Tip Of Drop Research Articles

Evaluating and Managing the Microbial Contamination of Eye Drops: A Two-Phase Hospital-Based Study.

The microbial contamination of eye drop tips and caps varies between 7.7% and 100%. In seeking patient protection and continuous improvement, the Pharmacy Department in the Sterile Ophthalmological and Oncological Preparations Unit at Cochin Hospital AP-HP, Paris, France, conducted a two-phase study to compare the antimicrobial efficiency and practical use of standard packaging and a marketed eye drop container incorporating a self-decontaminating antimicrobial green technology by Pylote SAS at the tip and cap sites. The first phase was conducted in situ to identify the microbial contaminants of eye drops used in the hospital and community settings. A total of 110 eye drops were included for testing. Staphylococcus species were the most prevalent bacteria. Candida parapsilosis was detected in only one residual content sample and, at the same time, on the cap and tip. The second phase was performed in vitro, according to JIS Z2801. Reductions above one log in Staphylococcus aureus and Pseudomonas aeruginosa counts were noted in Pylote SAS eye drop packaging after 24 h of contact. The practical tests showed satisfactory results. Pylote SAS antimicrobial mineral oxide technology exhibited promising effects that combined effectiveness, safety, and sustainability to protect the patient by preventing infections due to the contamination of eye drop containers.

Effect of saw-tooth ply drops on the mechanical performance of tapered composite laminates

Automated Fibre Placement (AFP) is a manufacturing technique to produce large, high quality composite parts, where preimpregnated carbon fibre tapes are laid side-by-side to generate the composite preform. Thickness changes within a component are realised through internal ply terminations, with the tapes being cut perpendicular to the fibre direction. In plies laid up at an angle to the taper direction, the AFP tape cuts create saw-tooth shaped ply drop tips. This increases the size of resin rich zones and enlarges the region where stress concentrations and cracks can develop, which amplifies the risk of failure. This study investigates the effect of simulated AFP saw-tooth ply drop tips, created by hand layup, on the mechanical properties and failure behaviour of carbon fibre/epoxy composites and compares its results to reference broad goods layups. The stiffness was found to be unaffected and the strength was reduced by ~ 10%. The failure mode was governed by delamination for the saw-tooth ply drop tips, whereas the reference specimens failed by rupture of the unidirectional fibres. A finite element modelling technique was used to select the layup to be tested. The models were then refined in the light of experimental results, to accurately predict the failure and explain the failure mode transition.

A molecular-dynamics study of sliding liquid nanodrops: Dynamic contact angles and the pearling transition

HypothesisThat the behavior of sliding drops at the nanoscale mirrors that seen in macroscopic experiments, that the local microscopic contact angle is velocity dependent in a way that is consistent with the molecular-kinetic theory (MKT), and that observations at this scale shed light on the pearling transition seen with larger drops. MethodsWe use large-scale molecular dynamics (MD) to model a nanodrop of liquid sliding across a solid surface under the influence of an external force. The simulations enable us to extract the shape of the drop, details of flow within the drop and the local dynamic contact angle at all points around its periphery. FindingsOur results confirm the macroscopic observation that the dynamic contact angle at all points around the drop is a function of the velocity of the contact line normal to itself, Ucmsinϕ, where Ucm is the velocity of the drop’s center of mass and ϕ is the slope of the contact line with respect to the direction of travel. Flow within the drop agrees with that observed on the surface of macroscopic drops. If slip between the first layer of liquid molecules and the solid surface is accounted for, the velocity-dependence of the dynamic contact angle is identical with that found previous MD simulations of spreading drops, and consistent with the MKT. If the external force is increased beyond a certain point, the drop elongates and a neck appears between the front and rear of the drop, which separate into two distinct zones. This appears to be the onset of the pearling transition at the tip of a macroscopic drop. The receding contact angle at the tip of the drop is far removed from its equilibrium value but non-zero and approaches a more-or-less constant critical value as the transition progresses.

Simulation of surfactant-mediated tipstreaming in a flow-focusing geometry

A surfactant coated drop is drawn through an aperture by a converging flow. Under suitable conditions that have also been investigated in a series of independently led experiments, the drop tip emits a thin tipstreaming thread that later breaks up into near-monodisperse droplets.

