Name: Glass-Based Devices to Generate Drops and Emulsions (Video) | JoVE Journal
Uploaded: 2022-04-05T13:00:00.000+00:00
Duration: 8 min 45 s
Description: Learn to fabricate glass microfluidic devices for generating stable emulsions with controlled drop sizes. This accessible protocol enables beginners to build versatile tools for applications in agriculture, cosmetics, and food industries.

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We are thankful to the ACS PRF (grant 60302-UR9), Agrobio S.L. (contract #311325), and MCIN/AEI/10.13039/501100011033/FEDER, UE (grant No. PID2021-122369NB-I00).

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<li>Guerrero, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Emission+modes+in+electro+co-flow%5BTitle%5D)+AND+%22Physics+of+Fluids%22%5BJournal%5D)">Emission modes in electro co-flow.</a> Physics of Fluids. 31 (8), 082009(2019).</li>
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<li>Castro-Hernández, E., Gundabala, V., Fernández-Nieves, A., Gordillo, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Scaling+the+drop+size+in+coflow+experiments%5BTitle%5D)+AND+%22New+Journal+of+Physics%22%5BJournal%5D)">Scaling the drop size in coflow experiments.</a> New Journal of Physics. 11, 075021(2009).</li>
<li>Godfray, H. C. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Food+Security%3A+the+challenge+of+feeding+9+billion+people%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">Food Security: the challenge of feeding 9 billion people.</a> Science. 327 (5967), New York, N.Y. 812-818 (2010).</li>
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<li>Muñoz-Cárdenas, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Generalist+red+velvet+mite+predator+%28Balaustium+sp.%29+performs+better+on+a+mixed+diet%5BTitle%5D)+AND+%22Experimental+%26+Applied+Acarology%22%5BJournal%5D)">Generalist red velvet mite predator (Balaustium sp.) performs better on a mixed diet.</a> Experimental & Applied Acarology. 62 (1), 19-32 (2014).</li>
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<li>Sullivan, M. T., Stone, H. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((The+role+of+feedback+in+microfluidic+flow-focusing+devices%5BTitle%5D)+AND+%22Philosophical+Transactions+of+the+Royal+Society+A%3A+Mathematical%2C+Physical%2C+and+Engineering+Sciences%22%5BJournal%5D)">The role of feedback in microfluidic flow-focusing devices.</a> Philosophical Transactions of the Royal Society A: Mathematical, Physical, and Engineering Sciences. 366 (1873), 2131-2143 (2008).</li>
<li>Shang, L., Cheng, Y., Zhao, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Emerging+droplet+microfluidics%5BTitle%5D)+AND+%22Chemical+Reviews%22%5BJournal%5D)">Emerging droplet microfluidics.</a> Chemical Reviews. 117 (12), 7964-8040 (2017).</li>
<li>Christopher, G. F., Anna, S. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Microfluidic+methods+for+generating+continuous+droplet+streams%5BTitle%5D)+AND+%22Journal+of+Physics+D%3A+Applied+Physics%22%5BJournal%5D)">Microfluidic methods for generating continuous droplet streams.</a> Journal of Physics D: Applied Physics. 40 (19), 319(2007).</li>
<li>Nunes, J. K., Tsai, S. S., Wan, J., Stone, H. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Dripping+and+jetting+in+microfluidic+multiphase+flows+applied+to+particle+and+fiber+synthesis%5BTitle%5D)+AND+%22Journal+of+Physics+D%3A+Applied+Physics%22%5BJournal%5D)">Dripping and jetting in microfluidic multiphase flows applied to particle and fiber synthesis.</a> Journal of Physics D: Applied Physics. 46 (11), 114002(2013).</li>
</ol>

The authors have nothing to disclose.

The protocol to fabricate three different glass-based devices has been described above. In the case of the device to generate simple drops, the flow rate and liquid properties are crucial to generate drops in a controlled manner. Drops will form at the tip in the dripping regime, or at the end of the jet in the jetting regime. The transition from dripping to jetting is parametrized by the dimensionless Weber number, We23. This number represents the ratio between inertial and surface tension forces...

