Plug Length Research Articles

Highly sensitive detection of five typical fluoroquinolones in low-fat milk by field-enhanced sample injection-based CE in bubble cell capillary.

Fluoroquinolones are a group of synthetic antibiotics with a broad activity spectrum against mycoplasma, Gram-positive, and Gram-negative bacteria. Due to the extensive use of fluoroquinolones in farming and veterinary science, there is a constant need in the analytical methods able to efficiently monitor their residues in food products of animal origin, regulated by Commission Regulation (European Union) no. 37/2010. Herein, field-enhanced sample injection for sample stacking prior the CZE separation was developed inside a bubble cell capillary for highly sensitive detection of five typical fluoroquinolones in bovine milk. Ethylenediamine was proposed as the main component of BGE for the antibiotics separation. The effect of BGE composition, injection parameters, and water plug length on the field-enhanced sample injection-based CE with UV detection was investigated. Under the optimized conditions, described field-enhanced sample injection-based CE-UV analysis of fluoroquinolones provides LODs varying from 0.4 to 1.3ng/mL. These LOD values are much lower (from 460 to 1500 times) than those obtained by a conventional CE in a standard capillary without bubble cell. The developed method was finally applied for the analysis of fluoroquinolones in low-fat milk from a Swiss supermarket. Sample recovery values from 93.6 to 106.0% for different fluoroquinolones, and LODs from 0.7 to 2.5μg/kg, were achieved. Moreover, the proposed ethylenediamine-based BGE as volatile and compatible with MS system, enabled the coupling of the field-enhanced sample injection-based CE with a recently introduced electrostatic spray ionization MS via an iontophoretic fraction collection interface for qualitative fluoroquinolones identification.

Effect of flow orientation on liquid–liquid slug flow in a capillary tube

• Frictional pressure drop is influenced by orientation of millichannel. • Frictional pressure drop highest for downflow orientation. • Influence of orientation most pronounced in slug flow. • Plug length increases and plug frequency decreases with increase in toluene velocity. • In-situ organic fraction highest for downflow orientation. Experiments using water and toluene have been performed to estimate the pressure drop characteristics in a 2 mm diameter 1 m long conduit. The measurements reveal a significant influence of conduit orientation on frictional pressure drop. The frictional gradient is highest for downflow followed by upflow and minimum for horizontal orientation of the test passage. The influence of orientation is more pronounced for slug flow as compared to dispersed, inverted dispersed and annular flow patterns. In order to explain these observations, an in-depth study of the slug flow pattern has been performed using image analysis. The influence of inlet phase velocities on slug characteristics namely length, frequency and velocity of toluene plugs, in-situ toluene fraction and relative slip between the phases has been investigated. The observations reveal that plug length increases with increasing toluene flow rate while its frequency increases with water flow rate. The in-situ toluene fraction increases with increasing toluene and decreasing water flow rate as expected and it is highest for downward liquid–liquid flow as compared to upward or horizontal flow for the same input conditions. This explains the significant influence of orientation on frictional pressure gradient during liquid–liquid slug flow in a 2 mm conduit.

Sample Stacking and On-Line Derivatization for the Analysisof Ampicillin and Amoxicillin by Microemulsion Electrokinetic Chromatography

A novel method using microemulsion electrokinetic chromatography (MEEKC) combining the preconcentration technique called reversed electrode polarity stacking mode (REPSM) with subsequent on-line derivatization has been developed for the quantitative determination of ampicillin and amoxicillin. It was found that the derivatization of antibiotics with 2,3-naphtalenedicarboxaldehyde (NDA) is accelerated greatly in microemulsion media, compared to the standard derivatization procedure, no heating being required. Various parameters that affect the ratio of derivatization, such as temperature, mixing and reaction time, concentration of NDA solution, and plug length were investigated and optimized. The combination of RESPM technique with the following derivatization allows the detection limits of ampicillin and amoxicillin to be decreased by a factor of 300 or more. The potential of the developed method was demonstrated by the determination of these antibiotics in a biological sample through the analysis of urine.

