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Order and Chaos: Collective Behavior of Crowded Drops in Microfluidic Systems

ORAL · Invited

Abstract

Droplet microfluidics, in which micro-droplets serve as individual reactors, has enabled a range of high-throughput biochemical processes. Unlike solid wells typically used in current biochemical assays, droplets are subject to instability and can break especially at fast flow conditions. Although the physics of single drops has been studied extensively, the flow of crowded drops or concentrated emulsions—where droplet volume fraction exceeds ~80%—is relatively unexplored in microfluidics.

 

This talk examines the collective behavior of drops in a concentrated emulsion by tracking the dynamics and the fate of individual drops within the emulsion. At the fast flow limit, we show that droplet breakup within the emulsion is stochastic, in stark contrast to the deterministic behavior in classical single-drop studies. In addition to capillary number and viscosity ratio, break-up probability is governed by a confinement factor which measures drop size relative to a characteristic channel length. The stochasticity arises from the time-varying packing configuration of the drops. Replacing surfactant by nanoparticles as droplet stabilizers suppresses break-up, and increases the throughput of droplet processing by >300%. Finally, strategic placement of an obstacle suppresses break-up by >103 fold.

 

At the slow flow limit of the concentrated emulsion, we observe an unexpected order, where the velocity of individual drops in the emulsion exhibits spatiotemporal periodicity. Such periodicity is surprising from both fluid and solid mechanics point of view. We show the phenomenon can be explained by treating the emulsion as a soft crystal undergoing plasticity, in a nanoscale system comprising thousands of atoms as modeled by droplets. Our results represent a new type of collective order not described before, and have practical use in on-chip droplet manipulation.

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Publication: 1. Alison D. Bick, Jian Wei Khor, Ya Gai, and Sindy K. Y. Tang*, "Strategic placement of an obstacle suppresses droplet break-up in the hopper flow of a microfluidic soft crystal", Proceedings of the National Academy of Sciences, 118, e2017822118, 2021.<br>2. Simon Schutz, Jian Wei Khor, Sindy K.Y. Tang, and Tobias Schneider, "Interaction and breakup of droplet pairs in a microchannel Y-junction", Physical Review Fluids, 5, 083605, 2020.<br>3. Alison Bick, and Sindy K. Y. Tang*, "Effect of Volume Fraction on Droplet Break-up in an Emulsion flowing through a Microfluidic Constriction", Applied Physics Letters, 115, 093702, 2019.<br>4. Ya Gai, Alison Bick, and Sindy K.Y. Tang*, "Timescale and spatial distribution of local plastic events in a two-dimensional microfluidic crystal", Physical Review Fluids, 4, 014201, 2019.<br>5. Jian Wei Khor, Neal Jean, Eric S. Luxenberg, Stefano Ermon, and Sindy K.Y. Tang*, "Using Machine Learning to Discover Shape Descriptors for Predicting Emulsion Stability", Soft Matter, 15, 1361-1372, 2019.<br>6. Chia M. Leong, Ya Gai, and Sindy K.Y. Tang*, "Internal flow inside droplets within a concentrated emulsion during droplet rearrangement", Physics of Fluids, 30, 032002, 2018. <br>7. Jian Wei Khor, Minkyu Kim, Simon Schutz, Tobias Schneider, and Sindy K.Y. Tang*, "Time-varying droplet configuration determines break-up probability of drops within a concentrated emulsion", Applied Physics Letters, 111, 124102, 2017. <br>8. Ya Gai†, Minkyu Kim†, Ming Pan, and Sindy K.Y. Tang*, "Amphiphilic nanoparticles suppress droplet break-up in a concentrated emulsion flowing through a narrow constriction", Biomicrofluidics, 11, 034117, 2017. <br>9. Chia Leong, Ya Gai, and Sindy K.Y. Tang*, "Internal flow in droplets within a concentrated emulsion flowing in a microchannel", Physics of Fluids, 28, 112001, 2016. <br>10. Ya Gai, Chia Leong, Wei Cai, and Sindy K. Y. Tang*, "Spatiotemporal periodicity of dislocation dynamics in a two-dimensional microfluidic crystal flowing in a tapered channel", Proceedings of the National Academy of Sciences, 113, 12082-12087, 2016 <br>11. Ya Gai, Jian Wei Khor, and Sindy K. Y. Tang*, "Confinement and viscosity ratio effect on droplet break-up in a concentrated emulsion flowing through a narrow constriction", Lab on a Chip, 16, 3058 - 3064, 2016. <br>12. Minkyu Kim, Ming Pan, Ya Gai, Shuo Pang, Chao Han, Changhuei Yang, and Sindy K.Y. Tang*, "Optofluidic ultrahigh-throughput detection of fluorescent drops", Lab on a Chip, 15, 1417-1423, 2015.<br>13. Liat Rosenfeld, Tiras Lin, Ratmir Derda, and Sindy K.Y. Tang, "Review and Analysis of Performance Metrics of Droplet Microfluidics Systems," Microfluidics and Nanofluidics, 16, 5, 921-939, 2014.<br>14. Liat Rosenfeld, Lin Fan, Yunhan Chen, Ryan Swoboda, and Sindy K.Y. Tang, "Break-up of Droplets in a Concentrated Emulsion Flowing through a Narrow Constriction," Soft Matter, 10, 421-430, 2014.<br>

Presenters

  • Sindy Tang

    Stanford University

Authors

  • Sindy Tang

    Stanford University