Capillary Axis Research Articles

Test of image acceleration by distant collision of proton microbeam

Charged particles in front of surfaces always accelerate towards the surface due to the attractive image force between the charge and the surface. It is shown that using capillary transmission by charged particle projectiles the image acceleration can be clearly studied without close collision with the inner surface. We present experimental studies of the deflection of the proton microbeam with energy of 1 MeV as a function of the entering beam position parallel to the capillary axis when they pass through the steel macro-capillary.

Dynamic guiding process of 10-keV O−ions transmitting through Al2O3nanocapillaries

In this Brief Report, we observed the dynamic guiding of 10-keV O${}^{\ensuremath{-}}$ ions (50 pA/mm${}^{2}$) transmitting through Al${}_{2}$O${}_{3}$ nanocapillaries. The transmitted O${}^{\ensuremath{-}}$ ions shift gradually to the direction of the capillary axis, and the angular FWHM also increases significantly. However, the transmission rate of O${}^{\ensuremath{-}}$ ions decreases. The data imply that the charges deposited in capillaries provide a sufficient guiding force to the incident O${}^{\ensuremath{-}}$ ions.

Cold atom guidance in a capillary using blue-detuned, hollow optical modes

We demonstrate guiding of cold 85Rb atoms through a 100-micron-diameter hollow core dielectric waveguide using cylindrical hollow modes. We have transported atoms using blue-detuned light in the 1st order, azimuthally-polarized TE01 hollow mode, and the 2nd order hollow modes (HE31, EH11, and HE12), and compared these results with guidance in the red-detuned, fundamental HE11 mode. The blue-detuned hollow modes confine atoms to low intensity along the capillary axis, far from the walls. We determine scattering rates in the guides by directly measuring the effect of recoil on the atoms. We observe higher atom numbers guided using red-detuned light in the HE11 mode, but a 10-fold reduction in scattering rate using the 2nd order modes, which have an r4 radial intensity profile to lowest order. We show that the red-detuned guides can be used to load atoms into the blue-detuned modes when both high atom number and low perturbation are desired.

Microwave discharge in a finite length vessel

A microwave surface-wave discharge at low pressure in a finite length vessel is experimentally investigated. Argon plasma is created in a dielectric capillary with length of 15 mm, the tube being an extension of an open-ended coaxial structure. Microwave power at frequency 2.45 GHz is coupled into the source applicator at power levels 6-15 W. The plasma column increases with the power applied and standing wave mode is realized when its length is equal to the capillary length. The electron temperature and plasma density are obtained simultaneously by passive optical emission spectroscopy using the line-ratio method. The surface waves phase-diagram shows that their wavelengths are much lower than the free-space wavelength. The input impedance of the plasma column - modelled as a lossy transmission line - has resonance behaviour when its length is equal to λg/2 or λg (λg being the surface waves wavelength). The results obtained for the plasma parameters, the surface waves wavelength and the vessel length show correlation to the power reflected, absorbed and radiated from the plasma column. The radiation pattern of the column shows that the main lobe is nearly perpendicular to the capillary axis.

Charging dynamics in electron transmission through Al2O3 capillaries

We have measured the transmission of low-energy (E=250eV) electrons through insulating Al2O3 nanocapillaries. We find that transmission at the incident energy is quickly suppressed with increasing tilt angle ψ of the capillary axis relative to the incoming electron beam while inelastic transmission becomes more important. For small angles of incidence charging dynamics lead to a decrease of total transmission (elastic and inelastic) with time. Our results are interpreted on the basis of a microscopic model for the electron transport within the capillary including also close interactions with the capillary wall.

Can positrons be guided by insulating capillaries?

Guiding of positrons and electrons by macroscopic insulating capillaries is studied both experimentally and via static 2D modeling of positron trajectories. Using sub-femtoamp beams, the transmitted intensities through a macroscopic tapered borosilicate glass capillary are measured as a function of capillary tilt angle and time. In agreement with previous guiding experiments, a portion of the entering beams are found to emerge in the same direction of the capillary axis and the transmitted intensities are found to decrease with increasing capillary tilt angle. But, a comparison with transmitted photon data implies that these features may result from purely geometric reasons. Our simulations imply that even sub-femtoamp beam intensities are sufficient to form charge patches and produce guiding within a short time and that too high intensities may lead to defocusing effects in tapered capillaries. But measurements of the transmitted positron and electron beams as a function of charge entering the capillary could not definitely confirm or reject guiding as no major increases in the transmitted intensity were observed. Thus, guiding of low-intensity positron beams is still in question.