Asymmetric heat transfer in liquid–liquid segmented flow in microchannels

Heat transfer in segmented flow in microchannels can be significantly enhanced by recirculating vortices, due to the presence of interfaces. The processes of heat transfer in segmented flow subjecting to asymmetric boundary conditions are studied. Two types of boundary conditions are considered, asymmetric constant surface temperature and asymmetric constant surface heat flux. The paths of heat flow and the effects of the thermal conductivity, the plug length, and the Peclet number are studied. The results show different features from those at symmetric boundary conditions. The heat transfer process at asymmetric boundary conditions is controlled by both thermal advection and diffusion at the mid-plane of the channel. The coupling effects between the adjacent plugs complicate the process by the heat transfer across plug–plug interfaces.

Experimental study of liquid/liquid second-dispersion process in constrictive microchannels

Abstract Liquid/liquid second-dispersion signifies the process of breaking dispersed droplets, for the purpose of further reducing the droplet size. This study introduces a systemic experimental study on the second-dispersion process in different constricted microchannels and presents the flowing behaviors of liquid plugs passing through them. Both breakup, non-breakup and their transition of plugs were observed at the operating conditions of Ca ranging from 0.0002 to 0.025 and Re ranging from 0.74 to 27.3 in the orifice. Main factors influencing the plug breakup process, such as surfactant concentration, plug length, flow rate, viscosity and channel geometry, were carefully examined. We found the liquid/liquid second-dispersion process, driven by the capillary pressure criterion, is both a statically confined and dynamical confined process. To realize second-dispersion, the hydraulic radius ratio of downstream microchannel to constrictive orifice should be larger than 1.89 and the operating condition should be lower than the critical capillary numbers (∼10 −3 ) of continuous phase and dispersed phase. We find these critical capillary numbers reduce as the rising of hydraulic radius of the orifice but changed little with the enlargement of downstream microchannel. The orifice geometry also dominates the daughter droplets, whose radii are at least two times of the orifice hydraulic radius.

Pressure-assisted electrokinetic injection stacking for verteporfin drug to achieve highly sensitive enantioseparation and detection in artificial urine by capillary electrophoresis

Pressure-assisted electrokinetic injection (PAEKI) was applied for negatively charged verteporfin (VER) overloading and inline stacking, which targeted highly sensitive enantioseparation by CE. The essential step of PAEKI is a constant pressure used to counterbalance the electroosmotic flow (EOF), consequently, the large amount of analyte could be permitted into capillary and concentrated at the motionless boundary of the sample zone and background electrolyte (BGE). Aiming to know the balance, the velocity of the whole BGE in capillary by the impetus of pressure (0.2–2.0psi), and the velocity of EOF depending on the length of sample plug and voltage (5.0–20kV) was investigated, respectively. The velocity of bulk flow in capillary has good linearity with the pressure or applied voltage. Through the pattern of EOF marked peak and analyte peaks (dissolved in pure water), the constant pressure (0.8psi) vs. the added voltage (−10.3kV) during PAEKI was confirmed to immobilize the bulk flow of BGE, thus the sample injection time could sustain 2.0min without compromising separation efficiency. The obtained LOD (S/N=3) of each isomer at UV detection (428nm) was around 10.3μg/L, which was improved to 116 and 39-fold in comparison with normal hydrodynamic injection (HDI) and electrokinetic injection (EKI). The LOD is far below the reported value with LIF detection of VER. The RSD (n=5) of migration time and peak area was, respectively, around 3.5% and 5.7% for the proposed PAEKI method. Finally, PAEKI was used for the detection of VER in artificial urine to investigate the matrix interference.

