Simulation of reactive mixing behaviors inside micro-droplets by a lattice Boltzmann method

Efficient Separation of Methanol Single-Micron Droplets by Tailing Phenomenon Using a PDMS Microfluidic Device

Microdroplet-based fluidic systems have the advantages of small size, short diffusion time, and no cross-contamination; consequently, droplets often provide a fast and precise reaction environment as well as an analytical environment for individual molecules. In order to handle diverse reactions, we developed a method to create organic single-micron droplets (S-MDs) smaller than 5 μm in diameter dispersed in silicone oil without surfactant. The S-MD generation microflow device consists of a mother droplet (MoD) generator and a tapered separation channel featuring multiple side channels. The tapered channel enhanced the shear forces to form tails from the MoDs, causing them to break up. Surface treatment with the fluoropolymer CYTOP protected PDMS fluid devices from organic fluids. The tailing separation of methanol droplets was accomplished without the use of surfactants. The generation of tiny organic droplets may offer new insights into chemical separation and help study the scaling effects of various chemical reactions.

Comparative investigation of a lattice Boltzmann boundary treatment of multiphase mass transport with heterogeneous chemical reactions

Multiphase reactive transport in porous media is an important component of many natural and engineering processes. In the present study, boundary schemes for the continuum species transport-lattice Boltzmann (CST-LB) mass transport model and the multicomponent pseudopotential model are proposed to simulate heterogeneous chemical reactions in a multiphase system. For the CST-LB model, a lattice-interface-tracking scheme for the heterogeneous chemical reaction boundary is provided. Meanwhile, a local-average virtual density boundary scheme for the multicomponent pseudopotential model is formulated based on the work of Li et al. [Li, Yu, and Luo, Phys. Rev. E 100, 053313 (2019)10.1103/PhysRevE.100.053313]. With these boundary treatments, a numerical implementation is put forward that couples the multiphase fluid flow, interfacial species transport, heterogeneous chemical reactions, and porous matrix structural evolution. A series of comparison benchmark cases are investigated to evaluate the numerical performance for different pseudopotential wetting boundary treatments, and an application case of multiphase dissolution in porous media is conducted to validate the present models' ability to solve complex problems. By applying the present LB models with reasonable boundary treatments, multiphase reactive transport in various natural or engineering scenarios can be simulated accurately.

Lattice Boltzmann modeling of interfacial mass transfer in a multiphase system

In the present study, a numerical model based on the lattice Boltzmann method (LBM) is proposed to simulate multiphase mass transfer, referred to as the CST-LB model. This model introduced continuum species transfer (CST) formulation by an additional collision term to model the mass transfer across the multiphase interface. The boundary condition treatment of this model is also discussed. In order to verify the applicability, the CST-LB model is combined with the pseudopotential multiphase model to simulate a series of benchmark cases, including concentration jump near the interface, gas dissolution in a closed system, species transport during drainage in a capillary tube, and multiphase species transport in the porous media. This CST-LB model can also be coupled with other multiphase LBMs since the model depends on the phase fraction field, which is not explicitly limited to specified multiphase models.

Secondary Flows, Mixing, and Chemical Reaction Analysis of Droplet-Based Flow inside Serpentine Microchannels with Different Cross Sections

Chemical bioreactions are an important aspect of many recent microfluidic devices, and their applications in biomedical science have been growing worldwide. Droplet-based microreactors are among the attractive types of unit operations, which utilize droplets for enhancement in both mixing and chemical reactions. In the present study, a finite-volume-method (FVM) numerical investigation is conducted based on the volume-of-fluid (VOF) applying for the droplet-based flows. This multiphase computational modeling is used for the study of the chemical reaction and mixing phenomenon inside a serpentine microchannel and explores the effects of the aspect ratio (i.e., AR = height/width) of rectangular cross-sectional geometries as well as three other cross-sectional geometries including trapezoidal, triangular, and circular, on consumption and production rates of chemical species. It is found that in these droplet bioreactors, the reaction begins from the forward section of the droplet. We investigate the secondary flows and chemical reactions inside the droplets in a serpentine microchannel with different cross-sectional geometries. Different transient Dean vortices and secondary flows in the presence and absence of the droplets are studied and explained based on the position of the droplets. It is found that as the droplets pass through the microchannel turns, the patterns and magnitude of the secondary flows change, depending on the cross-sectional geometry. Eventually, the results demonstrate that the AR = 2 rectangular cross-section is the most helpful geometry, whereas the trapezoidal cross-section takes into account the least efficient one between all geometries.

