Capillary Orifice Research Articles

High-Speed Imaging of Giant Unilamellar Vesicle Formation in cDICE.

Giant unilamellar vesicles (GUVs) are widely used as in vitro model membranes in biophysics and as cell-sized containers in synthetic biology. Despite their ubiquitous use, there is no one-size-fits-all method for their production. Numerous methods have been developed to meet the demanding requirements of reproducibility, reliability, and high yield while simultaneously achieving robust encapsulation. Emulsion-based methods are often praised for their apparent simplicity and good yields; hence, methods like continuous droplet interface crossing encapsulation (cDICE), which make use of this principle, have gained popularity. However, the underlying physical principles governing the formation of GUVs in cDICE and related methods remain poorly understood. To this end, we have developed a high-speed microscopy setup that allows us to visualize GUV formation in real time. Our experiments reveal a complex droplet formation process occurring at the capillary orifice, generating >30 μm-sized droplets and only in some cases GUV-sized (∼15 μm) satellite droplets. According to existing theoretical models, the oil-water interface should allow for the crossing of all droplets, but based on our observations and scaling arguments on the fluid dynamics within the system, we find a size-selective crossing of GUV-sized droplets only. The origin of these droplets remains partly unclear; we hypothesize that some small GUVs might be formed from large droplets sitting at the second interface. Finally, we demonstrate that proteins in the inner solution affect GUV formation by increasing the viscosity and altering the lipid adsorption kinetics. These results will not only contribute to a better understanding of GUV formation processes in cDICE but ultimately also aid in the development of more reliable and efficient methods for GUV production.

High-Speed Generation of Microbubbles with Constant Cumulative Production in a Glass Capillary Microfluidic Bubble Generator.

This work reports a simple bubble generator for the high-speed generation of microbubbles with constant cumulative production. To achieve this, a gas-liquid co-flowing microfluidic device with a tiny capillary orifice as small as 5 μm is fabricated to produce monodisperse microbubbles. The diameter of the microbubbles can be adjusted precisely by tuning the input gas pressure and flow rate of the continuous liquid phase. The co-flowing structure ensures the uniformity of the generated microbubbles, and the surfactant in the liquid phase prevents coalescence of the collected microbubbles. The diameter coefficient of variation (CV) of the generated microbubbles can reach a minimum of 1.3%. Additionally, the relationship between microbubble diameter and the gas channel orifice is studied using the low Capillary number (Ca) and Weber number (We) of the liquid phase. Moreover, by maintaining a consistent gas input pressure, the CV of the cumulative microbubble volume can reach 3.6% regardless of the flow rate of the liquid phase. This method not only facilitates the generation of microbubbles with morphologic stability under variable flow conditions, but also ensures that the cumulative microbubble production over a certain period of time remains constant, which is important for the volume-dominated application of chromatographic analysis and the component analysis of natural gas.

Generation of microbubbles via a tapered capillary

We propose a novel method for efficient production of microbubbles based on a tapered capillary with an interiorly attached filament. When gas–liquid displacement driven by an input pressure occurs in the capillary, the gas cone ruptures close to the orifice of the capillary. The generated microbubbles can be pushed out of the capillary and collected by a liquid tank when the pressure is appropriately selected. A liquid column is employed in the straight part of the capillary, which can sustain the liquid film near the capillary orifice and hence the bubble generation by transporting liquid along the filament. Within the working pressure range, increasing the input air pressure leads to a decrease in the microbubble diameter. The minimum diameter of the microbubbles is approximately equal to the orifice diameter of the tapered capillary. In our experiments, microbubbles with a minimum diameter of 1.56 μm can be realized. Theoretically, we derive a one-dimensional unsteady lubrication equation describing the evolution of the gas–liquid interface in a tapered tube. The bubble pinch-off is justified by the numerical solution of the lubrication equation. In particular, the predicted bubble diameters are in agreement with the experimental measurements.

High-speed imaging of giant unilamellar vesicle production in cDICE.

