We report an experimental analysis of path and shape oscillations of an air bubble of diameter d rising in water at high Reynolds number Re in a vertical Hele-Shaw cell of width h. Liquid velocity perturbations induced by the relative movement have also been investigated to analyse the coupling between the bubble motion and the wake dynamics. The confinement ratio h/d is less than unity so that the bubble is flattened between the walls of the cell. As the bubble diameter is increased, the Archimedes and the Bond numbers increase within the ranges 10 Ar 10 4 and 6 × 10 −3 Bo 140. Mean shapes become more and more elongated. They first evolve from in-plane circles to ellipses, then to complicated shapes without fore-aft symmetry and finally to semi-circular-capped bubbles. The scaling law Re = 0.5Ar is valid for a large range of Ar, however, indicating that the liquid films between the bubble and the walls do not contribute significantly to the drag force exerted on the bubble. The coupling between wake dynamics, bubble path and shape oscillations evolves and a succession of different regimes of oscillations is observed. The rectilinear bubble motion becomes unstable from a critical value Ar 1 through an Hopf bifurcation while the bubble shape is still circular. The amplitude of path oscillations first grows as Ar increases above Ar 1 but then surprisingly decreases beyond a second Archimedes number Ar 2 . This phenomenon, observed for steady ellipsoidal shape with moderate eccentricity, can be explained by the rapid attenuation of bubble wakes caused by the confinement. Shape oscillations around a significantly elongated mean shape start for Ar Ar 3 . The wake structure progressively evolves due to changes in the bubble shape. After the break-up of the fore-aft symmetry, a fourth regime involving complicated shape oscillations is then observed for Ar Ar 4 . Vortex shedding disappears and unsteady attached vortices coupled to shape oscillations trigger path oscillations of moderate amplitude. Path and shape oscillations finally decrease when Ar is further increased. For Ar Ar 5 , capped bubbles followed by a steady wake rise on a straight path.

An acridine derivative was covalently linked to the 5' end of a homopyrimidine oligonucleotide. Specific binding to a homopurine homopyrimidine sequence of duplex DNA was demonstrated by spectroscopic studies (absorption and fluorescence) and by "footprinting" experiments with a copper phenanthroline chelate used as an artificial nuclease. A hypochromism and a red shift of the acridine absorption were observed. Triple-helix formation was also accompanied by a hypochromism in the ultraviolet range. The fluorescence of the acridine ring was quenched by a stacking interaction with a GC base pair adjacent to the homopurine-homopyrimidine target sequence. The intercalating agent strongly stabilized the complex formed by the oligopyrimidine with its target duplex sequence. Cytosine methylation further increased the stability of the complexes. Footprinting studies revealed that the oligopyrimidine binds in a parallel orientation with respect to the homopurine-containing strand of the duplex. The intercalated acridine extended by 2 base pairs the region of the duplex protected by the oligopyrimidine against degradation by the nuclease activity of the copper phenanthroline chelate. Random intercalation of the acridine ring was lost due to the repulsive effect of the negatively charged oligonucleotide tail. Intercalation occurred only at those double-stranded sequences where the homopyrimidine oligonucleotide recognized the major groove of duplex DNA.