Friction forces of particulate plugs moving in vertical and horizontal pipes

This paper presents a comprehensive theoretical and experimental analysis of the friction forces of particulate plugs. Friction forces of particulate plugs play a key role in the estimation of the pressure gradient over a pneumatic conveying pipeline. Therefore, a theoretical model to calculate those forces for variable orientations of the pipe inclination (0≤α≤90) was developed. In particular, were analyzed the behavior of various parameters such as plug length, plug diameter, internal friction of the material, wall friction, the pressure gradient due to airflow through the plug, and the stress transmission coefficient. The agreement of the new correlations was found to be in the range of ±15% for the stress ratio values. Moreover, by substituting these correlations into the newly developed theoretical model and comparing its values to experimental results, an agreement of ±30% for the plug friction force calculation was achieved. The present study enables a better understanding of the characteristics of the two-phase (gas–solids) plug flow regime.

Wall friction of coarse grained sediment plugs transported in a water flow through a vertical pipe

Hydraulic transport is an energy effective means of transporting vast amounts of solid bulk goods suspended in a fluid flow, encountered in many fields in industry. The case of vertical hydraulic transport of suspended sediments and ore is of special interest in the fields of dredging, terrestrial mining and ocean mining. From field experience it is known that during vertical hydraulic transport over long distances, solid sediment plugs can develop that pose the risk of riser blockage. In this paper an investigation into wall friction of layered sediment plugs is set out. Knowledge about this specific type of plugs and their associated wall friction is necessary for the design of vertical transport systems. First a model is developed, and then an experiment is conducted to verify the model. The model input consists of soil mechanical parameters like the internal angle of friction of the sediment and the coefficient of friction between the sediment material and the riser wall, and geometrical properties such as plug length and riser diameter. By using common values for both the internal angle of friction and the kinetic friction coefficient, the model predicts the outcome of the experiments reasonably well.

Should maximum pressures in ore pipelines be computed out of system startups or power outages?

A key aspect of the design of long distance ore concentrate pipelines is the need to properly predict maximum pressures. This is traditionally done by means of transient analysis, thus predicting the possible impact of slurry hammers, which may occur during operation in a potentially uncontrolled manner in case of power outages. In this technical note, it is shown for typical ore slurry and pipeline characteristics, that in long distance systems with routes having inclined sections, the plug formation mechanism may become a dominant factor in system overpressures. A dimensionless number expressed as the ratio of the Joukowski and the plug overpressure value, suggests that a scale for the critical plug length above which maximum pressures are controlled by the plug mechanism rather than the transient flow is between about 150m and 500m, or a few percent points of the overall pipeline length in common long distance systems. A critical dependence on the solids initial concentration and the product of the static friction factor and the solids settled concentration is addressed.

Liquid–liquid segmented flows in polycarbonate microchannels with cross-sectional expansions

Liquid–liquid segmented flows in microchannels fabricated on polycarbonate test chips were investigated experimentally. Three different cross-sectional expansion ratios of 16, 4, and 2 from the injection to the test channels were used to observe the effect of different cross-sectional areas of the injection channel on the segmented flow regimes in the test channel. Flow regime maps and the transitions between flow regimes were determined for the different expansion ratios. The effect of the cross-sectional expansion ratio was determined by mutual comparison of these results. The measured length of the dispersed fluid segments was found to scale inversely with the carrier fluid volumetric flow ratio (βC). A simplified geometric model for the volume of the dispersed and the carrier fluid plugs provided good estimates of the carrier plug length, which scaled inversely with (1−βC). Velocity and pressure drop measurements of the liquid–liquid segmented flows were carried out for all flow regimes, and the associated trends were correlated with changes in the flow topology. Most previous homogeneous and separated flow models based on the viscous frictional pressure drop underestimated the measured pressure drops. Including the contribution of the capillary pressure drop in addition to the frictional pressure drop improved the agreement with the measured data.

