A Versatile Flow-Profile Engineering Method in the Stokes Flow Regime for Complex-Shaped Flows

Flow profiles are frequently engineered in microfluidic channels for enhanced mixing,reaction control,and material synthesis.Conventionally,flow profiles are engineered by inducing inertial secondary flow to redistribute the streams,which can hardly be reproduced in microfluidic environments with negligible inertial flow.The employed symmetric channel structures also limit the variety of achievable flow profiles.Moreover,each of the flow profiles specifically corresponds to a strictly defined flow condition and cannot be generalized to other flow environments.To address these issues,we present a systematic method to engineer the flow profile using inertialess secondary flow.The flow is manipulated in the Stokes regime by deploying a cascaded series of microsteps with various morphologies inside a microchannel to shape the flow profile.By tuning the shapes of the microsteps,arbitrary outflow profiles can be customized.A numerical profile-transformation program is developed for rapid prediction of the output profiles of arbitrary sequences of predefined microsteps.The proposed method allows the engineering of stable flow profiles,including asymmetric ones,over a wide range of flow conditions for complex microfluidic environmental prediction and design.

Self-synchronization of reinjected droplets for highefficiency droplet pairing and merging

Droplet merging serves as a powerful tool to add reagents to moving droplets for biological and chemical reactions.However,unsynchronized droplet pairing impedes high-effciency merging.Here,we develop a microfluidic design for the self-synchronization of reinjected droplets.A periodic increase in the hydrodynamic resistance caused by droplet blocking a T-junction enables automatic pairing of droplets.After inducing spacing,the paired droplets merge downstream under an electric field.The blockage-based design can achieve a 100%synchronization efficiency even when the mismatch rate of droplet frequencies reaches 10%.Over 98%of the droplets can still be synchronized at nonuniform droplet sizes and fluctuating reinjection flow rates.Moreover,the droplet pairing ratio can be adjusted flexibly for on-demand sample addition.Using this system,we merge two groups of droplets encapsulating enzyme/substrate,demonstrating its capacity to conduct multi-step reactions.We also combine droplet sorting and merging to coencapsulate single cells and single beads,providing a basis for high-efficiency single-cell sequencing.We expect that this system can be integrated with other droplet manipulation systems for a broad range of chemical and biological applications.

Oil-mediated high-throughput generation and sorting of water-in-water droplets

Aqueous two-phase system(ATPS)droplets have demonstrated superior compatibility over conventional water-in-oil droplets for various biological assays.However,the ultralow interfacial tension hampers efficient and stable droplet generation,limiting further development and more extensive use of such approaches.Here,we present a simple strategy to employ oil as a transient medium for ATPS droplet generation.Two methods based on passive flow focusing and active pico-injection are demonstrated to generate water-water-oil double emulsions,achieving a high generation frequency of~2.4 kHz.Through evaporation of the oil to break the double emulsions,the aqueous core can be released to form uniform-sized water-in-water droplets.Moreover,this technique can be used to fabricate aqueous microgels,and the introduction of the oil medium enables integration of droplet sorting to produce single-cell-laden hydrogels with a harvest rate of over 90%.We believe that the demonstrated high-throughput generation and sorting of ATPS droplets represent an important tool to advance droplet-based tissue engineering and single-cell analyses.

Gas marbles: ultra-long-lasting and ultra-robust bubbles formed by particle stabilization

Bubbles and foams are ubiquitous in daily life and industrial processes.Studying their dynamic behaviors is of key importance for foam manufacturing processes in food packaging,cosmetics and pharmaceuticals.Bare bubbles are inherently fragile and transient;enhancing their robustness and shelf lives is an ongoing challenge.Their rupture can be attributed to liquid evaporation,thin film drainage and the nuclei of environmental dust.Inspired by particle-stabilized interfaces in Pickering emulsions,armored bubbles and liquid marble,bubbles are protected by an enclosed particle-entrapping liquid thin film,and the resultant soft object is termed gas marble.The gas marble exhibits mechanical strength orders of magnitude higher than that of soap bubbles when subjected to overpressure and underpressure,owing to the compact particle monolayer straddling the surface liquid film.By using a water-absorbent glycerol solution,the resulting gas marble can persist for 465 d in normal atmospheric settings.This particle-stabilizing approach not only has practical implications for foam manufacturing processes but also can inspire the new design and fabrication of functional biomaterials and biomedicines.

Alginate-gelatin emulsion droplets for encapsulation of vitamin A by 3D printed microfluidics

Microfluidics is characterized by the manipulation of fluids in submillimeter channels and has great application potential in encapsulation.To further extend the application of microfluidics in food industries,a 3D printed microfluidic device is used to encapsulate vitamin A and improve its stability.Two natural macromolecules,sodium alginate and gelatin,are added to water as the continuous phase to generate monodisperse emulsion.Under different flow rate ratios,the diameter of droplets decreases with the increase of continuous flow rate.However,at the same flow rate ratio,varying the dispersed and continuous flow rates does not significantly change the diameter and size distribution of emulsion collected.The prepared O/W(oil/water)single emulsion can form microgel particles and avoid degradation of vitamin A by simulated gastric acid;the encapsulated vitamin A will not be released until particles reach simulated intestinal tract.In the simulated digestion in vitro,no vitamin A is released for 2 h in the acidic environment;under an alkaline or neutral environment such as those in intestinal fluids,vitamin A can be released from the microgel particles within 2.5 h.Using the presented approach,emulsions encapsulating vitamin A have been prepared and can potentially be applied to encapsulate other oil-soluble substances in the food industry.

Ultrafast laser-scanning time-stretch imaging at visible wavelengths

Optical time-stretch imaging enables the continuous capture of non-repetitive events in real time at a line-scan rate of tens of MHz—a distinct advantage for the ultrafast dynamics monitoring and high-throughput screening that are widely needed in biological microscopy.However,its potential is limited by the technical challenge of achieving significant pulse stretching(that is,high temporal dispersion)and low optical loss,which are the critical factors influencing imaging quality,in the visible spectrum demanded in many of these applications.We present a new pulse-stretching technique,termed free-space angular-chirpenhanced delay(FACED),with three distinguishing features absent in the prevailing dispersive-fiber-based implementations:(1)it generates substantial,reconfigurable temporal dispersion in free space(41 ns nm^(−1))with low intrinsic loss(o6 dB)at visible wavelengths;(2)its wavelength-invariant pulse-stretching operation introduces a new paradigm in time-stretch imaging,which can now be implemented both with and without spectral encoding;and(3)pulse stretching in FACED inherently provides an ultrafast all-optical laser-beam scanning mechanism at a line-scan rate of tens of MHz.Using FACED,we demonstrate not only ultrafast laser-scanning time-stretch imaging with superior bright-field image quality compared with previous work but also,for the first time,MHz fluorescence and colorized time-stretch microscopy.Our results show that this technique could enable a wider scope of applications in high-speed and high-throughput biological microscopy that were once out of reach.