We investigate the characteristics of the oscillatory motion and wake of confined bubbles freely rising in a thin-gap cell (h = 3.1 mm width). Once the diameter d of the bubble in the plane of the cell is known, the mean vertical velocity of the bubble V b is proportional to the gravitational velocity (h/d) 1/6 √ gd, where g is the gravitational acceleration. This velocity is used to build the Reynolds number Re = V b d/ν that characterizes the flow induced by the bubble in the surrounding liquid (of kinematic viscosity ν), and which determines at leading order the mean deformation of the bubble given by the aspect ratio χ of the ellipse equivalent to the bubble contour. We then show that in the reference frame associated with the bubble (having a fixed origin and axes corresponding to the minor and major axes of the equivalent ellipse) the characteristics of its oscillatory motion in the plane of the cell display remarkable properties in the range 1200 < Re < 3000 and h/d < 0.4. In particular, the velocity of the bubble presents along its path an almost constant component along its minor axis (fluctuations in time of approximately 5 %), given by V a /V b 0.92 for all Re. The dimensionless amplitude of oscillation of the angular velocity is also constant for all Re,rd/V b 0.75, while that of the transverse velocity of the bubble (along its major axis) is given byṼ t /V b 0.32χ, reaching values comparable to those of the axial velocity V a for the most deformed bubbles (χ ≈ 3). Furthermore, the frequency f of oscillation scales with the inertial time scale based on the transverse velocity of the bubbleṼ t , corresponding to a constant Strouhal number St * = fd/Ṽ t 0.27. Using high-frequency particle image velocimetry, we investigate in detail the properties of the wake associated with the oscillatory motion of sufficiently confined bubbles. We observe that vortex shedding occurs for a maximal transverse velocity V t of the bubble, corresponding to a maximal drift angle of the bubble. Furthermore, the measured vorticity of the vortex at detachment corresponds to the estimation V b χ 3/2 /d of the vorticity produced at the bubble surface. Three stages then emerge concerning the evolution in time of the wake generated by the bubble. For one to two periods of oscillation T x following the release of a vortex, a rapid decay of the vorticity of the released vortex is observed. Meanwhile, the released vortex located initially at a distance of approximately one diameter from the bubble centre moves outwards from the bubblepath and expands. At intermediate times, the vortex † Email address for correspondence: roig@imft.fr Oscillatory motion and wake of a bubble rising in a thin-gap cell 61 street undergoes vortex pairing. When viscous effects become predominant at a time of the order of the viscous time scale τ ν = h 2 /(4ν), the vortex street becomes frozen and decays exponentially in place.

This work reports an experimental investigation of the dispersion of a low-diffusive dye within a homogeneous swarm of high-Reynolds-number rising bubbles at gas volume fractions α ranging from 1 % to 13 %. The capture and transport of dye within bubble wakes is found to be negligible and the mixing turns out to result from the bubble-induced turbulence. It is described well by a regular diffusion process. The diffusion coefficient corresponding to the vertical direction is larger than that corresponding to the horizontal direction, owing to the larger intensity of the liquid fluctuations in the vertical direction. Two regimes of diffusion have been identified. At low gas volume fraction, the diffusion time scale is given by the correlation time of the bubble-induced turbulence and the diffusion coefficients increase roughly as α 0.4 . At large gas volume fraction, the diffusion time scale is imposed by the time interval between two bubbles and the diffusion coefficients become almost independent of α. The transition between the two regimes occurs sooner in the horizontal direction (1 % α 3 %) than in the vertical direction (3 % α 6 %). Physical models based on the hydrodynamic properties of the bubble swarm are introduced and guidelines for practical applications are suggested.

Interfacial mass transfer is known to be enhanced for confined bubbles due to the efficiency of the transfer in the thin liquid films between them and the wall. In the present experimental investigation, the mechanisms of gas–liquid mass transfer are studied for isolated bubbles rising at high Reynolds number in a thin gap. A planar laser induced fluorescence (PLIF) technique is applied with a dye the fluorescence of which is quenched by dissolved oxygen. The aim is to measure the interfacial mass fluxes for pure oxygen bubbles of various shapes and paths rising in water at rest. In the wakes of the bubbles, patterns due to the presence of dissolved oxygen are observed on PLIF images. They reveal the contrasted contributions to mass transfer of two different regions of the interface. The flow around a bubble consists of both two thin liquid films between the bubble and the walls of the cell and an external high‐Reynolds‐number in‐plane flow surrounding the bubble. Mass transfer mechanisms associated to both regions are discussed. Measurement of the concentration of dissolved oxygen is a difficult task due to the nonlinear relation between the fluorescence intensity and the concentration in the gap. It is however possible to accurately measure the global mass flux transferred through the bubble interface. It is determined from the fluorescence intensity recorded in the wakes when the oxygen distribution has been made homogeneous through the gap by diffusion. Assuming a reasonable distribution of oxygen concentration through the gap at short time also allows a measurement of the mass fluxes due to the liquid films. A discussion of the results points out the specific physics of mass transfer for bubbles confined between two plates as compared to bubbles free to move in unconfined flows. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2394–2408, 2017

