Droplet Merging Research Articles

Microdroplet Operations in Polymeric Microtubes.

Microsystem technologies allow a plethora of operations to be achieved for microemulsion- and microdroplet-based assays, providing miniaturized, yet large-throughput capabilities to assist experimentation in analytical chemistry, biology, and synthetic biology. Many of such approaches have been implemented on-chip, using microfluidic and lab-on-a-chip technologies. However, the microfabrication of such devices relies on expensive equipment and time-consuming methods, thus hindering their uptake and use by many research laboratories where microfabrication expertise is not available. Here, we demonstrate how fundamental water-in-oil microdroplet operations, such as droplet trapping, merging, diluting, and splitting, can be obtained using straightforward, inexpensive, and manually fabricated polymeric microtube modules. The modules are based on creating an angled tubing interface at the interconnection between two polymeric microtubes. We have characterized how the geometry and fluid dynamic conditions at this interface enabled different droplet operations to be achieved in a versatile and functional manner. We envisage this approach to be an alternative solution to expensive and laborious microfabrication protocols for droplet microfluidic applications.

Shaping electrospray deposition profile by a quadrupole: From circular to elliptical patterns

Thin films deposited by conventional electrospray of volatile solutions often give rise to rough morphology consisting of “coffee rings” formed by individual sessile droplets. This is due to the long average inter-droplet distance and low merging probability of deposited droplets within the sprayed circular area. We present a quadrupole configuration to form a squeezing electric field that compresses the circular electrospray pattern into a much slenderer elliptical shape. The design is validated by tracking the trajectories of all sprayed droplets using the Lagrangian method. Moreover, the statistical study of the deposited droplet merging process and experimental results prove that the quadrupole configuration can dramatically decrease inter-droplet distance and increase the merging probability of deposited droplets. The simple apparatus offers precise controls of the aspect ratio of the pattern and provides a feasible approach to produce smooth thin films without pin holes.

Acoustohydrodynamic tweezers via spatial arrangement of streaming vortices.

Acoustics-based tweezers provide a unique toolset for contactless, label-free, and precise manipulation of bioparticles and bioanalytes. Most acoustic tweezers rely on acoustic radiation forces; however, the accompanying acoustic streaming often generates unpredictable effects due to its nonlinear nature and high sensitivity to the three-dimensional boundary conditions. Here, we demonstrate acoustohydrodynamic tweezers, which generate stable, symmetric pairs of vortices to create hydrodynamic traps for object manipulation. These stable vortices enable predictable control of a flow field, which translates into controlled motion of droplets or particles on the operating surface. We built a programmable droplet-handling platform to demonstrate the basic functions of planar-omnidirectional droplet transport, merging droplets, and in situ mixing via a sequential cascade of biochemical reactions. Our acoustohydrodynamic tweezers enables improved control of acoustic streaming and demonstrates a previously unidentified method for contact-free manipulation of bioanalytes and digitalized liquid handling based on a compact and scalable functional unit.

Light-controlled versatile manipulation of liquid metal droplets: a gateway to future liquid robots.

The controlled actuation of liquid metal (LM) droplets has recently shown great potential in developing smart actuating systems for applications in robotics. However, there is a lack of a simple approach for the precise manipulation of multiple LM droplets in a 2D plane, which hinders the development of complex control over droplets for realizing useful robotic applications. To overcome this challenge, here, a versatile and powerful light-induced manipulation of LM droplets is presented. The key principle is to selectively activate phototransistors in an electrolyte using infrared laser beams to electrically control LM droplets via Marangoni forces. This approach shows the ability of inducing concurrent motion, splitting, and merging of multiple LM droplets simply using light without complex and bulky systems. Parameters affecting the manipulation of LM droplets are thoroughly investigated. Moreover, a vehicle carrier driven by wheels composed of multiple LM droplets for making a light-controlled relay is demonstrated. We believe such a light-induced control method for manipulating LM droplets has the potential for advancing the development of future field-programmable robotics and droplet-based soft collaborative robots.

Minimizing filling time for ultraviolet nanoimprint lithography with templates with multiple structures

Optimizing the locations and sizes of droplets is the key to reducing defects and increasing throughput of ultraviolet nanoimprint lithography. In practice the templates are composed of regions with different structures. The interface between structures will generate complicated fluid flow behavior that will slow the filling time. Here, we explore several strategies through simulations to distribute resist material according to a nonuniform pattern to reduce filling time and ultimately increase throughput. In order to mimic the complexity of a template, the interface between different pairs of template structures is considered and the spreading and merging of droplets are simulated. From these simulations, it is found that the volume and arrangement of droplets underneath strongly affect the imprint time. By distributing the correct amount of resist underneath the template, one can remove the unnecessary fluid transferring step in droplet spreading and reduce the total filling time. Furthermore, by optimally placing the resist droplets, one can delay merging events and accelerate the spreading speed. Finally, the advantage of hexagonal arrangements is explored.

