Impact of inlet channel geometry on microfluidic drop formation

Effect of the fluid injection configuration on droplet size in a microfluidic T junction

The effect of confinement on the droplet formation in T junctions was studied for three configurations of fluid injection. The sizes of the main droplets and the satellite droplets were measured in the squeezing and dripping regimes. The evolution of droplet sizes with capillary number in the continuous phase is similar to that in flow-focusing junctions, i.e., the size of the main droplets decreases with an increase of this capillary number, while the size of the satellite droplets increases with an increase of this capillary number. While in the range of flow rates investigated the injection configuration does not exhibit a significant effect on the main droplet sizes, it does have an effect on the size of the satellite droplets. The latter ones are smaller when the neck rupture of the droplet occurs on an angle of the microsystem.

Single-cell analysis and sorting using droplet-based microfluidics

We present a droplet-based microfluidics protocol for high-throughput analysis and sorting of single cells. Compartmentalization of single cells in droplets enables the analysis of proteins released from or secreted by cells, thereby overcoming one of the major limitations of traditional flow cytometry and fluorescence-activated cell sorting. As an example of this approach, we detail a binding assay for detecting antibodies secreted from single mouse hybridoma cells. Secreted antibodies are detected after only 15 min by co-compartmentalizing single mouse hybridoma cells, a fluorescent probe and single beads coated with anti-mouse IgG antibodies in 50-pl droplets. The beads capture the secreted antibodies and, when the captured antibodies bind to the probe, the fluorescence becomes localized on the beads, generating a clearly distinguishable fluorescence signal that enables droplet sorting at ∼200 Hz as well as cell enrichment. The microfluidic system described is easily adapted for screening other intracellular, cell-surface or secreted proteins and for quantifying catalytic or regulatory activities. In order to screen ∼1 million cells, the microfluidic operations require 2-6 h; the entire process, including preparation of microfluidic devices and mammalian cells, requires 5-7 d.

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.

Thermocapillary migration in small-scale temperature gradients: application to optofluidic drop dispensing

We experimentally investigate the thermocapillary migration induced by local laser heating of the advancing front of a growing droplet confined in a microfluidic channel. When heating implies an effective increase in interfacial tension, the laser behaves as a "soft door" whose stiffness can be tuned via the optical parameters (beam power and waist). The light-driven thermocapillary velocity of a growing droplet, which opposes the basic flow, is characterized for different types of fluid injection, either pressure or flow rate driven, and various channel aspect ratios. Measurements are interpreted using a simplified model for the temperature gradient at the interface, based on a purely diffusive, three-layer system. Considering the mean temperature gradient, we demonstrate that the classical large-scale temperature gradient behavior is retrieved in the opposite case when the thermal gradient length scale is smaller than the droplet size. We also demonstrate that the thermocapillary velocity is proportional to the smallest droplet curvature imposed by the channel confinement. This suggests that the thermocapillary velocity is in fact proportional to the mean temperature gradient and to the largest interface curvature radius, which both coincide with the imposed one and the spherical droplet radius in large-scale and unconfined situations. Furthermore, as used surfactant concentrations are largely above the critical micelle concentration, we propose a possible explanation, relying on state-of-the-art considerations on high-concentration surfactant-covered interfaces for the observed effective increase in interfacial tension with temperature. We also propose a mechanism for explaining the blocking effect at the scaling-law level. This mechanism involves the temporal evolution of hydrodynamic and thermocapillary forces, based on experimental observations. We finally show that this optocapillary interaction with a microfluidic droplet generator allows for controlling either the flow rate (valve) or the droplet size (sampler), depending on the imposed fluid injection conditions.

Droplet based microfluidics

Droplet based microfluidics is a rapidly growing interdisciplinary field of research combining soft matter physics, biochemistry and microsystems engineering. Its applications range from fast analytical systems or the synthesis of advanced materials to protein crystallization and biological assays for living cells. Precise control of droplet volumes and reliable manipulation of individual droplets such as coalescence, mixing of their contents, and sorting in combination with fast analysis tools allow us to perform chemical reactions inside the droplets under defined conditions. In this paper, we will review available drop generation and manipulation techniques. The main focus of this review is not to be comprehensive and explain all techniques in great detail but to identify and shed light on similarities and underlying physical principles. Since geometry and wetting properties of the microfluidic channels are crucial factors for droplet generation, we also briefly describe typical device fabrication methods in droplet based microfluidics. Examples of applications and reaction schemes which rely on the discussed manipulation techniques are also presented, such as the fabrication of special materials and biophysical experiments.

A new method of UV-patternable hydrophobization of micro- and nanofluidic networks

This work reports a new method to hydrophobize glass-based micro- and nanofluidic networks. Conventional methods of hydrophobizing glass surfaces often create particulate debris causing clogging especially in shallow nanochannels or require skilful handling. Our novel method employs oxygen plasma, silicone oil and ultraviolet (UV) light. The contact angle of the modified bare glass surface can reach 100° whilst the inner channels after treatment facilitate stable and durable water-in-oil droplet generation. This modified surface was found to be stable for more than three weeks. The use of UV in principle enables in-channel hydrophobic patterning.

Parallel synchronization of two trains of droplets using a railroad-like channel network

We present a simple method of water-in-oil droplet synchronization in a railroad-like channel network. The network consisted of a top channel, a bottom channel, and ladder-like channels interconnected between the two main channels. The presence of the pressure difference between the top and bottom channels resulted in the crossflow of carrier oil through the ladder network until the pressure in each channel was balanced automatically. The proposed model and method proved the feasibility of the parallel synchronization of two trains of droplets with up to 95% synchronization efficiency. Physical parameters that could improve the efficiency were investigated with the systematic variation of the droplet length and droplet generation frequency by controlling the flow rate in each channel. Under a subtle difference in the generation frequency, an unmatched droplet sandwiched between two matched droplets in the ladder network was switched and synchronized in turn. For perfect one-to-one droplet synchronization, the droplet length and the droplet generation frequency needed to be the same for both the top and bottom channels. In addition, one-to-multiple droplet synchronization was demonstrated by matching the product of the droplet length and the droplet generation frequency for both the top and bottom channels. The proposed method provides a simple unit operation for parallel synchronization of the trains of droplets that can be easily integrated with the conventional continuous-flow droplet-based microfluidic platform.

Guiding, distribution, and storage of trains of shape-dependent droplets

We present a simple method of guiding, distributing, and storing of a train of shape-dependent droplets by using side flows, cavity guiding tracks, and storage chambers. The squeezing flow makes a train of flattened droplets to align to one side of the wall and the pushing flow guides it to one of the designated guiding tracks. Then the guided droplets move along the guiding track due to the lowered surface energy when they flow along the track. In addition, simultaneous droplet guiding and storing process has been demonstrated. An array of storage chambers placed in each track could store each train containing differently concentrated droplets. The proposed method will be useful for distribution of droplets for further processes or storing for multiplex, large-scale, dynamic assays over time.

Statistical modeling of single target cell encapsulation

High throughput drop-on-demand systems for separation and encapsulation of individual target cells from heterogeneous mixtures of multiple cell types is an emerging method in biotechnology that has broad applications in tissue engineering and regenerative medicine, genomics, and cryobiology. However, cell encapsulation in droplets is a random process that is hard to control. Statistical models can provide an understanding of the underlying processes and estimation of the relevant parameters, and enable reliable and repeatable control over the encapsulation of cells in droplets during the isolation process with high confidence level. We have modeled and experimentally verified a microdroplet-based cell encapsulation process for various combinations of cell loading and target cell concentrations. Here, we explain theoretically and validate experimentally a model to isolate and pattern single target cells from heterogeneous mixtures without using complex peripheral systems.

Droplets formation and merging in two-phase flow microfluidics

Two-phase flow microfluidics is emerging as a popular technology for a wide range of applications involving high throughput such as encapsulation, chemical synthesis and biochemical assays. Within this platform, the formation and merging of droplets inside an immiscible carrier fluid are two key procedures: (i) the emulsification step should lead to a very well controlled drop size (distribution); and (ii) the use of droplet as micro-reactors requires a reliable merging. A novel trend within this field is the use of additional active means of control besides the commonly used hydrodynamic manipulation. Electric fields are especially suitable for this, due to quantitative control over the amplitude and time dependence of the signals, and the flexibility in designing micro-electrode geometries. With this, the formation and merging of droplets can be achieved on-demand and with high precision. In this review on two-phase flow microfluidics, particular emphasis is given on these aspects. Also recent innovations in microfabrication technologies used for this purpose will be discussed.

