Session T30: Micro/Nano scale Flows: Droplets and Sorting

Predicting the trajectories of particles in micro-centrifuge devices

In microfluidic devices, micrometer-scale channels and cavities are arranged to capture and sort particles (e.g. separating red blood cells from circulating cancer cells). Rational design of devices requires understanding how these parameters of shape and size determine particle trajectories. But, images of particle trajectories through these devices capture only the 2-dimensional dynamics of the system. Our goal is to be able to quickly and accurately reconstruct the full 3-dimensional trajectories. We present a data assimilation (Ensemble Transform Kalman Filter) study which incorporates a model of the background fluid flow in the device with very precise, partial measurements of the particle position. The framework is tested with observing system simulation experiments that use synthetic data from a hybrid asymptotic-numerical model where we account for the particle disturbance. The set up for this problem is simple as it requires the background flow of a device, the particle size, and partial measurements of the trajectory. The motivation for this project is to provide simpler ways to predict the trajectory of particles in future microfluidic devices.   

Microfluidic droplet flow regimes of ternary mixtures with solvent at ultralow interfacial tension

We experimentally investigate droplet pattern formation in coaxial microchannels using ternary mixtures of two immiscible fluids and a miscible solvent. The role of solvent concentration is examined through analysis of periodic flow patterns and scaling relationships are developed to calculate a wide range of interfacial tensions at short time-scales. In particular, low-viscosity aqueous and ethylic mixtures of isopropanol are injected into a viscous silicone oil using square microcapillaries. The flow maps obtained display dispersed flows, such as dripping and jetting regimes, at low solvent concentration and separated flows, including core-annular flows, at large concentration. A method based on analysis of droplet size and spacing in the stable dripping regime is implemented to measure extremely small values of interfacial tension in ternary fluid systems.   

Liquid-liquid dispersions in milli-scale symmetric confined impinging jets

This paper presents the emulsification process of two immiscible liquids and the flow of the droplets in symmetrical confined impinging jets (CIJs) with two 2mm ID outlets. To investigate the complex phenomena, experiments are combined with computational fluid dynamics (CFD) simulations. Compared to asymmetric CIJs with only one outlet, symmetrical CIJs demonstrate better emulsification performance at high flow rates which makes them preferrable for high-throughput processing. CIJs find many applications such as rapid precipitation, crystallization, extraction of metals, and pharmaceuticals. The flow patterns formed during the impingement of a kerosene oil and an aqueous stream were studied for flowrates ranging from 30-90 ml/min with high speed imaging and ultrasound sensors. Drop sizes were measured immediately after the jet impingement zone and further downstream the channel. The CFD approach implemented a model transition mechanism of volume of fluid (VOF) to discrete phase model (DPM). The CFD successfully captured the mechanism of jet break up and droplet formation at the impinging zone. Good agreement on drop size was found between the experiments and the modelling. Drop velocity profiles were also measured by the ultrasound doppler method. The results showed that drop sizes and drop velocity profiles are significantly influenced by the Reynolds number of the continuous phase.   

Using Droplet Microfluidics to Encapsulate Cancer Spheroids in Hydrogel Systems

Patient-derived tumor organoids have become more relevant models for drug screening, but the limitations of conventional production techniques result in models lacking a degree of reproducibility between batches. Thus, we aim to develop a more robust system for uniform organoid formation, by leveraging droplet microfluidic (DM) technology to maneuver pre-formed cancer spheroids (cell clusters) and individually house them in nanoliter hydrogel droplets, by water-in-oil emulsion. DM chips were fabricated through replica molding using molds made by soft lithography and stereolithography, with either double flow-focusing (DFF) or T-junction geometries. The continuous phase was mineral oil with surfactant. The dispersed phase was gelatin methacrylate and spheroids. Based on criteria previously developed by the team, an optimized design has shown success in generating uniform spheroid-laden droplets without breaking the spheroids apart. Spheroids did not maintain suspension in the inlet reservoirs past 2 min due to their higher density than the buffer solution, decreasing equal distribution into the device. Instead, settled spheroids entered the DM device in groups, but the DFF design aided in alignment and eventually single encapsulation was achieved. These preliminary studies showcase the capabilities of DM for encapsulating single spheroids successfully. Further work is assessing inlet techniques for higher system effectiveness, with mindfulness of spheroid fragility and size.   

Deformability-based separation of microgels using microfluidics

Droplet microfluidics has the potential to increase the throughput of biomolecule screening. However, current on-chip passive droplet sorting methods have throughputs significantly below standard active fluorescence activated droplet sorting techniques. Here, we present an asymmetric microfluidic T-junction to separate picoliter sized microgels based on their deformability. We experimentally characterize the motion and deformation of microscale agarose hydrogels transported at low Re through microchannels. Microgels of different stiffnesses are separated based on their deformation and relaxation after leaving the T-junction. With increasing rigidity, we observe that microgels shift their exit from the right outlet to the top outlet in the device at a constant flow rate. We demonstrate that sorting of microgels can be tuned by altering their viscoelasticity relative to the continuous oil phase. Our results can provide insights to the label-free, high-throughput, passive separation of monodisperse droplets for protein screening and directed evolution of enzymes.   

Effects of fluid rheology on particle-focusing behavior in a spiral microchannel

Passive particle manipulation in non-Newtonian fluids through microchannels has received scientific attention due to its biomedical and chemical applications requiring size–based focusing and separation of microparticles. In Non-Newtonian fluids through curved microchannels, particle-focusing is achieved due to the synergy between inertial lift, elastic lift, and dean drag force. In addition, the complex rheological properties exhibited by non-Newtonian fluids highly influence the focusing behavior. We investigate in this work the effects of fluid rheology on particle-focusing behavior in a spiral microchannel by employing polymer solutions with varying rheological properties. Focusing behavior is examined over a range of Reynolds and Weissenberg numbers at the outlet region of the microchannel. A systematic experimental approach aids in decomposing the fluid rheology effects on particle migration and focusing. This work explores the individual and combined effects of inertia, elasticity, and shear thinning property on particle-focusing behavior.