Electrolytic drops in an electric field: A numerical study of drop deformation and breakup.

The deformation and breakup of an axisymmetric, conducting drop suspended in a nonconducting medium and subjected to an external electric field is numerically investigated here using an electrokinetic model. This model uses a combined level set-volume of fluid formulation of the deformable surfaces, along with a multiphase implementation of the Nernst-Planck equation for transport of ions, that allows for varying conductivity inside the drop. A phase diagram, based on a parametric study, is used to characterize the stability conditions. Stable drops with lower ion concentration are characterized by longer drop shapes than those achieved at higher ion concentrations. For higher drop ion concentration, greater charge accumulation is observed at drop tips. Consequently, such drops break up by pinching off rather than tip streaming. The charge contained in droplets released from unstable drops is shown to increase with drop ion concentration. These dynamic drop behaviors depend on the strength of the electric field and the concentration of ions in the drop and result from the interplay between the electric forces arising from the permittivity jump at the drop interface and the ions in the bulk.

溶滴移行現象の数値解析

Metal transfer modes in GMA welding process are different from welding variables: current, voltage, shielding gas, electrode wire and etc. Essentially they are influenced by physical properties of molten metal and electro-magnetic force. Then we numerically investigated the time-change of liquid transfer process using calculation model developed. The numerical result agreed very well with experimental result of water drop transfer from cylindrical nozzle. Numerical analysis shows that liquid transfer modes depend on the pressure balance at the tip of drop. When the dynamic pressure induced by liquid flow is lower than capillary pressure due to surface tension, globular transfer occurs. In contrast, when the dynamic pressure of liquid flow is higher, spray transfer occurs. In the case that the electrical current flows in the liquid, electro-magnetic force significantly influences on the transfer mode and the drop detachment. As for liquid flow-out from the nozzle of 1.2 mm in diameter, calculated results with use of physical properties of molten mild steel show that electro-magnetic force induced by higher current than 250 A changes the transfer mode from globular to spray. Whereas in real GMAW with pure argon gas shielding the transfer mode changes at around 230 A. The behavior of drop detachment is varied by both spatial distribution and time-change of the electro-magnetic force.

Numerical Model of Metal Transfer Using an Electrically Conductive Liquid

In order to understand the mechanism of the metal transfer process, the numerical model, which enables to calculate the time-change of liquid transfer process, was constructed. By using the model, we numerically investigated the influence of liquid properties and electro-magnetic force on the modes of liquid transfer. It is shown that liquid transfer modes depend on the pressure balance at the tip of drop. When the dynamic pressure induced by liquid flow is lower than capillary pressure due to surface tension at the tip of liquid drop, globular transfer occurs. In contrast, when the dynamic pressure of liquid flow is higher, spray transfer occurs. In the case that the electrical current flows in the liquid, electro-magnetic force significantly influences on the transfer mode and the drop detachment. Calculated results using physical properties of molten mild steel show that electro-magnetic pinch force induced by higher current than 250 A changes the transfer mode from globular to spray, for liquid flow-out from the nozzle of 1.2 mm in diameter. Whereas in real GMAW with pure argon gas shielding the transfer mode changes at around 230 A. The behaviour of drop detachment is varied by both spatial distribution and time-change of the electro-magnetic force.

Effects of viscoelasticity on the retraction of a sheared drop

Effects of drop and matrix viscoelasticity on the retraction of a sheared drop are numerically investigated. Retraction of an Oldroyd-B drop in a Newtonian matrix is initially faster and later slower with increasing drop Deborah number. The observed behavior is explained using an ordinary differential equation model representing the dominant balance between various forces during retraction. The initial faster relaxation of viscoelastic drops is due to viscoelastic stresses pulling the drop interface at the tips inward. The later slower retraction is due to the slowly-relaxing viscoelastic forces at the equator, where they act against the capillary force. The drop inclination decreases substantially during retraction unlike in a Newtonian case. Matrix viscoelasticity slows the relaxation of a Newtonian drop because of the increasingly slow relaxation of highly stretched polymers near the drop tip with increasing Deborah number. Increasing the ratio of polymeric to total viscosity further accentuates the viscoelastic effects in both cases. For an Oldroyd-B drop in an Oldroyd-B matrix, a competition between the dispersed and the continuous phase elasticities, represented by their ratio, determines the dynamics; larger values of the ratio leads again to initial faster and later slower retraction.

A comparison of viscoelastic stress wakes for two-dimensional and three-dimensional Newtonian drop deformations in a viscoelastic matrix under shear

A recent experimental study of a Newtonian drop suspended in a viscoelastic matrix undergoing simple shear displays a transient overshoot in drop deformation which is qualitatively similar to two-dimensional (2D) numerical simulation results. Despite the similarity, an interpretation in light of the 2D result is misleading because the overshoot is absent in the fully three-dimensional (3D) simulation. This motivates a study of regimes where qualitatively different and interesting features such as overshoots in deformation occur for a 2D drop but not for a 3D drop. The influence of viscoelastic “wakes” that emanate from the drop tips is reported. The viscoelastic wakes are larger and of higher magnitude in 3D than in 2D, and lead to more deformation in 3D. During drop evolution, the less deformed drop is found to be aligned more with the flow direction. As the drop-to-matrix viscosity ratio increases from 1 to past 3, drop rotation is promoted, with accompanying retraction when the capillary number is sufficiently high. Thus, a 3D overshoot in deformation is promoted with increasing viscosity ratio.

Influence of viscoelasticity on drop deformation and orientation in shear flow: Part 1. Stationary states

The influence of matrix and droplet viscoelasticity on the steady deformation and orientation of a single droplet subjected to simple shear is investigated microscopically. Experimental data are obtained in the velocity–vorticity and velocity–velocity gradient plane. A constant viscosity Boger fluid is used, as well as a shear-thinning viscoelastic fluid. These materials are described by means of an Oldroyd-B, Giesekus, Ellis, or multi-mode Giesekus constitutive equation. The drop-to-matrix viscosity ratio is 1.5. The numerical simulations in 3D are performed with a volume-of-fluid algorithm and focus on capillary numbers 0.15 and 0.35. In the case of a viscoelastic matrix, viscoelastic stress fields, computed at varying Deborah numbers, show maxima slightly above the drop tip at the back and below the tip at the front. At both capillary numbers, the simulations with the Oldroyd-B constitutive equation predict the experimentally observed phenomena that matrix viscoelasticity significantly suppresses droplet deformation and promotes droplet orientation. These two effects saturate experimentally at high Deborah numbers. Experimentally, the high Deborah numbers are achieved by decreasing the droplet radius with other parameters unchanged. At the higher capillary and Deborah numbers, the use of the Giesekus model with a small amount of shear-thinning dampens the stationary state deformation slightly and increases the angle of orientation. Droplet viscoelasticity on the other hand hardly affects the steady droplet deformation and orientation, both experimentally and numerically, even at moderate to high capillary and Deborah numbers.

Effects of matrix viscoelasticity on viscous and viscoelastic drop deformation in a shear flow

The deformation of a Newtonian/viscoelastic drop suspended in a viscoelastic fluid is investigated using a three-dimensional front-tracking finite-difference method. The viscoelasticity is modelled using the Oldroyd-B constitutive equation. Matrix viscoelasticity affects the drop deformation and the inclination angle with the flow direction. Numerical predictions of these quantities are compared with previous experimental measurements using Boger fluids. The elastic and viscous stresses at the interface, polymer orientation, and the elastic and viscous forces in the domain are carefully investigated as they affect the drop response. Significant change in the drop inclination with increasing viscoelasticity is observed; this is explained in terms of the first normal stress difference. A non-monotonic change – a decrease followed by an increase – in the steady-state drop deformation is observed with increasing Deborah number (De) and explained in terms of the competition between increased localized polymer stretching at the drop tips and the decreased viscous stretching due to change in drop orientation angle. The transient drop orientation angle is found to evolve on the polymer relaxation time scale for highDe. The breakup of a viscous drop in a viscoelastic matrix is inhibited for smallDe, and promoted at higherDe. Polymeric to total viscosity ratio β was seen to affect the result through the combined parameter βDeindicating a dominant role of the first normal stress difference. A viscoelastic drop in a viscoelastic matrix with matched relaxation time experiences less deformation compared to the case when one of the phases is viscous; but the inclination angle assumes an intermediate value between two extreme cases. Increased drop phase viscoelasticity compared to matrix phase leads to decreased deformation.