Hydraulic modelling of a flat heat pipe with two different groove shapes and a small vapour section

This paper reports a hydraulic modelling of liquid and vapour flows occurring inside a working flat plate heat pipe. This specific grooved heat pipe, used for terrestrial applications, is composed of two kinds of grooves: triangular and drop-shape. It also presents the peculiarity of having a very thin vapour space. Based on classical conservation equations, the modelling approach is characterized by specific boundary conditions appropriate to such a system, which is highly sensitive to a variety of parameters (filling ratio, temperature, heat power applied, inclination…). Special attention was paid to resolution between the grooves in the upper part and grooves in the lower part.After comparison of the results with experimental data in terms of dry-out lengths, a parametric study is presented showing the evolution of pressure losses and liquid plug length as functions of working temperature for three working fluids: ethanol, acetone and n-pentane. Finally, due to the small dimensions of the vapour space, a specific study has been conducted on the liquid flow that may occur on both sides of the vapour channels. Even though flow assumptions are still poorly controlled, it is shown here that the length of the liquid plug is highly sensitive to the hydraulic behaviour occurring at the end of the condenser. This behaviour can have a significant impact on the thermal performances of heat pipes, especially when the length of the condenser is low, or of the same order of magnitude as the liquid plug.

Velocity Effect on Liquid Plug Length in a Capillary Tube

Velocity Effect on Liquid Plug Length in a Capillary Tube

Capillary zone electrophoresis of quantum dots dispersed in mixed micelles: New evidence of the concentration effect

CZE was applied to characterizing a nanoparticle/micellar system, in which CdSe nanoparticles dispersed in a mixture of non-ionic and ionic surfactants are introduced into a capillary to form the sample zone that migrates being sandwiched by the background electrolyte (BGE). Parameters that affect the migration of the surface-modified CdSe nanoparticles and conditions under which they are focused within the micellar zone (or released from it into bulk BGE) were explored, including the sample composition, sample plug length, and applied voltage. The observed migration behavior was analyzed within the framework of a concept of formation of a mixed pseudomicellar system, according to which a nanoparticle coated with an ionic surfactant is treated as a micelle-like entity. It was found out that the nanoparticles are subject to transformation that results in building-up a mixed pseudomicellar system (with regular micelles), with a consequence of exhibiting distinctive migration phenomena. Particularly observed and brought into focus is the event of focusing of the nanoparticles.

Computational modeling of unsteady surfactant-laden liquid plug propagation in neonatal airways

Surfactant-free and surfactant-laden liquid plug propagation in neonatal airways in various generations representing the upper and lower airways are investigated computationally using a finite-difference/front-tracking method. Emphasis is placed on the unsteady surfactant-laden plug propagation as a model for Surfactant Replacement Therapy (SRT) and airway reopening. The numerical method is designed to solve the evolution equations of the interfacial and bulk surfactant concentrations coupled with the incompressible Navier-Stokes equations. Available experimental data for surfactant Survanta are used to relate surface tension coefficient to surfactant concentration at the interface. It is found that, for the surfactant-free case, the trailing film thickness is in good agreement with Taylor's law for plugs with plug length greater than the airway width. Mechanical stresses that could be injurious to epithelial cells such as pressure and shear stress and their gradients are maximized on the front and rear menisci with increasing magnitudes in the lower generations. These mechanical stresses, especially pressure and pressure gradient, are diminished with the introduction of surfactants. Surfactant is absorbed onto the trailing film and thickens it, eventually leading to either plug rupture or, if totally consumed prior to rupture, to steadily propagating plug. In the upper airways, initially small plugs rupture rapidly and plugs with comparable initial plug length with the airway width persist and propagate steadily. For a more effective SRT treatment, we recommend utilization of plugs with initial plug length greater than the airway width. Increasing surfactant strength or increasing the initially instilled surfactant concentration is found to be ineffective.