Assembly of Multi-Spheroid Cellular Architectures by Programmable Droplet Merging.

Artificial multicellular systems are gaining importance in the field of tissue engineering and regenerative medicine. Reconstruction of complex tissue architectures in vitro is nevertheless challenging, and methods permitting controllable and high-throughput fabrication of complex multicellular architectures are needed. Here, a facile and high-throughput method is developed based on a tunable droplet-fusion technique, allowing programmed assembly of multiple cell spheroids into complex multicellular architectures. The droplet-fusion technique allows for construction of various multicellular architectures (double-spheroids, multi-spheroids, hetero-spheroids) in a miniaturized high-density array format. As an example of application, the propagation of Wnt signaling is investigated within hetero-spheroids formed from two fused Wnt-releasing and Wnt-reporter cell spheroids. The developed method provides an approach for miniaturized, high-throughput construction of complex 3D multicellular architectures and can be applied for studying various biological processes including cell signaling, cancer invasion, embryogenesis, and neural development.

Influence of droplet velocity, spacing, and inter-arrival time on line formation and saturation in binder jet additive manufacturing

Binder Jetting (BJ) is a low-cost Additive Manufacturing (AM) process that uses inkjet technology to selectively bind particles in a powder bed. BJ relies on the ability to control, not only the placement of binder on the surface but also its imbibition into the powder bed. This is a complex process in which picoliter-sized droplets impact powder beds at velocities of 1–10 m/s. However, the effects of printing parameters such as droplet velocity, size, spacing, and inter-arrival time on saturation level (fraction of pore space filled with binder) and line formation (merging of droplets to form a line) are unknown. Prior attempts to predict saturation levels with simple measurements of droplet primitives and capillary pressure assume that droplet/powder interactions are dominated by static equilibrium and neglect the impact of printing parameters. This study analyzes the influence of these parameters on the effective saturation level and conditions for line formation when printing single lines into powder beds of varied materials (316 stainless steel, 420 stainless steel, and alumina) and varied particle size (d50=10–47 µm). Results show that increasing droplet velocity or droplet spacing decreases effective saturation while droplet spacing, velocity, and inter-arrival time affect line formation. At constant printing velocity, the conditions for successful line printing are shown to be a function of droplet spacing and square root of the droplet inter-arrival time analogous to the Washburn model for infiltration into a porous media. The results have implications to maximizing build rates and improving quality of small features in BJ.

Magnetic field based actuation and amalgamation of ferrofluid droplets on hydrophobic surface: An experimental and numerical study

This article presents a detailed experimental analysis along with numerical simulations to provide the information about shape evolutions and mechanism of actuation and amalgamation of the ferrofluid droplets deposited on a hydrophobic surface by moving a permanent magnet. To validate the numerical methods used in this article, a benchmark phenomenon of a sessile droplet spreading under the effect of a non-uniform magnetic field is first simulated, and the results are compared with available experimental observations. To further ensure the accuracy of experimental and numerical techniques and to understand the wetting properties and spreading behavior of non-stationary ferrofluid droplets, a prototype demonstration of the merging droplets on a hydrophobic solid surface in the presence of a permanent magnet is designed. It is observed that for hydrophobic surfaces, the merging droplets entrap an air bubble at the time of first contact. Moreover, the physics behind the transient variations of droplet morphology and the effect of the state-of-the-art parameters on droplet actuation are also discussed. The force evaluation, energy variations, velocity contours, and velocity vectors of the moving droplet are provided to understand the internal behavior of droplet mobility. Experiments are performed several times with different speeds of moving a magnet to find the critical velocity when the droplet fails to follow the magnet motion. While doing so, we encounter an anomalous phenomenon of thread formation and daughter droplet generation at the receding end of the sliding droplet. A phase diagram is also provided in the end, which describes different regions of the sliding phenomenon.

Multiple Emissions of Splashes upon Drop Impact

For the first time, photo and video detection is carried out for all groups of small droplets (splashes) emitted when a drop of $$0.2 < D < 0.5$$ cm in diameter falls with a velocity of $$1 < U < 4.05$$ m/s into a pool with water. In the splash-formation mode, groups of droplets are ejected in echelons first from the tops of spikes on the veil around the primary contact area, then, from the spikes on the edges of the split shells of the crown, and, finally, from the teeth of its chevron. The first splashes fly out radially, and the next move inward around the cavity and vertically upward. The splash sizes grow with time from 0.1 to 1.0 mm. Their velocities decrease from 20 to 1 m/s, and the intervals between groups increase from 50 μs to 2.5 ms until the beginning of smoothing of the crown teeth ( $$t \approx 17$$ ms). The acceleration of the droplets is facilitated by the conversion of the available potential energy when the free surfaces of the merging droplets are annihilated.