Tip streaming from a liquid drop forming from a tube in a co-flowing outer fluid

Dynamics of formation of a drop of an incompressible Newtonian fluid of viscosity μ1 and density ρ1 from the tip of a tube of radius R1 into a co-flowing immiscible, incompressible Newtonian fluid of viscosity μ2 and density ρ2 that is enclosed in a concentric cylindrical tube of radius R2 are investigated under creeping flow conditions. Transient drop shapes, and fluid velocities and pressures, are calculated numerically by solving the governing Stokes equations with the appropriate boundary and initial conditions using the Galerkin/finite element method for spatial discretization and an adaptive finite difference method for time integration. In accord with previous studies, the primary effect of increasing the ratio of the volumetric injection rate Q̃2 of the outer fluid to that of the inner fluid Q̃1, Qr≡Q̃2∕Q̃1, is shown to be a reduction in the volume of primary drops that are formed. When Qr is small, calculations show that drop formation occurs in a slug flow regime where the primary drops that are about to be formed are elongated axially and occupy virtually the entire cross section of the outer tube. In this slug flow regime, the primary drops at breakup resemble cylinders that are terminated by hemispherical caps and their aspect ratios Lp∕Dp≫1, where Lp denotes their axial lengths and Dp≈2R2 their maximum diameters. As Qr increases, the dynamics are shown to transition to the dripping regime, where the primary drops are more globular, Lp∕Dp∼1, and their radii are of the order of or smaller than R1. As Qr increases, the importance of viscous stress exerted by the outer fluid relative to the surface tension or capillary pressure increases. Thus, the drop length measured from the tube exit to the drop tip at breakup increases while the primary drop volume decreases as Qr increases. When Qr is sufficiently large, viscous stress exerted by the outer fluid induces a recirculating flow within a forming drop. Once Qr exceeds a critical value Qrt, viscous stress exerted by the outer fluid becomes so large that the growing drop takes on a conical shape and a thin fluid jet with a radius that is a few orders of magnitude smaller than that of the radius of the inner tube emanates from its tip. This latter regime of drop breakup, which is henceforward referred to as tip streaming, is remarkably similar to electrohydrodynamic jetting that is seen from the tips of conical drops in electric fields and tip streaming that occurs from the pointed ends of surfactant-covered free drops subjected to linear extensional or shear flows, but takes place here in the absence of electric fields or surfactants. Scaling arguments for fixed a≡R2∕R1 show that for exterior viscous stress to overcome capillary pressure and cause tip streaming, Ca−1<mQr, where Ca≡μ1Q̃1∕γπR12 is the capillary number, γ is the interfacial tension, and m≡μ2∕μ1 is the viscosity ratio. In accordance with the scaling arguments, the computed predictions show that the critical value of the flow rate ratio signaling transition from dripping to tip streaming Qrt varies inversely with m for small to moderate m but becomes independent of both Ca and m as viscosity ratio grows without bound.

Two-dimensional study of drop deformation under simple shear for Oldroyd-B liquids

The effect of viscoelasticity on the deformation of a circular drop suspended in a second liquid in shear is investigated with direct numerical simulations. A numerical algorithm based on the volume-of-fluid method for interface tracking is implemented in two dimensions with the Oldroyd-B constitutive model for viscoelastic liquids. The code is verified against a normal mode analysis for the stability of two-layer flow in a channel; theoretical growth rates are reproduced for the interface height, velocity and stress components. Drop simulations are performed for drop and matrix liquids of different viscosities and elasticities. A new feature is found for the case of equal viscosity, when the matrix liquid is highly elastic and surface tension is low; hook-like structures form at the drop tips. This is due to the growth of first normal stress differences that occur slightly above the front tip and below the back tip as the matrix elasticity increases above a threshold value.