We have used alpha-oligomers as antisense oligonucleotides complementary to three different sequences of the rabbit beta-globin mRNA: a region adjacent to the cap site, a region spanning the AUG initiation codon or a sequence in the coding region. These alpha-oligonucleotides were synthesized either with a free 5' OH group or linked to an acridine derivative. The effect of these oligonucleotides on mRNA translation was investigated in cell-free extracts and in Xenopus oocytes. In rabbit reticulocyte lysate and in wheat germ extracts oligomers targeted to the cap site and the initiation codon reduced beta-globin synthesis in a dose-dependent manner, whereas the target mRNA remained intact. The anti-cap alpha-oligomer was even more efficient that its beta-counterpart in rabbit reticulocyte lysate. In contrast, only the alpha-oligomer, linked to the acridine derivative, complementary to the cap region displayed significant antisense properties in Xenopus oocytes. Therefore initiation of translation can be arrested by oligonucleotide/RNA hybrids which are not substrates for RNase-H.

We investigated experimentally the motion of elongated finite-length cylinders (length $L$, diameter $d$) freely falling under the effect of buoyancy in a low-viscosity fluid otherwise at rest. For cylinders with densities $\unicode[STIX]{x1D70C}_{c}$ close to the density $\unicode[STIX]{x1D70C}_{f}$ of the fluid ($\overline{\unicode[STIX]{x1D70C}}=\unicode[STIX]{x1D70C}_{c}/\unicode[STIX]{x1D70C}_{f}\simeq 1.16$), we explored the effect of the body volume by varying the Archimedes number $Ar$ (based on the body equivalent diameter) between 200 and 1100, as well as the effect of their length-to-diameter ratios $L/d$ ranging from 2 to 20. A shadowgraphy technique involving two cameras mounted on a travelling cart was used to track the cylinders along their fall over a distance longer than $30L$. A dedicated image processing algorithm was further implemented to properly reconstruct the position and orientation of the cylinders in the three-dimensional space. In the range of parameters explored, we identified three main types of paths, matching regimes known to exist for three-dimensional bodies (short-length cylinders, disks and spheres). Two of these are stationary, namely, the rectilinear motion and the large-amplitude oscillatory motion (also referred to as fluttering or zigzag motion), and their characterization is the focus of the present paper. Furthermore, in the transitional region between these two regimes, we observed irregular low-amplitude oscillatory motions, that may be assimilated to the A-regimes or quasi-vertical regimes of the literature. Flow visualization using dye released from the bodies uncovered the existence of different types of vortex shedding in the wake of the cylinders, according to the style of path. The detailed analysis of the body kinematics in the fluttering regime brought to light a series of remarkable properties. In particular, when normalized with the characteristic velocity scale $u_{0}=\sqrt{(\overline{\unicode[STIX]{x1D70C}}-1)gd}$ and the characteristic length scale $l_{0}=\sqrt{dL}$, the mean vertical velocity $\overline{u_{Z}}$ and the frequency $f$ of the oscillations become almost independent of $L/d$ and $Ar$. The use of the length scale $l_{0}$ and of the gravitational velocity scale to build the Strouhal number $St^{\ast }=fl_{0}/u_{0}$ allowed us to generalize to short ($0.1\leqslant L/d\leqslant 0.5$) and elongated cylinders ($2\leqslant L/d\leqslant 12$), the result $St^{\ast }\simeq 0.1$. An interpretation of $l_{0}$ as a characteristic length scale associated with the oscillatory recirculation thickness generated near the body ends is proposed. In addition, the rotation rate of the cylinders scales with $u_{0}/L$, for all $L/d$ and $Ar$ investigated. Furthermore, the phase difference between the oscillations of the velocity component $u$ along the cylinder axis and of the inclination angle $\unicode[STIX]{x1D703}$ of the cylinder is approximately constant, whatever the elongation ratio $L/d$ and the Archimedes number $Ar$.

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