A review of microfluidic concepts and applications for atmospheric aerosol science

Figures & data

Figure 1. Microfluidic channel masks (left) and SEM images of the corresponding PDMS channels (right) of a T-junction (top) and co-flow with flow focusing (bottom) geometries. In the mask images (left), arrows denoting continuous, dispersed, sheath, and emulsion flows show direction of flow and approximate relative velocities (proportional to arrow length) of the flow in each channel.

Figure 2. Relevant flow regimes in microfluidic flows (use mL hr⁻¹) and typical aerosol particle sampling (use liters per minute, LPM), for a given volumetric flow rate at a fixed hydraulic diameter. Microfluidic flow is considered in channels 100 µm tall ranging from 10 µm wide with ethanol () to 1 mm wide with heavy mineral oil (). Typical microfluidic flows have Re < 1. Typical aerosol particle sampling is considered for sample tubing from about 1/8 in. to 1 in. diameter for sampling particles in air ().

Table 1. Selected aerosol generation techniques with typical size distribution parameters. CV is coefficient of variance, , (Equation (7)[7] ).

Aerosol generation method	Median Diameter [μm]	CV [%]	Reference
Laboratory techniques
Capillary aerosol generator (CAG)	0.29–3.40^1	30–100	(Gupta et al. 2003)
Modified Collison atomizer	0.032–1.3^2	18–35	(Liu and Lee 1975)
DeVilbis nebulizer	2.8–4.2	80–90	(Hinds 1999)
Lovelace nebulizer	2.6–5.8	80–130	(Hinds 1999)
Vibrating orifice monodisperse aerosol generator (VOAG)	0.5–50^3	∼1.4	(Berglund and Liu 1973)
Electrohydrodynamic (EHD) atomization	∼10–2,000	–^4	(Cloupeau and Prunet-Foch 1994; Grace and Marijnissen 1994)
Sintered glass filter	60 nm	–^4	(Prather et al. 2013)
Piezo-activated drop-on-demand generator	10–100	<1	(Ulmke et al. 2001; Vaughn et al. 2016)
Piezoceramic dispenser/droplet chain technique	0.3–18	–^4	(Baldelli et al. 2016)
Microfluidic techniques
Microchannel emulsification	∼20	<2	(Kobayashi et al. 1999; Kawakatsu et al. 2001)
T-junction	∼30–200	<2	(Garstecki et al. 2006; Christopher et al. 2008)
Co-flow	20–200	<2	(Nisisako et al. 2006)
Flow-focusing	<1–600	<1	(Anna et al. 2003; Seo et al. 2007)
Microfluidic spray dryer^5	40 nm	–^4	(Thiele et al. 2011)
Microfluidic nebulator^5	14; 300 nm	–^4	(Amstad et al. 2015; 2017)

¹ Aerodynamic diameter;

² mobility diameter;

³ physical diameter;

⁴ no data given;

⁵ generates liquid droplets in air.

Table 2. Comparison of typical particle diameters for various aerosol experiment setups, sampling instruments, and atmospheric conditions. Although biphasic microfluidics is typically operated at droplet sizes at the large end of coarse mode aerosol and into the cloud droplet regime, more refinement of microfluidic droplet generation techniques will lower the smallest diameters accessible to microfluidic experiments.

Experimental framework	Particle diameter	Reference
Biphasic microfluidics	∼30–200 μm	this study, (Metcalf et al. 2016)
Electrodynamic Balance (EDB)	2–20 μm	(Colberg et al. 2004)
	6–40 μm	(Davis et al. 1990)
	14–44 μm	(Davies et al. 2012)
	20–65 μm	(Song et al. 2013)
	∼20–50 μm	(Pope et al. 2010)
	40–80 μm	(Steiner et al. 1999)
	100–250 μm	(Davis 1997)
Optical tweezers	0.025–10 μm	(Ashkin et al. 1986)
	1–10 μm	(Mitchem and Reid 2008)
	5–10 μm	(Bzdek et al. 2016)
Acoustic levitation	10 μm–5 mm	(Trinh 1985; Warschat and Riedel 2017)
Aerosol sampling instruments
Nanometer Differential Mobility Analyzer (Nano-DMA)	3–50 nm	(Chen et al. 1998)
Radial Differential Mobility Analyzer (RDMA)	3–200 nm	(Zhang et al. 1995)
Differential Mobility Analyzer (DMA)	5 nm–1 μm	(Knutson and Whitby 1975)
Electrical aerosol analyzer	6 nm–1 μm	(Liu and Pui 1975)
Aerodynamic lens	20–240 nm	(Liu et al. 1995)
Aerosol Particle Mass Analyzer (APM)	∼20–700 nm	(Ehara et al. 1996)
Couette Centrifugal Particle Mass Analyzer (CPMA)	50 – ∼600 nm	(Olfert et al. 2006)
Particle bounce measurements in cascade impactor	30–120 nm	(Virtanen et al. 2010)
Wide-range Particle Spectrometer	0.01–10 μm	(Liu et al. 2010)
Aerodynamic Particle Sizer (APS)	0.5–16 μm	(Chen et al. 2007)
Atmospheric conditions		(Hinds 1999; Pruppacher and Klett 2004; Seinfeld and Pandis 2016)
Nucleation mode (ultrafine particles)	<10 nm
Aitken mode (ultrafine particles)	10–100 nm
Accumulation mode (fine particles)	0.1–2 μm
Coarse mode (coarse particles)	2–50 μm
Cloud droplet	5–100 μm
Rain drop	0.05–10 mm