Evidence of blocking effects on 3-keV Ne7+ions guided through nanocapillaries in polycarbonate

We studied the dynamic properties of ion guiding through nanocapillaries etched in insulating polycarbonate (PC). Capillaries with diameters of 95 and 165 nm and a length of 10 mu m were used. In a further sample, the capillaries had a diameter of 175 nm and a length of 30 mu m. A few additional measurements were made using capillaries in polyethylene terephthalate (PET). The temporal evolution of the intensity and the angular distribution of the transmitted ions were studied by measuring transmission profiles as a function of the charge deposited on the sample surface. The tilt angle of the capillary axis was varied from 0 degrees to 5 degrees. The mean emission angle of the transmission profiles exhibit pronounced oscillations, similarly as in previous measurements using PET. However, for PC, nearly an order of magnitude more charge needs to be inserted into the capillaries to accomplish the oscillations. In contrast to PET, with PC, we observed a strong decrease of the profile intensities with irradiation time. This observation provides evidence of blocking effects on the ions, which are likely to be due to a repulsive field produced by significant charge deposition inside the PC capillaries.

Charge exchange of keV O− ions transmitted through Al2O3 nano-capillaries

The transmission of 10–18 keV O− ions through Al2O3 nano-capillaries has been investigated experimentally. From the angular distribution of the transmitted particles, O0 and O− and O+ ions can be distinguished in the mixed scattered beams. The fractions of the scattered ions increase with both the tilt angle and the incident energy, which indicates strongly that there is dynamical charge exchange in the scattering of negative ions on the insulator surface. Moreover, it is found that the scattered neutrals (O0) are only more or less guided along the capillary axis, which is greatly different from the case of the guiding effect of highly charged ions.

Charge evolution and energy loss associated with electron transmission through a macroscopic single glass capillary

Charge (time) evolution and the angular dependence of incident electrons in the range 300–1030 eV through a single macroscopic glass capillary was studied. Charge measurements were done at a sample tilt angle of ψ = 2˚ for observation angles θ = 0˚ and 0.5˚ (both ψ and θ were measured with respect to the incident beam direction) at incident energies of 520.7 and 824.5 eV using a parallel-plate spectrometer. After equilibrium of transmission, electrons had lower average centroid (mean) energies than the respective primary beam values. Centroid energies of transmitted electrons at the centroid of the angular distribution (where the observation angle θ is nearly equal to tilt angle of the sample ψ) were found to decrease exponentially with increasing sample tilt angles for all the measured electron energies. This energy loss is attributed to inelastic scattering of electrons with the inner wall of the sample close to the capillary entrance. Furthermore, the centroid energies of the transmitted electron angular distributions at 520.7 eV were found to lose energy for angular positions away from the capillary axis (angular centroid position) for all tilt angles, indicating a higher degree of inelastic scattering at the edges of the angular distributions.

Analysis of single nanoparticle detection by using 3-dimensionally confined optofluidic ring resonators

We theoretically analyze the ability of 3-dimensionally confined optofluidic ring resonators (OFRRs) for detection of a single nanoparticle in water and in air. The OFRR is based on a glass capillary, on which bottle-shaped and bubble-shaped ring resonators can form. The spectral position of the whispering gallery mode in the OFRR shifts when a nanoparticle is attached to the OFRR inner surface. For both ring resonator structures, the electric field at the inner surface can be optimized by choosing the right wall thickness. Meanwhile, different electric field confinement along the capillary longitudinal axis can be achieved with different curvatures. Both effects significantly increase the sensitivity of the ring resonator for single nanoparticle detection. It is found that the sensitivity is enhanced about 10 times, as compared to that of a solid microsphere biosensor recently reported, and that the smallest detectable nanoparticle is estimated to be less than 20 nm in radius for a Δλ/λ resolution of 10(-8). The high sensitivity and the naturally integrated capillary based microfluidics make the OFRR a very promising sensing platform for detection of various nano-sized bio/chemical species in liquid as well as in air.

Theoretical and experimental study of laser beam propagation in capillary tubes for non-symmetrical coupling conditions

A non-symmetrical theoretical model is used to describe the propagation of laser beams in dielectric capillary waveguides under non-ideal coupling conditions. The displacement of the laser beam focusing point from the capillary axis, the deviation of the transverse energy distribution from a symmetric one, and an angle of incidence different from zero modify, through the excitation and beating of several modes, the energy repartition during the propagation in the waveguides. The results of modeling are in very good agreement with experimental results, obtained with a low-intensity laser beam on a test bench, where good control of the laser energy distribution in the focal plane, the focusing point position, and the angle of incidence was achieved.

Motion of a drop along the centreline of a capillary in a pressure-driven flow

The deformation of a drop as it flows along the axis of a circular capillary in low Reynolds number pressure-driven flow is investigated numerically by means of boundary integral computations. If gravity effects are negligible, the drop motion is determined by three independent parameters: the size a of the undeformed drop relative to the radius R of the capillary, the viscosity ratio λ between the drop phase and the wetting phase and the capillary number Ca, which measures the relative importance of viscous and capillary forces. We investigate the drop behaviour in the parameter space (a/R, λ, Ca), at capillary numbers higher than those considered previously. If the fluid flow rate is maintained, the presence of the drop causes a change in the pressure difference between the ends of the capillary, and this too is investigated. Estimates for the drop deformation at high capillary number are based on a simple model for annular flow and, in most cases, agree well with full numerical results if λ ≥ 1/2, in which case the drop elongation increases without limit as Ca increases. If λ < 1/2, the drop elongates towards a limiting non-zero cylindrical radius. Low-viscosity drops (λ < 1) break up owing to a re-entrant jet at the rear, whereas a travelling capillary wave instability eventually develops on more viscous drops (λ > 1). A companion paper (Lac & Sherwood, J. Fluid Mech., doi:10.1017/S002211200999156X) uses these results in order to predict the change in electrical streaming potential caused by the presence of the drop when the capillary wall is charged.

Thin film coatings on capillary inner walls by microplasma

The effect of a strong static magnetic field on a capillary microplasma generated by RF (13.56 MHz) excitation using the parallel-plate electrodes was studied. If the magnetic field is perpendicular to both RF electric field and a capillary axis and also the Larmor radius of an electron r L is sufficiently smaller than both the electron mean free path λ e and a discharge gap d g , the oscillating E × B drift will appear effectively in a parallel direction with a capillary axis. It was found that a magnetized capillary microplasma generates under the condition of r L < λ e and r L < d g . Next, to examine the effect of a strong magnetic field on thin film deposition, SiO 2 thin film coating on the inner wall of a poly(propylene) (PP) narrow tube using tetraethoxysilane as the precursor was demonstrated under the condition with/without magnetic field of 0.4 T at atmospheric pressure. SEM images and XPS spectra showed that SiO 2 films with a smooth surface were prepared on the tube inner wall in the both cases.

Electron Guiding through Insulating Nanocapillaries

We simulate the electron transmission through insulating Mylar (polyethylene terephthalate, or PET) capillaries. We show that the mechanisms underlying the recently discovered electron guiding are fundamentally different from those for ion guiding. Quantum reflection and multiple near-forward scattering rather than the self-organized charge up are key to the transmission along the capillary axis irrespective of the angle of incidence. We find surprisingly good agreement with recent data. Our simulation suggests that electron guiding should also be observable for metallic capillaries.

Guiding of slowNe7+ions through nanocapillaries in insulating polyethylene terephthalate: Incident current dependence

The transmission of highly charged ions through nanocapillaries in insulating polyethylene terephthalate (PET) polymers was investigated. In experiments at laboratories in RIKEN (Japan) and HMI (Germany) different detection methods were applied to study the ion current dependence in a wide range covering two orders of magnitude. At HMI an electrostatic ion spectrometer was used and at RIKEN a two-dimensional position sensitive detector was implemented. New PET samples with parallel capillaries and low density were manufactured. For tilted capillaries, the ions are guided along the capillary axis, since the majority of ions are deflected in a charge patch created in the capillary entrance. The results provide insights into the mechanisms of capillary guiding. The fraction of transmitted ions was found to be nearly independent on the incident ion current indicating a sudden increase in the discharge current depleting the entrance charge patch. The experimental results were well-reproduced by model calculations based on a nonlinear (exponential) expression for the discharge current.

