Flow focusing

Last updated

Flow focusing in fluid dynamics is a technology whose aim is the production of drops or bubbles by straightforward hydrodynamic means. The output is a dispersed liquid or gas, frequently in the form of a fine aerosol or an emulsion. No other driving force is required, apart from traditional pumping, a key difference with other comparable technologies, such as electrospray (where an electric field is needed). Both flow focusing and electrospray working in their most extensively used regime produce high quality sprays composed by homogeneous and well-controlled-size droplets. Flow focusing was invented by Prof. Alfonso M. Gañan-Calvo (who now teaches at ETSI in Seville) in 1994, patented in 1996, and published for the first time in 1998.

Contents

Mechanism

The basic principle consists of a continuous phase fluid (focusing or sheath fluid) flanking or surrounding the dispersed phase (focused or core fluid), so as to give rise to droplet or bubble break-off in the vicinity of an orifice through which both fluids are extruded. The principle may be extended to two or more coaxial fluids; gases and liquids may be combined; and, depending on the geometry of the feed tube and orifices, the flow pattern may be cylindrical or planar. [1] [2] Both cylindrical and planar flow focusing have led to a variety of developments (see also the works of Peter Walzal).

A flow focusing device consists of a pressure chamber pressurized with a continuous focusing fluid supply. Inside, one or more focused fluids are injected through a capillary feed tube whose extremity opens up in front of a small orifice, linking the pressure chamber with the exterior ambient. The focusing fluid stream moulds the fluid meniscus into a cusp giving rise to a steady micro or nano-jet exiting the chamber through the orifice; the jet size is much smaller than the exit orifice, thus precluding any contact (which may lead to unwanted deposition or reaction). Capillary instability breaks up the steady jet into homogeneous droplets or bubbles.

The feed tube may be composed of two or more concentric needles and different immiscible liquids or gases to be injected, leading to compound drops. [3] On being suitably cured, such drops may lead to multilayer microcapsules with multiple shells of controllable thickness. Flow focusing ensures an extremely fast as well as controlled production of up to millions of droplets per second as the jet breaks up.

The role of the tangential viscous stress is essential in establishing a steady meniscus shape in flow focusing, as illustrated in the case of a simple liquid jet surrounded by a gas. In the absence of a sufficiently strong tangential stress, a round-apex meniscus is obtained. Both the inner liquid and the external gas flows would exhibit stagnation regions around the round apex. The surface tension stress σ/D would be simply balanced by an appropriate pressure jump across the interface. If one slowly pushes a liquid flow rate Q, the system would spit intermittently the excess of liquid to recover the round-apex equilibrium shape. However, when the tangential stress is sufficiently vigorous compared to σ /D, the surface can be deformed into a steady tapering shape, which allows the continuous and smooth acceleration of the liquid under the combined actions of the pressure drop ΔP and the tangential viscous stress τs on the liquid surface.

Applications

Flow focusing may be applied in the food, medicine, pharmaceutical, cosmetic, photographic and environmental industry, among other potential uses. The production of compound particles is an important field: drug encapsulation, dye-labeled particles and multiple-core particles can be cited. [4] [5] Other applications include flow cytometry [6] [7] and microfluidic circuits. [8] [9] Contrast agent such as droplets and Microbubbles can be produced in flow focusing microfluidics device.

Related Research Articles

Microfluidics refers to a system that manipulates a small amount of fluids ((10-9 to 10-18 liters) using small channels with sizes ten to hundreds micrometres. It is a multidisciplinary field that involves molecular analysis, biodefence, molecular biology, and microelectronics. It has practical applications in the design of systems that process low volumes of fluids to achieve multiplexing, automation, and high-throughput screening. Microfluidics emerged in the beginning of the 1980s and is used in the development of inkjet printheads, DNA chips, lab-on-a-chip technology, micro-propulsion, and micro-thermal technologies.

