Ya Gai

Spatiotemporal periodicity of dislocation dynamics in a two-dimensional microfluidic crystal flowing in a tapered channel

Significance Collective interactions in many-body systems can give rise to unexpected order. Such interactions underlie a wide range of complex phenomena such as swarming in animals and traffic patterns. This work reports an unexpected order in the flow of highly confined and highly concentrated water-in-oil drops, which can be explained and modeled as a soft crystal being extruded in the nanoscale. The findings are important to the understanding of out-of-equilibrium many-body systems, and the flow control of these drops used as microreactors in droplet microfluidics. Furthermore, contrary to the unpredictable microscale crystal deformation process, the discoveries here indicate that nanoscale crystal deformation can be highly ordered and predictable, and imply that the manufacturing of nanocrystalline materials may be easier than perceived. When a many-body system is driven away from equilibrium, order can spontaneously emerge in places where disorder might be expected. Here we report an unexpected order in the flow of a concentrated emulsion in a tapered microfluidic channel. The velocity profiles of individual drops in the emulsion show periodic patterns in both space and time. Such periodic patterns appear surprising from both a fluid and a solid mechanics point of view. In particular, when the emulsion is considered as a soft crystal under extrusion, a disordered scenario might be expected based on the stochastic nature of dislocation dynamics in microscopic crystals. However, an orchestrated sequence of dislocation nucleation and migration is observed to give rise to a highly ordered deformation mode. This discovery suggests that nanocrystals can be made to deform more controllably than previously thought. It can also lead to novel flow control and mixing strategies in droplet microfluidics.

Internal flow in droplets within a concentrated emulsion flowing in a microchannel

Droplet microfluidics has enabled a wide variety of high-throughput biotechnical applications through the use of monodisperse micro-droplets as bioreactors. Previous fluid dynamics studies of droplet microfluidics have focused on single droplets or emulsions at low volume fractions. The study of concentrated emulsions at high volume fractions is important for further increasing the throughput of droplet microfluidics, but the fluid dynamics of such emulsions in confined microchannels is not well understood. This paper describes the use of microscopic particle image velocimetry to quantify the flow inside individual droplets within a concentrated emulsion having volume fraction φ ∼ 85% flowing as a monolayer in a straight microfluidic channel. The effects of confinement (namely, the number of rows of droplets across the width of the channel) and viscosity ratio on the internal flow patterns inside the drops at a fixed capillary number of 10−3 and a Reynolds number of 10−2 to 10−1 are studied. The results s...

Microfluidic guillotine for single-cell wound repair studies

Significance Whereas tissues can repair wounds regularly, single cells—including muscle cells, neurons, and cancer cells—are increasingly recognized to have a wound response. Ability to understand and control single-cell repair holds the potential for new strategies to treat human diseases. Existing wounding methods and model organisms are insufficient for elucidating the biological mechanisms involved. This paper reports a microfluidic guillotine for the high-throughput bisection of hundreds of cells reproducibly. The fine time resolution achieved enables mechanistic studies critical for a molecular understanding of single-cell wound repair currently impossible with existing wounding methods. The work here will lay the foundation for understanding how single cells heal themselves, a fundamental feature distinguishing living from nonliving matter. Wound repair is a key feature distinguishing living from nonliving matter. Single cells are increasingly recognized to be capable of healing wounds. The lack of reproducible, high-throughput wounding methods has hindered single-cell wound repair studies. This work describes a microfluidic guillotine for bisecting single Stentor coeruleus cells in a continuous-flow manner. Stentor is used as a model due to its robust repair capacity and the ability to perform gene knockdown in a high-throughput manner. Local cutting dynamics reveals two regimes under which cells are bisected, one at low viscous stress where cells are cut with small membrane ruptures and high viability and one at high viscous stress where cells are cut with extended membrane ruptures and decreased viability. A cutting throughput up to 64 cells per minute—more than 200 times faster than current methods—is achieved. The method allows the generation of more than 100 cells in a synchronized stage of their repair process. This capacity, combined with high-throughput gene knockdown in Stentor, enables time-course mechanistic studies impossible with current wounding methods.

Amphiphilic nanoparticles suppress droplet break-up in a concentrated emulsion flowing through a narrow constriction

This paper describes the break-up behavior of a concentrated emulsion comprising drops stabilized by amphiphilic silica nanoparticles flowing in a tapered microchannel. Such geometry is often used in serial droplet interrogation and sorting processes in droplet microfluidics applications. When exposed to high viscous stresses, drops can undergo break-up and compromise their physical integrity. As these drops are used as micro-reactors, such compromise leads to a loss in the accuracy of droplet-based assays. Here, we show droplet break-up is suppressed by replacing the fluoro-surfactant similar to the one commonly used in current droplet microfluidics applications with amphiphilic nanoparticles as droplet stabilizer. We identify parameters that influence the break-up of these drops and demonstrate that break-up probability increases with increasing capillary number and confinement, decreasing nanoparticle size, and is insensitive to viscosity ratio within the range tested. Practically, our results reveal two key advantages of nanoparticles with direct applications to droplet microfluidics. First, replacing surfactants with nanoparticles suppresses break-up and increases the throughput of the serial interrogation process to 3 times higher than that in surfactant system under similar flow conditions. Second, the insensitivity of break-up to droplet viscosity makes it possible to process samples having different composition and viscosities without having to change the channel and droplet geometry in order to maintain the same degree of break-up and corresponding assay accuracy.

Internal flow inside droplets within a concentrated emulsion during droplet rearrangement

Droplet microfluidics, in which each droplet serves as a micro-reactor, has found widespread use in high-throughput biochemical screening applications. These droplets are often concentrated at various steps to form a concentrated emulsion. As part of a serial interrogation and sorting process, such concentrated emulsions are typically injected into a tapered channel leading to a constriction that fits one drop at a time for the probing of droplet content in a serial manner. The flow physics inside the droplets under these flow conditions are not well understood but are critical for predicting and controlling the mixing of reagents inside the droplets as reactors. Here we investigate the flow field inside droplets of a concentrated emulsion flowing through a tapered microchannel using micro-particle image velocimetry. The confining geometry of the channel forces the number of rows of drops to reduce by one at specific and uniformly spaced streamwise locations, which are referred to as droplet rearrangement...