Giant unilamellar vesicles (GUVs) are cell-sized containers that are commonly used as three-dimensional model membranes in biophysics, as in vitro model systems in synthetic biology, and even as cargo carriers in various other research fields. Despite their ubiquitous use, there is still no one-size-fits-all GUV production method. Over the years, numerous methods have been developed, attempting to meet the demanding requirements of robustness, reliability, and high yield while simultaneously achieving robust encapsulation. Double emulsion-based methods are often praised for their apparent simplicity and good yields; hence, methods like continuous droplet interface crossing encapsulation (cDICE) that make use of this principle, have gained popularity in recent years. In cDICE, aqueous droplets that originate from a capillary orifice are continuously forced through an oil-water interface by centrifugal force, thereby forming a lipid bilayer and thus GUVs. Although cDICE and related methods are frequently used in the field, the complexity of the underlying principles and fluid dynamics has not been considered previously, and how exactly the GUVs are being formed remains unknown. To elucidate the process of GUV formation in cDICE, we have developed a high-speed microscopy setup that allows us to visualize GUV formation in real time. We focused on the capillary orifice, where initial droplet formation occurs, and on the oil-water interface, where droplets are converted into GUVs. Our experiments reveal a complex droplet formation process at the capillary orifice and suboptimal droplet transfer through the water-oil interface, which we explain using fluid dynamics and theoretical modeling. Our results are a first step towards explaining the widely observed variation in encapsulation efficiency and size polydispersity in cDICE. Ultimately, these results will contribute to a better understanding of GUV formation processes in cDICE and in extension, in double emulsion-based methods in general.

Optical emission spectroscopy and Langmuir probe studies of an intermediate pressure, supersonic microplasma jet deposition source

Optical emission spectroscopy and double Langmuir probe studies were conducted on the supersonic expansion plume of a flow-through, intermediate pressure (∼10 Torr) Ar/N2 microplasma source used for material deposition. Emission from the first positive system of N2 (B3Πg → A3Σu+) was used to determine the expanding gas rotational and vibrational temperatures; both were strongly dependent on plasma drive current, and the gas (rotational) temperature could be tuned from 300 to 800 K. The effects of drive current, plasma circuit configuration, and O2 addition to the gas feed on the local electron temperature (Te) were investigated using a miniature double Langmuir probe. Electron temperatures of 1–2 eV were estimated and decreased slightly with higher O2 content in the feed and increased distance from the capillary orifice. Te dependence on drive current was more complicated, falling into two regimes, namely, a nonuniform “predischarge” regime at low currents (<12 mA) where Te varied greatly, and the normal hollow cathode regime at higher currents (>12 mA) where Te remained nearly constant as plasma current was increased. These phenomena are discussed in light of the IV characteristics of the discharge drive circuit.

Enhancement of electric field on bubble dispersion characteristics in leaky-dielectric liquid medium

Enhancement of the electric field on mixture has attached wide interests owing to its advantages in the terrestrial and space applications. In this paper, the dispersion characteristics of bubbles in leaky-dielectric liquid were investigated with a novel designed capillary electrode in the presence of a non-uniform electric field, employing high-speed photography and with a focus on the electrohydrodynamic (EHD) effects. The results show that the bubble dispersion is associated with three typical patterns in the manipulated parameter regions, including the dripping, mixed, and spraying patterns. With the increase of electrical Bond number (BoE), bubble size, characterized by the dimensionless bubble diameter ζ, is decreased significantly, while the evolution process of bubbles becomes faster and more complex. In particular, the size of the generated micro/nanobubbles in the spraying pattern is basically below 20 μm. The rise of the dimensionless flow rate (Q/Q0) increases ζ. It is found that the transition of bubble dispersion patterns depends strongly on the BoE, and the rise in Q/Q0 tends to slow down the transition process. In addition, an empirical correlation of ζ as a function of BoE and Q/Q0 is established for the dripping pattern of bubbles based on the present experiment data, which is believed to be of significance for the modeling of bubble dispersion under electric field. The bubble shapes, characterized by aspect ratios, show a diversity of distribution in the cross-section close to the capillary orifice, while terminally tending to follow the conventional formulation in the literature as the bubbles develop in the vertical direction.

Passive Flow Control for Series Inflatable Actuators: Application on a Wearable Soft-Robot for Posture Assistance

This paper presents a passive control method for multiple degrees of freedom in a soft pneumatic robot through the combination of flow resistor tubes with series inflatable actuators. We designed and developed these 3D printed resistors based on the pressure drop principle of multiple capillary orifices, which allows a passive control of its sequential activation from a single source of pressure. Our design fits in standard tube connectors, making it easy to adopt it on any other type of actuator with pneumatic inlets. We present its characterization of pressure drop and evaluation of the activation sequence for series and parallel circuits of actuators. Moreover, we present an application for the assistance of postural transition from lying to sitting. We embedded it in a wearable garment robot-suit designed for infants with cerebral palsy. Then, we performed the test with a dummy baby for emulating the upper-body motion control. The results show a sequential motion control of the sitting and lying transitions validating the proposed system for flow control and its application on the robot-suit.

