Numerical Simulation of Droplet Generation in Coaxial Microchannels

In this study,numerical simulations of the pinching-off phenomena displayed by the dispersed phase in a continuous phase have been conducted using COMSOL Multiphysics(level-set method).Four flow patterns,namely“drop flow”,“jet flow”,“squeeze flow”,and“co-flow”,have been obtained for different flow velocity ratios,channel diameter ratios,density ratios,viscosity ratios,and surface tension.The flow pattern map of two-phase flow in coaxial microchannels has been obtained accordingly,and the associated droplet generation process has been critically discussed considering the related frequency,diameter,and pinch-off length.In particular,it is shown that the larger the flow velocity ratio,the smaller the diameter of generated droplets and the shorter the pinch-off length.The pinch-off length of a droplet is influenced by the channel diameter ratio and density ratio.The changes in viscosity ratio have a negligible influence on the droplet generation pinching frequency.With an increase in surface tension,the frequency of generation and pinch-off length of droplets decrease,but for small surface tension the generation diameter of droplet increases.

A novel integrated microfluidic chip for on-demand electrostatic droplet charging and sorting

On-demand droplet sorting is extensively applied for the efficient manipulation and genome-wide analysis of individual cells.However,state-of-the-art microfluidic chips for droplet sorting still suffer from low sorting speeds,sample loss,and labor-intensive preparation procedures.Here,we demonstrate the development of a novel microfluidic chip that integrates droplet generation,on-demand electrostatic droplet charging,and high-throughput sorting.The charging electrode is a copper wire buried above the nozzle of the microchannel,and the deflecting electrode is the phosphate buffered saline in the microchannel,which greatly simplifies the structure and fabrication process of the chip.Moreover,this chip is capable of high-frequency droplet generation and sorting,with a frequency of 11.757 kHz in the drop state.The chip completes the selective charging process via electrostatic induction during droplet generation.On-demand charged microdroplets can arbitrarilymove to specific exit channels in a three-dimensional(3D)-deflected electric field,which can be controlled according to user requirements,and the flux of droplet deflection is thereby significantly enhanced.Furthermore,a lossless modification strategy is presented to improve the accuracy of droplet deflection or harvest rate from 97.49% to 99.38% by monitoring the frequency of droplet generation in real time and feeding it back to the charging signal.This chip has great potential for quantitative processing and analysis of single cells for elucidating cell-to-cell variations.

Lattice Boltzmann Simulation of Droplet Generation in a Microfluidic Cross-Junction

Using the lattice Boltzmann multiphase model,numerical simulations have been performed to understand the dynamics of droplet formation in a microfluidic cross-junction.The influence of capillary number,flow rate ratio,viscosity ratio,and viscosity of the continuous phase on droplet formation has been systematically studied over a wide range of capillary numbers.Two different regimes,namely the squeezinglike regime and the dripping regime,are clearly identified with the transition occurring at a critical capillary number Cacr.Generally,large flow rate ratio is expected to produce big droplets,while increasing capillary number will reduce droplet size.In the squeezing-like regime(Ca≤Cacr),droplet breakup process is dominated by the squeezing pressure and the viscous force;while in the dripping regime(Ca>Cacr),the viscous force is dominant and the droplet size becomes independent of the flow rate ratio as the capillary number increases.In addition,the droplet size weakly depends on the viscosity ratio in both regimes and decreases when the viscosity of the continuous phase increases.Finally,a scaling law is established to predict the droplet size.

