Tau. We conducted non-LTE radiative transfer modeling with the goal to derive the NH3 abundance in these objects' CSEs. For the latter two stars, Very Large Array imaging of NH3 radio-wavelength inversion lines were used to provide further constraints, particularly on the spatial extent of the NH3-emitting regions. Results. We find remarkably strong NH3 emission in all of our objects with the NH3 line intensities rivaling those obtained for the ground state H2O line. The NH3 abundances relative to H2 are very high and range from 2 x 10-7 to 3 x 10-6 for the objects we have studied. Our observations confirm and even deepen the circumstellar NH3 enigma. While our radiative transfer modeling does not yield satisfactory fits to the observed line profiles, it leads to abundance estimates that confirm the very high values found in earlier studies. New ways to tackle this mystery will include further Herschel observations of more NH3 lines and imaging with the Expanded Very Large Array.Comment: 4+2 page

First detection of Hydrogen Chloride towards protostellar shocks

We present the first detection of hydrogen chlorine in a protostellar shock, by observing the fundamental transition at 626 GHz with the Herschel HIFI spectrometer. We detected two of the three hyperfine lines, from which we derived a line opacity < 1. Using a non-LTE LVG code, we constrained the HCl column density, temperature and density of the emitting gas. The hypothesis that the emission originates in the molecular cloud is ruled out, as it would imply a too dense gas. Conversely, assuming that the emission originates in the 10"-15" size shocked gas previously observed at the IRAM PdB interferometer, we obtain: N(HCl)=0.7-2 x 10(13) cm-2, temperature > 15 K and density > 3 x 10(5) cm-3}. Combining with the Herschel HIFI CO(5-4) observations allows to further constrain the gas density and temperature, 10(5)-10(6) cm-3 and 120-250 K, as well as the HCl column density, 2 x 10(13) cm-2, and, finally, abundance: 3-6 x 10(-9). The estimated HCl abundance is consistent with that previously observed in low- and high- mass protostars. This puzzling result in the L1157-B1 shock, where species from volatile and refractory grains components are enhanced, suggests either that HCl is not the main reservoir of chlorine in the gas phase, against previous chemical models predictions, or that the elemental chlorine abundance is low in L1157-B1. Astrochemical modelling suggests that HCl is in fact formed in the gas phase, at low temperatures, prior to the occurance of the shock, and that the latter does not enhance its abundance.Comment: ApJ, in pres