<span class="mw-page-title-main">Electrospray ionization</span> Technique used in mass spectroscopy

Electrospray ionization (ESI) is a technique used in mass spectrometry to produce ions using an electrospray in which a high voltage is applied to a liquid to create an aerosol. It is especially useful in producing ions from macromolecules because it overcomes the propensity of these molecules to fragment when ionized. ESI is different from other ionization processes since it may produce multiple-charged ions, effectively extending the mass range of the analyser to accommodate the kDa-MDa orders of magnitude observed in proteins and their associated polypeptide fragments.

In fluid dynamics, the capillary number (Ca) is a dimensionless quantity representing the relative effect of viscous drag forces versus surface tension forces acting across an interface between a liquid and a gas, or between two immiscible liquids. Alongside the Bond number, commonly denoted , this term is useful to describe the forces acting on a fluid front in porous or granular media, such as soil. The capillary number is defined as:

<span class="mw-page-title-main">Marangoni effect</span> Physical phenomenon between two fluids

The Marangoni effect is the mass transfer along an interface between two phases due to a gradient of the surface tension. In the case of temperature dependence, this phenomenon may be called thermo-capillary convection.

The name electrospray is used for an apparatus that employs electricity to disperse a liquid or for the fine aerosol resulting from this process. High voltage is applied to a liquid supplied through an emitter. Ideally the liquid reaching the emitter tip forms a Taylor cone, which emits a liquid jet through its apex. Varicose waves on the surface of the jet lead to the formation of small and highly charged liquid droplets, which are radially dispersed due to Coulomb repulsion.

<span class="mw-page-title-main">Rheometer</span> Scientific instrument used to measure fluid flow (rheology)

A rheometer is a laboratory device used to measure the way in which a viscous fluid flows in response to applied forces. It is used for those fluids which cannot be defined by a single value of viscosity and therefore require more parameters to be set and measured than is the case for a viscometer. It measures the rheology of the fluid.

<span class="mw-page-title-main">Liquid chromatography–mass spectrometry</span> Analytical chemistry technique

Liquid chromatography–mass spectrometry (LC–MS) is an analytical chemistry technique that combines the physical separation capabilities of liquid chromatography with the mass analysis capabilities of mass spectrometry (MS). Coupled chromatography - MS systems are popular in chemical analysis because the individual capabilities of each technique are enhanced synergistically. While liquid chromatography separates mixtures with multiple components, mass spectrometry provides spectral information that may help to identify each separated component. MS is not only sensitive, but provides selective detection, relieving the need for complete chromatographic separation. LC-MS is also appropriate for metabolomics because of its good coverage of a wide range of chemicals. This tandem technique can be used to analyze biochemical, organic, and inorganic compounds commonly found in complex samples of environmental and biological origin. Therefore, LC-MS may be applied in a wide range of sectors including biotechnology, environment monitoring, food processing, and pharmaceutical, agrochemical, and cosmetic industries. Since the early 2000s, LC-MS has also begun to be used in clinical applications.

<span class="mw-page-title-main">Coffee ring effect</span>

In physics, a "coffee ring" is a pattern left by a puddle of particle-laden liquid after it evaporates. The phenomenon is named for the characteristic ring-like deposit along the perimeter of a spill of coffee. It is also commonly seen after spilling red wine. The mechanism behind the formation of these and similar rings is known as the coffee ring effect or in some instances, the coffee stain effect, or simply ring stain.

In fluid statics, capillary pressure is the pressure between two immiscible fluids in a thin tube, resulting from the interactions of forces between the fluids and solid walls of the tube. Capillary pressure can serve as both an opposing or driving force for fluid transport and is a significant property for research and industrial purposes. It is also observed in natural phenomena.

<span class="mw-page-title-main">Multiphase flow</span>

In fluid mechanics, multiphase flow is the simultaneous flow of materials with two or more thermodynamic phases. Virtually all processing technologies from cavitating pumps and turbines to paper-making and the construction of plastics involve some form of multiphase flow. It is also prevalent in many natural phenomena.