Study of droplet flow in a T-shape microchannel with bottom wall fluctuation

Droplet generation in a T-shape microchannel, with a main channel width of 50 μm, side channel width of 25 μm, and height of 50 μm, is simulated to study the effects of the forced fluctuation of the bottom wall. The periodic fluctuations of the bottom wall are applied on the near junction part of the main channel in the T-shape microchannel. Effects of bottom wall＇s shape,fluctuation periods, and amplitudes on the droplet generation are covered in the research of this protocol. In the simulation,the average size is affected a little by the fluctuations, but significantly by the fixed shape of the deformed bottom wall, while the droplet size range is expanded by the fluctuations under most of the conditions. Droplet sizes are distributed in a periodic pattern with small amplitude along the relative time when the fluctuation is forced on the bottom wall near the T-junction,while the droplet emerging frequency is not varied by the fluctuation. The droplet velocity is varied by the bottom wall motion,especially under the shorter period and the larger amplitude. When the fluctuation period is similar to the droplet emerging period, the droplet size is as stable as the non-fluctuation case after a development stage at the beginning of flow, while the droplet velocity is varied by the moving wall with the scope up to 80% of the average velocity under the conditions of this investigation.

Parametric study of droplet size in an axisymmetric flow-focusing capillary device

A conventional technique for microfluidic droplet generation is Co-axial Flow Focusing(CFF)in which a contraction zone is placed downstream of the dispersed phase nozzle.In this contraction zone,the dispersed-phase(dphase)fluid is pinched off by continuous-phase(c-phase)fluid to generate micro-droplets.Studying the influence of multiple parameters such as the fluids velocities and viscosities,the interfacial tension,and nozzle and orifice diameters on the droplet size is of great importance for the design and application of CFF devices.Thus,development of more complete numerical models is required.In this paper,we show our model is compatible with experimental data and then numerically investigate the effects of aforementioned parameters on the droplet generation in a CFF microfluidic device.Simulation results showed that the c-phase flow rate,viscosity and the interfacial tension had great impacts on the droplet size.The effect of the nozzle diameter on the generated droplet size was small compared to that of the orifice in a CFF device.Using the simulation results,a correlation was also developed and suggested which predicts the droplet size with less than 15%error in a wide range of the introduced dimensionless parameters.

High-throughput generation of uniform droplets from parallel microchannel droplet generators and the preparation of polystyrene microsphere material

To prepare uniform polystyrene particles with ten microns of diameter,a parallel scaling-up strategy for the capillary-assembled stepwise microchannel was developed,which created uniform droplets with high-throughput and formed a large amount of emulsion templates for the polymerization of styrene and cross-linker.The microchannel droplet generator was robust for the flow rate deviation of the continuous phase in the jetting flow,and droplet generation frequency up to 2.8 x 104 Hz was achieved with only four parallel droplet generators,which were much more efficient than the parallelly scaled microfluidic devices working in dripping flow.32-52μm average diameter droplets with 4.5%-8.4%diameter variation coefficients were successfully prepared from the microchannel device fabricated by low-cost 3D-print method,and the droplets were subsequently turned to solid particles via a two-step polymerization in the platform.The polystyrene particles were further reduced to 16.9-23.5μm with 5.0%-8.6%diameter variation coefficients due to the accompanying emulsion polymerization,and the working capacity of the platform reached hundred milligrams of particles per hour.

Recent developments of droplets-based microfluidics for bacterial analysis

Droplet-based microfluidics enables the generation of uniform microdroplets at picoliter or nanoliter scale with high frequency(∼kHz)under precise control.The droplets can function as bioreactors for versatile chemical/biological study and analysis.Taking advantage of the discrete compartment with a confined volume,(1)isolation and manipulation of a single cell,(2)improvement of in-droplet effective concen-trations,(3)elimination of heterogeneous population effects,(4)diminution of contamination risks can be achieved,making it a powerful tool for rapid,sensitive,and high-throughput detection and analysis of bacteria,even for rare or unculturable strains in conventional methods.This mini-review will focus on the generation and manipulation of micro-droplets and bacteria detection and analysis carried out by droplet-based microfluidics.Finally,applications with high potential of droplet-based bacteria analysis are briefly introduced.Due to the advantages of rapid,sensitive,high throughput,and compatibility with rare and unculturable bacteria in conventional methods,droplet-based microfluidics has tremendous potential of providing novel solutions for biological medicine,microbiological engineering,environmental ecology,etc.