Development of a capillary tube orifice for high pressure and a low flow rate in a launch vehicle

The purpose of an orifice is to reduce the flow rate and the pressure. A tiny hole with a 0.1 mm diameter is necessary when the up-stream pressure is very high at 22 MPa, and the required flow rate is very low at 1 g/s. The machining of such a tiny hole with a 0.1 mm diameter as the orifice has a poor processability, and the flow passage is easily clogged by particles when the orifice size is too small. A new capillary tube orifice was developed to resolve the above mentioned problems. The fabricated capillary tube orifices were tested at room and cryogenic temperatures to measure its performance with helium gas. The effective cross sectional area of the tested orifices was constant in various test conditions. The cross sectional area of the fabricated capillary orifices was observed with a microscope, and the friction factor was analysed.

Numerical Simulation and Experimental Validation of Liquid Metal Droplet Formation in a Co-Flowing Capillary Microfluidic Device.

A two-phase flow axisymmetric numerical model was proposed to understand liquid metal droplet formation in a co-flowing capillary microfluidics device based on a phase field model. The droplet detachment processes were observed in the experiment and are in good agreement with the simulation method. The effects of the viscosities and flowrates of the continuous phase fluid, interfacial tension as well as the wetting property of the metallic needle against the bulk liquid metal on the droplet formation and production rate were numerically investigated. It was found that the droplet diameter decreased with the increment of the viscosities and flowrates of the outer phase carrier fluid. The dispersed phase fluid with high interfacial tension tended to prolong the time for equilibrium between the viscous drag force and interfacial tension on the liquid–liquid fluid surface, delaying the droplet to be pinched off from the capillary orifice and causing large droplet diameter. Finally, the wetting performance of the metallic needle against the liquid metal was explored. The result indicate that the droplet diameter became less dependent on the contact angle while the size distribution of the liquid metal droplet was affected by their wetting performance. A more hydrophilic wetting performance were expected to prepare liquid metal droplet with more monodispersity. The numerical model and simulation results provide the feasibility of predicting the droplet formation with a high surface tension in a glass capillary microfluidic device.

Stable multi-jet behavior of ethanol under an electric field

Electrospray is a mass transfer technology in which charged liquid is broken into fine droplets/particles with ideal monodispersity, easy controllability, and high-deposition . In recent decades, with the rapid development of various micro precision instrument technologies, the research of electrospray has been widely applied to micro/nano-scaled thin film and particle preparation, air purification, micro-combustion, space micro-propulsion, mass spectrometry, and biotechnology. Previous researches mainly focused on the stable cone-jet mode that has the advantages for producing several micron-sized particles or droplets with good stability and monodispersity. However, electrospray in this cone-jet mode has a major disadvantage that restricts the practical application of electrospray. The droplet size in the cone-jet mode increases with an increase of the flow rate, and this means the production of tiny droplets limits the supply flow rate, and this limited flow rate cannot meet the requirements for atomization flow in practical application. Multi-jet is an important operation mode in electrospray, and sub-droplets of comparable size to the sub-droplets produced by cone-jet can be obtained on the basis of multiplication of the single capillary flow. However, multi-jet atomization is very unstable, and it is difficult to attain a stable jet atomization state; therefore, in-depth research on the evolution of its morphology and atomization characteristics is necessary to expand the practical application of electrostatic atomization technology. Based on high-speed photography technology with a high spatial-temporal resolution, the influence of capillary geometry, electrode spacing, liquid supply flow rate, and applied voltage on the multi-jet formation and evolution in electrospray was studied in detail. The electric bond number and the dimensionless flow rate were introduced to describe their action laws on the evolution behavior of the stable jet number, stable atomization range, and jet deviation angle in the stable multi-jet mode. The research results show that the range of the number of stable jets N of multi-jet is from 2 to 10. When 2≤ N N ≤10, the jets are ejected from the outer edge of the capillary orifice to form an edge jet mode, and it is particularly important to note that the occurrence frequency of the stable 5–8 strands is higher than other strands. In the edge mode, as the number of stable jets increases, the range of the electric bond number of the stable multi-jets and the critical minimum electric bond number gradually increases, and a voltage switching phenomenon of stabilizing multi-jet is also presented. As the dimensionless flow rate increases, the maximum value of the electric bond number under each number of stable jets shows a linear growth trend. The maximum value of the electric bond number increases with the increase of the stable jet number under the same flow rate, and a significant increase in the minimum value of the electric bond number is observed. Additionally, as the number of stable jets in the edge mode increases, the overall deviation angle of the jet increases, but the range of variation of the deviation angle of each stable jet number is controlled within 5°±0.3°. Moreover, as the electric bond number increases, the jet deviation angle gets closer to 45°, which indicates that the electric field force dominates the jet deviation as the electric field intensity increases.