<span class="mw-page-title-main">Capillary length</span>

The capillary length or capillary constant, is a length scaling factor that relates gravity and surface tension. It is a fundamental physical property that governs the behavior of menisci, and is found when body forces (gravity) and surface forces are in equilibrium.

Cell sorting is the process through which a particular cell type is separated from others contained in a sample on the basis of its physical or biological properties, such as size, morphological parameters, viability and both extracellular and intracellular protein expression. The homogeneous cell population obtained after sorting can be used for a variety of applications including research, diagnosis, and therapy.

Microfluidics in chemical biology is the application of microfluidics in the study of chemical biology.

<span class="mw-page-title-main">Analytical nebulizer</span> Type of apparatus that converts liquids into a fine mist

The general term nebulizer refers to an apparatus that converts liquids into a fine mist. Nozzles also convert liquids into a fine mist, but do so by pressure through small holes. Nebulizers generally use gas flows to deliver the mist. The most common form of nebulizers are medical appliances such as asthma inhalers or paint spray cans. Analytical nebulizers are a special category in that their purpose is to deliver a fine mist to spectrometric instruments for elemental analysis. They are necessary parts of inductively coupled plasma atomic emission spectroscopy (ICP-AES), inductively coupled plasma mass spectrometry (ICP-MS), and atomic absorption spectroscopy (AAS).

<span class="mw-page-title-main">Inkjet technology</span>

Inkjet technology originally was invented for depositing aqueous inks on paper in 'selective' positions based on the ink properties only. Inkjet nozzles and inks were designed together and the inkjet performance was based on a design. It was used as a data recorder in the early 1950s, later in the 1950s co-solvent-based inks in the publishing industry were seen for text and images, then solvent-based inks appeared in industrial marking on specialized surfaces and in the1990's phase change or hot-melt ink has become a popular with images and digital fabrication of electronic and mechanical devices, especially jewelry. Although the terms "jetting", "inkjet technology" and "inkjet printing", are commonly used interchangeably, inkjet printing usually refers to the publishing industry, used for printing graphical content, while industrial jetting usually refers to general purpose fabrication via material particle deposition.

Dimensionless numbers in fluid mechanics are a set of dimensionless quantities that have an important role in analyzing the behavior of fluids. Common examples include the Reynolds or the Mach numbers, which describe as ratios the relative magnitude of fluid and physical system characteristics, such as density, viscosity, speed of sound, flow speed, etc.

Droplet-based microfluidics manipulate discrete volumes of fluids in immiscible phases with low Reynolds number and laminar flow regimes. Interest in droplet-based microfluidics systems has been growing substantially in past decades. Microdroplets offer the feasibility of handling miniature volumes of fluids conveniently, provide better mixing, encapsulation, sorting, sensing and are suitable for high throughput experiments. Two immiscible phases used for the droplet based systems are referred to as the continuous phase and dispersed phase.

Self-propulsion is the autonomous displacement of nano-, micro- and macroscopic natural and artificial objects, containing their own means of motion. Self-propulsion is driven mainly by interfacial phenomena. Various mechanisms of self-propelling have been introduced and investigated, which exploited phoretic effects, gradient surfaces, breaking the wetting symmetry of a droplet on a surface, the Leidenfrost effect, the self-generated hydrodynamic and chemical fields originating from the geometrical confinements, and soluto- and thermo-capillary Marangoni flows. Self-propelled system demonstrate a potential as micro-fluidics devices and micro-mixers. Self-propelled liquid marbles have been demonstrated.

Microfluidics refers to the flow of fluid in channels or networks with at least one dimension on the micron scale. In open microfluidics, also referred to as open surface microfluidics or open-space microfluidics, at least one boundary confining the fluid flow of a system is removed, exposing the fluid to air or another interface such as a second fluid.