Figure 3. Selected microfluidic methods for producing temperature gradients, chemical gradients, and electric fields. (a) Reproduced from Stan, C. A., Schneider, G. F., Shevkoplyas, S. S., Hashimoto, M., Ibanescu, M., Wiley, B. J. and Whitesides, G. M. (2009). A Microfluidic Apparatus for the Study of Ice Nucleation in Supercooled Water Drops. Lab Chip, 9(16):2293–2305 with permission of The Royal Society of Chemistry. (b) Reproduced from Morel, M., Galas, J.-C., Dahan, M. and Studer, V. (2012). Concentration Landscape Generators for Shear Free Dynamic Chemical Stimulation. Lab Chip, 12(7):1340–1346 with permission of The Royal Society of Chemistry. (c) Reproduced from Fair, R. B., Khlystov, A., Srinivasan, V., Pamula, V. K. and Weaver, K. N. (2004). Integrated Chemical/Biochemical Sample Collection, Pre-Concentration, and Analysis on a Digital Microfluidic Lab-on-a-Chip Platform. Edited by Linda A Smith and Daniel Sobek. Proc. SPIE, 5591:113–124 with permission of SPIE.

Figure 4. Selected microfluidic chemical and physical measurement techniques. (a) Reprinted with permission from Metcalf, A. R., Boyer, H. C. and Dutcher, C. S. (2016). Interfacial Tensions of Aged Organic Aerosol Particle Mimics Using a Biphasic Microfluidic Platform. Environ. Sci. Technol., 50(3):1251–1259. Copyright 2016 American Chemical Society. (b) Reprinted with permission from Song, H., Tice, J. D. and Ismagilov, R. F. (2003). A Microfluidic System for Controlling Reaction Networks in Time. Angew. Chem., 115(7):792–796. Copyright 2003 WILEY-VCH Verlag GmbH & Co. (c) Reprinted from Damit, B. (2017). Droplet-Based Microfluidics Detector for Bioaerosol Detection. Aerosol Sci. Technol., 51(4):488–500 with permission of the publisher (Taylor & Francis Ltd, http://www.tandfonline.com) and The American Association for Aerosol Research, www.aaar.org.

Figure 5. Selected microfluidic droplet and particle sorting techniques. (a) Reprinted with permission from Maenaka, H., Yamada, M., Yasuda, M. and Seki, M. (2008). Continuous and Size-Dependent Sorting of Emulsion Droplets Using Hydrodynamics in Pinched Microchannels. Langmuir, 24(8):4405–4410. Copyright 2008 American Chemical Society. (b) Reprinted with permission from Squires, T. M. and Quake, S. R. (2005). Microfluidics: Fluid Physics at the Nanoliter Scale. Rev. Mod. Phys., 77(3):977–1026. Copyright 2005 by the American Physical Society. (c) Adapted by permission from Macmillan Publishers Ltd: Nature Protocols, Mazutis, L., Gilbert, J., Ung, W. L., Weitz, D. A., Griffiths, A. D., and Heyman, J. A. (2013). Single-Cell Analysis and Sorting Using Droplet-Based Microfluidics. Nat. Protoc., 8 (5):870 -891, copyright 2013.

Figure 6. Selected microfluidic trapping and manipulation techniques. (a) Reproduced from Dylla-Spears, R., Townsend, J. E., Jen-Jacobson, L., Sohn, L. L. and Muller, S. J. (2010). Single-Molecule Sequence Detection via Microfluidic Planar Extensional Flow at a Stagnation Point. Lab Chip, 10(12):1543–1549 with permission of The Royal Society of Chemistry. (b) Reprinted with permission from Tanyeri, M. and Schroeder, C. M. (2013). Manipulation and Confinement of Single Particles Using Fluid Flow. Nano Lett., 13(6):2357–2364. Copyright 2013 American Chemical Society. (c) Reprinted with permission from Shenoy, A., Rao, C. V. and Schroeder, C. M. (2016). Stokes Trap for Multiplexed Particle Manipulation and Assembly Using Fluidics. Proc. Natl. Acad. Sci. U. S. A., 113(15):3976–3981.

Table 3. Selected microfluidic techniques and potential aerosol science applications.

Microfluidic Technique	Potential Application to Aerosol Science
Temperature control/gradients	Ice nucleation studies
Chemical gradients	Bioaerosol studies; heterogeneous chemistry
Electric fields	Electrophoretic mobility
Droplet/particle sorting	Particle size separation
Trapping and manipulation	Single-particle studies; thermodynamic and physical characterization
Diffusional mixing	Heterogeneous chemistry
Designer emulsions	Generate aerosol standards

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