Title: Colliding-Droplet Microreactor: Rapid On-Demand Inertial Mixing and Metal-Catalyzed Aqueous Phase Oxidation Processes

Abstract

In-depth investigations of the kinetics of aqueous chemistry occurring in microdroplet environments require experimental techniques that allow a reaction to be initiated at a well-defined point in time and space. Merging microdroplets of different reactants is one such approach. The mixing dynamics of unconfined (airborne) microdroplets have yet to be studied in detail, which is an essential step toward widespread use and application of merged droplet microreactors for monitoring chemical reactions. Here, we present an on-demand experimental approach for initiating chemical reactions in and characterizing the mixing dynamics of colliding airborne microdroplets (40 ± 5 μm diameter) using a streak-based fluorescence microscopy technique. The advantages of this approach include the ability to generate two well-controlled monodisperse microdroplet streams and collide (and thus mix) the microdroplets with high spatial and temporal control while consuming small amounts of sample (<0.1 μL/s). Mixing times are influenced not only by the velocity at which microdroplets collide but also the geometry of the collision (i.e., head-on vs off-center collision). For head-on collisions, we achieve submillisecond mixing times ranging from ~900 μs at a collision velocity of 0.1 m/s to <200 μs at ~6 m/s. For low-velocity (<1 m/s) off-center collisions, mixing times were consistent withmore » the head-on cases. For high-velocity (i.e., > 1 m/s) off-center collisions, mixing times increased by as much as a factor of 6 (e.g., at ~6 m/s, mixing times increased from <200 μs for head-on collisions to ~1200 μs for highly off-center collisions). At collision velocities >7 m/s, droplet separation and fragmentation occurred, resulting in incomplete mixing. These results suggest a limited range of collision velocities over which complete and rapid mixing can be achieved when using airborne merged microdroplets to, e.g., study reaction kinetics when reaction times are short relative to typical bulk reactor mixing times. We benchmark our reactor using an aqueous-phase oxidation reaction: iron-catalyzed hydroxyl radical production from hydrogen peroxide (Fenton's reaction) and subsequent aqueous-phase oxidation of organic species in solution. In conclusion, kinetic simulations of our measurements show that quantitative agreement can be obtained using known bulk-phase kinetics for bimolecular reactions in our colliding-droplet microreactor.« less

Authors:

Davis, Ryan D. ^[1]; Jacobs, Michael I.  ^[2];

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• Search DOE PAGES for ORCID "0000-0003-3682-0409"
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Houle, Frances A.  ^[1];

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• Search DOE PAGES for ORCID "0000-0001-5571-2548"
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Wilson, Kevin R.  ^[1]

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• Search DOE PAGES for ORCID "0000-0003-0264-0872"
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+ Show Author Affiliations

1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Chemical Sciences Division
2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Chemical Sciences Division; Univ. of California, Berkeley, CA (United States). Dept. of Chemistry

Publication Date:

2017-10-30

Research Org.:

Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF)

OSTI Identifier:

1456985

Grant/Contract Number:  

AC02-05CH11231

Resource Type:

Accepted Manuscript

Journal Name:

Analytical Chemistry

Additional Journal Information:

Journal Volume: 89; Journal Issue: 22; Journal ID: ISSN 0003-2700

Publisher:

American Chemical Society (ACS)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY