Droplet based microfluidics is a rapidly growing interdisciplinary field of research combining soft matter physics, biochemistry and microsystems engineering. Its applications range from fast analytical systems or the synthesis of advanced materials to protein crystallization and biological assays for living cells. Precise control of droplet volumes and reliable manipulation of individual droplets such as coalescence, mixing of their contents, and sorting in combination with fast analysis tools allow us to perform chemical reactions inside the droplets under defined conditions. In this paper, we will review available drop generation and manipulation techniques. The main focus of this review is not to be comprehensive and explain all techniques in great detail but to identify and shed light on similarities and underlying physical principles. Since geometry and wetting properties of the microfluidic channels are crucial factors for droplet generation, we also briefly describe typical device fabrication methods in droplet based microfluidics. Examples of applications and reaction schemes which rely on the discussed manipulation techniques are also presented, such as the fabrication of special materials and biophysical experiments.

https://iopscience.iop.org/article/10.1088/0034-4885/75/1/016601/pdf

Droplet based microfluidics

This article presents a comprehensive review of numerical methods and models for interface resolving simulations of multiphase flows in microfluidics and micro process engineering. The focus of the paper is on continuum methods where it covers the three common approaches in the sharp interface limit, namely the volume-of-fluid method with interface reconstruction, the level set method and the front tracking method, as well as methods with finite interface thickness such as color-function based methods and the phase-field method. Variants of the mesoscopic lattice Boltzmann method for two-fluid flows are also discussed, as well as various hybrid approaches. The mathematical foundation of each method is given and its specific advantages and limitations are highlighted. For continuum methods, the coupling of the interface evolution equation with the single-field Navier–Stokes equations and related issues are discussed. Methods and models for surface tension forces, contact lines, heat and mass transfer and phase change are presented. In the second part of this article applications of the methods in microfluidics and micro process engineering are reviewed, including flow hydrodynamics (separated and segmented flow, bubble and drop formation, breakup and coalescence), heat and mass transfer (with and without chemical reactions), mixing and dispersion, Marangoni flows and surfactants, and boiling.

Numerical modeling of multiphase flows in microfluidics and micro process engineering: a review of methods and applications

In this review we discuss the state of the art of the optical whole-field velocity measurement technique micro-scale Particle Image Velocimetry (µPIV). µPIV is a useful tool for fundamental research of microfluidics as well as for the detailed characterization and optimization of microfluidic applications in life science, lab-on-a-chip, biomedical research, micro chemical engineering, analytical chemistry and other related fields of research. An in depth description of the µPIV method is presented and compared to other flow visualization and measurement methods. An overview of the most relevant applications is given on the topics of near-wall flow, electrokinetic flow, biological flow, mixing, two-phase flow, turbulence transition and complex fluid dynamic problems. Current trends and applications are critically reviewed. Guidelines for the implementation and application are also discussed.

Micro-Particle Image Velocimetry (microPIV): recent developments, applications, and guidelines.

Two-phase microfluidic flows

Multiphase flow microfluidics for the production of single or multiple emulsions for drug delivery

Bubble generation in a simple co-flowing micro-channel with a cross-sectional area of 169 × 007 mm2 was experimentally and numerically investigated Air and water were used as the gas and liquid, respectively Mixtures of water–glycerol and water–Tween 20 were also used to obtain the effects of viscosity and surface tension The experimental data show that the break-up process is periodic under certain operating conditions The break-up dynamics are also examined using three-dimensional incompressible two-phase flow numerical simulation based on the volume of fluid (VOF) method The simulation successfully predicts the flow behavior and provides a more detailed examination of the bubble shape The physics can be further explained by the detailed micro-PIV measurements, which show that the bubble is formed due to the velocity component perpendicular to the gas flow created by the sudden change of the liquid velocity distribution around the barrier The bubble length L is dependent on the liquid flow rate Ql and the gas flow rate Qg, and the ratio of L to the channel width w is a function of the ratio of gas and liquid flow rates Qg/Ql which is similar to that previously used in the T-junction case The formulation of bubble frequency f is derived under current conditions and it shows a good agreement with the experimental data at the low frequency region Different bubble shapes can be obtained at different liquid viscosities and surface tensions The ratio L/w can still be predicted by a modified equation which uses the real bubble width wb or an equivalent bubble length Le

http://iopscience.iop.org/article/10.1088/0960-1317/17/5/021/pdf

Formation of bubbles in a simple co-flowing micro-channel

Adiabatic gas-liquid flow patterns and void fractions in microchannels were experimentally investigated. Using nitrogen and water, experiments were conducted in rectangular microchannels with hydraulic diameters of 0.209mm, 0.412mm and 0.622mm, respectively. Gas and liquid superficial velocities were varied from 0.06–72.3m∕s and 0.02–7.13m∕s, respectively. The main objective is focused on the effects of microscale channel sizes on the flow regime map and void fraction. The instability of flow patterns was observed. Four groups of flow patterns including bubbly slug flow, slug-ring flow, dispersed-churn flow, and annular flow were observed in microchannels of 0.412mm and, 0.622mm. In the microchannel of 0.209mm, the bubbly slug flow became the slug flow and the dispersed-churn flow disappeared. The current flow regime maps showed the transition lines shifted to higher gas superficial velocity due to a dominant surface tension effect as the channel size was reduced. The regime maps presented by other author...

An experimental study of the size effect on adiabatic gas-liquid two-phase flow patterns and void fraction in microchannels

Flow characteristics of water in straight and serpentine micro-channels with miter bends

A new approach of numerically generating a microtube with three-dimensional random surface roughness is presented. In this approach, we combined a bi-cubic Coons patch with Gaussian distributed roughness heights. Two random roughness generation methods are studied. A computational fluid dynamic solver is used to solve the 3-D N–S equations for the flow through the generated rough microtubes with D = 50 μm and L = 100 μm. The effects of the peak roughness height, H, asperities spacing in the θ direction, S
 θ
 , and Z direction, S
 Z
 , standard deviation of the Gaussian distribution, σ, arithmetical mean roughness, R
 a, on the Poiseuille number, Po are investigated. It is found that when H/D < 5% the Po number can still be predicted by the conventional flow theory if the mean diameter of rough microtubes, D
 m, is used to be the hydraulic diameter D
 h. When H/D = 10%, the main flow is strongly affected by the roughness at Reynolds number Re = 1,500. The Po number increases with Re and deviates from the prediction up to 11.9%. The Po number does not change a lot with S
 θ
 and S
 Z
 because D
 m almost keeps constant when the spacing is changed. For the rough microtubes with different R
 a
 values, the Po numbers can be almost the same, which prove that only with the R
 a
 value we can not determine the friction in the rough microtube. The mean value μ, the maximum and minimum values of the random roughness are found to be critical to determine the Po number.

Renqiang Xiong

Papers

Formation of bubbles in a simple co-flowing micro-channel

An experimental study of the size effect on adiabatic gas-liquid two-phase flow patterns and void fraction in microchannels

Flow characteristics of water in straight and serpentine micro-channels with miter bends

Investigation of laminar flow in microtubes with random rough surfaces

Bubble generation and transport in a microfluidic device with high aspect ratio