Optoelectronic Manipulation, Trapping, Splitting, and Merging of Water Droplets and Aqueous Biodroplets Based on the Bulk Photovoltaic Effect

Optical and optoelectronic techniques for micro- and nano-object manipulation are becoming essential tools in nano- and bio-technology. A remarkable optoelectronic technique that has experimented a strong development in the last few years is the so called photovoltaic optoelectronic tweezers. It is based on the light-induced electric fields generated by the bulk photovoltaic effect in certain ferroelectrics such as LiNbO3. The technique is simple and versatile, enabling a successful manipulation of a large variety of micro- and nano-objects with only optical control, without the need of electrodes or power supplies. However, it is still a challenge for this tool, to handle objects immersed in aqueous solution due to the electric screening effects of polar liquids. This has hindered their application in biotechnology and biomedicine where most processes develop in aqueous solution. In this work, a new efficient route to overcome this problem has been proposed and demonstrated. It uses photovoltaic optoelectronic tweezers to manipulate aqueous droplets, immersed in a non-polar oil liquid, but hanging at the interface air-oil. In this singular configuration, the high electric fields generated in the photovoltaic substrate allow a simple and flexible manipulation of aqueous droplets controlled by the light. Droplet guiding, trapping, merging and splitting have been achieved and efficient operation with water and a variety of bio-droplets (DNA, sperm, and PBS solutions) have been demonstrated. The reported results overcome a main limitation of these tweezers to handle bio-materials and promises a high potential for biotechnological and biochemistry applications including their implementation in optofluidic devices.

Abstract 133: Micro-environment on demand (MOD): A versatile technology for cognate T-cell receptor - neoantigen discovery

Abstract NCI estimates that cancer will be the leading cause of death in 2030, worldwide. Checkpoint inhibitors and adoptive cell therapies (ACTs) cost up to ~$2 million/patient and have shown durable responses in a few patients while many patients experience life-threatening side-effects. So, there is a critical need to identify cost-effective therapies to treat cancer with minimal to no side-effects. Deep mapping of cancer antigens and their cognate immune cells is one approach that has the potential for a more universal/targeted treatment. However, systematic methods do not exist to identify cancer antigens and their cognate immune cells functionally and link phenotypic readouts to antigen and immune cell DNA sequences. Here we present a single cell handling and analysis platform based on Printed Droplet Microfluidics, a new droplet microfluidic technology that enables the precise construction of cell-cell interaction experiments with single cell resolution, followed by functional and genomic analysis. At the core of the technology is a fluorescence-activated flow sorter repurposed to deterministically sort and merge droplets containing single cells, beads, reagents, and drugs, collect and assay merged droplets and resort assay-positive droplets for downstream genomic analyses. The selectivity of the sorting/merging technology enables both the construction of experiments with up to one million precise cell combinations and the performance of flexible multi-step molecular biology workflows. We will show proof-of-concept for the MOD platform that integrates three technologies after fluorescently labeled single-cells are encapsulated in droplets - droplet-based cell-sorting and merging, single-cell-based cytokine assays and selective sequencing of T-cell receptor variable regions based on cytokine signals. The technology has the potential to rapidly accelerate the rate of immunotherapy discovery by enabling a larger number of experiments to be performed with higher precision, more modes of analysis, and less starting material than competing bulk technologies. Citation Format: Maithreyan Srinivasan, Justin Madrigal, Nathan Schoepp, Russell Cole. Micro-environment on demand (MOD): A versatile technology for cognate T-cell receptor - neoantigen discovery [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 133.

On Droplet Coalescence in Quasi-Two-Dimensional Fluids.

Coalescence of droplets plays a crucial role in nature and modern technology. Various experimental and theoretical studies explored droplet dynamics in three-dimensional (3D) and on 2D solid or liquid substrates. In this paper, we demonstrate the complete coalescence of isotropic droplets in thin quasi-2D liquids-overheated smectic films. We observe the merging of micrometer-sized flat droplets using high-speed imaging and analyze the shape transformations of the droplets on the timescale of milliseconds. Our studies reveal the scaling laws of the coalescence time, which exhibits a different dependence on the droplet geometry from that in the case of droplets on a solid substrate. A theoretical model is proposed to explain the difference in behavior.