Controlled generation of drops in the micron and nanoscale with a narrow size distribution is a challenging task. These drops are of interest for the engineering of soft materials with many applications in science and technology1,2,3,4,5,6.
The most common devices for the high production rate of drops are mixers7 and ultrasound emulsificators8. These methods are simple and low cost, but they typically result in polydisperse drops with a wide range of sizes. Hence, additional steps are required to produce monodisperse samples. Microfluidic devices can be designed differently to provide an efficient way to drop formation. Additionally, the usually low flow rates involved (i.e., low Reynolds number) allow for great control over the fluid flow.
While microfluidic devices are commonly made using lithographic techniques with poly(dimethyl) siloxane (PDMS), this manuscript focuses on glass-based capillary devices. PDMS devices are usually chosen for their ability to design complex channel patterns and because of their scalability. Glass devices, on the contrary, are rigid and have greater solvent resistance than their PDMS counterparts. Additionally, glass can be modified to change its wettability, which allows controlling the generation of complex emulsions. Being able to independently treat the nozzle and channel walls enables the formation of drops in a controlled and reproducible manner, while assuring the stability of the resultant emulsions if the drops were to touch the walls9; otherwise the drops may coalesce and accumulate at the wall. Another difference between these two types of devices is that in glass-based devices, the flow is three-dimensional, while it is planar in conventional PDMS devices. This fact minimizes the drop contact with the channel walls so that the influence of contact lines can be neglected10, thereby protecting the stability of multiple emulsion drops.
<img alt="Liquid flow dynamics diagram: inner/outer liquid interactions, multiphase flow, fluid mechanics." class="xfigimg" src="/files/ftp_upload/63376/63376fig01.jpg" data-altupdatedat="1788282455000" data-alt="Figure 1"> 
Figure 1: Different microfluidic device configurations. Sketches of (A) a T-junction, (B) a coflowing device, and (C) a flow-focusing device. <a href="https://www.jove.com/files/ftp_upload/63376/63376fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
There are three main geometries used, namely T-junction11, flow focusing12,13, and coflow14. In the T-junction geometry, the dispersed phase contained in the channel perpendicularly intersects the main channel which houses the continuous phase. The shear stress exerted by the continuous phase breaks the incoming dispersed liquid resulting in drops. The generated drops are limited in lower size by the dimensions of the main channel11. In the flow-focusing geometry, the two fluids are forced through a small orifice that is located in front of the injection tube. The result is the formation of a jet, which is much smaller than the injection tube12,13. Finally, the coflow geometry has a configuration characterized by the coaxial flow of two immiscible fluids14. In general, dripping and jetting can be observed depending on the operating conditions. The dripping regime occurs at low flow rates and the resulting droplets are very monodisperse and have a diameter proportional to the tip size. The drawback is its low production frequency. The jetting regime occurs at higher flow rates as compared to the dripping regime. In this case, the drop diameter is directly proportional to the diameter of the jet which can be much smaller than the diameter of the tip under the right conditions.
An alternative to these hydrodynamic approaches relies on the additional use of electric forces. Electrospray is a well-known and widely used technique for generating droplets. It is based on the principle that a liquid with a finite electrical conductivity will deform in the presence of a strong electric field. The liquid will eventually adopt a conical shape resulting from the balance between electric and surface tension stresses15. The process starts with the electric field inducing an electric current in the liquid that causes charges to accumulate at the surface. The presence of the electric field results in an electric force on these charges, which drags the liquid along, elongating the meniscus in the direction of the field. Under different conditions, the meniscus can either shed the charged drops or may emit one or several jets which then break into drops15. Although these electrically assisted microfluidic methods naturally allow the generation of small drops, they suffer from a lack of a steady-state operation that compromises the emulsion monodispersity. The resultant charged drops tend to discharge on the confining walls and/or anywhere in the device where the electric potential is lower than the imposed external voltage. Thus, the electrified meniscus becomes unstable, ultimately emitting drops in a chaotic way and causing their uncontrolled production and loss of monodispersity.
In electro-coflow, the electrical and hydrodynamic stresses are coupled in a coflow microfluidic device16 similar to the one used for generating double emulsions12. Two main features allow electro-coflow to be successful in reaching a steady-state emission regime: (i) the dispersed phase is ejected into another coflowing viscous liquid, and (ii) the use of a liquid counter-electrode or ground. Having a flowing outer liquid has proven to change the geometric properties of the drop emission process17. The liquid counter-electrode allows the discharge and extraction of the resultant drops, assuring the steady-state generation of drops. In addition, by exploiting the balance of electrical and hydrodynamic forces, the resultant drop sizes can potentially vary within a wider range than the sizes that can be covered by any of the previously mentioned techniques.
This detailed video protocol is intended to help new practitioners in the use and fabrication of glass-based microfluidics.