Immobilized capillary tyrosinase microreactor for inhibitor screening in natural extracts by capillary electrophoresis

A method for creating an immobilized capillary tyrosinase (TRS) reactor based on a layer-by-layer (LBL) assembly for inhibitor screening is described. Tyrosinase was immobilized on the surface of fused-silica capillary via ionic binding technique with cationic polyelectrolyte hexadimethrine bromide (HDB). Then, HDB solution with the same plug length as the TRS was injected again into the capillary to cover the immobilized enzyme by forming HDB–TRS–HDB sandwich-like structure. Then, the substrate of l-tyrosine was introduced into the capillary and on-line enzyme inhibition study was performed by capillary electrophoresis (CE). The enzyme activity was determined by the quantification of peak area of the product of l-DOPA. Enzyme inhibition can be read out directly from the reduced peak area of the product in comparison with a reference electropherogram obtained in the absence of any inhibitor. The immobilized enzyme could withstand 25 consecutive assays by only losing 12% activity. A known TRS inhibitor, kojic acid was employed as a model compound for the validation of the inhibitor screening method. Finally, screening 19 natural extracts of traditional Chinese drugs was demonstrated. The results indicated that inhibition activity could be straightforwardly identified with the system.

Transport of Wetting and Nonwetting Liquid Plugs in a T-shaped Microchannel

The transport of liquid plugs in microchannels is very important for many applications such as in medical treatments in airways and in extraction of oil from porous rocks. A plug of wetting and non-wetting liquids driven by a constant pressure difference through a T-shaped microchannel is studied numerically with lattice Boltzmann (LB) method. A two-phase flow LB model based on field mediators is built. Three typical flow patterns (blocking, rupture and splitting flow) of plug flow are obtained with different driving pressures. It is found that it becomes difficult for a plug with short initial plug length to leave the microchannel; the flow pattern of plug transport varies with the contact angle, especially from wetting to nonwetting; with the increase of interfacial tension, the front interface of plug moves faster; the front and rear interfaces of the plug with small viscosity ratio move faster in the microchannel than those of the plug with large viscosity ratio. The study is helpful to provide theoretical data for the design and scale-up of liquid-liquid reactors and separators.

Use of neutral capillaries for the enantioseparation ofN-benzoylated amino acids by capillary electrophoresis with bromobalhimycin as chiral selector

In this study, the partial filling technique on both polycationic polymer hexadimethrine bromide (HDB) modified capillary and eCAP neutral capillary were systematically compared in order to enhance the enantioseparation ability of bromobalhimycin as CE additive. The separation conditions, such as pH, the plug length, and the concentration of bromobalhimycin, etc., were optimized in order to obtain satisfactory separations. As expected, for all tested 28 N-benzoylated amino acids, up to five times higher enantioresolutions were obtained on the eCAP neutral capillary compared to that on the polycationic polymer hexadimethrine bromide modified capillary. Moreover, 26 of 28 tested racemic compounds were almost baseline- resolved without observing any interference from the front of the plug of bromobalhimycin. Although the limitation of longer running time on the neutral capillary, it allows the use of higher content of bromobalhimycin in the running buffer without any interference on the detection of analytes when enantioseparations are more difficult to obtain.

Droplet morphometry and velocimetry (DMV): a video processing software for time-resolved, label-free tracking of droplet parameters

Emerging assays in droplet microfluidics require the measurement of parameters such as drop size, velocity, trajectory, shape deformation, fluorescence intensity, and others. While micro particle image velocimetry (μPIV) and related techniques are suitable for measuring flow using tracer particles, no tool exists for tracking droplets at the granularity of a single entity. This paper presents droplet morphometry and velocimetry (DMV), a digital video processing software for time-resolved droplet analysis. Droplets are identified through a series of image processing steps which operate on transparent, translucent, fluorescent, or opaque droplets. The steps include background image generation, background subtraction, edge detection, small object removal, morphological close and fill, and shape discrimination. A frame correlation step then links droplets spanning multiple frames via a nearest neighbor search with user-defined matching criteria. Each step can be individually tuned for maximum compatibility. For each droplet found, DMV provides a time-history of 20 different parameters, including trajectory, velocity, area, dimensions, shape deformation, orientation, nearest neighbour spacing, and pixel statistics. The data can be reported via scatter plots, histograms, and tables at the granularity of individual droplets or by statistics accrued over the population. We present several case studies from industry and academic labs, including the measurement of 1) size distributions and flow perturbations in a drop generator, 2) size distributions and mixing rates in drop splitting/merging devices, 3) efficiency of single cell encapsulation devices, 4) position tracking in electrowetting operations, 5) chemical concentrations in a serial drop dilutor, 6) drop sorting efficiency of a tensiophoresis device, 7) plug length and orientation of nonspherical plugs in a serpentine channel, and 8) high throughput tracking of >250 drops in a reinjection system. Performance metrics show that highest accuracy and precision is obtained when the video resolution is >300 pixels per drop. Analysis time increases proportionally with video resolution. The current version of the software provides throughputs of 2-30 fps, suggesting the potential for real time analysis.