A surfactant-conserving volume-of-fluid method for interfacial flows with insoluble surfactant

An axisymmetric numerical method to simulate the dynamics of insoluble surfactant on a moving liquid–fluid interface is presented. The motion of the interface is captured using a volume-of-fluid method. Surface tension, which can be a linear or nonlinear function of surfactant concentration (equation of state), is included as a continuum surface force. The surfactant evolution is governed by a convection–diffusion equation with a source term that accounts for stretching of the interface. In the numerical method, the masses of the flow components and the surfactant mass are exactly conserved. A number of test cases are presented to validate the algorithm. Simulations of a drop in extensional flow, and its subsequent retraction and breakup upon cessation of the external flow, are performed. Even when the initial surfactant distribution is dilute, we observe that increases in surfactant concentration locally (i.e. at the drop tips) can result in a local deviation from the dilute limit. We show that this can lead to differences in effective surface tension, the Marangoni forces and the associated drop dynamics between results using the linear and nonlinear equations of state.

Self-similar recoil of inviscid drops

After capillary pinchoff of a fluid thread or drop, the newly created drop tips recoil due to the large local curvature. Similarity solutions for the postpinchoff recoil of an axisymmetric inviscid fluid of density ρ1 and surface tension γ immersed in a surrounding fluid of density ρ2 are obtained over a range of the density ratio D=ρ2/ρ1. The far-field shape of the two new drops and the far-field dipole potentials are prescribed from known prepinching solutions [D. Leppinen and J. R. Lister, Phys. Fluids 15, 568 (2003)] and the positions and self-similar shape of the two recoiling tips are calculated. The momentum of the prepinching flow makes a significant difference to the recoiling shapes. Capillary waves are observed, in agreement with previous two-dimensional studies and analytical calculations, and the wave frequency is found to increase with D. The recoil of a single axisymmetric drop (with a conical far-field shape) under surface tension is also studied as a function of D and the far-field cone angle θ0. Capillary waves are again observed, and the results for small values of θ0 are shown to agree well with previous asymptotic predictions. The related problem of violent jet emission, following the formation of a near-conical structure with very high curvature at its tip, is also discussed and its similarity with the recoiling cone problem investigated.

A new ac electrospray mechanism by Maxwell-Wagner polarization and capillary resonance.

We report a new high-frequency (>10 kHz) ac electrospray that is capable of generating micron-sized electroneutral drops. Unlike its dc counterpart, the drops are not ejected continuously from a sharp Taylor cone but intermittently from a resonating meniscus at the orifice. We attribute the resonant frequency to the capillary-inertia vibration time of the meniscus and the drop ejection to the Maxwell-Wagner electric stress at the drop tip, which is observed to reverse its direction across a crossover frequency. Above this frequency, the oppositely directed Maxwell-Wagner force causes the liquid to recede up the microneedle as an apparent electrowetting effect.

An Adsorption–Desorption-Controlled Surfactant on a Deforming Droplet

The effects of a sorption-controlled, monolayer-forming surfactant on a drop deforming in an extensional flow are studied numerically. Scaling arguments are presented for drops of 1 cm and 1 μm, indicating the applicability of these results. For all simulations, when mass transfer is slow compared to surface convection, the insoluble limit is recovered; when mass transfer is rapid, the drop behavior is the same as that for a surfactant-free drop. For a surfactant which forms a monolayer, there is an upper bound to the surface concentration, Γ∞. The surface tension reduction diverges as the surface concentration Γ approaches this limit, strongly altering the hydrodynamics.The drop deformation is studied relative to a surfactant-free drop in terms of the capillary number, Ca, the ratio of characteristic viscous stresses to surface tension. In the insoluble limit, for Γ ⪡ Γ∞, droplets deform more than in the absence of surfactants at a given Ca and break-up at lower Ca. When stable drop shapes are attained, stagnant caps form at the drop tips. Finite surfactant mass transfer rates eliminate these caps and diminish the deformation.For Γ approaching Γ∞in the insoluble limit, interfaces are strongly stressed for perturbative surface concentration gradients; Γ remains nearly uniform throughout the deformation process. Deformations are reduced at a given Ca. When stable drop shapes are attained, the surface is completely stagnated. Marangoni stresses force the surface velocity to zero to keep Γ below its upper bound. For soluble surfactants, as mass transfer rates increase, the magnitude of these stresses diminishes. Deformations change nonmonotonically with mass transfer rates and are not bounded by the limiting clean interface and insoluble limits.The drop contribution to the volume averaged stress tensor Σ is also calculated. The axial component Σzzincreases with the drop length; the radial component Σrrincreases with the drop breadth. Since the deformation is strongly influenced by the surfactant concentration and the mass transfer rates, so too is Σ.