<table><tbody><tr><td>2-[methoxy(polyethyleneoxy)6-9propyl] trimethoxysilane.</td><td>Gelest</td><td>SIM6492.7</td><td></td></tr><tr><td>Ceramic tile</td><td>Sutter</td><td>CTS</td><td></td></tr><tr><td>Ethylene glycol</td><td>Fisher</td><td>BP230</td><td>These can be found at other companies like Sigma-Aldrich</td></tr><tr><td>Hexane</td><td>Sigma- Aldrich</td><td>34859</td><td>Available in other vendors</td></tr><tr><td>ITW Polymers Adhesives Devcon 5 Minute Epoxy Adhesive 25 mL Dev-Tube</td><td>Ellsworth adhesives</td><td>470740</td><td></td></tr><tr><td>Microforge</td><td>Narishige</td><td>MF 830</td><td></td></tr><tr><td>Micropipette puller</td><td>Sutter</td><td>P97</td><td></td></tr><tr><td>Microscope slides</td><td>Fisher</td><td>12-544-1</td><td>Available in other vendors</td></tr><tr><td>Needle 20 Gauge, .0255&quot; ID, .0355&quot; OD, 1/2&quot; Long</td><td>McMaster</td><td>75165A677</td><td></td></tr><tr><td>SDS</td><td>Sigma-aldrich</td><td>428015</td><td>Surfactant</td></tr><tr><td>Silicone oil</td><td>Clearco</td><td>PSF-10cSt</td><td>The catalog number correspond to the 10cSt viscosity oil. Different viscosity oils can be found at this company</td></tr><tr><td>Span 80</td><td>Fisher</td><td>S0060500G</td><td>non-ionic surfactant</td></tr><tr><td>Square glass capillary 2mm ID (borosillicate 300 or 600 mm long)</td><td>VitroCom</td><td>S 102</td><td></td></tr><tr><td>Standard Glass Capillaries, 6 in., 2 / 1.12 OD/ID</td><td>World Precision instruments</td><td>1B200-6</td><td>These can be found at other companies like Sutter or Vitrocom</td></tr><tr><td>Syringe pump</td><td>Chemyx</td><td>FUSION 100-X</td><td>This model has a good quality/price ratio</td></tr><tr><td>Syringes (it will depend on the compatibility with the liquids)</td><td>Fisher</td><td></td><td>Catalog number will depend on the size</td></tr><tr><td>Trimethoxy(octyl)silane</td><td>Sigma- Aldrich</td><td>376221</td><td>Available in other vendors</td></tr><tr><td>Tubing ( it will depend on the compatibility with the liquids)</td><td>Scientific commodities</td><td>BB3165-PE/5</td><td>This reference is for polyethylene micro tubing. The size fits the needle size listed here</td></tr></tbody></table>

glass-based devices to generate drops and emulsions

Glass-Based Devices to Generate Drops and Emulsions

In this manuscript, three different step-by-step protocols to generate highly monodisperse emulsion drops using glass-based microfluidics are described. The first device is built for the generation of simple drops driven by gravity. The second device is designed to generate emulsion drops in a coflowing scheme. The third device is an extension of the coflowing device with the addition of a third liquid that acts as an electric ground, allowing the formation of electrified drops that subsequently discharge. In this setup, two of the three liquids have an appreciable electrical conductivity. The third liquid mediates between these two and is a dielectric. A&#160;voltage difference applied between the two conducting liquids creates an electric field that couples with hydrodynamic stresses of the coflowing liquids, affecting the jet and drop formation process. The addition of the electric field provides a path to generate smaller drops than in simple coflow devices and for generating particles and fibers with a wide range of sizes.

In this manuscript, three different devices have been designed to generate drops. We have generated drops with a size of (3.29 ± 0.08) mm (Figure 4B) and (1.75 ± 0.04) mm (Figure 4C) using the device described in step 1. The emulsion drops can be generated using the coflow and the electro-coflow devices. For the latter, we show dripping in Figure 9, while cone-jet and whipping modes are shown in Figure 10</stron...

Here, a protocol to manufacture glass-based microfluidic devices used for generating highly monodisperse emulsions with controlled drop size is presented.