Controlling droplet incubation using close-packed plug flow

Controlling droplet incubation is critical for droplet-based microfluidic applications; however, current techniques are either of limited precision or place strict limits on the incubation times that can be achieved. Here, we present a simple technique to control incubation time by exploiting close-packed plug flow. In contrast to other techniques, this technique is applicable to very short and very long incubation times.

Syringe-vacuum microfluidics: A portable technique to create monodisperse emulsions

We present a simple method for creating monodisperse emulsions with microfluidic devices. Unlike conventional approaches that require bulky pumps, control computers, and expertise with device physics to operate devices, our method requires only the microfluidic device and a hand-operated syringe. The fluids needed for the emulsion are loaded into the device inlets, while the syringe is used to create a vacuum at the device outlet; this sucks the fluids through the channels, generating the drops. By controlling the hydrodynamic resistances of the channels using hydrodynamic resistors and valves, we are able to control the properties of the drops. This provides a simple and highly portable method for creating monodisperse emulsions.

Predictive model for the size of bubbles and droplets created in microfluidic T-junctions

We present a closed-form expression that allows the reader to predict the size of bubbles and droplets created in T-junctions without fitting. Despite the wide use of microfluidic devices to create bubbles and droplets, a physically sound expression for the size of bubbles and droplets, key in many applications, did not yet exist. The theoretical foundation of our expression comprises three main ingredients: continuity, geometrics and recently gained understanding of the mechanism which leads to pinch-off. Our simple theoretical model explains why the size of bubbles and droplets strongly depends on the shape of a T-junction, and teaches how the shape can be tuned to obtain the desired size. We successfully validated our model experimentally by analyzing the formation of gas bubbles, as well as liquid droplets, in T-junctions with a wide variety of shapes under conditions typical to multiphase microfluidics.

Functional patterning of PDMS microfluidic devices using integrated chemo-masks

Microfluidic devices can be molded easily from PDMS using soft lithography. However, the softness of the resulting microchannels makes it difficult to photolithographically pattern their surface properties, as is needed for applications such as double emulsification. We introduce a new patterning method for PDMS devices, using integrated oxygen reservoirs fabricated simultaneously with the microfluidic channels, which serve as "chemo-masks". Oxygen diffuses through the PDMS to the nearby channel segments and there inhibits functional polymer growth; by placement of the chemo-masks, we thus control the polymerization pattern. This patterning method is simple, scalable, and compatible with a variety of surface chemistries.

Microfluidic device incorporating closed loop feedback control for uniform and tunable production of micro-droplets

Both micro- and nanofluidics are finding increasing use in the growing toolbox of nanotechnology; for the production of nanoparticles, and as micro-reactors for carefully controlled chemical reactions. These laboratories-on-a-chip hold vast potential for industrial application, however, only the most simple are truly starting to emerge as commercially viable, particularly in the area of droplet formation and emulsion creation. In order to automate droplet production with a desired size and dispersity, we have designed a microfluidic-based technology utilizing elementary microchannel geometries in combination with a closed loop feedback system to control the continuous- and dispersed-phase flow rates. Both the device geometry and control system have been optimized to allow for the production of a tunable emulsion. By utilizing discrete linear control theory, the device is able to produce the desired results with little to no prior knowledge of the fluid material properties to be used in either phase. We present our results from initial development using flow-focusing microfluidic geometry for droplet formation, computer-tethered syringe pumps to individually control the continuous and dispersed phase flow rates, a high-speed camera, and a controller and driver system for the optical measurements and pumps, respectively. We will show the efficacy of this technique for Newtonian and viscoelastic liquids, with and without the presence of surfactants. It can be envisioned that through careful control optimization, such a system can be developed to a point that will allow the production of "designer" emulsions with droplets eventually reaching the nanoscale.