Externally Pressurized Thrust Spherical Bearing Performance Under Variable Lubricant Viscosity (Un-recessed Clearance Type)

In a previous study handling this subject, a series of six papers has been published covering the fitted type (equal radii of this sort of bearings) with its configurations in details. The present study handles the same subject investigating the clearance type of this sort of bearings starting with the un-recessed one. The clearance type of bearings, where the sphere radius is smaller than that of the seat, provides plenty of complications and great divergence in its behavior compared with the fitted one. The study investigates theoretically the bearing behavior under the lubricant viscosity variation in the presence of the centripetal inertia due to the shaft rotation and the surface roughness. Solutions have been derived for the un-recessed clearance type of bearings with hemispherical and partial hemispherical seats in addition to its two special cases using capillary tube and orifice restrictors. Unlike other studies, using the conventional integration (i.e., without using the Sommerfeld substitution), only one equation with one form for the pressure gradient is derived to cover the positive and negative eccentricity ratios. Expressions for the pressure distribution, temperature distribution in turn the temperature rise, load carrying capacity; volume flow rate, frictional torque, friction factor, power losses and stiffness factor are obtained. The study shows the combined effects of the viscosity variation, the centripetal inertia and the surface roughness on the bearing performance. The optimum design (with constant viscosity) based on the minimum power losses, minimum flow rate and the optimal restrictor dimensions, in a previous study is checked where it is found out that designing this type of bearings on such basis only is not sufficient in despite of the bearing consistency. A natural dynamic phenomenon ignored in the previous study is revealed and briefly touched.

Study on bubble coalescence and bouncing behaviors upon off-center collision in quiescent water

Behaviors of bubble coalescence and bouncing upon off-center collision were experimentally studied with the collision between a rising bubble generated by a free moving capillary and a bubble resting under a fixed copper cap. The collision process was recorded with two high-speed cameras. The impact relative velocity and the impact parameter B were tuned by adjusting the vertical and horizontal distances between the capillary orifice and the resting bubble. The results show that the occurrence of bubble coalescence with off-center collision is much more than that with head-on collision. The critical Weber number (We) based on relative velocity is 0.16 when B = 0 and it increases to 0.42 when B = 0.72. It was found that the bubbles coalesce when We(1 − B2) < 0.16 + B/24, otherwise they bounce. In addition, the outcome of the two bubbles collision is related to the liquid film length formed during collision of the two bubbles. It was found that two bubbles coalesce when the maximum liquid film length during collision is below 0.43–0.45 mm, otherwise the two bubbles bounce off.

Experimental investigation of coalescence behaviour of bubble pairs forming at capillary orifices submerged in bacterial suspension

Coalescence behaviour of bubble pairs markedly affected methane bioconversion because methane is usually pumped into a bacterial suspension through an aeration system, forming bubbles at capillary orifices. In the present work, a bioreactor was developed to observe the coalescence behaviour of bubble pairs forming at capillary orifices submerged in bacterial suspension via a visualization experiment. The results indicated that in a bacterial suspension the inrush bubble number decreased and the inrush time shortened, but the contact time was prolonged and the maximum contact area increased compared to those in pure water. Moreover, with an increase in the gas flow rate, the maximum contact area decreased and the contact time shortened, while more trailing bubbles merged into the leading bubble. Furthermore, the increase in the centre distance was conducive to increasing the maximum contact area and prolonging the contact time but the bubble inrush was observed at a centre distance of 0.94 mm. In addition, the larger centre distance enhanced the oscillation amplitude and frequency of the aspect ratio, which had not significantly changed at different gas flow rates. The results of this study will contribute to the optimal design of an aeration system that enhances the performance of the bioreactor.

In Situ Imaging of Photocatalytic Activity at Titanium Dioxide Nanotubes Using Scanning Ion Conductance Microscopy.