Collector Droplet Behavior during Formation of Nanowire Junctions.

Formation of nanowire networks is an appealing strategy for demonstrating novel phenomena at the nanoscale, e.g., detection of Majorana Fermions, as well as an essential step in realizing complex nanowire-based architectures. However, a detailed description of mechanisms taking place during growth of such complex structures is lacking. Here, the experimental observations of gold-catalyzed germanium nanowire junction formation are explained utilizing phase field modeling corroborated with real-time in situ scanning electron microscopy. When the two nanowires collide head on during the growth, we observe two scenarios. (i) Two catalytic droplets merge into one, and the growth continues as a single nanowire. (ii) The droplets merge and subsequently split again, giving rise to the growth of two daughter nanowires. Both the experiments and modeling indicate the critical importance of the liquid-solid growth interface anisotropy and the growth kinetics in facilitating the structural transition during the nanowire merging process.

Programmable Digital Liquid Metal Droplets in Reconfigurable Magnetic Fields.

Gallium-based liquid metals exhibit excellent locomotion and deformation capabilities under external stimuli and has potential in developing intelligent robots. Programing the locomotion and morphology of the Liquid metal (LM) to endow it with functionalities and intelligence as robots is charming but remains challenging. In this study, we develop a programmable digital LM (PDLM) control platform that can realize versatile locomotion and morphological manipulation of magnetic LM (MLM) droplets using arrays of electromagnets. We demonstrate on-demand transportation, deformation, breakup, and merging of multiple MLM droplets simultaneously and precisely. We find that the intriguing behaviors of MLM under a magnetic field are due to the interplay of surface tension and magnetic forces. Furthermore, we present a functional cooperative droplet robot by equipping the MLM droplets with three-dimensionally printed microtool modules. We show that both the position and orientation of a rod-shaped object can be precisely manipulated by the cooperation of the MLM droplet robots. More interestingly, we explore the capability of the MLM droplet robots for cooperatively handling a copper wire to connect and disconnect electronic circuits. Finally, we demonstrate that the PDLM control platform is capable of programing a group of MLM droplets to accomplish a digital display task. We believe that the PDLM control system presents a promising potential in developing LM-based reconfigurable circuits, digital display systems, and biomimetic soft robotic systems with high controllability, multifunctionalities, and intelligence.

Programmable µChopper Device with On-Chip Droplet Mergers for Continuous Assay Calibration.

While droplet-based microfluidics is a powerful technique with transformative applications, most devices are passively operated and thus have limited real-time control over droplet contents. In this report, an automated droplet-based microfluidic device with pneumatic pumps and salt water electrodes was developed to generate and coalesce up to six aqueous-in-oil droplets (2.77 nL each). Custom control software combined six droplets drawn from any of four inlet reservoirs. Using our μChopper method for lock-in fluorescence detection, we first accomplished continuous linear calibration and quantified an unknown sample. Analyte-independent signal drifts and even an abrupt decrease in excitation light intensity were corrected in real-time. The system was then validated with homogeneous insulin immunoassays that showed a nonlinear response. On-chip droplet merging with antibody-oligonucleotide (Ab-oligo) probes, insulin standards, and buffer permitted the real-time calibration and correction of large signal drifts. Full calibrations (LODconc = 2 ng mL−1 = 300 pM; LODamt = 5 amol) required <1 min with merely 13.85 nL of Ab-oligo reagents, giving cost-savings 160-fold over the standard well-plate format while also automating the workflow. This proof-of-concept device—effectively a microfluidic digital-to-analog converter—is readily scalable to more droplets, and it is well-suited for the real-time automation of bioassays that call for expensive reagents.

Controlled Chemistry via Contactless Manipulation and Merging of Droplets in an Acoustic Levitator.

A unique, versatile, and material-independent approach to manipulate contactlessly and merge two chemically distinct droplets suspended in an acoustic levitator is reported. Large-amplitude axial oscillations are induced in the top droplet by low-frequency amplitude modulation of the ultrasonic carrier wave, which causes the top sample to merge with the sample in the pressure minimum below. The levitator is enclosed within a pressure-compatible process chamber to enable control of the environmental conditions. The merging technique permits precise control of the substances affecting the chemical reactions, the sample temperature, the volumes of the liquid reactants down to the picoliter range, and the mixing locations in space and time. The performance of this approach is demonstrated by merging droplets of water (H2O) and ethanol (C2H5OH), conducting an acid-base reaction between aqueous droplets of sodium hydroxycarbonate (NaHCO3) and acetic acid (CH3COOH), the hypergolic explosion produced via merging a droplet of an ionic liquid with nitric acid (HNO3), and the coalescence of a solid particle (CuSO4·5H2O) and a water droplet followed by dehydration using a carbon dioxide laser. The physical and chemical changes produced by the merging are traced in real time via complementary Raman, Fourier-transform infrared, and ultraviolet-visible spectroscopies. The concept of the contactless manipulation of liquid droplets and solid particles may fundamentally change how scientists control and study chemical reactions relevant to, for example, combustion systems, material sciences, medicinal chemistry, planetary sciences, and biochemistry.