Guiding of slow highly charged ions through insulating nanocapillaries in PET polymers

The transmission of highly charged ions through nanocapillaries in insulating PET polymers was investigated. The data clearly indicates capillary guiding for which the majority of ions are rendered along the capillary axis when the capillaries are tilted with respect to the incident beam direction. New PET samples with parallel capillaries were adopted in experiments using a variety of ions. The results yield new insights into the mechanisms of capillary guiding. The fraction of transmitted ions was found to be independent on the incident ion current indicating a sudden increase in the discharge current. The results are discussed in terms of the guiding power which specifies the ability of a material to guide ions. Combining the present work with previous studies, we observed a relation between the guiding power and the width of the transmission profile.

Ringlike cores of cylindrically confined nematic point defects

Nematic liquid crystals confined in a cylindrical capillary and subjected to strong homeotropic anchoring conditions is a long-studied fundamental problem that uniquely incorporates nonlinearity, topological stability, defects, and texture physics. The observed and predicted textures that continue to be investigated include escape radial, radial with a line defect, planar polar with two line defects, and periodic array of point defects. This paper presents theory and multiscale simulations of global and fine scale textures of nematic point defects, based on the Landau-de Gennes tensor order parameter equations. The aim of this paper is to further investigate the ringlike nature of point defect cores and its importance on texture transformation mechanisms and stability. The paper shows that the ringlike cores can be oriented either along the cylinder axis or along the radial direction. Axial rings can partially expand but are constrained by the capillary sidewalls. Radial rings can deform into elliptical structures whose major axis is along the capillary axis. The transformation between several families of textures under capillary confinement as well as their stability is discussed in terms of defect ring distortions. A unified view of nematic textures found in the cylindrical cavities is provided.

Effects of dissolved polymer on the transport of colloidal particles through a microcapillary

The effect of water-soluble polymer on the transport of latex particles through a microcapillary was investigated. Capillary hydrodynamic fractionation (CHDF) experiments were performed using polystyrene (PS) particles and poly(ethylene oxide) (PEO) solutions as the eluant. Generally, the average particle velocities were greater than those corresponding to a polymer-free eluant. A decrease in the sample axial dispersion was also observed using the PEO solutions. In addition, increasing the polymer molecular weight resulted in lower particle residence times in the capillary tube. The enhanced particle transport arises primarily from an increase in the particle diameter resulting from the adsorption of PEO onto the PS surfaces, and, more importantly, from the migration of particles toward the capillary axis due to the normal stress of the PEO solution.

Simulation of heavy-ion guiding in insulators

Guiding of charged particles through insulating target structures (capillaries, glass cones, parallel glass plates) has become a topic of considerable interest. It has been shown for insulating capillary targets that highly charged ions are deflected along the capillary axis while remaining in their initial charge state. Similar results have been reported for the focusing of ion beams by glass cones. We have developed a classical transport theory relating microscopic charge-up with macroscopic material properties. For insulating capillary transmission good agreement with experiments is found. Currently investigated experimental systems will be accessible to theory within the same framework.

Interaction of slow highly-charged ions with metals and insulators

Interaction of slow highly charged ions with insulator as well as metallic surfaces is discussed. In addition to the usual flat surface targets, studies with thin foils having a multitude of straight holes of ∼100nm in diameter (micro-capillary foil) are introduced, which provide various unique information on the above surface interaction. In the case of an insulator micro-capillary foil, a so-called guiding effect was observed, where slow highly charged ions can transmit through the capillary tunnel keeping their initial charge state even when the capillary axis is tilted against the incident beam. A similar guiding effect has recently been found for slow highly-charged ions transmitted through a single tapered glass capillary. In both cases, the guiding effects are expected to be governed by a self-organized charging and discharging of the inner-wall of the insulator capillary. One of the prominent features of this guiding effect with the tapered capillary is the formation of a nano-size beam, which can be applied in various fields of science including surface nano-modification/analysis, nano-surgery of living cells, etc.