Marangoni effects on drop deformation in an extensional flow: The role of surfactant physical chemistry. I. Insoluble surfactants

The shape of a drop centered in an axisymmetric extensional flow is determined by the viscous stresses that deform the drop and surface tension γ′ that resists the deformation. The ratio of these stresses is given by the capillary number, Ca. When Ca is small enough, the drop attains a steady shape. However, above a threshold value, Cacr, the drop elongates continuously, and no steady shape is attained. When surfactants are present on the drop interface, the surface tension is determined by the surface concentration profile, which varies throughout the deformation process. Initially, the surface tension is given by γeq′, in equilibrium with the uniform surface concentration Γeq′. When the flow is initiated, surfactant is swept toward the drop tips, reducing the surface tension there, and altering the interfacial stress balance tangentially through Marangoni stresses and normally through the Laplace pressure. In this paper, the effects of an insoluble surfactant on drop deformation are studied. In previous work, either a surface equation of state for the surface tension γ′ that is linear in the surface concentration Γ′ was used, an approximation that is valid only for dilute Γ′, or Γ′ sufficiently dilute for the linear approximation to be valid were studied. In this paper, a nonlinear surface equation of state that accounts for surface saturation and nonideal interactions among the surfactant molecules is adopted. The linear framework results are recovered for Γ′ that are sufficiently dilute. As Γ′ is increased, the effects of saturation and surfactant interactions are probed at constant initial Γeq′ and at constant initial γeq′. Finally, the case of strong intersurfactant cohesion is treated with a first-order surface phase transformation model. At moderate surface concentrations, these nonlinear phenomena strongly alter the steady drop deformations and Cacr relative to the uniform surface tension and linear equation of state results.

Simulating the microbond technique with macrodroplets

In this study, the microbond technique has been simulated by the use of macrodroplets. These larger scale specimens consisted of cone-shaped epoxy droplets of length 6 mm, on 1 mm diameter steel wire. The specimens were formed reproducibly in a mold to provide 30 ° and 45 ° contact angles between fiber and resin. The model geometry was used to examine the effects of contact angle, loading position, and loading type (point versus axisymmetric loading) on the debonding strength. The results indicated that a higher debonding strength was achieved at the higher contact angle and at loading positions farther from the tip of the cone-shaped drop. When the load was applied near the tip of the drop, which is typical in microbond experiments, there was no statistical difference in debonding strength for point loading and axisymmetric loading. Finally, with the macrodroplets, it was possible to visually observe a three-stage debonding process.

Shapes and stability of pendant and sessile dielectric drops in an electric field

Axisymmetric equilibrium shapes and stability of linearly polarizable dielectric (ferrofluid) drops of fixed volume which are pendant/sessile on one plate of a parallel-plate capacitor and are subjected to an applied electric (magnetic) field are determined by solving simultaneously the free boundary problem comprised of the Young-Laplace equation for drop shape and the Laplace equation for electric (magnetic) field distribution. When the contact angle that the drop makes with the plate is fixed to be 90° and the distance between the plates is infinite, the results are identical to those of a free drop immersed in a uniform field and resolve discrepancies between previously reported theoretical predictions and experimental measurements. Remarkably, regardless of the value of the ratio of the permittivity (permeability) of the drop to that of the surrounding fluid, κ, drop shapes develop conical tips as drop deformation increases. However, three types of behaviour are found, depending on the value of κ. When κ < κ1, the drop deformation grows without bound as field strength rises. On the other hand, when κ > κ2 > κ1, families of equilibrium drop shapes become unstable at turning points with respect to field strength. Beyond the turning points, the unstable families terminate: the mean curvature at the virtually conical drop tip grows without bound. However, in the range κ1 < κ < κ2, the new results predict that drop deformation exhibits hysteresis, in accord with experiments of Bacri, Salin & Massart (1982) and Bacri & Salin (1982, 1983). Such hysteresis phenomena have been surmized previously on the basis of approximate theories, though they have not been calculated systematically until now. Moreover, detailed computations reveal the importance of varying the drop size and plate spacing, and whether, along the three-phase contact line, the contact line is fixed or the contact angle is prescribed.