Continuous biphasic chemical processes in a four-phase segmented flow reactor

A four-phase segmented flow regime for continuous biphasic reaction processes is introduced, characterized over 1500 automatically conducted experiments, and used for biphasic ligand exchange of CdSe quantum dots.

Tuning the Splitting Behavior of Droplet in a Bifurcating Channel through Wettability-Capillarity Interaction

We present a comprehensive computational physics-based study of the influence of surface wettability on the displacement behavior of a droplet in a three-dimensional bifurcating channel. Various surface wettability configurations for the daughter branches are considered to gain insight into the wettability-capillarity interaction. Also, the influence of initial droplet size on the splitting dynamics for different wettability configurations is investigated. Time evolution of the droplet displacement behavior in the bifurcating channel is discussed for different physicochemical parameters including capillary number and wettability. Three distinct flow regimes are identified as the droplet interacts with the bifurcating tip of the channel, namely, splitting, nonsplitting, and oscillating regimes. Furthermore, the occurrence of Rayleigh-Plateau instability in different wettability scenarios is discussed. Additionally, the intricacies associated with the droplet dynamics are elucidated through the temporal evolution of the droplet surface area and mass outflow of the continuous phase. A flow regime map based on the capillary number and wettability contrast of the daughter branches is proposed for a comprehensive description of the droplet dynamics.

Mixing Efficiency Analysis on Droplet Formation Process in Microchannels by Numerical Methods

Liquid–liquid two-phase flow in microchannels has attracted much attention, due to the superiority of mass transfer enhancement. One of the biggest unresolved challenges is the low mixing efficiency at the microscale. Suitable mixing efficiency is important to promote the mass transfer of two-phase flow in microchannels. In this paper, the mixing efficiency in three junction configurations, including a cross-shaped junction, a cross-shaped T-junction, and a T-junction, is investigated by the volume of fluid (VOF) method coupled with user-defined scalar (UDS) model. All three junction configurations are designed with the same hydraulic diameter of 100 μm. Mixing components are distributed in the front and back parts of the droplet. The mixing efficiency in the droplet forming stage and the droplet moving stage are compared quantitatively. Results show that different junction configurations create very different mixing efficiencies, and the cross-shaped T-junction performs best, with relatively lower disperse phase fractions. However, with an increase of the disperse phase fraction, the cross-shaped junction is superior.

Continuous- versus Segmented-Flow Microfluidic Synthesis in Materials Science

Materials science is a fast-evolving area that aims to uncover functional materials with ever more sophisticated properties and functions. For this to happen, new methodologies for materials synthesis, optimization, and preparation are desired. In this context, microfluidic technologies have emerged as a key enabling tool for a low-cost and fast prototyping of materials. Their ability to screen multiple reaction conditions rapidly with a small amount of reagent, together with their unique physico-chemical characteristics, have made microfluidic devices a cornerstone technology in this research field. Among the different microfluidic approaches to materials synthesis, the main contenders can be classified in two categories: continuous-flow and segmented-flow microfluidic devices. These two families of devices present very distinct characteristics, but they are often pooled together in general discussions about the field with seemingly little awareness of the major divide between them. In this perspective, we outline the parallel evolution of those two sub-fields by highlighting the key differences between both approaches, via a discussion of their main achievements. We show how continuous-flow microfluidic approaches, mimicking nature, provide very finely-tuned chemical gradients that yield highly-controlled reaction–diffusion (RD) areas, while segmented-flow microfluidic systems provide, on the contrary, very fast homogenization methods, and therefore well-defined super-saturation regimes inside arrays of micro-droplets that can be manipulated and controlled at the milliseconds scale. Those two classes of microfluidic reactors thus provide unique and complementary advantages over classical batch synthesis, with a drive towards the rational synthesis of out-of-equilibrium states for the former, and the preparation of high-quality and complex nanoparticles with narrow size distributions for the latter.

Passive Mixing inside Microdroplets

Droplet-based micromixers are essential units in many microfluidic devices for widespread applications, such as diagnostics and synthesis. The mixers can be either passive or active. When compared to active methods, the passive mixer is widely used because it does not require extra energy input apart from the pump drive. In recent years, several passive droplet-based mixers were developed, where mixing was characterized by both experiments and simulation. A unified physical understanding of both experimental processes and simulation models is beneficial for effectively developing new and efficient mixing techniques. This review covers the state-of-the-art passive droplet-based micromixers in microfluidics, which mainly focuses on three aspects: (1) Mixing parameters and analysis method; (2) Typical mixing element designs and the mixing characters in experiments; and, (3) Comprehensive introduction of numerical models used in microfluidic flow and diffusion.