Scott S. H. Tsai

Dripping and jetting in microfluidic multiphase flows applied to particle and fiber synthesis

Dripping and jetting regimes in microfluidic multiphase flows have been investigated extensively, and this review summarizes the main observations and physical understandings in this field to date for three common device geometries: coaxial, flow-focusing and T-junction. The format of the presentation allows for simple and direct comparison of the different conditions for drop and jet formation, as well as the relative ease and utility of forming either drops or jets among the three geometries. The emphasis is on the use of drops and jets as templates for microparticle and microfiber syntheses, and a description is given of the more common methods of solidification and strategies for achieving complex multicomponent microparticles and microfibers.

Inclined to splash: triggering and inhibiting a splash with tangential velocity

When a liquid drop impacts a smooth, solid, dry surface, the drop forms a radially spreading lamella, which can lead to a splash. Previous studies have focused almost exclusively on impacts perpendicular to a surface; yet it is common for drops to impact on angled or moving surfaces. The asymmetry of such impacts leads to an azimuthal variation of the ejected rim, and under certain conditions only part of the rim breaks up to form droplets. We show that the tangential component of impact can act to enhance or suppress a splash. We develop a new model to predict when this type of splashing will occur. The model accounts for our observations of the effects of tangential velocity and agrees well with previous experimental data.

One‐Step Two‐Dimensional Microfluidics‐Based Synthesis of Three‐Dimensional Particles

Synthesis of three-dimensional anisotropic microparticles using a simple one-step microfluidic-based method is described. The method exploits the nonuniformity of the polymerizing UV light, UV absorption by opaque nanoparticles in the precursor solution, and discontinuous photomask patterns to make magnetic and non-magnetic microparticles in a twodimensional microchannel. Numerical simulations of monomer conversion in the microfluidic channel are performed to predict the manufactured particle shape.

Conformal coating of particles in microchannels by magnetic forcing

We present a co-flow microfluidic method to coat paramagnetic beads with a thin layer of fluid as the beads are pulled across a liquid-liquid interface by an external magnetic field. We show that the coating thickness can be controlled by the magnitude of the flow speed. Also, the number of beads aggregated within a single coating can be adjusted by varying the strength of the magnetic field or the liquid-liquid interfacial tension.

Water-in-Water Droplets by Passive Microfluidic Flow Focusing.

We present a simple microfluidic system that generates water-in-water, aqueous two phase system (ATPS) droplets, by passive flow focusing. ATPS droplet formation is achieved by applying weak hydrostatic pressures, with liquid-filled pipette tips as fluid columns at the inlets, to introduce low speed flows to the flow focusing junction. To control the size of the droplets, we systematically vary the interfacial tension and viscosity of the ATPS fluids and adjust the fluid column height at the fluid inlets. The size of the droplets scales with a power law of the ratio of viscous stresses in the two ATPS phases. Overall, we find a drop size coefficient of variation (CV; i.e., polydispersity) of about 10%. We also find that when drops form very close to the flow focusing junction, the drops have a CV of less than 1%. Our droplet generation method is easily scalable: we demonstrate a parallel system that generates droplets simultaneously and improves the droplet production rate by up to one order of magnitude. Finally, we show the potential application of our system for encapsulating cells in water-in-water emulsions by encapsulating microparticles and cells. To the best of our knowledge, our microfluidic technique is the first that forms low interfacial tension ATPS droplets without applying external perturbations. We anticipate that this simple approach will find utility in drug and cell delivery applications because of the all-biocompatible nature of the water-in-water ATPS environment.

Detection of trace arsenic in drinking water: challenges and opportunities for microfluidics

Arsenic contamination of drinking water is a major global problem, with contamination in Bangladesh deemed most serious. Although the current World Health Organisation (WHO) maximum contamination limit (MCL) for arsenic in drinking water is 10 μg L−1, due to practical and economic constraints, the standard limit in Bangladesh and many other developing nations is 50 μg L−1. We propose that an ideal arsenic sensor, designed for routine monitoring, will have five essential qualities: sensitivity and selectivity for arsenic; speed and reliability; portability and robustness; reduced health and environmental risks; and affordability and ease of use for local technicians. It is our opinion that many of these characteristics can be accentuated by microfluidic systems. We describe candidate colorimetric, electrochemical, biological, electrophoretic, surface-sensing, and spectroscopic methods for arsenic detection; and comment on their potential for portable microfluidic adaptation. We also describe existing developments in the literature towards the ultimate creation of microfluidic total analysis systems (μTASs) for arsenic detection. The fundamental purpose of this review is to highlight the need for better portable arsenic contamination detection, and describe how microfluidic technology may be developed to address this need.