Magnetic resonance imaging of the development of fronts during spontaneous imbibition

Abstract Magnetic resonance imaging (MRI) was used to monitor the displacement of refined oil by spontaneous imbibition of brine into fully oil-saturated cylindrical cores with either one end open (OEO) or two ends open (TEO) to flow. The rocks were outcrop Whitestone Upper Zone (UZ) limestone, Berea and a Bentheim sandstone/Rordal chalk composite. In all cases, the wetting state was very strongly water-wet (VSWW). For OEO imbibition, high resolution imaging showed that brine invasion started at localized points and grew hemi-spherically outwards. The invaded regions increasingly overlapped until, after invasion of about one-third of the length of a standard core plug, a well-defined imbibition front formed that progessed as a remarkably sharp front at a velocity proportional to the square root of time. This behavior was usually observed for different boundary conditions and a range of permeabilities. For imbibition tests on a single core with TEO, imbibition started from the same core face no matter whether the core was horizontal, vertical or also vertical but inverted, showing that core heterogeneity dominated over the effect of gravity. The effect of heterogeneity was investigated further by imaging imbibition fronts in a TEO composite core formed from chalk butted against sandstone to provide a large difference in pore size. The fronts in the composite core moved at different times and velocities. Results are explained in terms of the capillary pressure gradients that develop within the composite core and, in particular, the capillary pressure at the open face(s). None of the behavior observed for a wide range of conditions can be modeled by the standard method which assumes relative permeabilities and capillary pressure relationships to give self similar fronts that expand as the front advances. The development of distinct fronts supports the modeling of imbibition, after the initial stages, as a piston-like displacement process.

Local Nusselt number enhancement during gas–liquid Taylor bubble flow in a square mini-channel: An experimental study

Taylor bubble flow takes place when two immiscible fluids (liquid–liquid or gas–liquid) flow inside a tube of capillary dimensions within specific range of volume flow ratios. In the slug flows where gas and liquid are two different phases, liquid slugs are separated by elongated Taylor bubbles. This singular flow pattern is observed in many engineering mini-/micro-scale devices like pulsating heat pipes, gas–liquid–solid monolithic reactors, micro-two-phase heat exchangers, digital micro-fluidics, micro-scale mass transfer process, fuel cells, etc. The unique and complex flow characteristics require understanding on local, as well as global, spatio-temporal scales. In the present work, the axial streamwise profile of the fluid and wall temperature for air–water (i) isolated single Taylor bubble and, (ii) a train of Taylor bubbles, in a horizontal square channel of size 3.3 mm × 3.3 mm × 350 mm, heated from the bottom (heated length = 175 mm), with the other three sides kept insulated, are reported at different gas volume flow ratios. The primary aim is to study the enhancement of heat transfer due to the Taylor bubble train flow, in comparison with thermally developing single-phase flows. Intrusion of a bubble in the liquid flow drastically changes the local temperature profiles. The axial distribution of time-averaged local Nusselt number (Nu¯z) shows that Taylor bubble train regime increases the transport of heat up to 1.2–1.6 times more as compared with laminar single-phase liquid flow. In addition, for a given liquid flow Reynolds number, the heat transfer enhancement is a function of the geometrical parameters of the unit cell, i.e., the length of adjacent gas bubble and water plug.