In this letter, in situ imaging of the photocatalytic activity of titanium dioxide (TiO2) nanotubes for the degradation of an organic pollutant (i.e., Rhodamine B (RhB)) is realized with nanometer resolution using scanning ion conductance microscopy (SICM). Upon illumination, the separated electrons and holes at the nanotubes induce oxidation of RhB to produce the more positively charged Rhodamine 123 (Rh 123), which leads to increased ionic current through the capillary orifice and an elevated apparent altitude in the SICM image. Active sites with higher activity on the nanotubes exhibit a significant high spatial-resolution character. The successful imaging of the photocatalytic activity of TiO2 nanotubes should provide an in situ approach for local investigation of the photocatalytic process at the catalyst.

Experimental investigation on bubble dynamic behavior in a confined space bioreactor with bacterial suspension

AbstractIn this work, a confined space bioreactor with bacterial suspension was developed to experimentally investigate the adsorption of microorganisms at the gas–liquid interface and the effects of inner diameter, inclined angle, and gas flow rate on bubble dynamic behavior. The results indicated that the adsorption of microorganisms at the gas–liquid interface was affected by the bacterial chemotaxis and the gas–liquid interface expansion, and the confined space was conducive to accelerate the detachment of smaller bubble and exacerbate the fluctuation of gas–liquid interface. Moreover, the larger inner diameter induced that the bubble was squeezed by the 2 side walls of the confined space bioreactor, restricting the bubble shocking and slowing down the bubble rising velocity for increasing the bubble residence time. Furthermore, the smallest bubble departure size and the shortest inrush time were observed at horizontal capillary orifice, but the largest inrush bubble number was obtained at down‐45° capillary orifice. In addition, the inrush bubble number increased with increasing gas flow rate, but the bubble departure sizes were not much different. The results can be used to optimize the design of aeration system for improving the performance of the bioreactor.

Molecular rheotaxis directs DNA migration and concentration against a pressure-driven flow

In-line preconcentration techniques are used to improve the sensitivity of microfluidic DNA analysis platforms. The most common methods are electrokinetic and require an externally applied electric field. Here we describe a microfluidic DNA preconcentration technique that does not require an external field. Instead, pressure-driven flow from a fluid-filled microcapillary into a lower ionic strength DNA sample reservoir induces spontaneous DNA migration against the direction of flow. This migratory phenomenon that we call Molecular Rheotaxis initiates in seconds and results in a concentrated DNA bolus at the capillary orifice. We demonstrate the ease with which this concentration method can be integrated into a microfluidic total analysis system composed of in-line DNA preconcentration, size separation, and single-molecule detection. Paired experimental and numerical simulation results are used to delineate the parameters required to induce Molecular Rheotaxis, elucidate the underlying mechanism, and optimize conditions to achieve DNA concentration factors exceeding 10,000 fold.

Bubble dynamic behaviours at various capillary orifices in a bioreactor with methane oxidizing bacteria suspension

AbstractBubble dynamic behaviour has a significant influence on methane bioconversion because a gas mixture including methane and oxygen is usually supplied into a bioreactor with a methane oxidizing bacteria suspension via capillary orifice, forming bubbles. In the work, bubble dynamic behaviours at various capillary orifices were experimentally visualized, as well as the movement of microorganisms was observed during bubble formation evolution. The experimental results showed the chemotaxis effect and the bubble expansion as two key factors affecting the movement of microorganisms in the bioreactor with methane oxidizing bacteria suspension. At the incline angle of 90°, the equivalent departure diameter and departure volume were the smallest, while the departure time was the shortest. With increasing inner diameter, the equivalent departure diameter and departure volume increased, but the waiting time was the longest and the departure time was the shortest at the inner diameter of 0.3 mm. In addition, the successive inrush of bubbles forming at the capillary orifice with 90° incline angle and 0.1 mm inner diameter resulted in the shock of the gas‐liquid interface, affecting the bubble dynamic behaviour. The results in this work contribute to the design of a high‐productivity bioreactor with an optimized aeration system for enhancing mass transfer of methane from the bubble to the bacteria suspension and subsequent methane bioconversion.