Microfluidic conformal coating of non-spherical magnetic particles

We present the conformal coating of non-spherical magnetic particles in a co-laminar flow microfluidic system. Whereas in the previous reports spherical particles had been coated with thin films that formed spheres around the particles; in this article, we show the coating of non-spherical particles with coating layers that are approximately uniform in thickness. The novelty of our work is that while liquid-liquid interfacial tension tends to minimize the surface area of interfaces-for example, to form spherical droplets that encapsulate spherical particles-in our experiments, the thin film that coats non-spherical particles has a non-minimal interfacial area. We first make bullet-shaped magnetic microparticles using a stop-flow lithography method that was previously demonstrated. We then suspend the bullet-shaped microparticles in an aqueous solution and flow the particle suspension with a co-flow of a non-aqueous mixture. A magnetic field gradient from a permanent magnet pulls the microparticles in the transverse direction to the fluid flow, until the particles reach the interface between the immiscible fluids. We observe that upon crossing the oil-water interface, the microparticles become coated by a thin film of the aqueous fluid. When we increase the two-fluid interfacial tension by reducing surfactant concentration, we observe that the particles become trapped at the interface, and we use this observation to extract an approximate magnetic susceptibility of the manufactured non-spherical microparticles. Finally, using fluorescence imaging, we confirm the uniformity of the thin film coating along the entire curved surface of the bullet-shaped particles. To the best of our knowledge, this is the first demonstration of conformal coating of non-spherical particles using microfluidics.

Electric field induced sheeting and breakup of dielectric liquid jets

We report experimental observations of the controlled deformation of a dielectric liquid jet subjected to a local high-voltage electrostatic field in the direction normal to the jet. The jet deforms to the shape of an elliptic cylinder upon application of a normal electrostatic field. As the applied electric field strength is increased, the elliptic cylindrical jet deforms permanently into a flat sheet, and eventually breaks-up into droplets. We interpret this observation—the stretch of the jet is in the normal direction to the applied electric field—qualitatively using the Taylor-Melcher leaky dielectric theory, and develop a simple scaling model that predicts the critical electric field strength for the jet-to-sheet transition. Our model shows a good agreement with experimental results, and has a form that is consistent with the classical drop deformation criterion in the Taylor-Melcher theory. Finally, we statistically analyze the resultant droplets from sheet breakup, and find that increasing the appl...

Classification of biological cells using a sound wave based flow cytometer

A flow cytometer that uses sound waves to determine the size of biological cells is presented. In this system, a microfluidic device made of polydimethylsiloxane (PDMS) was developed to hydrodynamically flow focus cells in a single file through a target area. Integrated into the microfluidic device was an ultrasound transducer with a 375 MHz center frequency, aligned opposite the transducer was a pulsed 532 nm laser focused into the device by a 10x objective. Each passing cell was insonfied with a high frequency ultrasound pulse, and irradiated with the laser. The resulting ultrasound and photoacoustic waves from each cell were analyzed using signal processing methods, where features in the power spectra were compared to theoretical models to calculate the cell size. Two cell lines with different size distributions were used to test the system: acute myeloid leukemia cells (AML) and melanoma cells. Over 200 cells were measured using this system. The average calculated diameter of the AML cells was 10.4 ± 2.5 μm using ultrasound, and 11.4 ± 2.3 μm using photoacoustics. The average diameter of the melanoma cells was 16.2 ± 2.9 μm using ultrasound, and 18.9 ± 3.5 μm using photoacoustics. The cell sizes calculated using ultrasound and photoacoustic methods agreed with measurements using a Coulter Counter, where the AML cells were 9.8 ± 1.8 μm and the melanoma cells were 16.0 ± 2.5 μm. These results demonstrate a high speed method of assessing cell size using sound waves, which is an alternative method to traditional flow cytometry techniques.

Floating and sinking of self-assembled spheres on liquid-liquid interfaces: Rafts versus stacks

The floating and sinking of objects on fluid-fluid interfaces occurs in nature and has many important implications in technology. Here, we study the stability of floating self-assembled spheres on an oil-water interface, and how the sphere deposition geometry affects the size limits of the assemblies before they collapse and sink through the interface. Specifically, we compare the critical size of particle rafts to particle stacks. We show that, on liquid-liquid interfaces, monolayer rafts and stacked spheres exhibit different scaling of the critical number of spheres to the Bond number—the dimensionless ratio of buoyancy to interfacial tension effects. Our results indicate that particle stacks will sink with a lower threshold number of particles than particle rafts. This finding may have important implications to engineering applications where interfacial assemblies are not monolayers.