Individual Control and Quantification of 3D Spheroids in a High-Density Microfluidic Droplet Array

SummaryAs three-dimensional cell culture formats gain in popularity, there emerges a need for tools that produce vast amounts of data on individual cells within the spheroids or organoids. Here, we present a microfluidic platform that provides access to such data by parallelizing the manipulation of individual spheroids within anchored droplets. Different conditions can be applied in a single device by triggering the merging of new droplets with the spheroid-containing drops. This allows cell-cell interactions to be initiated for building microtissues, studying stem cells’ self-organization, or observing antagonistic interactions. It also allows the spheroids’ physical or chemical environment to be modulated, as we show by applying a drug over a large range of concentrations in a single parallelized experiment. This convergence of microfluidics and image acquisition leads to a data-driven approach that allows the heterogeneity of 3D culture behavior to be addressed across the scales, bridging single-cell measurements with population measurements.

Microdroplet event recognition and volume detection system based on flexible printed circuit electrode

Changes of microdroplet volume and the occurrence of different droplet events will exert a significant influence on droplet-based microfluidics. Droplet volume is closely related to reagent consumption and hydrodynamic resistance while droplet events reflect actual microchannel conditions. Based on current research, this work was dedicated to fabricate a reliable microdroplet sensing system with event recognition and volume detection function. Signal acquisition was on the basis of capacitance coupled detection and the coplanar fork sensing electrode was fabricated by flexible printed circuit(FPC) which was tens of times cheaper than photolithography. Wave peaks will appear while ink droplets traverse over the fork electrode from above. Shape features of waveform were used to recognize some simple droplet events and the period was employed to calculate droplet volume. Through experimental verification, droplet generation and droplet merging events could be recognized and distinguished by the system. In addition, the detection error between the theoretical volume calculated from formula and the actual volume derived from image analysis was less than 0.5 nL. The sensing system possesses the potential to achieve high throughput detection and surely can be employed to recognize more complicated droplet events in the future.

Microfluidic Study of the Electrocoalescence of Aqueous Droplets in Crude Oil.

In electrocoalescence, an electric field is applied to a dispersion of conducting water droplets in a poorly conducting oil to force the droplets to merge in the direction of the field. Electrocoalescence is used in petroleum refining to separate water from crude oil and in droplet-based microfluidics to combine droplets of water in oil and to break emulsions. Using a microfluidic design to generate a two-dimensional (2D) emulsion, we demonstrate that electrocoalescence in an opaque crude oil can be visualized with optical microscopy and studied on an individual droplet basis in a chamber whose height is small enough to make the dispersions two dimensional and transparent. From reconstructions of images of the 2D electrocoalescence, the electrostatic forces driving the droplet merging are calculated in a numerically exact manner and used to predict observed coalescence events. Hence, the direct simulation of the electrocoalescence-driven breakdown of 2D emulsions in microfluidic devices can be envisioned.

Interfacial dynamics with soluble surfactants: A phase-field two-phase flow model with variable densities

In this work, we present a hydrodynamics coupled phase-field surfactant model with variable densities. Two scalar auxiliary variables are introduced to transform the original free energy functional into an equivalent form, and then a new thermodynamically consistent model can be obtained. In this model, evolutions of two phase-field variables are described by two Cahn-Hilliard-type equations, and the fluid flow is dominated by incompressible Navier-Stokes equation. The finite difference method on staggered grid is used to solve the above model. Then a classical droplet rising case and a droplet merging case are used to validate our model. Finally, we study the effect of surfactants on droplet deformation and merging. A more prolate profile of droplet is observed under a higher surfactant bulk concentration, which verifies the effect of surfactant in reducing the interfacial tension. Increases in surface Peclet number and initial surfactant bulk concentration can enhance the non-uniformity of surfactant distribution around the interface, which will arise the Marangoni force. The Marangoni force acts as an additional repulsive force to delay the droplet merging. Cited as : Zhu, G., Li, A. Interfacial dynamics with soluble surfactants: A phase-field two-phase flow model with variable densities. Advances in Geo-Energy Research, 2020, 4(1): 86-98, doi: 10.26804/ager.2020.01.08