Dynamic behavior of bubble forming at capillary orifice in methane oxidizing bacteria suspension

In this work, the dynamic behavior of bubble forming at capillary orifice was investigated using a developed bioreactor with methane oxidizing bacteria suspension as well as the movement and adsorption of microorganism at the gas-liquid interface were observed experimentally. The effect of gas flow rate from 0.8 to 1.45mL/min on bubble formation evolution and departure process as well as the variations of equivalent departure diameter, departure volume and departure time of the leading bubble and the trailing bubbles were also explored during successive bubble formation. The experimental results showed that the chemotaxis effect and bubble expansion affected the movement and adsorption of microorganism at the gas-liquid interface, decreasing equivalent departure diameter and departure volume of the leading bubble as well as inrush number of the trailing bubble, shortening departure time of the leading bubble and inrush time of the trailing bubble, and prolonging waiting time of the leading bubble. Moreover, the increase of gas flow rate from 0.8 to 1.45mL/min contributed to the increase of inrush number of the trailing bubble and the decrease of departure time of the leading bubble, but equivalent departure diameter and departure volume of the leading bubble were not much different under various gas flow rates. In addition, the trailing bubble merged into the leading bubble resulted in a significant fluctuation at the gas-liquid interface, affecting the bubble dynamic behavior and improving the mass transfer of gas mixture. These findings are beneficial to enhance the performance of the bioreactor by the optimal design of aeration system for bioconversion of methane fed into methane oxidizing bacteria suspension.

Coupling of gas chromatography and electrospray ionization high resolution mass spectrometry for the analysis of anabolic steroids as trimethylsilyl derivatives in human urine

In this study, gas chromatography (GC) was interfaced with high resolution mass spectrometry (HRMS) with electrospray ionization source (ESI) and the relevant parameters were investigated to enhance the ionization efficiency. In GC-ESI, the distances (x-, y- and z) and angle between the ESI needle, GC capillary column and MS orifice were set to 7 (x-distance), 4 (y-distance), and 1 mm (z-distance). The ESI spray solvent, acid modifier and nebulizer gas flow were methanol, 0.1% formic acid and 5 arbitrary units, respectively. Based on these results, analytical conditions for GC-ESI/HRMS were established. In particular, the results of spray solvent flow indicated a concentration-dependent mechanism (peak dilution effect), and other parameters also greatly influenced the ionization performance. The developed GC-ESI/HRMS was then applied to the analysis of anabolic steroids as trimethylsilyl (TMS) derivatives in human urine to demonstrate its application. The ionization profiles of TMS-derivatized steroids were investigated and compared with those of underivatized steroids obtained from gas chromatography-electrospray ionization/mass spectrometry (GC-ESI/MS) and liquid chromatography-electrospray ionization/mass spectrometry (LC-ESI/MS). The steroids exhibited ionization profiles based on their structural characteristics, regardless of the analyte phase or derivatization. Groups I and II with conjugated or unconjugated keto functional groups at C3 generated the [M+H]+ and [M+H-TMS]+ ions, respectively. On the other hand, Groups III and IV gave rise to the characteristic fragment ions [M+H-TMS-H2O]+ and [M+H-2TMS-H2O]+, corresponding to loss of a neutral TMS·H2O moiety from the protonated molecular ion by in-source dissociation. To the best of our knowledge, this is the first study to successfully ionize and analyze steroids as TMS derivatives using ESI coupled with GC. The present system has enabled the ionization of TMS derivatives under ESI conditions and this method has potential as a novel ionization tool. It is also useful for the simultaneous analysis of steroids as TMS derivatives.

Role of local geometry on droplet formation in axisymmetric microfluidics

An unsteady model of droplet formation in co-flow and flow-focusing microfluidics is developed and numerically analyzed to investigate the dynamic behaviors of droplet formation in axisymmetric microfluidics with a focus on the role of local geometry. The effects of capillary number and local geometry on the droplet formation regimes, droplet sizes and monodispersity as well as droplet generation frequency are examined and analyzed. Once identified, a drop formation regime diagram is provided to quantitatively describe the respective regime of dripping, dripping-jetting transition, and jetting in axisymmetric microfluidics, depending on the Capillary number and orifice radius. The results indicate that, the existence of focusing orifice induces a strong hydrodynamic focusing effect, causing the droplet formation behaviors in flow-focusing microfluidics depart from the co-flow one. The dripping-jetting transition regime occurs at a smaller Capillary number in flow-focusing microfluidics, and the droplets produced by flow-focusing microfluidics are smaller than those in co-flow system with wider size distribution and higher frequency. Interestingly, the droplet formation in flow-focusing microfluidics is significantly affected by the orifice radius while it is insensitive to the orifice length. In addition, when the orifice radius is sufficiently small or the Capillary number is larger than 0.3, only the jetting regime is observed. As the orifice radius increases, the regions of Capillary number for both the dripping-jetting transition regime and dripping regime turn to be larger.