In this manuscript, three different step-by-step protocols to generate highly monodisperse emulsion drops using glass-based microfluidics are described. ...

glass-based-devices-to-generate-drops-and-emulsions

Research

JoVE Journal

Engineering

Droplet-based microfluidic platforms are promising candidates for high throughput experimentation since they are able to generate picoliter, self-compartmentalized vessels inexpensively at kHz rates. Through integration with fast, sensitive and high resolution fluorescence spectroscopic methods, the large amounts of information generated within these systems can be efficiently extracted, harnessed and utilized.

Fluorescence detection methods for microfluidic droplet platforms

Drop impact of non-Newtonian fluids is a complex process since different physical parameters influence the dynamics over a very short time (less than one tenth of a millisecond). A fast imaging technique is introduced in order to characterize the impact behaviors of different non-Newtonian fluids.

Fast Imaging Technique to Study Drop Impact Dynamics of Non-Newtonian Fluids

Microfluidic double emulsions generation typically involves devices with patterned wettability or custom-fabricated glass components. Here we describe the fabrication and testing of an all polydimethylsiloxane (PDMS) double emulsion generator that does not require surface treatment or complicated fabrication processes, and is capable of producing double emulsions down to 14 µm.

Double Emulsion Generation Using a Polydimethylsiloxane (PDMS) Co-axial Flow Focus Device

Here, we demonstrate a simple production method for size-controllable, monodisperse, water-in-oil (W/O) microdroplets using a capillary-based centrifugal microfluidic device. This method requires only a small sample volume and enables high-yield production. We expect this method will be useful for rapid biochemical and cellular analyses.

Capillary-based Centrifugal Microfluidic Device for Size-controllable Formation of Monodisperse Microdroplets

The assembly and positioning of the mitotic spindle depend on the combined forces generated by microtubule dynamics, motor proteins and cross-linkers. Here we present our recently developed methods in which the geometrical confinement of spherical emulsion droplets is used for the bottom-up reconstitution of basic mitotic spindles.

Reconstitution of Basic Mitotic Spindles in Spherical Emulsion Droplets

Lab-in-a-drop reaction systems allow the versatile implementation of complex reactions in a microfluidic scale. An automated actuation platform consisting of a 3 x 3 matrix of electromagnetic coils was developed and successfully used to merge two 10 µL microreactors and thereby initiate an enzymatic reaction in the resulting liquid marbles.

Aqueous Droplets Used as Enzymatic Microreactors and Their Electromagnetic Actuation

Here, we present a protocol to illustrate the fabrication processes and verifying experiments of a semi-three-dimensional (semi-3D) flow-focusing microfluidic chip for droplet formation.

Generation of Size-controlled Poly (ethylene Glycol) Diacrylate Droplets via Semi-3-Dimensional Flow Focusing Microfluidic Devices

A phase-inversion co-flow device is demonstrated to generate monodisperse high-viscosity droplets above 1 Pas, which is difficult to realize in droplet microfluidics.

Fabricating High-viscosity Droplets using Microfluidic Capillary Device with Phase-inversion Co-flow Structure

The overall goal of the protocol is to prepare over one million ordered, uniform, stable, and biocompatible femtoliter droplets on a 1 cm2 planar substrate that can be used for cell-free protein synthesis.

A Femtoliter Droplet Array for Massively Parallel Protein Synthesis from Single DNA Molecules

Water-in-oil droplet assays are useful for analytical chemistry, enzyme evolution, and single cell analysis, but typically require microfluidics to form the droplets. Here, we describe particle templated emulsification, a microfluidic-free approach to perform droplet assays.

Particle Templated Emulsification enables Microfluidic-Free Droplet Assays

This article describes protocols for high-throughput gelatin methacryloyl microgel fabrication using microfluidic devices, converting microgels to resuspendable powder (micro-aerogels), the chemical assembly of microgels to form granular hydrogel scaffolds, and developing granular hydrogel bioinks with preserved microporosity for 3D bioprinting.

Gelatin Methacryloyl Granular Hydrogel Scaffolds: High-throughput Microgel Fabrication, Lyophilization, Chemical Assembly, and 3D Bioprinting

Learn to fabricate glass microfluidic devices for generating stable emulsions with controlled drop sizes. This accessible protocol enables beginners to build versatile tools for applications in agriculture, cosmetics, and food industries.

Glass-Based Devices to Generate Drops and Emulsions

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