Capillary breakup rheometry is an experimental technique used to assess the extensional rheological response of low viscous fluids. Unlike most shear and extensional rheometers, this technique does not involve active stretch or measurement of stress or strain but exploits only surface tension to create a uniaxial extensional flow. Hence, although it is common practice to use the name rheometer, capillary breakup techniques should be better addressed to as indexers.

References

  1. ↑ Gañán-Calvo, Alfonso M. (1998-01-12). "Generation of Steady Liquid Microthreads and Micron-Sized Monodisperse Sprays in Gas Streams". Physical Review Letters. American Physical Society (APS). 80 (2): 285–288. Bibcode:1998PhRvL..80..285G. doi:10.1103/physrevlett.80.285. hdl:11441/103221. ISSN   0031-9007.
  2. ↑ Gañán-Calvo, Alfonso M.; Gordillo, José M. (2001-12-11). "Perfectly Monodisperse Microbubbling by Capillary Flow Focusing". Physical Review Letters. American Physical Society (APS). 87 (27): 274501. Bibcode:2001PhRvL..87A4501G. doi:10.1103/physrevlett.87.274501. hdl:11441/103230. ISSN   0031-9007. PMID   11800883.
  3. ↑ Utada, A. S. (2005-04-22). "Monodisperse Double Emulsions Generated from a Microcapillary Device". Science. American Association for the Advancement of Science (AAAS). 308 (5721): 537–541. Bibcode:2005Sci...308..537U. doi:10.1126/science.1109164. ISSN   0036-8075. PMID   15845850. S2CID   2410489.
  4. ↑ Martín-Banderas, Lucía; Flores-Mosquera, María; Riesco-Chueca, Pascual; Rodríguez-Gil, Alfonso; Cebolla, Ángel; Chávez, Sebastián; Gañán-Calvo, Alfonso M. (2005). "Flow Focusing: A Versatile Technology to Produce Size-Controlled and Specific-Morphology Microparticles". Small. Wiley. 1 (7): 688–692. doi:10.1002/smll.200500087. ISSN   1613-6810. PMID   17193506.
  5. ↑ Dendukuri, Dhananjay; Doyle, Patrick S. (2009-11-06). "The Synthesis and Assembly of Polymeric Microparticles Using Microfluidics". Advanced Materials. Wiley. 21 (41): 4071–4086. doi:10.1002/adma.200803386. ISSN   0935-9648. S2CID   12136073.
  6. ↑ Chung, S.; Park, S. J.; Kim, J. K.; Chung, C.; Han, D. C.; Chang, J. K. (2003-10-01). "Plastic microchip flow cytometer based on 2- and 3-dimensional hydrodynamic flow focusing". Microsystem Technologies. Springer Science and Business Media LLC. 9 (8): 525–533. doi:10.1007/s00542-003-0302-2. ISSN   0946-7076. S2CID   110440257.
  7. ↑ Ward, Thomas; Faivre, Magalie; Abkarian, Manouk; Stone, Howard A. (2005). "Microfluidic flow focusing: Drop size and scaling in pressureversus flow-rate-driven pumping". Electrophoresis. Wiley. 26 (19): 3716–3724. doi:10.1002/elps.200500173. ISSN   0173-0835. PMID   16196106. S2CID   17632023.
  8. ↑ Takeuchi, S.; Garstecki, P.; Weibel, D. B.; Whitesides, G. M. (2005-04-18). "An Axisymmetric Flow-Focusing Microfluidic Device". Advanced Materials. Wiley. 17 (8): 1067–1072. doi:10.1002/adma.200401738. ISSN   0935-9648. S2CID   14514523.
  9. ↑ Huebner, Ansgar; Sharma, Sanjiv; Srisa-Art, Monpichar; Hollfelder, Florian; Edel, Joshua B.; deMello, Andrew J. (2008). "Microdroplets: A sea of applications?". Lab on a Chip. Royal Society of Chemistry (RSC). 8 (8): 1244–1254. doi:10.1039/b806405a. ISSN   1473-0197. PMID   18651063.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.